CONCRETE PIPE AND PORTAL CULVERT HANDBOOK
PIPES, INFRASTRUCTURAL PRODUCTS AND ENGINEERING SOLUTIONS DIVISION
PREFACE TO 2006 REVISION
Concrete pipes and portal culverts are the most frequently used and accepted products for stormwater drainage, culverts, outfall sewers and many other applications. To meet these needs South Africa’s concrete pipe industry has grown tremendously over the past eighty years. Modern technology and the acceptance of SANS (SABS) standards ensure that products with consistently high quality are produced. Provided sound design and installation methods are followed, these products will give the desired hydraulic and structural performance over a long service life. This handbook is intended to cover all aspects of concrete pipe and portal culvert selection, specification, and testing. As a handbook it does not attempt to replace textbooks or codes, but rather to complement them by providing the information needed for quick site decisions and guidance for designers to ensure that all aspects of product use are considered. A companion publication ‘The Concrete Pipe and Portal Culvert Installation Manual‘ deals with product installation. Publications by the American Concrete Pipe Association have been used freely and acknowledgement is hereby made to this organisation. The Concrete Pipe, Infrastructural Products and Engineering Solutions (PIPES) Division of the Concrete Manufacturers Association has had this handbook prepared for the guidance of specifying bodies, consultants and contracting organisations using concrete pipes and portal culverts manufactured in accordance with the relevant SANS (SABS) standards. The Division expresses appreciation to A.R. Dutton & Partners for the preparation of the original Concrete Pipe Handbook to which additions and amendments have been made to produce this publication.
P roduced by: P IP E S cc P O B ox 12519 C lubview 0014
TABLE OF CONTENTS 1. INT R ODUC T ION .................................................................................................................... 1 1.1. OB J E C T IV E ........................................................................................................................ 1 1.2. S C OP E ............................................................................................................................... 1 2. P R ODUC T C LAS S IF IC AT ION ................................................................................................ 1 2.1. S T ANDAR DS ...................................................................................................................... 1 2.2. C ONC R E T E P IP E S ............................................................................................................ 2 2.3. P OR T AL C ULV E R T S ......................................................................................................... 4 2.4. MANHOLE S ........................................................................................................................ 6 3. HY DR AULIC S ........................................................................................................................ 7 3.1. C ONDUIT C LAS S IF IC AT ION ............................................................................................. 7 3.2. HY DR AULIC LE NG T H........................................................................................................ 8 3.3. P R E S S UR E P IP E LINE S ..................................................................................................... 9 3.4. S E W E R S AND S T OR MW AT E R OUT F ALLS .................................................................... 10 3.5. HY DR AULIC S OF S T OR MW ATE R C ULV E R T S .............................................................. 11 3.6. P OR OUS P IP E S ............................................................................................................... 17 4. LOADS ON B UR IE D P IP E LINE S ......................................................................................... 19 4.1. INT R ODUC T ION .............................................................................................................. 19 4.2. E AR T H LOADS ................................................................................................................ 19 4.3. T R AF F IC LOADING .......................................................................................................... 25 5. C ONC R E T E P IP E S T R E NG T HS .......................................................................................... 28 5.1. E X T E R NAL LOADS .......................................................................................................... 28 5.2. INT E R NAL P R E S S UR E .................................................................................................... 29 5.3. S AF E T Y F AC TOR S .......................................................................................................... 29 5.4. S E LE C T ION OF T HE C ONC R E TE P IP E C LAS S ............................................................. 29 6. B E DDING ............................................................................................................................. 32 6.1. G E NE R AL ........................................................................................................................ 32 6.2. T R E NC H AND NE G AT IV E P R OJ E C T ION INS T ALLAT IONS ........................................... 33 6.3. P OS IT IV E P R OJ E C T ION INS T ALLAT IONS ..................................................................... 36 6.4. S OILC R E T E B E DDING .................................................................................................... 38 6.5. J AC K ING C ONDIT IONS ................................................................................................... 38 7. P IP E J OINT ING .................................................................................................................... 39 7.1. J OINT T Y P E S ................................................................................................................... 39 7.2. B UT T AND INT E R LOC K ING J OINT P IP E S ...................................................................... 39 7.3. S P IG OT AND S OC K E T J OINT S ....................................................................................... 39 7.4. IN-T HE -W ALL J OINT S ..................................................................................................... 40 8. F LOATAT ION ....................................................................................................................... 41 8.1. G E NE R AL ........................................................................................................................ 41 8.2. F LOATAT ION B E F OR E B AC K F ILLING ............................................................................ 41 8.3. F LOATAT ION AF T E R B AC K F ILLING ............................................................................... 41 9. S E W E R C OR R OS ION.......................................................................................................... 42 9.1. C OR R OS ION ME C HANIS M ............................................................................................. 42 9.2. C OR R OS ION P R E DIC T ION AND C ONT R OL .................................................................. 43 9.3. DE V E LOP ME NT S IN S OUT H AF R IC A............................................................................. 44 9.4. DE S IG N AND DE TAIL C ONS IDE R AT IONS ..................................................................... 46 9.5. P IP E MAT E R IAL C HOIC E F OR S E W E R S ....................................................................... 47 9.6. S AC R IF IC IAL T HIC K NE S S AND ALLOW AB LE C R AC K W IDT HS ................................... 49 10. P OR T AL C ULV E R T S T R E NG T HS ................................................................................... 51 10.1. G E NE R AL......................................................................................................................... 51 10.2. DE T E R MINING P OR T AL C ULVE R T S T R E NG T HS ......................................................... 51 10.3. P OR T AL B AS E S LAB S ..................................................................................................... 54 11. F IE LD T E S T ING ............................................................................................................... 55 11.1. W ATE R T E S T ................................................................................................................... 55 11.2. AIR T E S T ING ................................................................................................................... 55 11.3. S OIL DE NS IT Y T E S T ....................................................................................................... 56 BIBLIOGRAPHY 59
1. INTRODUCTION 1.1. OBJECTIVE T he purpos e of this handbook is to give the us ers , designers , specifiers and ins tallers of precas t concrete pipe and portal culverts the bas ic guidelines for the correct us e, selection and s pecification of thes e products . A companion publication “T he C oncrete P ipe and P ortal C ulvert Ins tallation Manual” gives details of how thes e products s hould be ins talled.
1.2. SCOPE T he content of this handbook covers the pre-cons truction activities as s ociated with precas t concrete pipe and portal culverts , namely thos e undertaken by the designer of the project. Des criptions are given of the basic theory needed for determining: v product size v product s trength v product durability v s pecial product features T he basic formulae, diagrams and tables s upport this . T his information is adequate for mos t product applications . However, the theory given is by no means rigorous. T he reader is advised to cons ult the relevant textbooks or codes , s hould a detailed analysis be required. A lis t of us eful publications is given at the end of this handbook.
2. PRODUCT CLASSIFICATION 2.1. STANDARDS T here are three groups of s tandards which are applicable to precas t concrete pipe and portal culverts , namely: v C odes of practice that detail how product size, s trength and durability should be s elected. v P roduct s tandards that pres cribe what product requirements have to be met. v C ons truction standards that pres cribe how products s hould be ins talled. T he S outh African B ureau of S tandards (S AB S ) has been res tructured. T he division dealing with the production of s tandards is S tandards S outh Africa (S tanS A). All the previously des ignated S AB S s tandards are to be renamed as S outh African National S tandards (S ANS ) and will retain their numbers . T his document us es the latter. T he divis ion dealing with the is s uing of manufacturing permits and the auditing pf production facilities is G lobal C onformity S ervices (G C S ). T he products covered by this publication comply with the requirements of relevant (S ANS ) document. T hes e are performance s pecifications that detail the properties of the finis hed products needed to ens ure that they are s uitable for their required application. All thes e s tandards have the s ame bas ic layout, namely: v S cope v Normative references v Definitions v Materials us ed v R equirements to be met v S ampling and compliance v Ins pection and tes t methods v Marking
v Normative and informative annexures . Mos t factories operated by the P IP E S Division member companies have approved quality management s ys tems to ens ure that products comply with the relevant S ANS s pecifications . In addition to this G C S , does frequent audits to check that s tandards are being maintained. T hese s tandards are periodically reviewed to ens ure that marketplace requirements are met.
2.2. CONCRETE PIPES 2.2.1.Standards C urrently there are two S outh African national s tandards applicable to concrete pipe: S ANS 676 - R einforced concrete pres s ure pipes S ANS 677 - C oncrete non-pres s ure pipes T he code of practice for the s election of pipe strength is : S ANS 10102 - P art 1: S election of pipes for buried pipelines : G eneral provisions - P art 2: S election of pipes for buried pipelines: R igid pipes T here are no s tandards for determining the size or durability of concrete pipe. If the reader requires more detail than given in this publication, reference s hould be made to the appropriate literature, s ome of which is detailed at the end of this publication. T he s tandards for the ins tallation of concrete pipe are included as s ections in S ANS 1200 S tandardized s pecification for civil engineering cons truction. T hes e s ections are: S ANS 1200 DB - E arthworks (pipe trenches) S ANS 1200 L
- Medium pres s ure pipe lines
S ANS 1200 LB - B edding (pipes ) S ANS 1200 LD - S ewers S ANS 1200 LE – S torm water drainage S ANS 1200 LG - P ipe jacking
2.2.2.Pipe classes Non-pressure pipe P ipes are clas sified in terms of their crushing s trength when s ubjected to a vertical knifeedge tes t-load. T he two alternative crushing load tes t configurations are s hown in F igure 1 (a) & (b).
(a) T wo edge bearing tes t (b) T hree edge bearing tes t FIGURE 1: CRUSHING LOAD TEST CONFIGURATIONS FOR CONCRETE PIPE T he three edge-bearing tes t is preferred as the pipe is firmly held in place by the bottom two bearers before and during the tes t. W ith the two-edge bearing tes t there is the danger that the pipe could slip out of the tes ting apparatus or might not be perfectly s quare when tes ted.
The proof load is defined as the line load that a pipe can sus tain without the development of cracks of width exceeding 0.25 mm or more over a dis tance exceeding 300 mm, in a two or three edge bearing tes t. Non-reinforced pipes are not permitted to crack under their proof load. The ultimate load is defined as the maximum line load that the pipe will s upport in a two or three edge-bearing tes t and s hall be at least 1.25 times the proof load. T he s tandard crushing load s trength designation is the D-load (diameter load). T his is the proof load in kilonewtons per metre of pipe length, per metre of nominal pipe diameter. T he s tandard D-load clas ses with their proof and ultimate loads are given in T able 1. T AB LE 1: S T ANDAR D D-LOAD C LAS S IF IC AT ION F OR C ONC R E T E P IP E S Pipe Class D-Load 25D 50D 75D 100D
Proof load kN/m 25xD 50xD 75xD 100xD
Ultimate loadkN/m 31.25xD 62.50xD 93.75xD 125.00xD
Example F or a 1050 mm diameter 75D pipe proof load = 1.05 x 75 = 78.75 kN/m ultimate load = 1.05 x 93.75 = 98.44 kN/m
P ipes made in accordance to S ANS 677 are divided into two types , v S C pipes for s tormwater and culvert applications v S I pipes for s ewer and irrigation applications . S C pipes are used in applications where there is no internal pres s ure. A s mall s ample (p2%) of pipes is s ubjected to the crus hing s trength tes t to prove that they meet the s trength required. S I P ipes , on the other hand, are us ed in applications where there could be internal pres s ure under certain conditions (as when blockages occur). T o ens ure that the pipes will meet this pos sible condition and ens ure that the joints are watertight, a s mall s ample of pipes is hydros tatically tes ted to a pres s ure of 140 kilopas cals in addition to the crus hing s trength tes t. T able 2 gives proof loads of the preferred nominal diameters given in S ANS 676 and 677. T AB LE 2: P R E F E R R E D C ONC R E T E P IP E DIAME T E R S AND P R OOF LOADS IN- K N/M Notes Nominal Pipe D Loads in Kilonewtons/m Diameter-mm 1) P ipes with diameters 25D 50D 75D 100D s maller than 300 mm, or 300 15.0 22.5 30.0 larger than 1 800 mm are 375 18.8 28.1 37.5 made at s ome factories. 450 22.5 33.8 45.0 2) S trengths greater than 525 13.1 26.3 39.4 52.5 100D can be produced to 600 15.0 30.0 45.0 60.0 order. 675 16.9 33.8 50.6 67.5 3) Mos t pipes are made in moulds with fixed outside 750 18.3 37.5 56.3 75.0 diameters . T he designer 825 20.6 41.3 62.0 82.5 s hould check minimum 900 22.5 45.0 67.5 90.0 the internal diameters to 1 050 26.3 52.5 78.8 105.0 ens ure that requirements 1 200 30.0 60.0 90.0 120.0 are met. 1 350 33.8 67.5 101.3 135.0 1 500 37.5 75.0 112.5 150.0 1 800 45.0 90.0 135.0 180.0
Pressure pipe P res s ure pipes are clas sified in terms of their hydraulic s trength when s ubject to an internal pres s ure tes t under factory conditions. Hydraulic strength is defined as the internal pres s ure in bar that the pipe can withs tand for at leas t 2 minutes without s howing any s ign of leakage. T he standard hydraulic s trength designation is the tes t (T ) pres s ure. T he S ANS 676 pres sure clas s es are given in T able 3. T AB LE 3: S T ANDAR D P R E S S UR E C LAS S E S F OR P IP E Pipe class T2 T4 T6 T8 T 10
Test pressure Bars Kilopascals 2 200 4 400 6 600 8 800 10 1 000
Special-purpose pipe Many pres s ure pipelines are ins talled at a nominal fill and where they are not s ubject to traffic loads . Under these circums tances the hydraulic s trength designation, given in T able 3, is adequate. However, when a pipeline is s ubject to the simultaneous application of internal pres sure and external load, the pipes will need to s us tain a higher hydraulic pres s ure and crus hing s trength than when s ervice loads are applied s eparately. Under thes e conditions the pipes will be clas s ified as s pecial-purpos e pipes and the required hydraulic tes t pres s ure and crushing s trength to meet the required ins talled conditions will have to be calculated. T hes e pipes mus t be s pecified in terms of both their D-load and T -pres s ure values.
2.3. PORTAL CULVERTS 2.3.1.Standards T he s tandard for precas t concrete culverts is S ANS 986, precas t reinforced concrete culverts . T here is no National code of practice for the s election of portal culvert size or s trength. However, the bigges t single group of us ers , the national and provincial road authorities , require that portal culverts under their roads meet the s tructural requirements of T MH7, the C ode of P ractice for the Design of Highway B ridges and C ulverts in S outh Africa. T he local authorities generally adhere to the requirements of this code. T his document als o gives guidelines for product durability. If more detail than provided in this document is required, reference should be made to the appropriate literature, s ome of which is lis ted at the end of this publication. T he s tandards for the installation of precas t portal culverts are included in s ections 1200DB and 1200LE of the S ANS 1200 s eries .
2.3.2.Portal Culvert Classes P recas t portal culverts are clas sified in terms of their crushing s trength, when s ubjected to a combination of loading cas es involving vertical and horizontal knife-edge tes t-loads under factory conditions . T he proof and ultimate loads are defined in the s ame way as for pipes with the ultimate loads being 1.25 times the proof loads for the particular loading configurations .
T he s tandard crus hing s trength des ignation us ed is the S -load. (S pan-crushing load) T his is the vertical component of the proof load in kilonewtons that a 1metre length of culvert will withs tand, divided by the nominal span of the portal culvert in metres . T here are three different loading configurations that are applied to precas t portal culverts to model the ins talled conditions , namely: v Deck bending moment and s way v Deck s hear v Inner leg bending moment and s hear T hes e configurations are shown res pectively in F igure 2(a), (b) and (c) below and the s tandard S -load clas s es with their proof load requirements are given in T able 4. Ph
PV
PS
P hl
(a) Deck bending moment and s way
(b) Deck s hear
(c) Inner leg bending moment &s hear
FIGURE 2: LOAD TEST CONFIGURATIONS FOR PRECAST PORTAL CULVERTS T AB LE 4: S T ANDAR D S -LOAD C LAS S IF IC AT ION F OR P OR T AL C ULV E R T S Proof loads - kN/m of length Vertical Horizontal 75S 75 x S 30 100S 100 x S 30 125S 125 x S 30 150S 150 x S 30 175S 175 x S 30 200S 200 x S 30 Note: S is the nominal s pan in metres .
Culvert class S-Load
Leg Proof loads - kN/m of length Height > S/2 Height = S 0.4 x 75 x S 0.60 x 75 x S 0.3 x 100 x S 0.50 x 100 x S 0.2 x 125 x S 0.45 x 125 x S 0.2 x 150 x S 0.43 x 150 x S 0.2 x 175 x S 0.40 x 175 x S 0.2 x 200 x S 0.40 x 200 x S
T able 5 gives the vertical and horizontal proof loads obtained by applying the clas sification in T able 4 to the preferred portal culvert dimens ions given in S ANS 986. A table s imilar to T able 5 can be obtained by application of the values in T able 4 to obtain the inner leg bending moments and s hears . It s hould be noted that there will be two different values of the horizontal load for each culvert s pan and clas s, i.e. when 0.5 < H/S < 1.0 and H/S = 1.0. W hen H/S < 0.5 no horizontal leg load is required.
T AB LE 5: P R E F E R R E D P OR T AL C ULV E R T DIME NS IONS AND P R OOF LOADS Culvert span mm 450 600 750 900 1200 1500 1800 2100 2400 3000 3600
Vertical proof loads in kN/m of length Culvert class 75S 100S 150S 175S 200S 90.0 120.0 131.3 157.5 180.0 150.0 135.0 157.5 180.0 225.0 270.0 -
Horizontal proof load all classes kN/m
30
2.4. MANHOLES 2.4.1.Standards T he s tandard for precast concrete manhole s ections , slabs , lids and frames is S ANS 1294. T he s tandard manhole dimens ions are hard metric, namely: v 750 mm diameter - us ed as s haft sections v 1 000 mm diameter - normally used as chamber s ections v 1 250 mm diameter - us ed as chamber s ections v 1 500 mm diameter - us ed as chamber s ections v 1 750 mm diameter - us ed as chamber s ections T hes e s ections are available in lengths of 250 mm, 500 mm, 750 mm and 1 000 mm. In the pas t manholes were produced in s oft metric dimens ions. Hence when components have to be replaced it is es s ential that actual details and dimens ions be checked before ordering replacements as old s izes are no longer available and it may be neces s ary to replace the whole manhole. C urrently S ANS 1294 is being revis ed. W hen this s tandard is releas ed, a detailed s ection on manholes will be added to this publication.
3. HYDRAULICS 3.1. CONDUIT CLASSIFICATION C onduits conveying fluids are clas sified by various parameters , namely, whether: v T hey flow as open channels or clos ed conduits v T he flow is uniform, in which cas e the flow depth, velocity and dis charge along the whole length of the conduits are cons tant. If not uniform, the flow is varied v T he flow is s teady in which cas e the flow pas t a given point has a cons tant depth, velocity and dis charge. If not s teady, the flow is uns teady. A pipeline conveying potable water or other fluids generally flows full and operates under pres s ure and the flow is both uniform and s teady. T he total energy in such a s ys tem will have three components , namely conduit height or diameter, velocity head and pres s ure head as s hown in F igure 3. T otal energy line Hydraulic grade line
hf v2 2g
S treamline
hp
P ipe invert
T he total energy at any point along a conduit operating under pres s ure can be defined by B ernoulli’s equation: H = z + d/2 + hp + v2/2g W here z - height of invert above datum in d - conduit height or diameter in m v - velocity in m/s g - gravitational cons tant in m/s /s hp -pres s ure head in pipeline in m hf -energy los s due to friction in m
z
Datum
FIG 3: CONDUIT FLOWING FULL As there is pres s ure in s uch a conduit, the fluid can be carried uphill provided the value of “hp” s tays positive. S uch a s ys tem is clas sified as a pres s ure pipeline. On the other hand, a conduit conveying s tormwater or s ewage generally flows partly full and the flow is frequently both varied and uns teady. T here is an air/fluid interface and therefore, no pres sure component to the total energy as s hown in F igure 4. T otal energy line
W ater surface P ipe invert
hf v2 2g
T he total energy at any point along a conduit flowing partly full can be defined by the E nergy equation: H = y + v2/2g W here y - depth of flow in m v - velocity in m/s g - gravitational cons tant in m/s /s
Datum
FIG 4: CONDUIT FLOWING PARTLY FULL As there is no pres s ure in s uch a conduit, the fluid can only flow downhill and the s ys tem is clas sified as a gravity pipeline.
F igures 3 and 4 s how s ys tems where the pipe invert, hydraulic grade line or water s urface and the total energy line are all parallel. T his is called uniform flow and the only energy los s es are due to friction. However if there are any transitions s uch as changes in vertical or horizontal alignment, or the cros s ectional s hape of the conduit then thes e will also caus e energy los s es due to the liquid expanding or contracting. T he means of determining the hydraulic properties of conduits flowing under pres s ure and thos e flowing partly full, as open channels are unders tandably different. A further factor that needs to be considered is the hydraulic length of the conduit.
3.2. HYDRAULIC LENGTH T he hydraulic length of a conduit is determined by the relations hip between the energy los s es due to friction and those due to transitions . W hen the energy los s es due to friction exceed thos e due to transitions then the conduit is clas sified as hydraulically long. W hen thos e due to transitions exceed thos e due to friction then the conduit is clas sified as hydraulically s hort. In general a pipeline is hydraulically long whereas a culvert cros sing is hydraulically s hort. T he energy los s es due to friction are determined us ing one of the friction formulae, s uch as Manning, to calculate the velocity through the conduit. Manning’s equation is given below: v = 1/n(R ) 2/3S 1/2 where v - velocity n m/s n - Manning’s roughnes s coefficient R - hydraulic radius S - gradient of conduit T he energy los s es due to transitions in a conduit can be determined theoretically by comparing flow areas before and after the trans ition. F or mos t applications the us e of a coefficient as shown in the formula below, is adequate: H L = k(v2/2g) where H L - head los s in metres (m) k - a coefficient, usually between 0.0 and 1.0 dependent upon trans ition details v - velocity in metres per second (m/s ) g - the gravitational constant in metres per s econd per s econd (m/s /s ) C ommonly us ed energy los s coefficients are given in T able 6 below. T AB LE 6:E NE R G Y LOS S C O E F F IC IE NT S F OR P IP E LINE F LOW Entrance or outlet detail Entrance Outlet P rotruding 0.80 1.00 S harp 0.50 1.00 B evelled 0.25 0.50 R ounded 0.05 0.20 T he friction slope of a pipeline that has no transitions is the energy difference between inlet and outlet, divided by the pipeline length. If there are any trans itions in the pipeline, the energy los ses due to the transitions will reduce the amount of energy available to overcome friction.
3.3. PRESSURE PIPELINES T he hydraulic performance (velocity and dis charge) of a pres s ure pipeline is determined by using one of the friction formulas s uch as Manning, in combination with the continuity equation and energy los ses at transitions . T he continuity equation is Q = Av W here Q - dis charge in cubic metres per s econd (m3/s ) A - cros s -s ectional area in square metres (m2 ) v - velocity in metres per second (m/s ) Mos t low-pres s ure pipelines flow under gravity and have no additional energy inputs, i.e. no us e is made of additional energy to lift the water. If pres s ure is added to the pipeline by a pump, the energy is increas ed. An alternative approach to determining the hydraulic properties of a pipeline is to us e a chart for a pipe flowing full as given in F igure 5 and to add any energy inputs or subtract any energy los s es at transitions . If the pipeline is flowing under pres sure the friction s lope s hould be us ed, as this will probably be different from the pipeline gradient that could vary along the length of the pipeline.
FIGURE 5: FLOW CHART FOR CIRCULAR PIPES BASED ON MANNINNG FORMULA
3.4. SEWERS AND STORMWATER OUTFALLS Mos t s ewer and s torm water outfalls consis t of s ections of hydraulically long conduit flowing party full between transitions (manholes ). If the pipeline is flowing partly full then the slope of the energy line and the pipeline gradient will be the s ame. Under thes e circums tances the s ections of pipeline between manholes can be evaluated by us ing the chart for pipes flowing full, F igure 5 and then adjus ting the values using proportional flow as given in F igure 6 that gives the relations hip between the relative depth d/D and the other parameters as hydraulic radius , velocity and dis charge. E xamples of the combined us e of thes e figures are given below F igure 6.
F IG 6: R E LAT IV E F LOW P R OP E R T IE S OF C IR C ULAR P IP E F LOW ING P AR T LY F ULL Example 1:Given a 600 mm internal diameter (D) concrete pipeline at a slope of 1 in 1 000 and a discharge of 120 litres per second (Vs), determine velocity and flow depth. Use n = 0.011. F rom the flow chart intersecting the co-ordinates of diameter (600) and s lope (1 in 1 000) we obtain: Q =240 I/s and V =0,82 m/s T hen Q/Q full = 120/240=0.5 and F igure 6 gives d/D=0.5x600=300 mm and v/vfull =1.0x 0.82 = 0.82 m/s
Example 2: Given a flow of 200 l/s and a slope of 1 m in 2 000 m, determine the diameter of a concrete pipe to flow half full. Use n = 0,011 F rom F igure 6 for d/D = 0.5 ; Q full = Q/0.5 = 200/0.5 = 400 l/s and from F igure 5 for Q = 400 l/s and a s lope of 1 m in 2 000 m, D = 900 mm.
3.5. HYDRAULICS OF STORMWATER CULVERTS T he capacity of hydraulically s hort conduits , such as s tormwater culverts is predominantly dependent upon the inlet and outlet conditions . T hes e conduits s eldom flow full and the energy los s es at inlets and outlets due to s udden transitions far exceed any los s es due to friction. Under these circums tances , the charts for pipes flowing full s hould not be us ed. F or s tormwater culverts the mos t important hydraulic considerations are: v Headwater level at the entrance that will determine ups tream flooding. v R oadway overtopping neces sitating road closure. v Outlet velocity that could caus e downs tream eros ion. T he various factors that will influence the flow through a hydraulically s hort conduit, s uch as a culvert under a road are illus trated in F igure 7 below.
H HW INLE T
D
B AR R E L
S 0, S LOP E
OUT LE T
TW
L FIGURE 7: FACTORS INFLUENCING FLOW THROUGH CULVERTS W here HW - headwater or energy level at inlet in m T W - tailwater or energy level at outlet in m H - total energy los s between inlet and outlet in m D - internal diameter or height of conduit in m L - length of conduit in m S0 - culvert gradient in m/m T here are s everal different types of culvert flow, depending on whether the control is located at the inlet, along the barrel or at the outlet. Inlet control occurs when the inlet s ize, s hape and configuration controls the volume of water that can enter the culvert. In other words when the capacity of the inlet is les s than the capacity of the barrel and there is a free dis charge downs tream of the culvert.
HW
TW
HW
TW
(a) uns ubmerged inlet (b) s ubmerged inlet FIGURE 8: INLET CONTROL CONDITION AND VARIATIONS T his happens when the slope of the culvert is s teeper than the critical slope. W hen the conduit flows with an uns ubmerged inlet, the flow pas s es through critical depth at the entrance to the culvert. W hen the culvert flows with a s ubmerged inlet, which will occur when HW /D > 1.5, the inlet will act as an orifice and the flow will contracted as if flowing through a sluice gate. T he major energy los s will be at the culvert inlet. T he total energy through the culvert and the outlet velocity can be calculated from the critical or contracted depth at the entrance.
Barrel control occurs when the barrel s ize, roughnes s and s hape controls the volume of water that which can flow through the culvert. In other words when the capacity of the barrel is les s than the capacity of the inlet and the dis charge downs tream of it is free. T his happens when the s lope of the culvert is flatter than critical s lope and the cons triction at the entrance is drowned out by the flow through the barrel. T he major energy los s will be at the outlet. T he water s urface will pas s through critical depth at the outlet and the outlet energy level and velocity can be calculated from this , as des cribed below.
HW
H
HW
H TW
TW
(a) Uns ubmerged inlet (b) S ubmerged inlet FIGURE 9: BARREL CONTROL CONDITION AND VARIATIONS Outlet control occurs when the water level downs tream of the culvert controls the volume of water that can flow through the culvert by drowning out either inlet or barrel control conditions . In other words when the capacity of the barrel or the inlet cannot be realised becaus e there is no free dis charge downs tream of the culvert.
H HW
H TW
HW
TW
(a) Uns ubmerged inlet (b) S ubmerged inlet FIGURE 10: OUTLET CONTROL CONDITION AND VARIATIONS T he water s urface will not pas s through critical depth at any section of the culvert hence there are no s ections where there is a fixed depth dis charge relations hip. T he major energy los s will be at the outlet. T he capacity and headwater depths for the different types of culvert flow can be determined by calculation or from nomographs .
3.5.1.Capacity and Headwater Depth for Hydraulically Short Conduits W hen gradients are steep and the flow of water at the outlet of the pipe is partially full, the control will be at the inlet. In other words , more water can flow through the culvert than into it. T he capacity and headwater levels for a circular concrete pipe culvert operating under inlet control can be determined using the nomograph given in F igure 11. W hen gradients are very flat or the outlet of the culvert is s ubmerged, the control will be either through the barrel or at the outlet. In other words , more water can flow through the entrance to the culvert than through the barrel. T he capacity and headwater levels for a circular concrete pipe culvert operating with either barrel or outlet control can be determined using the nomograph given in F igure 12. However, the outlet velocity for the flow through culverts needs to be calculated. T he capacity and headwater levels for a rectangular concrete culvert operating under inlet control can be determined using the nomograph given in F igure 13 and that for a rectangular concrete culvert operating with outlet control is given in F igure 14.
FIGURE 11: HEADWATER DEPTH: CONCRETE PIPE CULVERTS: INLET CONTROL
FIGURE 12: HEADWATER DEPTH: CONCRETE PIPE CULVERTS: OUTLET CONTROL
FIGURE 13: HEADWATER DEPTH: RECTANGULAR CULVERTS: INLET CONTROL
FIGURE 14: HEADWATER DEPTH: RECTANGULAR CULVERTS: OUTLET CONTROL
3.5.2.Outlet Velocity for Hydraulically Short Conduits Outlet velocity is seldom calculated for culverts , yet it is this that caus es downs tream erosion and was h-a-ways that can res ult in recurring maintenance cos ts . T he exact calculation of outlet velocities is difficult. However, cons ervative es timates can be made us ing the procedures that follow. F or culverts flowing with inlet or barrel control, the outlet velocity can be calculated by identifying the control point at the entrance or outlet where the depth dis charge relations hip is fixed. F or a culvert of any cross -s ectional s lope, the critical depth will occur when Q 2T / gA 3 = 1 W here: Q - dis charge in m3/s T - flow width in m G - gravitational cons tant in meters /s econd per s econd (m/s /s ) A - flow area in m2 F or a rectangular section this reduces to dc = vc 2 / g W here: dc - the critical depth in m vc - the critical velocity in m/s T here is no s imple equation for the relationship between critical depth and velocity in a circular pipe. However, the use of the above equation will over es timate the velocity by about 10%. Hence, it will be adequate for mos t s tormwater drainage applications . F or the inlet control condition with an unsubmerged inlet, the outlet velocity can be calculated from the critical energy level at the inlet to the culvert. If the inlet is s ubmerged, the outlet velocity can be calculated from the energy level at the inlet, which is obtained by s ubtracting the inlet energy los s from the headwater depth. T his is calculated us ing the relevant coefficient from T able 6. F or the barrel control condition, the flow will pas s through critical depth at the outlet and the outlet velocity can be calculated from this . F or the outlet control condition, outlet velocity s hould not be a problem as it is the downs tream conditions that drown the flow through the culvert. If the outlet is not s ubmerged, the outlet velocity can be calculated by as s uming that the flow depth is the average of the critical depth and the culvert height in diameter. If the outlet is s ubmerged, the outlet velocity will be the dis charge divided by culvert area.
3.6. POROUS PIPES P orous pipes are us ed as a means of s ubs oil drainage and have the following applications : v S ubs urface drainage under roads and railways where the pres ence of seepage water from a high water table would be detrimental to the foundations of the road or railway v Under res ervoirs and other water retaining s tructures where the effects of leaks and uplift can be minimis ed and controlled by s ubsoil drainage v Under large areas s uch as parks , airports and agricultural holdings , where the s ubs oil mus t be well drained. Des igning a s ubs oil drainage s ys tem is bas ed on the s ame hydraulic principles as normally us ed for determining pipe sizes . T he primary problem is determining the flow, which is dependent on soil characteris tics , the area to be drained and rainfall. T he flow in the s ubs oil drainage s ys tem will depend on the judgement of the des igner. T able 7 below gives some guidelines .
T AB LE 7:AP P R OXIMAT E F LOW LIT R E S /S E C P E R HE C T AR E :V AR IOUS C ONDIT IONS Soil Type
Rainfall per annum – mm 750 – 1000 1000 – 1200
<750 C lays Loams S andy s oils
0.45 0.60 0.85
0.55 0.80 1.10
>1200
0.75 1.00 1.50
1.20 1.70 2.40
T he optimum s pacing and depth of a s ubs oil drain is largely dependent on the type of s oil. W here large areas are to be drained T able 8, that gives the capacity of porous pipes and T able 9, that gives a guide to s pacing in metres for various s oils and drain ins tallation depths can be us ed to es timate the size and spacing of pipes for a s ubs oil drainage s ys tem. T AB LE 8: F LOW C AP AC IT Y OF P OR OUS P IP E S IN LIT R E S P E R S E C OND Internal diameter (mm) 100 150 200 300
0.001
Slope of pipe in m/m 0.005 0.01 0.05
0.10
1.2 3.6 8.3 25.8
2.7 8.1 18.3 57.8
12.2 36.4 82.8 258.3
3.9 11.4 26.1 81.9
8.6 25.8 58.9 183.3
Although a slope of 0.001 is theoretically pos sible, slopes of les s than 0.005 are not practical. T he s pacing of drains , not hydraulic cons iderations , normally controls the des ign of a s ys tem. T AB LE 9: P OR OUS P IP E S P AC ING IN ME T R E S F OR DIF F E R E NT S OIL T Y P E S Pipe depth in m
Clays
Loams
Sandy clay
0.6 – 0.9 0.9 – 1.2
7 – 10 9 – 12
10 – 12 12 - 15
12 – 25 25 – 30
Although the tables only indicate sizes up to 300 mm in diameter, larger s izes may be available from certain pipe manufacturers . As there is no S outh African s tandard for thes e pipes the porosity s tandards from B S 1194, as given in T able 10 are us ed. T he manufacturers s hould be as ked for details of the crus hing s trengths for porous pipes . T AB LE 10: P OR OS IT Y V ALUE S IN LIT R E P E R S E C P E R ME T R E OF P IP E LE NG T H Pipe diameter in mm
100
150
200
300
P oros ity litre per s ec per metre length
1.0
2.0
2.5
5.0
4. LOADS ON BURIED PIPELINES 4.1. INTRODUCTION E very buried pipeline is s ubjected to loads that caus e s tres s es in the pipe wall. T hes e loads can be broadly defined as primary loads and s econdary loads . P rimary loads can be calculated and include : v mas s of earth fill above pipe v traffic loading v internal pres s ure loading. Other primary loads are pipe and water mas s es that can be ignored, except in critical s ituations . S econdary loads are not eas y to calculation as they are variable, unpredictable and localis ed. T hey can however caus e considerable damage to a pipeline due to differential movements between pipes . It is therefore es s ential that their potential impact be recognis ed and that where neces s ary that precautions are taken. E xamples of factors that could caus e s econdary loads are: v V olume changes in clay s oils due to variations in mois ture content v P res s ures due to growth of tree roots v F oundation and bedding behaving unexpectedly v S ettlement of embankment foundation v E longation of pipeline under deep fills v E ffects of thermal and mois ture changes on pipe materials and joints v E ffects of mois ture changes and movements on bedding v R es traints caused by bends , manholes etc. It is preferable to avoid or eliminate the caus es of thes e loads rather than attempt to resis t them. W here this is not pos sible, particular attention mus t be paid to pipe joints and the interfaces between the pipeline and other s tructures, s uch as manholes to ens ure that there is s ufficient flexibility. T he reader is referred to the section of this handbook dealing with joints . W here pipelines operate in expos ed conditions s uch as on pipe bridges or above ground, the pipes will be s ubject to thermal s tres s es and longitudinal movement. T he thermal s tres s es are caus ed by temperature differences between the inside and outside of the pipe that alternate between night and day res ulting in the pipe walls cracking due to cyclical s trains . T his is generally not a problem when the pipe walls are les s than 100mm thick. T he longitudinal movement is caused by the expansion and contraction of the pipeline due to temperature changes. T he design of the pipe and pipeline for s uch conditions s hould be dis cus s ed with a competent manufacturer or s pecialis t cons ultant s o that the neces sary precautions can be taken to cope with these effects and ens ure that the pipeline will operate s atis factorily. T hes e are beyond the s cope of this handbook.
4.2. EARTH LOADS T he calculation of earth loads on a buried conduit from firs t principles is complex. F or a thorough unders tanding, reference s hould be made to the s pecialis t literature and S ANS 10102 P arts 1 and 2. T he prime factors in es tablis hing earth loads on buried conduits are: v ins tallation method v fill height over conduit v backfill density v trench width or external conduit width
T o us e the tables in this handbook, it is neces s ary to unders tand the various methods of ins talling buried conduits . T he two bas ic ins tallation types and the corresponding loading conditions are the trench and the embankment conditions . T hes e are defined by whether the frictional forces developed between the column of earth on top of the conduit and thos e adjacent to it reduce or increas e the load that the conduit has to carry. A us eful concept is that of the geos tatic or pris m load. T his is the mas s of earth directly above the conduit as s uming that there is no friction between this column of material and the columns of earth either s ide of the conduit. T he geos tatic load will have a value between that of the trench and embankment condition. T hes e loading conditions are illus trated in F igure 13 below.
F riction acts upwards reducing load
F riction zero
F riction acts downwards increasing load
FIGURE 15: COMPARISON OF TRENCH, GEOSTATIC AND EMBANKMENT LOADING
4.2.1.Trench condition T he trench condition occurs when the conduit is placed in a trench that has been excavated into the undis turbed s oil. W ith a trench ins tallation the frictional forces that develop between the column of earth in the trench and the trench walls act upwards and reduce the load that the conduit has to carry. As a res ult the load on the conduit will be les s than the mas s of the material in the trench above it. T he load on the conduit is calculated from the formula: W = C t w B t2 W here: W - load of fill material in kN/m w - unit load of fill material in kN/m3 B t - trench width on top of conduit in m C t - coefficient that is function of fill material, trench width and fill height T he formula indicates the importance of the trench width B t that s hould always be kept to a practical minimum. As the trench width is increas ed s o is the load on the conduit. At a certain stage the trench walls are s o far away from the conduit that they no longer help it carry the load. T he load on the conduit will then be the same as the embankment load. If the trench width exceeds this value the load will not increas e any more. T his limiting value of B t at which no further load is transmitted to the conduit, is called the transition width. T he determination of the transition width is covered in the s pecialis t literature. It is s afe to as s ume that any trench width that gives loads in exces s of thos e given by the embankment condition exceeds the transition width. E arth loads due to trench loading on circular pipe where the trench widths and nominal pipe diameters are s pecified are given in T able 11. E arth loads due to trench loading on conduits where the trench widths are s pecified but the conduit dimens ions are not are given in T able 12. T AB LE 11: T R E NC H LOADS ON C IR C ULAR P IP E IN K N/M; NON-C OHE S IV E S OIL (G R OUP NO 1 S ANS 10102 P AR T 1); T R E NC H W IDT HS S ANS 1200 DB .
Diameter mm 225 300 375 450 525 600 675 750 825 900 1050 1200 1350 1500 1650 1800 Notes
Trench width m 0.859 0.945 1.031 1.118 1.204 1.290 1.376 1.663 1.749 1.835 2.208 2.380 2.620 2.800 2.980 3.360
Height of backfill above top of pipe in metres 0.6 9 10 11 13 14 15 16 19 20 21 26 28 31 33 35 39
1.0 15 17 18 20 22 23 25 31 32 34 42 45 50 53 57 65
1.5 21 23 26 28 31 33 36 44 47 50 61 66 73 78 84 95
2.0 26 29 32 36 39 42 46 57 61 64 79 86 95 102 109 125
2.5 30 34 38 42 47 51 55 69 73 77 96 104 116 125 134 153
3.0 34 39 43 48 53 58 63 80 85 90 112 122 136 147 157 180
3.5 37 42 48 54 59 65 70 90 95 101 127 138 155 167 180 206
4.0 40 46 52 58 64 71 77 99 105 112 141 154 173 187 201 231
5.0 44 51 59 66 74 81 89 115 123 131 167 183 207 224 242 279
6.0 48 56 64 72 81 90 99 129 139 148 190 209 237 258 278 323
7.0 50 59 68 77 87 97 107 141 152 163 210 233 264 288 312 363
1) F or nominal pipe diameters b 1200mm the external diameter has been taken as 1.15 times the nominal diameter; for larger sizes 1.2 times the nominal diameter. 1. T able 11 for non-cohesive s oil; gravel or s and; density = 20 kN/m3 and K µ = 0,19. 2. T he table is bas ed on the trench widths recommended in S ANS 1200DB . 3. If the s oil unit weight is known, the loads from the table may be adjus ted as follows : Load on pipe = load from table x unit weight of s oil / 20 4. T his P rocedure valid only if the s oil properties other than unit weight do not change. T AB LE 12: LOADS ON ANY C ONDUIT IN K N/M F OR G IV E N T R E NC H W IDT HS Trench Height of Backfill above top of pipe in metres Width 0.6 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0 7.0 in m 0.75 8 13 18 22 25 28 30 32 36 38 39 1.00 11 18 25 31 37 42 46 50 56 61 64 1.25 14 23 32 41 49 56 62 68 78 86 92 1.50 17 28 40 51 61 70 79 87 100 112 122 2.00 23 38 55 70 85 99 112 125 147 167 184 2.50 29 47 69 90 110 129 147 164 195 223 249 3.00 35 57 84 110 135 159 181 203 243 281 315 3.50 41 67 99 130 160 188 216 242 292 339 382 4.00 47 77 114 150 185 218 250 282 342 397 450 5.00 59 97 144 190 234 278 320 361 440 515 587 Note that T able 12 is for the s ame ins tallation conditions s oil properties us ed in T able 11.
4.2.2.Embankment condition In this condition the conduit is ins talled at ground level and is covered with fill material. All the earth s urrounding the conduit is homogeneous and the compaction is uniform. W ith
an embankment ins tallation the frictional forces that develop between the column of earth directly above the conduit and the columns of earth adjacent to the conduit, act downwards and increas e the load that the conduit has to carry. T he load on the conduit will be greater than the mas s of the material directly above it due to the frictional forces that develop. In addition the founding material under the conduit could yield and partly reduce the load that it has to carry. T he load on a conduit is calculated from the formula: W = w C e Bc 2 W here W - load on pipe in kN/m w - unit load on fill material in kN/m3 B c - overall diameter of pipe C e-coefficient that is function of fill material, conduit outside width, fill height, projection ratio, and founding conditions T he projection ratio is a meas ure of the proportion of the conduit over which lateral earth pres s ure is effective. It is calculated from p = x / B c , where x -height that conduit projects above or below the natural ground level T he s ettlement ratio, designated as rs , is a meas ure of the amount that the founding material under the conduit s ettles . V alues of this parameter are given in table 13 below. T AB LE 13: V ALUE S OF S E T T LE ME NT R AT IO Material type S ettlement ratio, rs
Rock or 1.0
Unyielding soil 1.0
Normal soil 0.7
Yielding soil 0.3
T he various types of embankment condition, illus trated in F igure 16 are: v P os itive projection where top of the conduit projects above the natural ground level. v Zero projection where the top of conduit is level with natural ground. T he load on the pipe is the geos tatic load. T his als o applies if the s ide fill to a s ub-trench is compacted to the s ame dens ity as the undis turbed soil in which the trench has been dug. v Negative projection where top of the conduit is below the natural ground level. As the trench depth increas es , this condition approaches a complete trench condition.
H
H
H x
x
(a) P ositive projection
BC
(b) Zero projection
(c) Negative projection
FIGURE 16:TYPES OF EMBANKMENT INSTALLATION. E arth loads due to embankment loading on circular pipes are given in T able 14 below.
T AB LE 14: P OS IT IV E P R OJ E C T ION E MB ANK ME NT LOADING IN K N/M ON A B UR IE D C ONDUIT ; NON-C OHE S IV E MAT E R IAL; DE NS IT Y 20 K N/M3, K M = 0.19; P R S = 0.7 Diameter mm
0.6
1.0
Height of backfill above top of pipe in metres 1.5 2.0 2.5 3.0 3.5 4.0 5.0
6.0
7.0
225 5 9 13 17 22 26 31 35 44 52 61 300 6 12 17 23 29 35 41 47 58 70 82 375 7 14 22 29 36 44 51 58 73 87 102 450 8 15 26 35 44 52 61 70 87 105 122 525 9 17 30 41 51 61 71 82 102 122 143 600 10 18 32 47 58 70 82 93 117 140 163 675 11 20 35 52 66 79 92 105 131 157 184 750 12 22 37 56 73 87 102 117 146 175 204 825 13 23 39 59 80 96 112 128 160 192 224 900 14 25 42 61 85 105 122 140 175 210 245 1050 16 28 46 68 92 121 143 163 204 245 286 1200 18 32 51 74 100 129 163 187 233 280 327 1350 21 37 58 83 111 142 177 216 274 329 383 1500 23 40 64 90 119 151 187 228 304 365 426 1650 25 44 69 97 127 161 199 240 335 402 468 1800 27 47 74 104 136 171 210 252 348 438 511 Notes : 1) T able 14 compiled for non-cohesive material with density of 20 kN/m3 and prs = 1.0 2) T able can be us ed for other s oil densities by multiplying load by actual density /20 3) T able can be us ed for different values of prs as follows : (a) If load value falls in s haded area, it may be us ed irres pective of the prs value. (b) If load value to the right of s haded area, multiply the value by following factors : Prs F actor
1.0 1.00
0.7 0.94
0.5 0.90
0.3 0.83
0.1 0.74
Example 1. Determination of backfill load under the following conditions: Embankment installation, positive projection. Pipe D = 525 mm; Projection ratio: x/D = 0.7; Foundation material: rock (rs = 1); Density of fill: 1 750 kg/m3; Height of fill above top of pipe: 3.5 m. prs = 0,7 *1 = 0.7; T able 14 applicable with correction for density only. F or D = 525 mm and height = 3.5 m, Load on pipe = 68.0 kN/m. Applying density correction, the actual load on pipe, W = 68(1750/2000) = 59.5 kN/m. Example 2Determination of backfill load under the following conditions: Embankment installation, positive projection; Pipe D = 750 mm; Projection ratio = 0.70; Foundation material: ordinary soil: (rs = 0.7); Density of fill: 1 600 kg/m3; Height of fill above top of pipe = 2.5 m; prs = 0.7 x 0.7 = 0.49 (say 0.5) F rom T able 14 for D = 750 mm and height = 2.5; Load on pipe = 67 kN/m; Applying dens ity correction, W = 67(1600/2000) = 53.6 kN/m. S ince prs = 0.5 and the value of load falls to the right of the heavy line, actual load on pipe is : W = 53.6 x 0.95 = 50.9 kN/m
4.2.3.Induced Trench Installation T he induced trench ins tallation is a s pecial technique used to increas e the height of the fill that can be carried by s tandard s trength conduits under very high embankments (s ee F igure 15(a)). T he procedure followed is to: v Ins tall the conduit as normally done in an embankment ins tallation v B ackfill over it to the required height v Dig a trench of the s ame width as the outside dimens ion of the conduit down to p 300mm from the top of the conduit v F ill the s ub-trench with a compres s ible material as s traw or s awdus t v C omplete backfilling up to formation level as for a s tandard embankment ins tallation. T he yielding material in the s ub-trench s ettles and thus produces frictional forces that reduce the load on the conduit. T he deeper the s ub-trench the higher the frictional forces developed and hence the greater the reduction in load to be carried by the conduit. Under very high fills , where s tandard pipe/bedding clas s combinations or portal culvert clas s es are inadequate to cope with the earth loads s tandard product clas s es are us ed and the s ub-trench depth is adjus ted to reduce the load to the required value. An important fact to appreciate with this type of ins tallation is that the s ettlement in the s ubtrench mus t not be s o great that the top of the formation s ettles . In other words there mus t be s ufficient fill over the conduit to allow a plain of equal s ettlement to form below the top of the formation. Details of this are s hown in F igure 17(a) below H
x
R eduction in load due to friction between the columns of backfill and compres s ible material C ompres s ible material in s ub-trench
(a) Induced trench (a) Induced trench
R eduction in load due to friction and cohes ion between columns of original material G rout between pipe and tunnel (b) J acked ins tallation (b) J acked
FIGURE 17: SPECIAL INSTALLATIONS T he procedure for calculating the depth of s ub-trench is given in S ANS 10102 P art I. T he des igner should not us e this procedure without firs t doing a detailed s tudy.
4.2.4.Jacked Installation W hen conduits are to be placed under exis ting roadways , railways or other areas that are already developed trench digging can be extremely dis ruptive and the indirect cos ts enormous . An alternative to this is the jacking ins tallation technique. W hen a conduit is jacked the mas s of the earth above the pipe is reduced by both friction and cohesion that develop between the columns of earth directly on top of the conduit and those columns of earth either side of it.
T his technique involves : v v v v
E xcavating a pit at the begging and end of the propos ed line. C ons tructing a launching pad in the entry pit P us hing a jacking s hield against the face of the pit T unnelling through the s oil while being protected by the jacking s hield by making an excavation slightly larger than the s hield jus t ahead of it v P us hing conduits into the tunnel as it progres ses v G routing the s pace left between the outside of the conduit and the tunnel. W ith a jacked ins tallation the vertical load on the conduits will be significantly les s than that experienced in a trench ins tallation. T his is becaus e the load is dependant on the outs ide dimens ion of the conduit and not the trench width and as the s oil above the conduits is undis turbed the load is reduced by both cohesion and friction. Once the fill height over the conduit exceeds about 10 times its outs ide width full arching will take place and no matter how much higher the fill there will be no further increas e in the load that the conduit has to carry.
4.3. TRAFFIC LOADING W here conduits are to be ins talled under trafficked ways details of the vehicles using them s hould be determined in terms of: v Axle s pacing and loads v W heel spacing, loads and contact areas T he type of riding s urface and height of fill over the conduits s hould als o be determined. Mos t concrete pipes and portals that are subject to live loads are thos e us ed under roads. In this handbook two types of design vehicle have been considered, namely a typical highway vehicle that has two s ets of tandem axles and the NB 36 vehicle, as sociated with abnormal loads on national highways (as des cribed in T MH7). As the typical highway vehicle may be overloaded or involved in an accident it is not s uitable as a des ign vehicle under public roads . T he design loads as given in T MH7 s hould be us ed for the design of all s tructures under major roads . Under mos t conditions the loading from the NB 36 vehicle is the mos t critical for buried s torm water conduits . T he typical legal vehicle would be us ed for the design of conduits in areas outs ide public jurisdiction. T he mos t s evere loading will occur when two s uch vehicles pas s , or are parked next to each other. F igure 18 illus trates the wheel configuration of these vehicles . 1.0
r1.8 1.0
r0.9 1.0
r1.8
r1.2 2.0
(a) 40kN wheel loads – legal limit
6.0 to26.0
2.0
(b) NB 36 loading – 90kN wheel loads
FIGURE 18: TRAFFIC LOADING ON ROADS F or the NB loading, 1 unit = 2.5 kN per wheel = 10 kN per axle and = 40 kN per vehicle. F or the NB 36 vehicle = 90 kN per wheel = 360 kN per axle.
W hen the effect of thes e loads is considered on buried conduits an allowance for impact for impact s hould be made. F or the typical highway vehicle this is us ually taken as 1.15. W here greater impact is expected due to a combination of high speed, rough s urface and hard s us pension, an impact factor up to 1.4 could be applied. T he effective contact area for thes e wheels is taken as 0.2 m x 0.5 m in direction of and trans vers e to direction of travel respectively. T he loads on pipes due to 40 kN wheel loads with the configuration s hown in F igure 16(a) are given in T able 15. T he table can be used for any wheel load (P ) provided that the wheel arrangement is the s ame and the load multiplied by P /4. T AB LE 15: LOADS IN K N/M ON B UR IE D C ONDUIT F R OM G R OUP OF 40 K N W HE E LS Pipe I/D mm
0.6 8.1 10.2 12.2 14.2 16.3 18.3 20.4 22.4 24.5 28.5 32.6 38.3 42.6 46.8 51.1
1.0 4.78 5.97 7.16 8.36 9.55 10.7 11.9 13.1 14.3 16.7 19.1 22.4 24.9 27.4 29.9
1.5 2.8 3.5 4.2 4.9 5.7 6.4 7.1 7.8 8.5 9.9 11.4 13.3 14.8 16.3 17.8
Fill height over pipes in m 2.0 2.5 3.0 3.5 4.0 1.8 1.3 1.0 0.7 0.6 2.3 1.6 1.2 0.9 0.7 2.8 2.0 1.5 1.1 0.9 3.3 2.3 1.7 13.3 1.0 3.7 2.7 2.0 1.5 1.2 4.2 3.0 2.2 1.7 1.4 4.7 3.3 2.5 1.9 1.5 5.2 3.7 2.7 2.1 1.7 5.6 4.0 3.0 2.3 1.8 6.6 4.7 3.5 2.7 2.1 7.5 5.3 4.0 3.1 2.5 8.8 6.3 4.7 3.6 2.9 9.8 7.0 5.2 4.0 3.2 10.8 7.7 5.7 4.4 3.5 11.8 8.4 6.3 4.9 3.9
5.0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.4 1.7 1.9 2.2 2.4 2.6
6.0 0.3 0.3 0.4 0.5 0.6 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.7 1.9
7.0 0.2 0.2 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.6 1.4
300 375 456 525 600 675 750 825 900 1 050 1 200 1 350 1 500 1 650 1 800 Notes : 1. No impact factor has been included. 2. Impact s hould certainly be considered for low fills (
T AB LE 16: LOADS IN K N/M ON B UR IE D P IP E S F R OM NB 36 G R OUP OF W HE E LS FILL HEIGHT OVER PIPES IN M PIPE I/D PIPE OD NB36 PT mm mm 0.6 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0 7.0 LOAD 300 0.345 26 12 7 4 3 2 1 1 1 1 0 114 375 0.431 31 15 8 5 3 2 2 1 1 1 0 115 456 0.518 35 17 10 6 4 3 2 2 1 1 1 116 525 0.604 39 19 11 7 5 3 2 2 1 1 1 117 600 0.690 43 22 12 8 5 4 3 2 1 1 1 118 675 0.776 46 24 14 9 6 4 3 2 2 1 1 120 750 0.863 49 25 15 9 6 5 4 3 2 1 1 121 825 0.949 52 27 17 10 7 5 4 3 2 1 1 123 900 1.035 55 29 18 11 8 6 4 3 2 2 1 125 1 050 1.208 60 33 21 13 9 7 5 4 3 2 1 129 1 200 1.380 64 36 24 15 10 8 6 5 3 2 2 133 1 350 1.620 67 40 28 18 12 9 7 5 4 3 2 138 1 500 1.800 67 43 31 20 14 10 8 6 4 3 2 144 1 650 1.980 68 46 34 22 15 11 9 7 5 3 3 149 1 800 2.160 69 49 37 24 17 13 10 8 5 4 3 156 Notes 1. T he NB 36 vehicle travels slowly and generally no impact needs to be considered. 2. Under certain conditions the NB 24 vehicle could be us ed for minor roads.
5. CONCRETE PIPE STRENGTHS 5.1. EXTERNAL LOADS T he size of circular pipes is defined by one dimens ion only. T his simplifies the relations hip between the load to be carried and the s trength required to do s o. F or rigid pipes as concrete the s trength is us ually determined by using what is called the direct method. Us ing the information from the previous s ections the required concrete pipe s trength can be determined by dividing the installed load by a bedding factor. F actory tes t loads and reactions are concentrated. T he field loads and reactions have a parabolic or radial dis tribution around a pipe. However it is as s umed that the loads are uniformly dis tributed over the pipe and that the bedding reactions have either a parabolic or uniform dis tribution dependant upon the bedding material us ed. A comparis on of these loads and reactions is s hown in F igure 16
a) T hree edge bearing tes t
b) Uniform reaction
c) P arabolic reaction
FIGURE 19 – FACTORY STRENGTH AS MODEL OF INSTALLED LOAD ON PIPE B edding factors have been derived for standard bedding clas s es and are des cribed in detail in S ection 6 that follows . T he bedding factors for a trench ins tallation as s ume that there is a vertical reaction only and no lateral s upport to the pipe. F or an embankment ins tallation lateral s upport is taken into account and hence the embankment bedding factors are s omewhat higher than those us ed for a trench ins tallation. F or mos t ins tallations the bedding factors given in T able 17 below are adequate. T AB LE 17: B E DDING F AC T OR S F OR C ONC R E T E P IP E Bedding details Material R einforced concrete C oncrete G ranular G ranular G ranular
Installation details Trench Embankment 3.4 4.8 2.6 3.9 2.0 2.4 1.5 2.0 1.1 1.2
Class Angle A 180 A 180 B 180 C 60 D 0 Note: 1) C las s D bedding s hould only be us ed when s uitable bedding material is not available. 2) C las s A bedding s hould not be us ed unles s there are s pecial requirements to be met. 3) F or zero and negative projection ins tallations us e trench bedding factors . F or positive projection conditions , where greater accuracy is required the bedding factors can be calculated using the procedure des cribed in S ection 6.
5.2. INTERNAL PRESSURE W here a pipeline is required to work under internal pres s ure, two conditions mus t be cons idered: v T he s tatic head of water in the pipe, excluding the los s es due to friction. v Dynamic factors that can caus e pres s ure s urges above and below the s tatic or working head. T he factors to be considered are: v W hether or not the flow can be unexpectedly s topped and if s o whether the s toppage is gradual or ins tantaneous v W hether the s urges below the working head can give ris e to negative pres s ures. F or pres s ure pipes a factor of s afety of 1.5 is normally us ed where only the working pres s ure is known. W here the pres s ures along the pipeline have been accurately calculated, taking into account s urge and water hammer effects , the line is us ually divided into pres s ure zones or reaches . T he factor of s afety at the lowes t s ection of any zone is us ually taken as 1.0. W hen concrete pipes are us ed for a pres s ure pipeline it is us ually a gravity s ys tem or a s iphon where surges cannot develop. Hence s pecifying a factory hydros tatic tes t pres s ure that is 1,5 times the maximum s tatic or operating head is adequate.
5.3. SAFETY FACTORS T he choice and application of s afety factors is left to the dis cretion of the designer. It is s ugges ted that either each load be considered independently and a factor of s afety ranging from 1.0 to 1.5, applied directly to the value of the load, or the required pipe s trength. R ecommended values are given in T able 18. T he determination of a factor of safety to be us ed in designing a pipe depends on: v F ield working conditions v Degree of s upervision v Height of fill above a pipe v W hether or not there are corrosive elements in the trans ported fluid, or the groundwater. T AB LE 18: R E C OMME NDE D S AF E T Y F AC T OR S F OR V E R T IC AL LOADS Pipe Application S torm water drainage S ewer pipes without s acrificial layer P ipes laid in corrosive ground condition S ewer pipes with s acrificial layer
Factor of safety Reinforced
Non-reinforced
1.0 1.3 1.3 1.0
1.3 1.7 1.7 1.3
5.4. SELECTION OF THE CONCRETE PIPE CLASS As the size of circular pipes is defined by one dimens ion only, the relations hip between the load to be carried and the pipe s trength required, is s implified. T he s trengths can be determined by using an indirect approach. T his means that the ins talled loads are connected into a factory tes t load by using a bedding or s afety factor.
5.4.1.External load T he relations hip between the factory tes t load and ins talled field load is given by the equation developed by Mars ton and S pangler, namely: W T = W I F S /B F W here W T - required proof load for 0.25 mm crack W I - external load (kN/m) F S - s afety factor B F - bedding factor T he pipe clas s is s elected s o that: WT < S W here S - proof load of a s tandard D-load clas s pipe (kN/m)
5.4.2.Internal pressure T he s election of the pres s ure clas s is made as follows : t = p x FS W here t - required tes t pres s ure (kP a) p - design pres sure in pipeline F S - factor of s afety T he pipe clas s is s elected s o that t
5.4.3.Combined internal pressure and external load W here pipes are to be s ubjected to combined external load and internal pres s ure the following formula is us ed for pipe s election: T = t / (1-(W T /S ) 2) W hen s electing a pipe for thes e conditions , a balance between T and S s hould be found. A pipe s hould not be s elected that is required to withs tand a very high pres s ure and a very low vertical load or vice vers a, as such a pipe would be uneconomical. Example 1 Determine the strength of a 900mm internal diameter storm water pipe under the following conditions: Trench installation, using trench width in accordance with SANS 1200 DB. Backfill material: dry sand (w = 1 600kg/m3). Height of fill on top of pipe: 3.5m. Traffic loading: NB 36. Bedding: Class B. Pipeline in corrosive soil conditions. F rom T able 8, load due to fill = (1600/2000) x 84 = 67.2 kN/m and from T able 11 NB 36 loading = 4,0 kN/m S ince pipe is in corrosive conditions a s afety factor of 1.3 should be applied to total load. T he clas s B bedding factor is 2.0, therefore required minimum proof load, S will be: W T = ((earth load + live load) x S F )/ B F = ((67.2+4) x 1.3) / 2.0 = (71.2x1.3) / 2.0 = 92.6 / 2.0 = 46.3 kN/m A clas s 75 D (67.5 kN/m proof load) will therefore be adequate. If an economic evaluation of the ins tallation is required the pipe bedding clas s combinations that are adequate as well as their cos ts are needed s o that the total cos t can be calculated.
Bedding class
Bedding
Required
Required
Standard
C B A non reinforced A reinforced
factor
Test
D-load
D-load
1.5 2.0 2.6 3.4
61.7 46.3 35.6 27.2
68.6 51.4 39.6 30.3
75 75 50 50
Example 2 Determination of strength of a 1 200 mm internal diameter pipe culvert to be installed under the following conditions: Embankment installation Positive projection: projection ratio p = 0.7 Foundation material: rock (rs = 1) Height of fill above top of pipe: 2.5 m Backfill density: 1 650 kg/m3. Light traffic conditions are expected, assume 4 000 kg maximum wheel load Class B bedding Non-corrosive conditions F or a value of prs = 0.7 x 1.0 = 0.7, T able 10 gives a backfill load = (1650/2000) x 147 = 121.3 kN/m and from T able 12 traffic load = 4.1 kN/m. T he factor of s afety is 1 (S ee 4.6) T he required proof load, will be: W T = ((earth load + live load) x S F )/ B F =((121.3 + 4.1) x 1)/ 2.4 = 52.3 kN/m A clas s 50D pipe (60 kN/m load s hould be s pecified. Example 3 A 300 mm internal diameter pressure pipeline is to be installed in a trench under the following conditions: The maximum pressure expected in the line including surge and water hammer is: 150 kPa Trench width : 900 mm Height of fill: 1.5 m Material: wet sand (density 2 000 kg/m3) Bedding: Class C Non-corrosive conditions F rom T able 4 for trench width 900 mm and height 1.5 m; Load on pipe = 20 kN/m C las s C bedding factor = 1.5 and F actor of safety = 1.0 (S ee section 4.6) R equired pipe s trength W T = ((20 / 1.5) x 1 = 13.3 kN/m As s ume a C las s 50D pipe is us ed (15 kN/m proof load). T o determine the minimum res istance to internal hydraulic pres s ure, the following formula is applied: T = t / (1-(W T / S ) 2)
(s ee P ar 4.7.3)
where t = 150 kP a, W T = 13.3 kN/m and S = 15.0 kN/m T herefore T = 150 / (1-(13.3 / 15) 2) = 700 kP a T he pipe s pecification s hould be C las s T 8 (tes t pres s ure 800 kP a) and C las s 50D. Alternative clas s es could be determined by s tarting with a 100D pipe (30 kN/m) T = 150 / (1-(13.3 / 30) 2) = 187 kP a In this design the pipe s pecification would be T 2 (200 kP a) and C las s 100D that would probably be more economic than the firs t alternative.
6. BEDDING 6.1. GENERAL T he bedding s upporting a pipe trans fers the vertical load on the pipe to the foundation. It als o provides a uniform s upport along the pipeline and prevents any load concentrations on the pipe due to irregularities in the foundation. T he ability of a rigid pipe to carry field loads that are larger than the tes t load depends on the degree of s upport given to the pipe by the bedding. T he ratio between the load that a pipe can s upport on a particular type of bedding, and the tes t load is called the bedding factor. W hen s electing granular materials for C las s B , C and D beddings the des igner mus t cons ider the interface between the bedding material and the surrounding natural material. P recautions mus t be taken to prevent the ingres s of fine material into the bedding layer, as this will res ult in a los s of support to the pipe.
F ormation level
Main backfill
B edding blanket
B edding cradle R eworked foundation T rench bottom
Bedding Factor - Bf
FIGURE 20: TERMINOLOGY FOR PIPE BEDDING
Bedding Angle - è FIGURE 21: RELATIONSHIP BETWEEN BEDDING FACTOR AND BEDDING ANGLE
6.2. TRENCH AND NEGATIVE PROJECTION INSTALLATIONS 6.2.1.General T he pipe weight and the loads on it are trans ferred to the foundation through the bedding. T he amount the bedding yields under this load determines the pres s ure dis tribution of the reaction between bedding and pipe. F or trench ins tallations no allowance is made for lateral earth pres sure. P res s ures are as s umed to act on the pipe in the vertical direction only. Loos e granular beddings are flexible and will yield more than a pipe deforms under load. T he pres sure dis tribution of the reaction from this type of bedding is parabolic. A rigid bedding with the s ame flexural s tiffnes s as the pipe will deform the s ame amount as the pipe under load and the pres sure dis tribution of reaction between pipe and bedding will be rectangular and uniform. F igure 21 gives the relations hip between the bedding factor and the angle of bedding s upport for uniform and parabolic reactions . T he maximum bending moment occurs at the invert of the pipe under thes e loading conditions . In negative projection ins tallations, where the limits are the trench condition and the zero friction condition, the development of lateral soil pres s ures is ignored, as it is difficult to obtain adequate compaction of the backfill in confined s paces . W here the design corres ponds to one of the bedding clas s es given below, the bedding factor for that clas s s hould be us ed. T he key to the materials used is given in T able 19 below. Alternatively, F igure 21 may us e to obtain an appropriate bedding factor. T AB LE 19: K E Y T O MAT E R IALS US E D Ins itu Material
Lightly compacted backfill
S elected granular material
Main backfill
Densely compacted backfill
F ine granular fill material
Loos e backfill
R eworked foundation
C ompacted granular material
6.2.2.Class A beddings T he concrete beddings commonly us ed are given in F igure 22. T he bedding width s hall not be les s than B c + 200 mm but may extend the full width of the trench. S teel reinforcement if us ed mus t not be les s than 0.4 % of the concrete cros s -s ection and mus t be placed trans versely beneath the pipe and as clos e to it as pos sible allowing for the minimum cover required for reinforced concrete. T he concrete shall have a 28-day cube s trength of not les s than 20 MP a.
r300
r300
Bc
B c /4
Bc
v
v B c /4
B c /4
B c+200 min (a) C las s A (non-reinforced)
Bc B c /4
(b) C las s A (reinforced)
FIGURE 22: CLASS A TRENCH BEDDINGS UNDER PIPES
B c+200 min
(c) F or W et C onditions
B c+200 min r100
Bc B c /2 B c /4
W hen clas s A bedding is placed over under pipes it s hould have a minimum thicknes s of B c/4. If this cannot be achieved then the concrete should be reinforced.
(a) C oncrete Arch
r100
B c+200 min
Bc B c /2 B c /4
(b) R einforced C oncrete Arch
FIGURE 23: CLASS A TRENCH BEDDINGS OVER PIPES T he clas s A bedding factors are: Unreinforced 2.6
0.4% Reinforcement 3.4
1.0% Reinforcement 4.8
T hes e factors are slightly higher than the values given in F igure 21 as it is as s umed that the C las s A concrete bedding is s tiffer than the pipe it s upports. As a res ult the pres sure under the pipe will have an inverse parabolic dis tribution, giving a lower bending moment at the pipe invert than the uniform dis tribution.
6.2.3.Class B Beddings
r300
r300
Bc
r300
B cB c
Bc B c /4
B c /4 0,7 Bc
(a) G ranular B edding
(b) S haped S ubgrade
(c) F ully E ncased
FIGURE 24: CLASS B TRENCH BEDDINGS T he C las s B G ranular bedding commonly us ed is s hown in F igure 24(a). T he bedding angle is 180 o and the pres s ure dis tribution under the pipe is as s umed to be parabolic. T he s election, placement and compaction of the granular material mus t be carried out s o that this as s umption is not compromis ed. T he cons truction detail of the s haped s ub-grade bedding with a granular curtain is s hown in F igure 24(b). T he width of the bedding is 0.7 B c (90 o bedding angle) and the pres s ure dis tribution under the pipe is as sumed to be uniform. T he depth of the fine granular blanket mus t not be les s than 50 mm and the s ide fill mus t be well compacted. T he C las s B bedding factors are: Granular Bedding 2.0
Shaped Sub-grade 2.0
Fully Encased 2.2
6.2.4.Class C beddings A reduced bedding factor is as s umed, to allow for a poorer quality of bedding cradle material and compaction and a s maller bedding angle than used with C las s B beddings . T he s election, placement and compaction of the granular material mus t be carried out s o that this as s umption is not compromis ed. Details are given in F igure 25 below.
r150
r150
Bc
r150
Bc
Bc
B c /8
B c /8 0,5 Bc
(a) G ranular cradle
(b) S haped s ub-grade
(c)S elected granular
FIGURE 25: CLASS C BEDDINGS W hen a granular cradle is us ed the bedding angle is 90o and the pres s ure dis tribution is as s umed to be parabolic. T he cons truction detail of this s haped s ub-grade bedding is shown in F igure 25(b). T he bottom of the trench is compacted, levelled and s haped s o as to s upport the pipe barrel over a width of 0.5 B c (60 o bedding angle). No blanket is provided and the backfill around the pipe is lightly compacted. T he cons truction detail of the flat granular bedding is s hown in F igure 25(c). It is as s umed that the pipe barrel penetrates the bedding material to achieve a s upport angle of angle of at eas t 45 o with a uniform pres s ure dis tribution under the pipe. A s uitable material for this type of bedding is a s ingle sized gravel or aggregate consis ting of rounded particles that can flow easily. C rus hed aggregates containing a high percentage of angular particles , are more s table and will minimis e the settlement of the pipe into the bedding material. It is important that the properties of the material are matched to the size and acceptable s ettlement of the pipe. T he bedding factors for clas s C granular beddings are: Granular support angle 60o 1.5
Shaped sub-grade 1.5
Uncompacted granular 1.5
Granular support angle 90o 1.7
6.2.5.Class D beddings No s pecial precautions are required for this clas s of bedding except that the s ub-grade mus t fully s upport the pipe in the longitudinal direction and that holes mus t be excavated in the floor of the trench to accommodate s ockets or joints that have a diameter greater than that of the pipe barrel. Load concentrations on the pipe mus t be avoided. T his clas s of bedding is not s uitable in situations where the founding conditions consis t of very hard or very soft insitu material s uch as rock, hard gravel or s oft clay.
r150
r150
Bc
Bc
(a) F lat s ub-grade
(b) C ompacted granular material
FIGURE 26: CLASS D TRENCH BEDDINGS T he cons truction detail of the flat s ub-grade bedding is s hown in F igure 26(a). W here the flat s ub-grade s urface is not s uitable as bedding it s hould be improved by compacting and levelling a layer of s uitably graded granular material. T his layer will provide uniform s upport along the length of the pipe, without the ris k of load concentrations occurring (see F igure 26(b)). T he type D beddings s hould only be us ed for s maller diameter pipes where the pipe cos t is much les s than the total ins tallation cos t. T he bedding factor for clas s D beddings in a trench or negative projection installation is 1.1
6.3. POSITIVE PROJECTION INSTALLATIONS 6.3.1.General In positive projection ins tallations , where the limits are the zero friction or geos tatic condition and the complete projection condition, active lateral s oil pres sures develop in the fill and these help to carry the vertical load on the pipe. T he bedding factors us ed for thes e ins tallations are therefore higher than thos e us ed for trench and negative projection ins tallations . T he bedding clas s es are the same as thos e us ed in negative projection ins tallations . T he enhanced values of the bedding factors as given below are determined by us ing S pangler’s method.
6.3.2.Spangler’s Method T he bedding factor applicable to positive projection ins tallations is calculated using formula below.
Bf
A N x q
where A - 1.431 for circular pipes N - is obtained from T able 14 x - is obtained from T able 15 q - is calculated from formula below mK H m q s Cc Bc 2 W here q - ratio of total lateral pres s ure to total vertical load K - R ankine’s coefficient of active earth pres sure, us ually taken as 0.33 C c - fill load coefficient for positive projection m - proportion of B c over which lateral pres s ure is effective. S ee F igure 22. H - fill height over pipe
T AB LE 14 – V ALUE S OF N F OR P OS IT IV E P R OJ E C T ION B E DDING S Type of bedding C las s C las s C las s C las s C las s
Value of N
A – res trained A – unres trained B C D
0.421 0.505 0.707 0.840 1.310
Note: R einforced or plain concrete beddings cast agains t s table rock, are res trained
T AB LE 15: V ALUE S OF x F OR P OS IT IV E P R OJ E C T ION B E DDING S Value of m 0.0 0.3 0.5 0.7 0.9 1.0
Concrete 0.150 0.743 0.856 0.811 0.678 0.638
Other 0.000 0.217 0.423 0.594 0.655 0.638
Note: T he parameter x is a function of the proportion of the pipe over which active lateral pres s ure is effective.
T he s tandard embankment bedding details are s hown in F igures 27 and 28 below.
mB c
Bc
Bc
v r0.3B c B c/4 (a) C las s A concrete
Bc
mB c vB c/4 B c/4
B c/8
(b) C las s B granular
(c) C las s C granular
FIGURE 27: EMBANKMENT CLASS A, B AND C BEDDINGS
Bc
mB c
Bc
mB c
B c/8 (a) C las s D granular
(b ) C las s D natural material
FIGURE 28: EMBANKMENT CLASS D BEDDINGS
mB c
6.4. SOILCRETE BEDDING S oilcrete or s oil-cement as it is s ometimes called is an alternative bedding material that is us ed under certain circums tances s uch as when there are: v v v
concerns about bedding material was hing away and causing piping next to pipeline time res traints on the installation trenches that are narrow and side compaction is difficult.
S oilcrete consis ts of a granular material that has between 3% and 6% of cement added to it and is made as a flowable mix with a slump of >200mm. T here s hould be no organic material in the s oil used and ideally the clay content s hould be minimal. T he s oilcrete is s tronger that soil, having a s trength between 0.5 and 1.0 MP a. T his material can be us ed in two ways , namely as a gap filler or as a bedding as illus trated in F igures 29 (a) and (b).
S oilcrete (a) S oilcrete as gap filler
(b) S oilcrete as bedding
FIGURE 29: USE OF SOILCRETE AROUND PIPES T he purpos e of the S oilcrete is to trans fer the load on the pipe to the surrounding soil. As it is s tronger than s oil it does not matter if there are s mall cracks in it. T he important is sue is that the material is s table and s upports the pipes. T o ens ure that there is s upport all around the pipe this material needs to be flowable and vibrated once placed. T o prevent floatation the s oilcrete is placed in two s tages , the firs t should not be higher than p a sixth of the pipe OD. T he s econd s tage can be placed as s oon as the initial s et has taken place. (W hen a man can walk on it.) for installation details reference s hould be made to the Ins tallation Manual that is a companion publication to this one. W hen s oilcrete is used as a gap filler the dis tance between the pipe and excavated material s hould be p 75 mm. W hen it is us ed as bedding the dimens ions s hould be the s ame as thos e used for concrete bedding. T he bedding factors for s oilcrete beddings will depend on the bedding angle and can be taken from the curve for concrete on F igure 21.
6.5. JACKING CONDITIONS W hen the pipes are jacked the excavation is s lightly larger than the external diameter of the pipe. However, the proces s of ins talling a pipe ens ures that positive contact is obtained around the bottom portion of the pipe and that ideal bedding conditions are obtained. If the pipe carries all or part of the vertical earth load, the us e of the trench bedding factors is appropriate. T hes e will depend on the width of contact between the outs ide of the pipe and the material through which the pipe is being jacked. As this will us ually be at leas t 120o a value of 1.9 can be us ed. W hen determining the bedding factor, the behaviour of the insitu material after the jacking is completed and the pos t ins tallation treatment given to the void between the pipe and the excavation s hould be considered. If this is grouted, a value of 3 can be us ed.
7. PIPE JOINTING 7.1. JOINT TYPES T he function of the joint is to provide flexibility and s ealing for the pipeline. J oints are des igned to cope with the movement that occur due to the s econdary forces within the s oil mas s . T here are four types of pipe joints , namely, butt (or plain ended), interlocking (or Ogee), spigot and s ocket and in-the-wall joints . T hes e are us ed for different applications that are determined by the amount of movement to be tolerated and the importance of keeping the pipeline sealed.
(a) B utt
(b) Interlocking
(c) S pigot and s ocket
(d) In-the-wall
FIGURE 31: JOINT TYPES FOR CONCRETE PIPE
7.2. BUTT AND INTERLOCKING JOINT PIPES B utt ended and interlocking pipe joints are not intended to prevent infiltration and exfiltration of water hence they are only us ed for s tormwater drainage and culvert pipe. B utt ended pipes are s eldom us ed as they do not have any means of s elf-centering when being jointed. If there is a potential problem with the los s of bedding material into the drainage s ys tem the joints s hould be s ealed either with mortar or s ealing tape. W hen s tormwater drains are placed on s teep slopes and the flow velocity exceeds 4 or 5 m/s it is advis able to us e one of the joints that can be s ealed with a rubber ring to prevent the high velocity water going through the joints and s couring cavities in the s oil around the pipes. P ipes for s ewers or pres sure pipelines s hould have s pigot and s ocket or in-the-wall joints that include seal in the form of either a rolling ring or confined ring.
7.3. SPIGOT AND SOCKET JOINTS P ipes with this joint type are the mos t commonly used for sewers . T hey are designed to s eal as well as tolerate movements in three directions , namely: v Draw or longitudinal movement. v Deflection or radial movement. v R elative s ettlement or dis placement of a pipe relative to the adjacent ones . In addition to this thes e joints take into cons ideration tolerances on concrete s urfaces , laying procedures and s eal dimens ions . T he rubber ring enables this type of joint to be deflected as s hown in F igure 32 s o that pipes can be laid around curves and s till remain watertight. A
A
FIG 32 – ANGULAR DEFLECTION OF SPIGOT AND SOCKET PIPES
T he amount of movement that can be tolerated at a joint will depend on the pipe size and the manufacturer’s details . T he radius of the curve is dependent on the angular deflection that is permitted for each pipe size. T ypical deflections and curve radii are given in T able 29. S pecific projects s hould be dis cus sed with the manufacturer concerned. T AB LE 29: ANG ULAR DE F LE C T IONS AND C UR V E R ADII Nominal Pipe Diameter - mm
Permissible Degrees
Minimum Radius - m
300 - 375
2.00
70
450 - 600
1.50
93
675 - 900
1.00
140
1 050 - 1 200
0.75
186
1 350 – 1 800
0.50
280
T he radius of curve that can be negotiated is directly proportional to the pipes ' effective length. T he values in this table were calculated us ing an effective pipe length of 2.44m. If a different length is used the radius from the table s hould be corrected by the ratio of the lengths . W here s harp curves in exces s of thes e values are required s pecial pipes with deflected s pigots or s ockets , or radius pipe can be produced. T his s hould be dis cus s ed with the manufacturers . W hen a curve is being negotiated, the pipes mus t firs t be fully jointed in a s traight line and only then deflected. T he s pigot and s ocket pipe has traditionally been made with a rolling rubber ring. T he S outh African s tandard for rubber rings is S ANS 974-1: Rubber joint rings (non-cellular) Part 1: Joint rings for use in water, sewer and drainage systems.
7.4. IN-THE-WALL JOINTS W ith large diameter pipes the wall is so thick that a rubber ring joint can be accommodated within the wall thicknes s. W ith this type of joint nibs on the jointing s urfaces or a groove in the spigot confine the s eal. As the seal remains in a fixed position the s ocket slides over this s o both the s eal and the s ocket of the pipe to be jointed s hould be s hould be thoroughly lubricated before the joint is made. T his type of joint is s ometimes called a confined or sliding rubber ring joint. P articular attention s hould be paid to lubricating the lead-in s ection of the s ocket that makes the firs t contact with the s eal. T he advantage of this type of joint is that the outside diameter of the pipe remains cons tant making the pipe ideally suited for jacking. F or jacking pipes from 900 mm in diameter and larger use this joint type. However for s ewers this type of joint is s eldom us ed for pipes of les s than 1500 mm in diameter. Mos t pipes larger than 1800 mm in diameter are made with this type of joint. T he joint is designed to cope with the same criteria as the s pigot and s ocket joint, but as it is s horter than the s pigot and s ocket joint the amount of movement that it can tolerate will in general be a little les s . T hese joints can, in general, cope with a deflection of 0.5 degrees and be us ed to negotiate curves if required to do s o. F or details of how pipes s hould be jointed the reader is again referred to the Concrete Pipe and Portal Culvert Installation Manual or the pipe s upplier.
8. FLOATATION 8.1. GENERAL Any buried pipeline, even when full of water will weigh les s than the s oil that it dis placed. Hence there will be a tendency for pipelines to lift rather than s ettle. W hen the groundwater level is higher than the bottom of the pipeline the buoyancy forces can lift the pipeline due to. If thes e conditions can occur either during the ins tallation or operation of the pipeline the designer should check that the pipeline will not float off its bedding. S AB S 0102 P art II (? , p51) lis ts s everal conditions that could give rise to this , namely: v F looding of trench to cons olidate backfill v P ipelines in flood plains or under man-made lakes that will be below groundwater level v S ub aqueous pipelines v P ipelines in other areas that may be s ubject to a high water table If any of thes e exis t the designer s hould calculate the forces to es tablis h whether or not floatation will be a problem. T hes e forces are: v W eight of pipe v W eight of water dis placed by pipe v W eight of load carried in pipe v W eight of any backfill over the pipe T wo floatation conditions can occur, namely: v P ipeline is s ubmerged partly or fully before backfilling v P ipeline becomes s ubmerged after backfilling
8.2. FLOATATION BEFORE BACKFILLING T he weight of the dis placed water in kN/m of pipeline, ww is calculated from: ww = Gw L 1 A 1 Gw - dens ity of water in kN/m3 L 1 - length of pipeline in m A 1 – cros s -s ectional area of pipeline below water s urface in m2 T he pipeline will float if ww > wp wp - the pipeline mas s in kN/m
(23)
(24)
8.3. FLOATATION AFTER BACKFILLING T he vertical s oil load acting on the pipeline in kN/m of length, wb can be calculated from: wb = G’ B c H (25) ’ 3 G – s ubmerged density of s aturated backfill in kN/m s ee formula (4) below B c - outside diameter of pipeline in m H – fill height over pipeline in m G’ = Gw – (G 1 – 1)/(1 + e) (26) G 1 – s pecific gravity of s oil particles e – void ratio of s oil T he pipeline will float if ww > wp + wb (27)
9. SEWER CORROSION 9.1. CORROSION MECHANISM C oncrete is the mos t frequently used material for the manufacture of outfall sewers . Under certain conditions concrete s ewers may be s ubject to corrosion from s ulphuric acid (H 2S O 4) formed as a res ult of bacterial action. T he physical appearance of corrosion is firs t detected as a white efflores cence above the water line, and it takes s everal months before this s tarts. T hereafter deterioration may be rapid in which cas e the concrete s urface becomes s oft and putty-like and there is aggregate fallout. T here are three s ets of factors contributing to this phenomenon, thos e res ulting in the generation of the gas hydrogen s ulphide (H 2S ) in the effluent thos e resulting in the release of H 2S from the effluent and thos e res ulting in the biogenic formation of H 2S O 4 on the s ewer walls . T hes e are illus trated in figure 33 below.
H 2S O 4 F OR MAT ION
H 2S R E LE AS E
H 2S G E NE R AT ION
FIGURE 33: CORROSION MECHANISM T he mos t important factors contributing to H 2S generation in the effluent are: v R etention time in s ewer v V elocities that are not self cleansing v S ilt accumulation v T emperature v B iochemical oxygen demand (B OD) v Dis s olved oxygen (DO) in effluent v Dis s olved S ulphides (DS ) in effluent v E ffluent pH. T he mos t important factors contributing to H 2S releas e from the effluent are: v C oncentration of H 2S in effluent v High velocities and turbulence T he mos t important factors contributing to H 2S O 4 formation on the s ewer walls are: v C oncentration of H 2S in s ewer atmos phere v R ate of acid formation v Amount of mois ture on s ewer walls v R ate of acid runoff
If there is ins ufficient oxygen in the effluent the bacteria that live in the slimes layer on the s ewer walls s trip the oxygen from the s ulphates in the effluent to form s ulphides . T he firs t s et of factors influence the rate at which this occurs . W hen there is an imbalance of H 2S in the s ewage and the sewer atmos phere this gas will come out of s olution s o that there is equilibrium. T he s econd set of factors influence this . T he H 2S releas ed into the s ewer atmos phere is abs orbed into the mois ture on the s ewer walls and is oxidised by another s et of bacteria to H 2S O 4. T his is influenced by the third set of factors . T he acid formed then attacks the cement in the concrete above the water line, as it is alkaline. If an inert aggregate is us ed there is aggregate fallout when the binder corrodes . T his expos es more of the binder that in turn is corroded by the acid. T he deterioration of the pipe wall is rapid. If concrete is made us ing a calcareous aggregate, which is alkaline, the acid attack is s pread over both binder and aggregate, the aggregate fallout problem is minimis ed and the rate at which the s ewer wall deteriorates is reduced.
9.2. CORROSION PREDICTION AND CONTROL R es earch by P omeroy and K ienow [8] led to the development of a quantitative method for predicting the rate of s ulphide generation and the res ultant rate of concrete corrosion. T his later became know as the Life F actor Method (LF M). In 1984 the American C oncrete P ipe As s ociation (AC P A) publis hed the “Design Manual S ulfide and C orrosion P rediction and C ontrol”[9]. T his quantified the LF M [10] by giving equations for predicting the corrosion in concrete sewers bas ed on the biological compos ition of the effluent, the s ystem hydraulics and the alkalinity of the concrete us ed. T he final output is the required additional cover to reinforcement, referred to as “sacrificial layer” in S outh Africa, for a concrete pipe to ens ure that it will remain s erviceable for its des ign life. T he theoretical prediction of H 2S generation in the s ewerage is based on an analysis of the effluent and is beyond the s cope of this document. If the reader requires the procedure reference s hould be made to reference 10. Once the DS in the effluent has been determined the rate of H 2S releas e from effluent, called the H 2S flux can be calculated from:
Fs f = 0,69 (s v) 3/8J [DS ]
(1)
Fs f - H 2S flux from s tream s urface, g/m /h 2
s - energy gradient of was tewater s tream, m/m v - s tream velocity, m/s J - fraction of DS pres ent as H 2S as function of pH [DS ]-average annual dis s olved sulphide concentration in was tewater, mg/l (0,2 to 0,3 mg/l les s than the total s ulphide concentration) T he abs orption of this H 2S into the mois ture layer on the wall of the s ewer is determined from a modification of the above equation:
Fs w = 0,69 (s v) 3/8 J [DS ] (b/P ’)
(2)
Fs w - H 2S flux to the pipe wall, g/m2/h b/P ’- ratio of was tewater s tream width to perimeter of pipe wall above water s urface. T his as s umes that all the H 2S that is releas ed is absorbed into the mois ture layer. T he concrete corrosion rate can be es timated by calculating the rate at which the H 2S flux to the pipe wall will be oxidis ed to H 2S O 4. “34g of H 2S are required to produce s ufficient H 2S O 4 to neutralis e 100g of alkalinity expres s ed as calcium carbonate (C aC O 3) equivalent. (3p23) If all the Fs w is oxidis ed the annual corrosion rate for the concrete can be predicted from:
C avg = (11.5k/A) Fs w (3) C avg - average corrosion rate, mm/year K - efficiency coefficient for acid reaction bas ed on the es timated fraction of acid remaining on s ewer wall. May be as low as 0,3 and will approach 1,0 for a complete acid reaction. A - Alkalinity of the cement-bonded material expres s ed as its calcium carbonate (C aC O 3) equivalent; It varies from p 0,16 for s iliceous aggregate concrete to p 0,9 for calcareous aggregate concrete; 0,4 for mortar linings. 11,5 - converts Fs w in g/m2/h, into C avg, in mm/year W hen combined with the equation for the flux of H 2S to the wall of a pipe equation is expres s ed as :
[11]
the LF M
Az = 11.5 k Fs w L (4) z - additional concrete cover, required over reinforcement, (mm) (s acrificial layer) L - required design life of s ewer in years T here are three options for preventing or minimis ing the corrosion in concrete sewers : v preventing acid formation v modifying concrete v protecting concrete. Acid formation can be prevented or minimized by adjus ting the hydraulic design of the s ewer. However, due to physical cons traints this is not always pos s ible and s ome corrosion can be anticipated. F or mos t s ewers modifying the concrete by changing the concrete components and/or providing additional cover to reinforcement is the mos t cos t effective option. P rotecting concrete by using an inert lining or coating is effective, but only economically jus tified when s evere corros ion is predicted.
9.3. DEVELOPMENTS IN SOUTH AFRICA S ince the 1960’s mos t concrete s ewers in S outh Africa have been made us ing calcareous aggregates , usually dolomitic and the principle of a sacrificial layer. T his s olution followed the recommendations of a 1959 C S IR publication[3]. T he experience in J ohannes burg where the first calcareous pipes were laid in 1960 and other cities and towns in S outh Africa indicates that this results in a cons iderable increas e in a s ewer’s life. As far as can be es tablis hed, there have been no reports of s erious problems on s ewers made us ing this approach. However, concern had been expres s ed by local authorities and cons ultants that the “dolomitic aggregate” s olution might not be adequate for certain s ewers in the long-term. T his concern was s ubs tantiated by application of the Life F actor Model (LF M) [4], developed in the US A by P omeroy and P arkhurs t that quantifies the biogenic corrosion of concrete, to s everal s ewers where s evere corros ion was anticipated. W hen the C S IR undertook a literature s earch during the 1980s , no reference could be found to field trials es tablis hed to calibrate the actual performance of various s ewer materials . [5] F ollowing s everal meetings of interes ted parties, a s teering committee was formed and a decision taken to include an experimental s ection, with a bypas s , as part of a s ewer being ins talled at V irginia in the F ree S tate. T he LF M indicated that the conditions anticipated in this s ewer were s o corrosive that the traditional s olution would be uns uitable and a cementitious pipe would require an inert lining or coating. T here have been three phas es to monitoring material performance and conditions in this s ewer.
v
P has e One was undertaken by the C S IR to monitor the conditions in the s ewer and the performance of traditional s ewer pipe materials in the sewer and subject to pure acid attack in a laboratory. v P has e T wo was undertaken by the University of C ape T own (UC T ) to continue monitoring the conditions in the s ewer and to inves tigate ways of s imulating these conditions in a laboratory. v P has e T hree is being undertaken jointly by UC T and an independent cons ultant as a continuation of the second phas e and involves meas uring the actual corrosion that occurred during the firs t two phas es ; s upervising the rehabilitation of the experimental s ection; and meas uring the actual corrosion on various new materials to be calibrated for us e in the LF M. At the time of writing the experimental s ection of s ewer has been rehabilitated and the actual corrosion on the s amples ins talled during phase one has been determine and is s ummaris ed in T able 25 below. F rom this table it can be s een that meas ured average corrosion rates after 14 years for all the materials was s omewhat greater than the es timates made following earlier ins pections . As thes e meas urements were taken on the s amples removed from the s ewer and the actual wall thicknes ses could be meas ured, giving greater accuracy. T o date the LF M has been applied to P C concretes only. T he corrosion rates meas ured in this experimental s ection of s ewer mean that the LF M can now be applied where other concretes are us ed. T he effective alkalinity of alternative concretes can now be allocated values in exces s of unity. In particular the effective alkalinity of an inert material can be taken as infinity. T he LF M can now be us ed to calculate the required s acrificial layer thicknes s by incorporating a material factor, MF that is the ratio of corros ion rate for the alternative material being cons idered and a s tandard concrete made from P C and s iliceous aggregate. T AB LE 25: ME AS UR E D & E S T IMAT E D C OR R OS ION AND MAT E R IAL F AC T OR S Material 5 year es timate 12 year es timate 14 year meas ured Material (cement/ T otal Ave T otal Ave T otal Ave factor*** Aggregate) P C /S IL >30 >6,0 >64 >6,0 > 105 > 7.5 1.000 P C /DOL 10 – 15 2 – 3 20 – 30 1,7 – 2,5 43 3.1 0.400 C AC /S IL 5 – 10 1–2 10 – 15 0,8 – 1,2 26 1.9 0.250 FC 10 - 12 2 + 20 - 25 1,7 – 2,1 0.320 C AC /DOL * 3,0 0,6 7,2 0,6 8,4 0,6 0.085 0.025 C AC /ALAG £ ** * V alues es timated on basis of other materials and performance of s amples in s ewer. **Much les s than C AC /DOL-no mas s los s 17 months in s ewer and pH on s urface >6,4 ***Average of maximum los s at side divided by corres ponding value for P C /S IL. B y applying the LF M as des cribed in equation 4 above to a particular s ewer and as suming an ‘A’ value of 0.16 as would be appropriate for a s tandard siliceous aggregate concrete the required s acrificial layer can be es tablis hed. T he sacrificial layer thicknes s for another material can be calculated by multiplying this value by the appropriate material factor, MF from T able 25. z ANOT HE R = 72 MF kFs wL (5) MF - Material F actor for chos en material and is obtained from T able 25.
T his extens ion to the LF M has been called the Material F actor Model (MF M). T he application of this and how it can be us ed to determine the mos t cos t effective pipe material for a given s ewer is des cribed in s ection 10.5 that follows . B as ed on the 5 year findings from the V irginia S ewer the concept of making a host pipe of one type of concrete to provide the s trength and an additional layer of another concrete to cope with the corrosion was inves tigated. A effective technique for doing this was developed and since 1997has been us ed on many of the major outfall s ewers in S outh Africa. T he mos t commonly us ed combination of materials has been a hos t pipe made of P C /S IL concrete and a s acrificial layer of C AC /DOL. W hen s uch pipes are made an allowance of 3 to 5 mm is made for the interface between the two concretes .
9.4. DESIGN AND DETAIL CONSIDERATIONS T here are many publications on s ewer corros ion. However few have been written in a S outh African context and to date as far as can be establis hed none have quantified the corrosion rates of non-P C concretes as des cribed above. T hes e do however addres s the is s ues of the hydraulic design and detailing of s ewers . T he following points s hould be cons idered when des igning and detailing a s ewer: v T he longer the s ewage s tays in the s ewer the greater are the chances that it will turn s eptic and the rate of s ulphide generation increas e. W here practical retention times s hould be kept to a minimum. v S low flows inhibit the abs orption of oxygen into fres h s ewage caus ing an increas e in s ulphide generation. In addition slow flows could res ult in thicker slimes layers and silt build-up that in turn increas e H 2S generation. F low velocities at minimum dis charge s hould be at leas t 0.8m/s v Mois ture condens ation on s ewer or manhole s urfaces provides the habitat for the bacteria that produce H 2S O 4. T aking s teps to reduce mois ture condensation are not always pos sible. v J unctions between s ewers with different velocities can obs truct in the sewers with the s lower flows causing long retention times in them. W hen s ewers are joined the ups tream gradients s hould be adjus ted s o that the entry velocities are as close as pos s ible to each other. v Mos t junctions are affected at manholes. E nergy los s es and turbulence are as s ociated with the releas e of H 2S and the pos s ibility of local corrosion. T he inverts of s uch manholes s hould be carefully benched with s mooth trans itions to minimis e energy los ses and turbulence. v If a fas t flowing lateral dis charges into a manhole benching as s hown in F igure 33(a) and at the s ame invert level as that of the collector, the flow in the collector can obs tructed causing long retention times ups tream of the junction. T he fas t flows s hould enter above, and in the direction of the collector s ewer, not at the s ame level and at as s mall an angle as pos sible, as s hown in F igure 33(b).
(a) S harp junction
(b) T ransitioned junction
FIGURE 34:CONNECTION BETWEEN COLLECTOR AND LATERAL.
T he rate of H 2S generation in rising mains and s iphons is much greater than in s ewers flowing partly full becaus e the slimes layer extends around the full pipe circumference, none of the gas generated es capes and there is no oxygen enrichment of the s ewage. S evere corrosion can occur in s ewers downs tream of thes e es pecially when s ewage retention times exceed much more than an hour. W hen the sewage dis charges into the gravity s ection of sewer the accumulated H 2S is liberated and can caus e s evere local corrosion. P rocedures for minimis ing retention times and the res ultant corrosion are: v Us e the s malles t practical pipe diameter for the full flowing s ection of s ewer v Make the s ection as s hort as pos sible v Operate pumps frequently, particularly in early years of the s ys tem where low flows could res ult in the s ewage upstream of the full flowing s ection becoming s eptic. S ewage with a high B OD us ually res ults in higher s ulphide content and this could result in the corrosion of the s tructures at the purification works . V arious meas ures that can be taken to reduce this are: v If the B OD is very high, greater than 1 000 mg/I, pre-treat the s ewage v Lay the feed line to the dosing tank below the hydraulic gradient to exclude oxygen. In s pecial cases the addition of hydrated lime to increas e the s ewage pH, or alternatively ventilating the outfall us ing a forced draught should be considered. C areful hydraulic design and attention to detail has a positive contribution in reducing s ewer corrosion. However they cannot eliminate the problems that could aris e if the corrosion potential is s evere and has not been identified by doing the neces s ary corrosion analysis . T he above considerations s hould be us ed in combination with an application of the LF M and MF M when designing and detailing s ewers ; not as a s ubs titute an analysis .
9.5. PIPE MATERIAL CHOICE FOR SEWERS T here are several concrete pipe alternatives that could be us ed for s ewers . T hes e are: v Hos t pipe and s acrificial layer made from P C /S IL v Hos t pipe and s acrificial layer made from P C /DOL v Hos t pipe made from P C /DOL or P C /S IL and s acrificial layer made from C AC /S IL v Hos t pipe made from P C /DOL or P C /S IL and s acrificial layer made from C AC /DOL v Hos t pipe made from P C /DOL or P C /S IL and an HDP E lining cas t in. T he relative corrosion rates of thes e s acrificial layer materials are given in T able 1above. B y applying the LF M and the MF M as des cribed above a technically sound solution that is als o the mos t cos t effective alternative for a s ewer operating under a particular s et of circums tances can be s elected. As the primary function of a s ewer is to convey was tewater the firs t item that s hould be addres s ed is the pipe s ize required. Ideally this s hould be bas ed on two limiting values of velocity namely: v A minimum value (0,7m/s ) at low flow that will ens ure s elf-cleansing. v A maximum value of 0,8 times the critical velocity to prevent exces s ive turbulence. T he internal diameter (ID) and the hydraulic properties obtained from thes e calculations s hould be us ed in combination with the effluent properties to predict the potential corrosion for the required design life as s uming a P C /S IL concrete. T he relative corrosion rates of other types of concrete being considered for the project s hould then be calculated bas ed on the details given in T able 25 above. T he s acrificial layer thicknes s with an appropriate allowance for an interface if the sacrificial layer and hos t pipe are made from different concretes s hould be added to the required internal diameter to give the host pipe internal diameter.
F rom the ins tallation conditions do a preliminary as s es s ment of the pipe clas s that will be required bas ed on the wors t-cas e s cenario as given in table xy above. If the pipe clas s indicated were 75D or 100D then the outside diameter (OD) of the pipe would be 1.2 times the indicated hos t pipe ID. If the pipe clas s indicated was 50D or les s then the outs ide diameter would be 1.14 times the hos t pipe ID. T he manufacturers brochures s hould be cons ulted to determine the neares t actual external diameter that would give at least the external diameter as indicated by the calculations done following the above procedure. T his s hould be done for each of the s olutions being evaluated as when s evere corros ion is predicted there will be a s ignificant difference between the minimum required hos t pipe OD’s and this could mean that the pipes using a different corrosion control meas ures would be made in moulds of different OD’s . T his is illus trated in the example that follows . Once the mould OD’s for the different s olutions have been es tablis hed the exercis e s hould be repeated for each of these alternatives but in the revers e order namely: v F or the required OD determine the pipe s trength and clas s required to handle the ins talled conditions . v Add the required s acrificial layer or lining thicknes s to the hos t pipe ID to determine the actual pipe ID. v C heck the hydraulics of the s ewer using the actual ID. T he designer is now in a position to get budget prices from the s uppliers s o the alternatives can be compared on an economic bas is. Example: determine the most cost effective pipe with an actual ID of 900mm for a range of Az values, namely 5, 10, 20 and 40. Assume that the required pipe class is 100D. MAT E R IAL Az V ALUE P IP E IDS AC R L HOS T ID P IP E OD HOS T - kg S AC R L - kg T OT AL - kg % HOS T P R IC E
pc/s il
MAT E R IAL Az V ALUE P IP E IDS AC R L HOS T ID P IP E OD HOS T - kg S AC R L - kg T OT AL - kg % HOS T P R IC E
pc/dol
5 900 30 960 1152 822 226 1046 145
5 900 12 1100 1320 757 89 845 117
pc/dol 10 900 60 1020 1224 928 467 1395 193
20 5 10 900 900 900 125 12 24 1150 924 948 1380 1108.8 1137.6 1179 757 803 1038 89 178 2217 845 981 307 117 136
10 900 24 910 1116 803 178 981 136
cac/dol 5 900 5 930 1140 738 37 775 123
20 900 50 1000 1200 892 385 1277 177
40 900 100 1100 1320 1079 811 1889 262
cac/dol 20 5 900 900 50 5 1000 910 1200 1092 892 738 385 37 1277 775 177 123
10 900 9 918 1102 753 66 819 132
20 900 15 918 1116 771 111 882 153 hdpe
10 900 9 950 1102 753 66 819 132
20 900 15 950 1116 771 111 882 153
40 900 25 950 1140 805 187 992 189
all 900 0 905 905 787 0 0 178
This table clearly illustrates the impact of the corrosion potential on the cost effectiveness of the various materials commonly used as corrosion control measures for sewers in
South Africa. As the corrosion potential increases the solutions that are more costly to produce actually become more cost effective solutions. The following shows this: v If there is any corrosion potential at all the PC/SIL solution will be the most costly and the PC/DOL solution where the host pipe and sacrificial layer is made from th asame material is the most cost effective. v Where corrosion potential becomes greater (15 < Az <30) the CAC/DOL sacrificial layer and a host pipe of a standard concrete will be the most cost effective. v Where corrosion potential becomes severe (Az >30) the HDPE lining cast into the host pipe will be the most cost effective It should be noted that the costs used in this exercise are hypothetical and that do make this comparison on an actual project it would be necessary to obtain actual prices from the pipe suppliers. Although a lining of C AC /S IL would be technically sound it would not be cost effective unles s it was very expens ive to trans port dolomitic aggregate to the manufacturing plant. F rom the above example it can be s een that all s ewer pipes and manholes s hould be manufactured us ing calcareous aggregates even if no corrosion is expected. T he concrete made for thes e s hould contain not more than 25% ins olubles when tes ted in hydrochloric acid. (Details of the tes t method are given in S ANS 676.) In s ome parts of S outh Africa aggregates are available with ins olubility levels of 12% to 18%. If available the lowes t practical level s hould be s pecified. T he s tandard s acrificial layer thicknes s es us ed in S outh Africa are 13 mm for pipes up to 1050 mm in diameter and 19 mm for diameters larger than this . If the corrosion analysis indicates that thes e thicknes s es are inadequate and a more cos tly material cannot be jus tified then a thicker sacrificial layer s hould be s pecified. T o ens ure that the hydraulic requirements will be met the minimum internal diameter and the s acrificial layer thicknes s s hould be s pecified. W hen the s acrificial layer and hos t pipe are made from different concretes an allowance s hould be made for the interface between the two concretes . Under thes e circums tances it would be realis tic to consider the des ign values for the s tandard s acrificial layers as being minimum values of 10 and 15 mm ins tead of nominal values of 13 and 19 mm.
9.6. SACRIFICIAL THICKNESS AND ALLOWABLE CRACK WIDTHS W here a increas ed sacrificial layer thicknes s is s pecified the allowable crack width s hould be increas ed in proportion to the increas e in concrete cover to s teel. T he allowable crack width can be calculated from the formula (6) that is given in S ANS 677. r = q(t-x) / (t-x-C 2)
(6)
r – allowable crack width for a s acrificial layer thicknes s of C 2 in mm q – allowable crack width for a pipe with s tandard cover to s teel in mm t – total wall thicknes s of pipe in mm x – dis tance from the outs ide s urface of pipe to the neutral axis in mm C2 = C - C1
(7)
C – total concrete cover to inner s teel reinforcement cage in mm C 1 – s tandard specified concrete cover to inner s teel reinforcement cage in mm T he neutral axis of the pipe can be taken as being half the hos t pipe wall thicknes s . T he relations hip between thes e s ymbols is s hown in F igure 35 below.
P ipe outs ide x R einforcement t
C1 C
C2
q r
P ipe inside
FIGURE 35: RELATIONSHIP BETWEEN CRACK WIDTH AND SACRIFICIAL LAYER Example: If a 900 mm diameter concrete pipe with a standard wall thickness of 93 mm has a sacrificial layer of 20 mm, what is the allowable crack width at proof load? Standard cover to steel is 10 mm. Neutral axis, x = 93/2 = 46.5 mm C = C1 + C2 = 10+ 20 = 30 mm and r = 0.25(113 – 46.5)/(113 – 46.5 – 20) = 0.36 mm T here are two practical factors that s hould be cons idered when s acrificial layers that are thicker than s tandard ones are s pecified, namely: v If the s acrificial layer is thicker than one third of the wall thicknes s the reinforcement will be clos e to the centre of the pipe wall and will not be effective in controlling cracks . v If the s acrificial layer thicknes s is more than twice the s tandard concrete cover to reinforcement the crack widths that could be accepted if equation (6) were blindly applied could be exces s ive and allow aggres s ive elements to enter the cracks and move the corros ion front clos er to the reinforcement. T he AC P A C oncrete P ipe Handbook (4,p57? ) s tates that problems have not been experienced with pipes that have cracks in them of up to 0.5 mm when the concrete cover to reinforcement is 25 mm. As this cannot be s ubs tantiated by any s cientific s tudy it is recommended that the s erviceability limit for crack widths be limited 0.4 mm even if equation (6) above indicates a larger value. B y applying the correct procedure for predicting corrosion and then choosing the pipe material that cos t effectively meets the requirements the above problems will in general be avoided.
10. PORTAL CULVERT STRENGTHS 10.1. GENERAL T he terms us ed with portal culvert ins tallations are detailed in F igure 29 below.
F ormation level
C ompacted s elected material
Main backfill
C rown Unit
B as e s lab B linding layer T rench bottom FIGURE 29: TERMONOLOGY FOR PORTAL CULVERTS As portal culverts are rectangular two dimens ions determine their s ize. Hence, the relations hip between the load to be carried and the required s trength cannot be s implified as it can with pipes . Hence, the s trength required is determined by us ing a direct approach. T he procedure adopted is : v Determine the s tructural properties of the portal v C alculate loads and load combinations v C alculate the bending moments and s hear forces generated in the portal by the various load combinations v Determine the bending moment and s hear force envelopes that cover all the loading cas es v Determine combinations of tes t loads to model the ins talled bending moment and s hear force envelopes . T his procedure can be followed by using ultimate values for both the ins talled and tes t loading conditions or by factoring the ins talled parameters and determining the proof load parameters that match them.
10.2. DETERMINING PORTAL CULVERT STRENGTHS As mentioned at the beginning of this handbook, there is no National S tandard for determining the loads on or S trengths of P ortal C ulverts. In T MH7, the C ode of P ractice for the Design of Highway B ridges and C ulverts in S outh Africa, C laus e 2.3.3.1, provision has been made for three s olution levels , namely: “General: With due recognition of the complexity of the problem of determining loading on culverts, but also of the need for simple procedures which can be used routinely, provision is made for the three-fold approach, viz: The application of simple design rules that can be applied to rigid and flexible culverts but that require the use of increased partial safety factors which allow for the approximate nature of the formulae used. The application of more sophisticated design theories to rigid and flexible culverts that take into account the type of culvert, the properties of the undisturbed ground and the fill materials as well as the effects of the actual
width of excavation, and the positive or negative projection. These theories also allow for the use of reduced partial safety factors. (In positive and negative projecting culverts, the tops of the structures are above and below undisturbed ground level respectively.) The application of sophisticated design theories or the design techniques based on the phenomenological approach to flexible and special types of culvert that required more accurate assessments of soil-structure interaction. This Code covers the first approach only, which is an extension of the AASHO1 and CPA2 formulae. The designer shall use his discretion in deciding on the best applicable method for any particular case and is referred to publications on the subject.” In the simplified approach the earth loading has been reduced to four combinations of foundation and installation conditions , namely: C ondition 1: C ulverts in trench on unyielding foundation with no projection. C ondition 2: C ulverts untrenched on yielding foundation. C ondition 3: C ulverts untrenched on unyielding foundation for H>1.7B C ondition 4: C ulverts untrenched on unyielding foundation for H<1.7B W here H - fill height in metres B - if trenched overall trench width, or if untrenched overall culvert width, in metres . C onditions 1 and 2 corres pond to the geos tatic loading condition and 3 and 4 to the pos itive projection ins tallation condition with an rs d p ratio of 1. Approximate methods for determining the effects of traffic loading on rigid conduits are given in C laus e 2.6.6 of T MH7. T his combination of the earth and traffic loading was applied to the s tandard portal culvert dimens ions to determine the product s trengths required. T hese s trengths were compared with thos e of the s tandard S -load culverts and the appropriate clas s es selected. T he relationship between s tandard portal culvert clas s es and maximum fill heights for T MH7 loading conditions applied to the s tandard sizes is given in T able 29 below. T he as s umptions , and claus es from T MH7 P arts 1 and 2 us ed to compile this table are: v T he table is applicable to rectangular portal culverts only v W hen s izes other than given in this table the manufacturer s hould be contacted. v A minimum fill height of 300 mm over the culvert units . W here this cannot be achieved a 100 mm reinforced concrete slab mus t be us ed. v S tandard traffic loading (S N A, B and C ) as des cribed in C lause 2.6.1.2 v F ill material unit weight 20 kN/m3 [C laus e 2.3.1] v C oncrete unit weight 24 kN/m3 [C laus e 2.2.1] v Horizontal earth pres s ure 7,8 kN/m2 per metre depth [C lause 2.4.2] v Ultimate Limit S tate load factors T able 7. If portal culverts are required where the fill over them is les s than 300 mm or more than the amount s tated in this table the loads mus t be calculated using the procedures in T MH7 and the s trength by following the procedure given at the end of s ection 10.1 above.
T AB LE 29: MAXIMUM F ILLS : S -LOAD P OR T AL C ULV E R T S UNDE R T MH7 LOADING . Culvert span x height in mm 600 600 600 750 750 750 750 900 900 900 900 900 1200 1200 1200 1200 1200 1500 1500 1500 1500 1500 1800 1800 1800 1800 1800 2100 2100 2100 2100 2100 2100 2400 2400 2400 2400 2400 2400 3000 3000 3000 3000 3000 3000 3600 3600 3600 3600 3600 3600
X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X
300 450 600 300 450 600 750 300 450 600 750 900 300 450 600 900 1200 450 600 900 1200 1500 600 900 1200 1500 1800 600 900 1200 1500 1800 2100 600 900 1200 1500 1800 2400 900 1200 1500 1800 2400 3000 900 1200 1500 1800 2400 3000
Installation conditions 1&2 200 S
175 S
175 S
150 S
100 S
75 S
75 S
75 S
75 S
75 S
10.2 11.0 12.0 8.7 9.2 10.0 10.5 8.6 9.0 9.5 10.0 10.2 7.1 7.4 7.7 8.2 8.8 4.7 4.9 5.3 5.6 6.0 3.3 3.6 3.8 4.0 4.3 3.3 3.5 3.7 3.9 4.1 4.3 3.2 3.4 3.5 3.7 3.8 4.0 3.2 3.3 3.4 3.5 3.7 3.1 3.1 3.2 3.3 3.3 3.5 3.5
Installation conditions 3&4 200 S
175 S
175 S
150 S
100 S
75 S
75 S
75 S
75 S
75 S
5.6 6.0 6.4 4.9 5.2 5.5 5.8 4.8 5.1 5.3 5.6 5.8 4.1 4.3 4.4 4.7 5.0 3.0 3.1 3.3 3.5 3.7 2.4 2.5 2.7 2.8 2.9 2.4 2.6 2.7 2.8 2.9 3.0 2.4 2.5 3.5 3.7 3.8 4.1 2.5 2.6 2.7 2.7 2.9 2.7 2.5 2.6 2.6 2.7 2.8 2.7
10.3. PORTAL BASE SLABS Mos t pre-cas t portal culverts are placed on cas t in place bas e s labs . T hese s hould be des igned to take the actual loads that will be applied to them. It is important to realise that the moments and s hears generated by the ins talled loads on bas e s labs are different from thos e generated on the portal unit. T he total load on the portal unit will be trans ferred down the legs to the bas e s lab. T he moments and s hears will then be trans ferred through the bas e slab to the founding material. If this material is unyielding the load will be trans ferred directly through the slab generating s hear but no bending moments . If however, the material under the s lab is yielding, both s hear forces and bending moments will be generated. T he ins talled loads on the portal crown are as s umed to be dis tributed over the whole width of the portal, except for the very low fill heights where there are concentrated loads from the traffic. T he tes t loads on the portal crown are concentrated live loads . If there were no factoring of installed loads , the tes t loads would be about half the ins talled ones . Hence, the tes t loads model the ins talled loads on the portal crown, but do not model the ins talled loads on the bas e slab. If pre-cas t bas es are to be us ed under crown units, a check s hould be done to ens ure that they are sufficiently s trong to take the impos ed loads .
11. FIELD TESTING 11.1. WATER TEST P ipelines consis t of pipes and joints . C oncrete pipes used for s ewers and low-pres sure pipelines are load and hydros tatically tested at the factory before delivery to s ite to ens ure that they will meet the s tructural requirements s pecified. As the pipes are jointed on site they need to be tested on site to ensure that the pipeline will meet its operating requirements . Apart from a vis ual ins pection the only field-testing needed on a concrete pipeline is one for leakage. T his gives the as s urance that the ins talled pipeline will meet the water tightnes s requirements . T he water tes t is carried out as follows : v C los e the s ection of pipeline to be tes ted with bulkheads or plugs . As thes e will be s ubject to considerable forces they should be designed and ins talled to ens ure that they can withs tand thes e with an adequate s afety factor. v Open the air valves and s lowly fill the test s ection with water to ens ure that all the air es capes . v K eep the tes t s ection under a slight pres s ure for 3 to 5 days to allow the pipes to abs orb water v If pipes were expos ed for more than a month additional time may be needed for this . v During this period check the s ealed ends and joints for leaks and the rate at which water has to be added to maintain the pres sure. v W hen the rate of adding water s tabilis es increas e the pres s ure to the required value. S AB S 1200-LD pres cribes that s ewers s hould be tes ted with a water head of not les s than 1.2 m and not more than 6.0m. T he los s allowance pres cribed is not more than 6 litres/100mm of diameter/100m/hour. P res s ure pipelines are tes ted in the s ame way but the requirements are more s tringent. A tes ting s chedule that gives the pres s ure for each s ection s hould be compiled to ens ure that the lower clas s pres s ure pipes are not overs tres s ed. T he full-s cale water tes ting of large diameter s ewers and pipelines is a difficult and cos tly exercis e. W hen available, s pecial joint tes ting equipment that applies water pres sure to one joint at a time is us ed. T his equipment has to be us ed with care and it s hould be appreciated that it is not tes ting the joint that has already been factory tes ted, but the jointing that has been done on site. Hence the pres s ures us ed are not the s ame as thos e for which the pipeline is rated. In mos t cas es when a s ewer is man entry ( 900 mm in diameter) and below the water table as frequently occurs with this size, it can be phys ically inspected to check for leaks . C oncrete has the property of autogenous healing and hair cracks or damp s pots s hould not be caus e for rejection, as this type of leakage will be s top within days of the pipe s urface being expos ed to a mois t environment.
11.2. AIR TESTING T he water tes ting of s ewers is s eldom practical es pecially in a country as S outh Africa where water is s carce and may not be available for the tes ting of s ewers . Air testing of concrete s ewers is an effective way for identifying isolated s ections that are leaking as poor joints or damaged pipes . As air and water have different properties this test is not an indicator of the water tightnes s of the pipe wall. T his tes ting can therefore be us ed as an acceptance tes t but not as jus tification for rejection. If there is a dispute the final acceptance or rejection of a s ewer s hould be bas ed on a water tes t.
T his tes t is conducted in a similar way to the water tes t. However as the intention of this is to find is olated problems the air pres sure inside the s ection being tes ted is only jus t above atmos pheric. T he procedure followed is : v S eal the ends of the s ection to be tes ted with bulkheads or plugs ; making s ure that the s afety factor of blow out to tes t pres s ure is at leas t 2. v One of the bulkheads is fitted with connections to an air s ource, a pres s ure releas e valve and a pres s ure gauge or monometer. v Air is added to the tes t s ection to increase the internal pres s ure to a pres cribed amount above atmos pheric. T his mus t allow s ufficient time for this to s tabilis e, as there may be differences between the air and pipe wall temperatures . v Once the air pres s ure within the tes t s ection has s tabilis ed the air s upply is s topped and the time in s econds that it takes for a given pres sure drop is meas ured. T he rate of air los s is then calculated. T he s ewer is then inspected to determine whether there are any joints or damaged s ections that are leaking. T hes e leaks can us ually be identified by the sound of es caping air. If no localis ed leaks are identified and the rate of pres sure drop is unacceptable the expos ed s ewer is s prayed with s oapy water to help find any problem areas . Leaking joints or damaged s ections of pipe mus t be repaired us ing means that are approved by the project engineer. S ection 7 of S AB S 1200-LD pres cribes the pres s ures and procedures that s hould be us ed for the air tes ting of s ewers namely: v An initial pres s ure of 3.75kP a(375mm of water) v Once the pres s ure s tabilis es , reduce it to 2.5kP a(250mm of water) v S witch off the machine and meas ure how long it takes for the pres s ure to drop to 1.25kP a(125mm of water) v T he minimum acceptable time for this drop to take place is 2 minutes /100mm diameter W henever pos sible defects s hould be repaired with the pipes in place. Only when pipes have been incorrectly ins talled or there has been damage due to s oil movements s hould the replacement of pipes be cons idered. If this is neces s ary it mus t be done from manhole to manhole s o that the whole ins tallation is redone and the pos sibility of relative s ettlement between s ections of s ewer is eliminated. S hould this s paying of s oapy water on the expos ed pipe s how s ections of pipe were bubbles form this will probably be due the pipes having dried out as a res ult of being expos ed for prolonged period (in exces s of 6 weeks ). W hen thes e pipes are expos ed to the mois t s ewer atmos phere the concrete will take up mois ture and the micros tructure will s eal.
11.3. SOIL DENSITY TEST T his needs to be checked W here s pecifications call for minimum dens ities of backfill or bedding material, thes e are normally given as a percentage of the Modified P roctor Density. T he tes t is carried out in the following way: v S amples of the various materials to us ed are obtained v E ach s ample is dried and then prepared at various mois ture contents . v F or each of the mois ture contents five layers are compacted in a 0.95 litre mould. v E ach layer receives 25 blows from a 4.54 kg hammer falling from 457 mm v T he optimum mois ture content is the mois ture content corresponding to maximum dry dens ity. T his maximum s oil density is referred to as the Modified P roctor Density.
Dens ity tes ts are done on the compacted backfill or bedding material on the site and then compared to the Modified P roctor Density to check that thes e materials have been placed to the required densities. M A T E R I A L D E N S I T Y P E R C E NT AG E MOIS T UR E C ONT E NT FIG 35: MOISTURE CONTENT AND DENSITY RELATIONSHIP
BIBLIOGRAPHY 1. American C oncrete P ipe As s ociation: Concrete Pipe Handbook, Virginia USA, 1981. 2. C larke N W B . Buried Pipelines: a manual of structural design and installation. London, Maclaren, 1968 3. P ortland C ement As s ociation: Handbook of Concrete Culvert Pipe Hydraulics. S kokie, 1964 4. S ANS 10102-2: Selection of pipes for buried pipelines Part 2: Rigid pipes 5. S ANS 1294: P recas t C oncrete Manhole s ections and slabs 6. S ANS 676: R einforced C oncrete P res s ure P ipes 7. S ANS 677: C oncrete non-pres s ure pipes 8. S ANS 986: P recas t reinforced concrete culverts 9. S ANS 10102: T he s election of pipes for buried pipelines . Part 1: General Provisions 10. S ANS 10102: T he s election of pipes for buried pipelines . Part 2: Rigid Pipes 11. S AB S 1200 DB : E arthworks (P ipe T renches ) 12. S AB S 1200 L: Medium-pres s ure P ipelines 13. S AB S 1200 LB : B edding (P ipes ) 14. S AB S 1200 LE : S tormwater Drainage 15. S AB S 1200 LD: S ewers 16. S AB S 1200 LG : P ipe J acking 17. Hart-Davis , Adam. What the Victorians did for us. Headline B ook P ublis hing, London, 2001, pp.59–61. 18.Ibid, p142. 19.C ouncil for S cientific and Indus trial R es earch (C S IR ), Division of B uilding T echnology. Corrosion of concrete sewers. C S IR , P retoria. S eries DR 12. 1959. 20.B ealey, Mike, Duffy, J ohn J , P reuit, R us s ell B , S tuckey, R obert E . Concrete pipe handbook. American C oncrete P ipe As s ociation, V irginia, US A, 1981, pp. 7-22 – 734. 21.C ouncil for S cientific and Indus trial R es earch (C S IR ), Division of B uilding T echnology. Report on phase 1 of sewer corrosion research: The Virginia sewer experiment and related research. C S IR , Division of B uilding T echnology, P retoria, 1996, p.40. 22.G oyns , A. Virginia sewer rehabilitation. Progress report no 1. A project being undertaken by the P ipe and Infras tructural P roducts Division of the C MA. P IP E S C C C enturion, 2003, pp.12 – 14. 23.G oyns , A. Virginia sewer rehabilitation. Progress report no 2. A project being undertaken by the P ipe and Infras tructural P roducts Division of the C MA. P IP E S C C C enturion, 2004. 24.K ienow, K K , P omeroy,R D. C orrosion resis tant des ign of sanitary sewer pipe. ASCE Convention and exposition, C hicago, US A, 1978. 25.McLaren, F rederick R . Design manual: sulfide and corrosion prediction and control. American C oncrete P ipe As s ociation, V irginia, US A, 1984. 26.Ibid, p.4-4. 27.B owker, R obert P G , S mith, J ohn M, W ebs ter, Neil A. Design manual: Odor and corrosion control in sanitary sewerage systems and treatment plants. C entre for E nvironmental R esearch Information, US E nvironmental P rotection Agency, C incinnati, 1985, p.23.
28. McLaren, F rederick R . Design manual: sulfide and corrosion prediction and control. American C oncrete P ipe As s ociation, V irginia, US A, 1984, p.4-4. 29. C ouncil for S cientific and Indus trial R es earch (C S IR ), Division of B uilding T echnology. Report on phase 1 of sewer corrosion research: The Virginia sewer experiment and related research. C S IR , Divis ion of B uilding T echnology, P retoria, 1996. 30. G oyns , A. Virginia sewer rehabilitation. Progress report no 1. A project being undertaken by the P ipe and Infras tructural P roducts Division of the C MA. P IP E S C C C enturion, 2003, pp.9–14. 31. C ouncil for S cientific and Indus trial R es earch (C S IR ), Division of B uilding T echnology. Report on phase 1 of sewer corrosion research: The Virginia sewer experiment and related research. C S IR , Divis ion of B uilding T echnology, P retoria, 1996, p.6. 32. Ibid, p.46, p.102. 33. F ourie, C W . Biologically induced sulphuric acid attack on concrete samples in the experimental sewer section at Virginia. Department of C ivil E ngineering, Univers ity of C ape T own, 2002. 34. Dumas , T H. P rivate communication, Lyon, F rance, J une 1994. 35. S and, W . B ock, E . W hite, D.C . Biotest system for rapid evaluation of concrete resistance to sulphur-oxidising bacteria. Materials P erformance, V ol26, No3, March 1987, pp. 14-17. 36. G oyns , A. Virginia sewer rehabilitation. Progress report No 1. A project being undertaken by the P ipe and Infras tructural P roducts Division of the C MA. P IP E S C C C enturion, 2003, pp.7–14. 37. F ourie, C .W . Biologically induced sulphuric acid attack on concrete samples in the experimental sewer section at Virginia. Department of C ivil E ngineering, Univers ity of C ape T own, 2002, p.1. 38. C ouncil for S cientific and Indus trial R es earch (C S IR ), Division of B uilding T echnology. Report on phase 1 of sewer corrosion research: The Virginia sewer experiment and related research. C S IR , Divis ion of B uilding T echnology, P retoria, 1996, p.93. 39. F ourie, C W . Biologically induced sulphuric acid attack on concrete samples in the experimental sewer section at Virginia. Department of C ivil E ngineering, Univers ity of C ape T own, 2002, p.9. 40. Ibid, p.2, p.6. 41. Ibid, p.11. 42. Ibid, p.8. 43. McLaren, F rederick R . Design manual: sulfide and corrosion prediction and control. American C oncrete P ipe As s ociation, V irginia, US A, 1984, pp.5-2–5-4. 44. B owker, R obert P G , S mith, J ohn M, W ebs ter, Neil A. Design manual: Odor and corrosion control in sanitary sewerage systems and treatment plants. C entre for E nvironmental R esearch Information, US E nvironmental P rotection Agency, C incinnati, 1985, p.25. 45. Ibid, p.25. 46. C ouncil for S cientific and Indus trial R es earch (C S IR ), Division of B uilding T echnology. Report on phase 1 of sewer corrosion research: The Virginia sewer experiment and related research. C S IR , Divis ion of B uilding T echnology, P retoria, 1996, p.58.
