r •CHAPTER 6
Empirical methods of design
It is the mark of an educated manto lookfor lookfor precision precision in eachelas s-ofthings s-ofth ings just so far- as-the- natu reof re ofth the e------------------ . subject admits.
Aristotle
Empirica l design methods methods relate practical experience gained on previous projects to the conditions anticipated at a proposed site. Rock mass classifications form the backbone of the empirical design approach and are widely employed in rock engineering. In fact, on many projects, the classification approach serves as the only practical basis for the design of complex underground structures. Most of the tunnels constructed at present make use ofso
98 Empirical methods o f design design Classification systems in ro '’ 'vgmeering 99 Table 6.1. 1. Majo r rock classifications classifications currently in use Name of ' ..classification
Originator and date
Country of origin
Rock loads
Terzaghi, 1946
USA
Stand-up time
Laufler, 1958
Rock quality designation Intact rock
Deere, 1964
Austria USA
Deere Deere & Miller
... -:
strength RSR concep conceptt
Wickham, et al. 1972 .... Bieniawski, 1973
Geomechanics Classification (RM R system system) Q-system
Tunnels with steel steel supports Tunneling . Core logging, tunneling Communication
USA
Tunneling
S. Africa & USA
Tunnels, mines, foundations
Norway
Barton, et al. 1974 Franklin, 1975
Canada
Tunneling, large chambers Tunneling
ISRM, 1981.............
International
General
Strength/block . size Basic geotechnical geotechnical classification
USA
Applications
o f each groupgroup J ° pr° vide 3 basls for understanding the characterist ics of c) To yield quantitative data for engineering engineering design; design; and d) T o provide a common basis for communicatio communication. n. ’ It tr ib ut ™5 0411 ^ fUlfilfed lfed ^ enSUring enSUring th3t th3t 3 c!assification c!assification s?stem has has the following a)
majority of the rock mass classification systems. It is a necessary parameter because the strength of the rock material constitutes the strength limit of the rock mass. The uniaxial compressive strength of rock material can be determined in the field indirectly by means of the point load strength index (Franklin, 1975). The second parameter most commonly employed is the rock quality designation (RQD). This is a quanti tative index based on a modified modified core recovery procedure procedure which incorporates only sound pieces of core which are 100 mm or greater in length. The RQD is a measure of drill core quality or fracture frequency, and disregards the influence of joint tightness, orientati on, continuity contin uity and.gouge imfilli ng) Conseqiip.ntly seqiip.ntly--the RQD does not fully describe a rock mass. Other classification parameters used in current rock mass classifications are: spacing of discontinuities, condition of discontinuities (roughness, continuity, sepa ration, joint-wall weathering, infilling), orientation of discontinuities, groundwater conditions (inflow, pressure), and stress field. An excellent discussion of the methods for quantitative description of discontinuities in rock masses masses can be found in a recent IS RM document (ISRM (IS RM , 1981). It is believed that in the case of surface excavations and those near-surface underground rock excavations which are controlled by the structural geological features, the following classification parameters are important: strength of intact rock material, spacing of discontinuities, condition of discontinuities, orientation of discontinuities and groundwater conditions. In the case of deep underground excavations where the behavior of rock masses is stress controlled, knowledge of the virgin stress field or the changes in stress can be of greater significance than the geological parameters. Most civil engineering projects, such as tunnels and subway chambers, will fall into the first category of geologically controlled rock mass structures. Rock classifications may be conveniently divided into two groups: intact rock classifications and rock mass classifications.
It is simple, easily easil y remembered, remembered, and understandabl e;
geotogtts*1te™ geotogt ts*1te™ 1SdCar SdC ar 3nd the iermin0l 0gy Used is widel>' accepted by engineers engineers and c) The most significant properties o f the rock masses are included; d) It is based on measurable parameters which can be determined by releva nt tests quickly and cheaply in the field; ^au ues ts. e) 11 “ based on a rati ng system system that can weigh the relative relat ive importa nce of the classification parameters; and f) It is functional by providing quantitative data for the design of rock support Classification parameters .-
An importan imp ortantt issue.,in rock roc k classifications is the selection of the parameters of greatest 7 * pr s ,o b t • » * " ■ » » « « o. o. i . * , -¡ -¡e h r s r s q antitativ ely describe a jointed rock mass ,for engineering engineering purposes purposes Various
Intact rock classifications classifications
The subject of intact rock strength classification is a fairly controversial topic since a number of classifications for rock material strength have been proposed. For . completeness, pleteness, they are compared i n Table Tabl e 6.2. The engineering classificat ion proposed by Deere and Miller (1966) has been widely recognized as particularly realistic and convenien convenientt for useinth e field of rock mechanics mechanics.. Recently, the IS RM Commission on Rock Classification has recommended different ranges of values for intact rock strength strength (ISRM (IS RM , 1981). 1981). The main reason for the the new IS R M ranges was the opinion t hat the Deere-Miller classification did not include differentiation in the strength in the range below 25 MPa, It should also be noted that this led to a recommendation that the convenient convenient value o f 1 M Pa (145 Ibf/in2) for the the uniaxial compressive strength may be taken as the lowest strength limit for rock materials. Hence, the materials with a strength lower than 1 M Pa should be considered aass soils and described described in accordance accordance
100
Err
Terzaghi’s rock load classification
' ^cal. methods o f design
~
specifically for tunnels and chambers while the Geomechanics Classification, although also initially developed for tunnels, has been applied to rock slopes and foundations, ground rippability assessment, as well as to mining problems (Laubscher, 1975, Ghose
Table 6.2 Various strength classifications for intact rock
0.5 0 7
2
i
3
_J
» i i »LL
4 5 6 78 ,
20
1—1—I—.Li ? I I
-
30 40 50
_ J _ __ __ _I _ I
Very weak
Moderately
Weak
Soil — —
Extremely low
strength
200
-J
300 4 00
__ _
St rong
Medium
High
Very high
strength
strength
strength
Moderately
Strong
and Raju, 1981, Kendorski et al, 1983).
V er y st rong
Low
____ strong
700
1 -J...J-1J
strength
____
weak
100
I t i l i
W ea k
Very low strength
Very weak
70
I
TERZAGHI’S ROCK LOAD CLASSIFICATION
Very
Extremely
„strong_
__ __stropg______
Geological Society
Rock
Very low strength
low strength
Very soft
Medium strength
High strength
Soft rock
rock
Very l ow strength
Very low
Low strength
1
I
2
3
I
T TT T T T
4 .5 6 7 8
10
i
20
•Extremely
Very high strength
Broch andFrankGn
strength
Terzaghi (1946) formulated the first rational method of evaluating rock loads -appropriate- to..theldesign_at steel sets. Thi s .ym an jm jw rta nt development because support by steel sets has been the most commonly used system for containing rock tunnel excavations during the past 50 years. It must be emphasized, however, that whil e this classification is appropriate for the purpose for Which it was evolved, Le., for estimating rock loads for steel-arch supported tunnels, it is not so suitable for modern
Extremely hard rock
rock
rock
I*1
Low
Medium
High
Very "high
strength
strength
strength
strength
Moderate
Medium
High
•Very high
i i n Ti li 70 100
30 40 50
—i
--
200
SURFACE ISRM 1979
r—i i n
300 400
700
Uniaxial compressive strength, MPa
Ro ck m ass classifications
Of the many rock mass classification systems in existence today, si x require special attention because they are most commonly known, namely, those proposed by: Terzaghi (1946), Lauffer (1958), Deere (1964), Wickham, Tiedemann and Skinner (1972), Bieniawski (1973), and Barton, Lien and Lunde (1974). The rock load-classification of Terzaghi (1946), was the first practical classification system introduced and has been dominant in the United States for over 35 years, proving very successful for tunneling with steel supports. Lauffer’s classification (1958) was based on the work of Stini (1950) and was a considerable step forward in the art of tunneling since it introduced the concept of the stand-up time of the active span in a tunnel, which is highly relevant in determining the type and amount of tunnel support Deere’s classification (1964) introduced the rock quality designation (RQD) index, which is a simple and practical method of describing the quality of rock core from boreholes. The concept of rock structure rating (R SR) , developed in the United States by Wickham, Tiedemann, and Skinner (1972, 1974), was the first system featuring classification ratings for weighing the relative importance of classification parameters. The Geomechanics Classification ( RM R system) proposed by Bieniawski (1973) and the Q-system proposed by Barton, Lien and Lunde (1974) were developed independently
Figure 6.1. Simplified diagram of tunnel rock-load (after Terzaghi, 1946). During construction of a tunnel, some relaxation of the rock mass will occur above and^on the sides of the tunnel The loosened rock with in the area acdb will tend to move in towards the tunnel. This movement w II be resisted by friction forcjS along the lateral boundaries ac and bd and these friction forces transfer the major portion of the overburden weight If onto the material on either s.de of the tu n n e l; ro rf a n d s,de of the tunnel are required only to support the balance which is equivalent to a h^'ght
^ °
^
Terzaghi’s rock
J ü î f ,6'3:, ^ rZaghi’S roc)c load Ossificat ion of 1946. 1.5(5
' “ feet ° f r0Ck ° n ,Unnel r0° r Wi,h wid‘h B ( ft) and height H,( f I at depth of more than
Rock condition
Rock load H in feet
1. Hard and intact 2. Har d stratified or schistose
s e r g £ u n t i c c ) ^ a a r p m c y F s ( <
Light lining required only if spallrng or popping occurs. Light support, mainly for protec tion against spalls. Load may change erratically from point -to-pointr~-------- v“----—
0 to 0.55
3. Mas sive, moderately jointed 4. Mod erate ly blocky and seamy 5. Very blocky and seamy 6. Completely crushed
Tabl e'6.4. Terzagh i’s rock load classification as modified by Deere et al., 1970
Remarks
Zero
0 to 0.255 0.25B to 0.35(5 + H )
Little or no side pressure.
9C
towards bottom of tunnel
8. Squeez ing rock, great depth 9. Swelling rock
(2.10 to 4.50) (B + jy ()
0
0
U -
Lining only ifspalling orpopping
0.255
^ ^ O c 3 2 w (U O »-<+J
Spalling common
0
0.5B
2 g, g 6
Side pressure if strata inclined, some spalling
0
0.25B to 0.35C
0 to 0.6C
0.35 C to 1.1C
3. Massive, mode rately jointed 4.-Moderatelyblocky andseamy
-20
Heavy side pressure, invert struts
75
required. Circular ribs are recommended.
Up to 250 feet, irrespective of the value of (B +ff)
0
9.
Considerable side pressure.
require either continuous
(<■10 to 2.10)( S +H ,)
Remarks
Initial Final
stratified or schistose
No side pressure.
support for lower ends of ribs or circular ribs.
Rock load, H
9
Softening effects of seepage
7. Squeezing rock, moderate ' depth
Rock condition
1. H ard and intact
-50
(0.35 to 1.I0)(5 4-H) ¡■IO(B + ff J
’ classificatori !103
- 10
5. Ve ry blocky, seamyand shattered
_i i
Circular ribs are required. In
25
extreme cases use yielding
6. Completely crushed
support.
2>*§ 2 8 O op g g p fl
Little or no side pressure Considerableside pressure. I f seepage, continuoussupport
1.1C
10
Definitions:
- ........... .............................
-5 2 blasting. This is L I as a condition involving the spontaneous iB d
“7
^
^
h~
7. Grav el and sane
* * *
^ T reS'Stance against sePa«tion along the " ttned b* ^nsverse joints. In such rock, the
^ J f a d o n ot ^
both spalling and popping conditions may te ^counter ed
separated from s up po rt . ■
T
^
boundaries between stra ta'T h^ ra^ ma yTr m'th ' 'f h spalling condition is quite common * "
together or L i nt im at el y t a e S d S l at “
*
-2
8. Squeezing,
b ct Weer lj oi nf s a re l oc a, |y g ™ 1™ SUPP°r i' In f° CkS ° f this type’
t a ^ S ^ J t S r i t T J h ^ k r0Ck fragn,“ (S WhiCh “ P « 'O ' in t er lo c ke d . I n s uc h r oc k, v er ti ca l wal ls m ay r equ ir e l at er al
moderate depth TJ
i n a e r k e a e h ò W c
■ ■9. Squeezing,
greatdepth — 1
0. Swelling the properties of a water-bearing sand.
r° ° k below the water tabh exhibits
~ minerals with a low swell ing capacity.
•
0.54C to 1.2C
0.62C to 1.38C
0.94C to 1.2C
1.08C to 1.38C
Loose
1.1C to 2.1C
Heavy side pressure. Continuous support required
Dense Side pressure Ph= 0.3y(0.5H, + Ifp)
2.1C to 4.5C up to 250ft.
Use circular support. In extremecases:yielding support
„
icroscopic particle s of micaceous minerals or of clay
—
...................
..
. .
........ ...... .
.......
..
RSR (Rock Structure Rating) concept
104 Empiricr
105
thods o f design
tunneling methods using shotcrete and rockbolts. After detailed studies, Cecil (1970) concluded that Terzaghi’s classification was too general to permit an objective evaluation of rock quality and that it provided no quantitative information on the properties of rock masses. The main features of Terzaghi’s classifi cation are depicted in Figu re 6.1 and are listed in Tables 6.3 and 6.4. The latest revision of Terzaghi’s rock-load coefficients was presented by Rose (1982).
RQD, percent
Ro'ck quality
<25 25-50 50-75 75-90 90-100
Very poor Poor Fair Good Excellent
Hendronrand-Deere (-1972) attempted to relate the R Q D index to Terzaghi s rock load factor. They found a reasonable correlation for steel-supported tunnels but not for openings supported by rockbolts. This supports the opinion that Terzaghi s rock load concept should be limited to tunnels supported by steel sets (Cording and Cording,
1.AUFFER-PACHER CLASSIFICA TION ' ■ The 1958 classification by Lauffer has its foundation in the earlier work on tunnel geology by Stini (1950) who is considered the father of the ‘Austrian School of tunneling and rock mechanics. Stini emphasized the importance of structural defects in rock masses. Lauffer proposed that the stand-up time for any active unsupported rock span is related to the various rock mass classes. An active unsupported span is the width of the tunnel or the distance from the face to the support if this is less than the tunnel width. The stand-up time is the period of time that a tunnel will stand unsupported after excavation. It should be noted that a number of factors may aflect the stand-up time, such as orientation of tunnel axis, shape of cross-section, excavation method and support method. Lauffer’s original classification is no longer used since it has been modified a number of times by other Austrian^engineers, notably by Pacher, •von Rabcewicz and Golser (1974). Pacher’s contributions were particularly notable and are well summarized by Edeling and Maidl (1980). The main significance of the Laufler-Pacher classification is that an increase in tunnel span leads to a major reduction in the stand-up time. This means, f o r example, that while a pilot tunnel having a small span may b e s u c c e s s f u l l y constructed full lace in fair rock conditions, a large span opening in this same rock may prove impossible to support in terms of the stand-up time. Only with a system of s m a l le r headings an benches or multiple drifts can a large cross-section tunnel be constructed m such roc C0' i russ ifica tion introduced the stand-up time and the span asr e l e v a n t parameters in determining the type and amount of tunnel support, and it has influenced the development of more recent rock mass classification systems. DEE RE’S ROCK QUALITY DESIGNATION
,
Deere proposed in 1964 a quantitative index based on a modified core recovery procedure which incorporates only sound pieces of core that are 100 mm or greater in .length. This rock qualit y designation (RQ D) has been widely used and has been found very useful for the selection of tunnel support, seeTable 6.5. Fo r R QD determination,, the International Society for Rock Mechanics recommends a core size of at least NX diameter (54 mm) drilled with double-tube diamond drilling equipment. The following
Deere 1972) . Merritt (1972) found that the R QD could be of considerable value in estimating support requirements for rock tunnels (see Table 6.5) bu t pointed out a limitati on of the RQD index in areas where, the joint s contai n thin clay fillings o r weathered material. The influence of clay seams and fault gouge on tunnel stability was discussed by Brekk e and Howard (1972). . . , Although the RQD is a quick and inexpensive index, it has limitati ons such as ^ disregarding of joint orientation, tightness, and gouge (infilling): mateua Consequently, while it is-a practical parameter for core quality estimation, it is no sufficient on its own to provide an adequate description of a rock mass.
RSR (ROCK STRUC TURE RATING) CONCEPT The RSR concept, a ground support prediction model, was developed in the United States in 1972 by Wickham, Tiedemann, and Skinner. The concept presents a quantitative method for describing the qua lity of a rock mass and for se ectmg I e appropriate ground support. I t was the f i r s t complete rock mass classification syste proposed since that introduced by Terzaghi in 1946. . The RSR concept was a step forward in a number of respects fi s y, quantitative classification, unlike Terzagh.’s qualitative one; secondly, it was a rock mass classification inc orporat ing many parameters unlike the R QD index that is limitedto core quality; thi rdly, it was a complete classificat ion Having an mpu an output unlike a Laufier-type classification that relies on practical experience to decide on a rock mass class, which then gives an output in terms of the stand-up time and span The main contribution of the RSR concept was that it introduced a rating system rock'masses. This was the sum of the weighted values of the individual- parameters considered in this classification system. In other words, the relative i mportance o the various classification parameters could be assessed. Tim rating system was determined on the basis of case histories as well as reviews of various books and technical paper dealing with different aspects of ground support it tunneling. ••
Ta blç 6.1 Sup port recomm enda tions for tunnels in rock (6 m to 12m diameter) based on RDQ (after Deere et al„ 1970)
Rock quality Tun neling me thod . . ...
Airprri**;,,~ Altern ative sup po rt systems ;Steel sets2
Excellent1 R Q D > 90
. Boring machine Conventional
Good1 75 c R Q D < 9 p
Boring machine
Conventional Fair 50 < R Q D < 7 5
Boring machine
C o n v e n t io n a l Poor2 25 < RQ D < 50
Boring machine
Conventional
N o n e to oc c. li gh t se t. R oc k l o a d ( 0 . 0 - 0 . 2 ) j B N o n e to oc c. lig ht se t. R oc k l o a d (0.0-0'.3)B Occ. light sets to pattern on 5ft to 6-ft ctr. Rock load (0 0 . to 0.4 )B Light sets, 5-ft to 6-ft ctr. Rock load (0.3 to 0.6)5 ■Light to medium sets, 5-ft to 6-ft ctr. Rock load (0 4-
Rockbolts3
N on e to o cc as io na l N on e to o cc as io n al
Occasional to pattern on 5-ft to 6-ft centers Pattern 5-ft to 6-ft centers
Pattern, 4-ft to 6-ft ctr.
Sho terete
N o n e to oc c. lo ca l ap p lic at io n N o n e to oc c. io ca i ap p h c at io n 2 in. to 3 in. N o n e to oc c. lo ca l ap p li ca tio n 2 in. to 3 in. Occ. local application 2 in. to 3 in. 2 in. to 4 in. crown
1. 0 ) £
Light to medium sets, 4-ft to 5-ft ctr. Rock load (0 61.3)5 Medium circular sets on 3-ft to 4-ft ctr. Rock load (l Q1.6)B
M e d i u m t o h e a v y s e ts o n 2 - f t
Pattern 3-ft to 5-ft ctr.
Pattern, 3-ft to 5-ft ctr.
4 i n . o r m o r e c r o w n a n d s id e s
4 in. to 6 in. on crown and sides. Com bine with bolts
110
EmPirical methods of design
i?£R (Rock Structure Rf
Support requirement charts have been prepared that provide a means of determining typical ground support systems based on RSR prediction as to the quality of the rock mass through which the tunnel is to be driven. C harts for 3 m, 6 m, 7m, and 10 m diameter tunnels are available, an example being given in Figure 6.3. The three steel rib curves reflect typical sizes used for the particular tunnel size. The curves for rockbolts and shotcrete are dashed to emphasize that they are based on assumptions and were not derived from case histories. The charts are applicable to either circular or horseshoe-shaped tunnels of comparable widths. ___The RSR concept is a very useful method fox.selecting^teeJ_rib_sjipportJor rock tunnels. As with any empirical approach, one should not apply the concept beyond the range of the sufficient and reliable data used for developing it. For this reason, the RSR concept is not recommended for selection of rockbolt and shotcrete support. It should be noted that, although definitions of the classification parameters were not explicitly stated by the proposers, most of the input data needed would be normally included in a standard joint survey; however, the lack of definitions (e.g., ‘slightly faulted’ or ‘folded’ rock) may lead to some confusion. A practical example using the RSR concept follows: Consider a 6m diameter tunnel to be driven in a slightly faulted strata featuring medium hard granite. The joint spacing is 2 ft and the joints are open. The estimated water inflow is 250gal/min per 1000 ft of the tunnel length. The tunnel will be driven against a dip of 45 degrees and perpe ndicular to the jointing.
The SUPP°rt m s distnbuted as follows;
Sections with steel ribs sections with rockbolts
147 14 3
ctlons with shotcrete Total support Total unsupported
164 26
Total
( M£%) ( 100.0%)
- wsections v u ym 190
^
^
T
s
s
i h
he
t h e b S Tah ^
m h ° rs « was adequate ouf interaction between adiar ^ aCted in te™ '°no nly ft w T ^ thatMci^ age for- d by the b0fe I f ™ . bt o * or for an a s s ^ ^ either for mck loads were dewl tunnels. Nevertheless th r , .MmPress,°n arch
«
■
,0«, '„ ( 2 4 » 0?
8 rel‘ " M « * " » ' « 2 5 » I S
S
"
Spacing (ft) = 24 W
■
’
t
'
*
1'
-
!
«
,
'
,
,
,
»
,
,
No correlation rn„M u t wuiD/ft •
t =
where
i +_!!_
'1concept 111
,6 I >
.
1.25 0 r i =j | ( 6 5 - R SR ) ( 6.2)
108 Em nirical methods o f design
.
RSR (Rock Structure Ratin g) concept 1f
geologic parameters were: a) rock type, b) joint pattern (average spacing of joints), c) joint orientations (dip and strike), d) type of discontinuities, e) major faults, shears, and folds, f) rock material properties, and g) weathering or alteration. Some of these factors were treated separately; others were considered collectively. The authors pointed out that in some instances it would be possible to define accurately the above factors, but in others, only general approximations could be made. The construction parameters were: a) size of tunnel, b) direction of drive, and c) method of excavation. All the above factors were grouped by Wickham, Tiedemann, and Skinner (1972) into three basic parameters, A, B, and C (Tables 6.6,6.7, and 6.8, respectively), which in themselves were evaluations as to the relativ e effect of various geological factors on the support requirements. These three parameters were as follows: a) Parameter A: General appraisal of a rock structure is on the basis of: 1. Rock type origin (igneous, metamorphic, sedimentary). 2. Rock hardness (hard, medium, soft, decomposed). 3. Geologic structure (massive, slightly faulted/folded, moderat ely faulted/folded, intensely faulted/folded). b) Parameter B: Effect of discontinuity pattern with respect to the direction of tunnel drive is on the basis of: 1. Jo int spacing. 2. Joi nt orientation (strike and dip). 3. Direction of tunnel drive. c) Parameter C: Effect of groundwater infl ow is based on: 1. Overall rock mass quality due to parameters A and B combined. 2. Join t condition (good, fair, poor). 3. Amount of water inflow (in gallons per minute per 1000 feet of the tunnel). The RSR value of any tunnel section is obtained by summarizing the weighted numerical values determined for each parameter. This reflects the quality of the rock mass with respect to its need for support. Since a lesser amount of support was expected for machine-bored tunnels than when excavated by drill and blast methods, it was suggested that RSR values be adjusted for machine-bored tunnels in the manner given in Figure 6.2.
Table 6.7. Roc k structure rating-P aramet er Average joint spacing
joint pattern, direction ofdrivefafter Wickh am et al., 1974) Strike || to axis
Strike 1 to axis
Direction of drive
Direction of drive. Both
With dip
Dip of prominent joints* Flat Dipping Vertical 1. Ve ry closely jointed <2in. . 2. Clos ely jointed 2-6 in. 3. Moderately jointed 6-12 in. 4. Moderate to blocky 1-2 ft. 5. Block y to massive 2-4 ft 6. Massive > 4ft..
Against dip
Both
Dipping Vertical
Dip of prominent joints* Flat Dipping Vertical
_9
-] -]
13
13
16
19
10 15
12 17
14
14
11
23
24
28
19
22
23
23
19
30
32
36
25
28
30
28
24
36 40
38 43
40 45.
33 37
35 40
36 40
34 38
28 34
-
Table 6.8. Rock structure rating - Parameter C : ground water, joint'condition (after Wickh am et al., 1974) Anticipated water ■(gpm/1000ft)
None Slight < 200 gpm Modera te 200-1000 gpm Hea vy > 1000 gpm
Sum of parameters A + B 13-44
45-75
Joint condition** Good
Fair
Poor '
Good
Fair
Poor
22 19 15' 10
18 15 11 8
12 9 7 6
25 23 21 18
22 19 16 14
18 14 12 10
•Dip: flat: 0-20deg; dipping: 20-50deg; and vertical: 50-90deg. •»Joint condition: Good = tight or cemented; Fair = slightly weathered or altered; Poor = severely Table 6.6. Rock structure rating - Parameter A: general area geology (after W ickham et al., 1974) Basic rock type Hard Med. Soft Igneous Metamorphic Sedimentary Type 1
1 1 2
2 2 3
3 3 4
weathered, altered, or open.
Geological structure Decomp. Massive 4 4 4 30
Slightly
Moderately
. Intensely
faulted or faulted or ! folded folded
faulted or ■ folded
22
9
15
It should be noted that Tables 6.6,6.7, and 6.8 are reproduced not from the original 1972 reference but from a report published two years later. T he RS R ratings were changed in 1974 and the latter report represents the latest information available. A total of 53 projects were evaluated, but since each tunnel was divided into typical
113
Geomechanics Classification (RMR system)
112 Empirical methods o f design Solutio n: Fr om Table 6.6: Fo r igneous rock of medium hardness (basic rock type 2) in slightly faulted rock, parameter A = 20. From Table 6.7: For moderate to blocky jointing, with strike perpendicular to the tunnel axis and witha drive against the dip of 45 deg, parameters = 25..From Table 6.8: For A + B = 45, poor joint condition and moderate water flow, parameter C = 12. Thus: RS R = A + B + C = 57. From Figure 6.3, the support requirements for a 6 In dia. tunnel with RSR =57 (estimated rock load 1.5kips/sq ft) will be 6H20 steel ribs at 6-ft spacing.
GEOMEC HANICS CLA SSIFICATION (RMR SYSTEM) The Geomechanics Classification or the rock mass rating (RM R) systemwas developed by Bie niaw ski in 1973. This engineering classification of rock masses, utilizes the following six parameters, all of which are measurable in the field and can also be obtained from borehole data: a) Uniax ial compressive strength of intact rock material; b) Rock quality designation (R QD ); c) Spacing of discontinuities; d) Condition o f discontinuities; e) Groundwater conditions; f) Orientation of discontinuities. To apply the geomechanics classification, the rock mass along the tunnel route is divided into a number of structural regions, i.e., zones in which certain geological features are more or less uniform within each region. The above six classification parameters are determined for each structural region from measurements in the field and entered into the standard input dat a sheet as shown in Chapter 5 (Fig. 5,17). The Geomechanics Classification is presented in Table 6.9. In Section A~of Table 6.9, the first five parameters are grouped into five ranges of values. Since the various parameters are not equally important for the overall classification of a rock mass, importance ratings are allocated to the different value ranges of the parameters, a higher rating indicating better rock mass conditions. These ratings were determined from 49 case histories (Bieniawski, 1976). Once the classification parameters are determined, the importance ratings are assigned to each parameter accor ding to Table 6.9, Section A. In this respect, the typical rather than the worst conditions are evaluated. Furthermore, it should be noted that the importance ratings, which are given for discontinuity spacings, apply to rock masses having three sets of discontinuities. Thus, when only two sets of discontinuities are present, a conservative assessment is obtained. After the importance ratings of the classification parameters are established, the ratings for the five parameters listed in Section A of.Table 6.9 are summed to yield the basic rock mass rating for the structural region under consideration. At this stage, the influence of the strike and dip of discontinuities is included by adjusting the basic rock mass rating according to Section B of Table 6.9. This step is treated separately because the influence of discontinuity orientation depends upon
STAND-UP TIME, hr ^ Í5~f\] jrA j C'l __ . 5 = 0 ' Figure6.4. Geomechanics Classification of rock masses: output for mining amr tunneling; • = histories of roof falls in mining; □ = tunneling roo f falls; contour lines = limits of applicability.
but by qualitative descriptions such as ‘favorable’. To facilitate a decision whether strike and dip orientati ons are favorable o r not,"reference should be made to Table 6.10, which is based on studies by Wickham, Tiedemann, and Skinner (1972). In the case of civil engineeringprojects, an adjustment for disconti nuity orientations wil l suffice. For mining applications, other adjustments may be called for such as the stress at depth or a change in stress (Kendorski et aL, 1983). After the adjustment for discontinuity orientations, the rock mass is classified according to Section C of Table 6.9, which groups the final (adjusted) rock mass ratings (RMR) into five rock mass classes. Note that the rock mass classes are in groups of twenty ratings each. Next,'Section D of Table 6.9 gives the practical meaning of each rock mass class by relating it to specific engineering problems. In the case of tunnels and chambers, the output from the.Geomechanics Classification is the stand-up time and the maximum stable rock span for a given rock mass rating, as depicted in Figure 6.4. Support load can be determined from the Geomechanics Classification as (Unal, 1983): • „
100-RMR n
,
P = --------- yB = yh,
100
.
,, r,
(6.3
Tab le 6.9. Geo me chanic s Classifica tion of rock A . C l a s s i fi c a t io n p a r a m e t e r s a n d t h e i r r a t in e s , - ' PARAMETER Strength
RANGES
Point-load strength index
intact rock material
o f
v a l u e s
For this low.range — u ni a x ia l c o m p r e s sive test is preferred
2 - 4 MPa
Uniaxial compressive strength
qualit y RQD
5-25
1-5
< i
MPa
MPa
MPa
90% - 100%
Rating S p a c i n g o f d i s c o n t i n ui t é :60 mm
Rating
Condition of discontinuities
i t. V e r y r o u g h s u r f a c e s . Not continuous No seperatlon Uhweathered wall rock.
(Slightly rough surfaces. xSepjy^tion < Tmm Slightly weathered walls
Slightly rough surfaces. Separation < 1 m m Highly weathered walls
Rating nfJoW per 10 m tunnel length ‘ Ground water
r
joint water in Pfesaure major principal _ _________ s t r e s s
None
<10 l i t r e s/ m i n
10-25 Utres/mii
Slickensided ¡surfaces,
t/*er
¡ Go u g e < ' 5 m m t h i c k ¡
3f t g o u g e > 5 m m . t h i c k
S e p a r a t i o n 1 -5 m m . j C o n t i n u o u s _______
Separation > 5 m m . Continous
R
i
25 - 125 l l t r e s/ r n l / i
OR ',
■ >
118 Empirical m ethods o f design
Geomechanics Classification ( f
system) 119
Table 6.12. Adjustments to the Geomechanics Classification for mining applications Strength of intact rock
Blasting damage adjustment AB
Rating: 0-15
0.8-1.0
Discontinuity density RQD: 0-20 Spacing: 0-20
Discontinuity orientation adjustment
In-situ stress & change of stress adjustment A
Rating: 0-40 Basic RMR 0-100
Discontinuity condition Rating: 0-30
Adjusted RM R
Groundwater condition Rating: 0-15
R M R x / t ^ x A j X S ........... —
max. 0.5
Support recommendations
design of slopes near the tunnel portals as well as allow estimates of the deformability of foundations for such structures as bridges and dams. , in the case of rock foundations, the rock mass rating R M R from the Geomechanics Classification has been related(Bieniawski, 1978) to the in situ modulus of deformation in the manner shown in Chapter 5, Figure 5.12.
116 Emp ! ~~nl methods o f design
v '
'
Geomechanics Classification (RMR system)
Table 6.10. E lle d of discontinui ty strike and dip orientations in tunneling )!
¡' p i
Strike perpendicular to tunnel axis
©
Drive with dip Dip 45°—90°
Dip 20°-45°
Drive against dip Dip 45"—90'
Very favorable
Favorable
Fair
O '
Dip 20°-45‘ 16000
Strike parallel to tunnel axis -Dip -20p —45—---- --- --D ip-45— 90sFair
O OD
Unfavorable
v m 14000 / / r J ---- o ^ P k a ,
Irrespective of strike nip o- a -----------
Very unfavorable
: : c » .
L E N N U 6 T / A O - F < O
-Se-' h e s t e>n\.á¿ 'a s ca g e
100-RM R
B
is the rock-load height in meters'
(6.4)
m
where B is the tunnel width in meters;
R M R is the rock mass rating from the Geomechanics Classification; y is the density of the rock, kg/m3.
The variation of the rock-loads from equation (6.3) for various rock classes as a function of roofspan is presented in Figure 6.5. The Geomechanics Classification provides guidelines for the selection of roof support ■ to ensure long-term stability of various rock mass classes, as given in Table 6.11. These guidelines depend on such factors as the depth below surface (in situ stress), tunnel size and shape, and the method of excavation. It should be noted that the support measures given in Table 6.11 represent the permanent and not the primary support. Hence, additional concrete lining is not required for structural purposes. However, to ensure full structural stability it is recommended that tunnel monitoring during construction be undertaken to provide a check on stabili zation of ro ck movements. The Geomechanics Classifi cation has been used extensively in mining, particul arly in 1 the United States. Initially, Laubscher and Taylor (1976) applied the Geomechanics Classification in asbestos mines in Africa specifically to assess cavability of ore, while Ferguson (1979) extended this classification to mining tunnels and haulages. Since mining is a dynamic process, additional adjustments to the classification parameters were introduced, such as in-situ stresses, as shown in Table 6.12. Most recently, the Geomechanics Classification was applied to coal mining in the United States (Bi enia wski etal., 1980, Newman, 1981, Unai, 1983) and in India (Ghos e and Raju, 1981) as well as to hard rock mining in the USA (Cummings et aL, 1982, Kendorski et aL, 1983). Furt her details of mining applicati ons are given in Chapt er 10 both for
12000
10000 H T G N E L ^ C T a I z N e U
8000
R s E s P a D A O L K C O R
6 00 0
4000
2000
10
15
20
25
SPAN , m Figure 6.5. Vari atio n of rock-load as a function of roof span in different rock classes in the Geomechanics Classification (after Unal, 1983). .
122 Empirical methods of design Qrsystem
Table 6,13. Clarif icati on ratings for Q-systera (after Barton, 1976)
123
Table 6.13 (continued) 1. Descriptions and ratings parameternRQ w for the — ,— y uD Rock quality designation A. Very poor B. Poor C. Fair D. Good E. Excellent
Co Vf lisA -
4. Descriptions and ratings for the parameter J a
"
(RQD , %)
Joint alteration number a) Rock wall contact
0-25 25-50 50-75 .
75-90_ 90-100
•
V.)
A. Tightly healed, hard, non-softening, impermeable filling i.e. quartz or epideteB. Unaltered joint walls, surface staining only C. Slightly altered joint walls. Non-softening mineral coatings, sandy
(approx) 0.75 1.0
_particles,-day-free-disintegrated-roek-tec;---------- --------------~20 D. Silty-, or sandy-clay coatings, small clay fraction (non-soft.) 3.0 E. Softening or low friction clay mineral coatings, i.e. kaolinit e or mica. Also chlorite, talc, gypsum, graphite etc., and small quantities of ■ swelling clays. 40 b) Rock wall contac t before 10cm shear
Join t set number A. Massive, no or few joints B. One join t set C. D. E. F. G.
F. San dy particles', clay-free disintegrated rock etc. G. Strongly oveNconsoIidated non-softening clay mineral fillings (continu ous, but < 5 mm thickness) H. Medium or low over-consolidation, softening, clay mineral fillings (continuous but < 5 mm thickness) J. Swelling-clay fillings, i.e. montmorillonite (continuous, but <5 mm thickness). Value of J a depends on percent of swelling clay-size particles, and access to water etc. c) No rock wall contact when sheared K. Zones or bands of disintegrated
W 0.5-1.0
One join t set plus random Two joint sets Two joint sets plus random Three joint sets Three joint sets plus random
2 3 -4~ 6
9
H. Fou r or more joint sets, random, heavily jointed “ sugar-cube" etc. J. Crushed rock, earthlike •
12 15
or crushed rock and clay (see G, H, J for description of clay condition)
20
Note:
L. Zones or bands of silty- or sandy-clay, small clay fraction (non-softening) M. Thick, continuous zones or bands of clay (see G, H, J for
(i) F or intersections use (3.0 x J ). (ii) F or portals use (2.0 x J J
description of clay conditi on)
H ' (25-35°) (25-^30°)----(20-25°)
-
. 2. Descriptions and ratings for the parameter ./„
W,)
(8-16°)
40 ■ gQ
(25-30°) (16-24°)-
gp
'.(12-16°)
g _j 2
6 8 or g _j2 5.0
( 6 - 12°)
(6-24°)
H
jq 13 or 13-20 '
(6-24°)
3: 'Descri ptions and ratings for the parameter J, 5. Descriptions and ratings for the parameter J w
Joint roughness number a) R ock wall contact and A. BC. D.
Joint water reduction factor
b) Ro ck wall contact before 10cm shear Discontinuous joints Rough or irregular, undulating Smooth, undulating Slickensided, undulating
E. Rough or irregular, planar F. Smooth, planar. G. Slickensided, planar ; : ■ Note:
C/J 4 ^ -?.■
"•
A. Dry excavations or minor inflow, i.e. < 5 1/miu. locally B. Medium inflow or pressure, occasio nal outwash of joint fillings C. Large inflow or high pressure in competent rock with unfilled joints D. Large inflow or high pressure, considerable outwash of joint fillings
1.5 .ii? 1,0 •" 0.5
!ii;le
r.
.
.
Exceptionally high inflow or water pressure continuing without notice able,decay
'. ■
0.2-O.i
>10
CU 0.05
>10
Note:
c) No rock wall contact when sheared
Note:
.
To
(i) Factors C to F are crude estimates. Increase J w if drainage measures are installed. (ii) Special problems caused by i ce formation ar e not considered. 6. Description and ratings for parameter SR F
Apprux. water pres. (kg/cm2) <1 1-2.5 2.5-10 2.5-10
;q 0.66 0.5 0.3
E. Exceptionally high inflow or water pressure at blasting, decaying with
(i) ^ t i o ^ r e f o to small scale features and intennediate scale features, in that order
H : Z°n e containing clay minerals thick enough to prevent rock wall contact ' j 0 J. Sandy, gravelly or crushed zone thick enough to prevent rock wall contact
(j j
I
Kihplrlcal mctlwils o f design
' "Mn '■"
I
]-system
*:l,“ m°*,lon r,lln“ ' r°r Q-IJWom (after Barton, 1976)
I>o«crl,.Udnl and ratings for the parameter RQ D
Rock quality designation A. Vciy poor
( V t^ o r
Co Yd îsA
■
4. Descriptions and ratings for the parameter J 0
(RQD, % )
II, Poor
0-25
C, Fair
25-50
D. Good nxccllcnt
50-75 75-90
Joint alteration number a) Rock wall contact
'> y ■
0 ). v t,' v c b
ài-.
< .
ij
W,) (approx.)
A. Tightly healed, hard, non-softening, impermeable filling i.e. quartz or epldete' . 90-100
Q? j
B. Unaltered joint walls, surface staining only j q C. Sli ghtly altered joint walls. Non-softening mineral costings, sandy ------ -i a« icfes,-day-free-dismtegrated-roek-tec:---------- ----------------20 D. Silty-, or sandy-clay coatings, small clay fraction (non-soft.) 30 E. Softening or low friction clay mineral coatings, i.e. kaolinite or mica. Also chlorite, talc, gypsum, graphite etc., and small quantities of swelling clays, 40 b) Rock wall contact before 10cm shear
B
Ofc\\\£c'C<û
Descriptions and ratings for the parameter J n
Join t set number A. Massive, no or few joints B. One joint set
(*U 0.5-1.0
c. One join t set plus random
2 3 -4~ 6
D. Two joint sets E. Two joint sets plus random F. Three joint sets G. Three joint sets plus random H. Fou r or more joint sets, random, heavily join ted sugar-cube" etc. J. Crushed rock, earthlike •.
912 15 ..
Note:
20
W F or intersections use (3.0 x J ). (ii) Fo r portals use (2.0 x J J
F. Sandy particles’, clay-free disintegrated rock etc. ' G. Strongly over-consolidated non-softening clay mineral fillings (continu ous, but < 5 mm thickness)
Descriptions and ratings for the parameter J r '
............'
(continuous but < 5mm thickness) J. Swelling-clay fillings, i.e. montmorillonite (continuous, but <5 mm thickness). Value of J, depends on percent of swelling clay-size particles, and access to water etc. c) No rock wall contact when sheared K. Zones or bands of disintegrated
80 '
or crushed rock and clay (see G, H, J for description of clay condition) L. Zones or bands of silty- or sandy-clay, small clay fraction (non-softening) M. Thick, continuous zones or bands of clay (see G, H, J for
6 8 or g jp 5.0
Joint water reduction factor (/r) 4 3
D. Slickensided, undulating E. Rough or irregular, planar . F. Sm ooth planar..
. ■
G. Slickensided, planar
"•
1.5 1.5 1,0' •• • 0.5
' Note:
; r r r ,“
7 -
^
< * •* -
Note:
A. B. C. D. E.
Dry exca vations or minor inflow, i.e. < 5 l/min. locally Medium inflow or pressure, occasional outwash of joint fillings Large inflow or high pressure in competent rock with unfilled joints Large inflow or high pressure, considerable outwash of joint fillings Exception ally high inflow or water pressure at blasting, decaying with time' '
(8-16°) (25-30°) (16-24°)’ ’ (12-16°)
( 6- 12°)
(6-24°)
B
!0 13 or 13-20 '
io¡0
(JJ
(6-24°)
0.5 0.3 0 2 0.1
> 10
0.1-0.05
> 10
1.0 0. 66 -
—
.
(1) Factor s C to F ar e crude estimates. Increase Jw if drainage measures are installed. (ii) Special problems caused by ice for mation are not considered. 6. Description and ratings for paramete r SR F
Approx. water pres. (kg/cm2) <1 1-2.5 2.5-10 2.5-10
.
F. Exception ally high inflow or water pressure continuing without noticeable, decay Note: :
-
f : Sandy, s a ltgravelly v T 8orclay T r * “ eD0Ugil t0 pr™ 'roct m|1 “ w crushed zone thick enough to prevent rock wall contact
J.
“(25-30°) ---(20-25°)
5. Descriptions and ratings for the parameter J n.
a) Rock wall contact and b) Rock wall contact before 10cm shear A. Discontinuous joints B. Rough or irregular, undulating C. Smooth, undulating
S
g_(2
..................
Joint roughness number
* ? N
gg
H. Me dium or low over-consolidation, softening, clay mineral fillings
description of clay conditi on) -3 :
40 ■
’
(25-35°)
_______
(«) RQD intervals of 5, i.e. 100, 95,^e ^ c. Îr^ suffic^nt"ySar a ra T ™ ' Va'Ue ^ ^ “ T * ‘° eVSlUateQ' 2.
123 '
Table 6.13 (continued)
124 E n r!-ical methods of design The ESR is related to the use for which the excavation is intended and the degrSi Table 6.13 (continued)
safety demanded, as shown below.
B. Single weak ness zones containing clay or chemically disintegr ated rock (deptb of
D. E. F. G.
A. B
2.5
excavation > 50 m) Multiple shear zones in competent rock (clay-free), ioose surrounding rock (any depth) Single shear zones in competent rock (clay-free) (depth of excavati on < 50 m) S ingle shear zones in competent rock (clay-free)'(depth of excavat ion > 50m) Loose open joints, heavily jointed or ‘sugar cube’ etc. (any depth)
7i 5Æ 2.55.0
excavation. b) Comp etent rock , rock stress problems
L. Mild rockburst (massive rock) M. Heavy rockburst (massive rock)
«■>! > 200 200-10 10-5 5-2.5 <2.5-
>13 13-0.66 0.66-.33 ■0.33—.16 <0.16
D.
E.
0.5-2 5-10 10-20
■
~ --- ------ ---- — ---- ------
Power , stations, major
highway or railroa d
1.0
Underg round nuclear power stations,
0.8
The relationship between the index Q and the equivalent dimension "I an
nv..li»ii
determines the appropriate support measures. Barton etal.(19 M) i.... 1,1 ■ " n
categories which pve estimates of permanent support. Fo. i - v determination,either Q is increased to 5Q or ES R is increased to i 1 1
(SR F)
--
for large excavations Storage rooms, water treatment plants, minor
railroad stations, factories
5 10 10 ® --
-
2.0 i.6
tunnels, civil defense chambers, portals, inter s e c t i o n s * ^ « ^ cV W W OYij F.
N. Mild squeezing rock pressure O. Heav y squeezing rock pressure d) Swe lling r ock: chemical swelling activity depending on presence of water
2 5
highway-and railroad tunnels, surge chambers, i " “ , r£ . i- p / C i i w ' © f a & c>
1.0
increase from 2.5 to 5for such cases (see H). c) Squeezing rock: plastic flow of incompetent rock under the influence of high rock pressure
-Mild.swelling-rock pressure---R. Hea vy swelling rock pressure
. Circular section
2
penstocks), pilot tunnels, drifts, and headings
Note: 10, reduce a( and c. to 0.8 c( (ii) Fo r strongly anis otropic virgin stress field (if measured): when 5
10, reduc e
- P,
Temporary mine openings Vertical shafts:
--------hydropower-(exGltiding-hig-h«ptess.ure
(SRF) 2.5 ‘
cJOl
No. of cases
3-5
rectangular/square section C. Permanent mine openings, water tunnels for
•Note :" . . . (i) Reduce these' values of SR Fb y 25-5 0% if the relevant shear zones only influence but do not intersect the
H. Low stress, near surface I Medium stress K. High stress, very tight structure (usually favorable to stability , may be unfavorable for wall stability)
ES R
Excavation category
excavation <, 50 m) C. Single weakness zones containing clay or chemically disintegrate d rock (depth of.
’"'I’I""
of the support measures using the Q-system, the reader should .ill ......... paper by Barton et al. (1974) or the book by Hoek and Bro wn ( l™ l) -The maximum unsupported span can be obtained as foll ow» ....
-
10-15
Maximum span (unsupported) = 2(ESR)Q0'4
Additional notes on the use of Table 6.13
The relationship between the Q value and the permanent .......... '*«'
When making estimates of the rock mass quality (Q) the following guidelines should be followed, in addition
calculated from the following equation:
to the notes listed above: ' . 1. Wh en bor ecore is unavailable, R QD can be estimated from the number of joints per unit volume, in which the number of joints per meter for each join tset are added. A simplerelation can be used to convert this number to RQD for the case of clay-free rock masses:
■P
*roof
= % j
^
(U )
If the number of jointfets is less than three, the equation is cxpirv.nl .r. n
;„ = total number of joints per m3; (RQD = 100 for J„< 4.5) 2. The parameter J„ representing the number of joint sets will often be affected by foliation, schistocity, ■slately cleavage or bedding, etc. If strongly developed these parallel ‘joints’ should obviously be counted as a complete joint set. However, if there are few ‘joints’ visible, or only occasional breaks in borecore due to these features, then it will be more appropriate to count them as 'random joints’ when evaluating J„. 3. The parameters f and J a (representing shear strength) should be relevant to the weakest significant
,
w
RQ D = 115 -3.3 ./„(approx.) where
.....
■roof
( 6 .9 )
_2 Jm - J- IQ - W 3
a
J r
V
4 When a rock mass contains clay, the factorSRF appropriate to.loosening I " ....
...
nln'uld be cviluited.
128 Empirical methods of design
Recent developments
etermine the ratings of the six classification parameters from Table 6.13 Step and calculate the Q value from equation (6.6). Step 4: Select the excavation support ratio (ESR) Step 5: Determine the suppor t measures for the Q value and the tunnel span/E SR ratio from a paper by Barton et al. (1974). Step 6: Estimate the possible maximum unsupported span from equation (6.7). Step 7; F or comparison purposes, determine the support pressure from equation (6.8) or (6.9). A correlation has been provided between the RMR and the Q-yalue (Bieniawski, 1976). A total of 117 case histories were analyzed involving 68 Scandinavian cases, 28 "Sout h Afri can cases,lmcr2 Pother documented case historiesTrom the United States, Canada, Australia, and Europe. The results are plotted in Figure 6.6 from which it will be seen that the following relationship is applicable: . R M R = 9In Q + 44
(6.10)
Rutledge (1978) determined in New'Zealand the following correlations between the three classification systems: ' R M R = 13.5logQ + 43
(standard deviation =9.4)
(6.11)
RS R = 0.77 RM R + 12.4 (standard deviation = 8.9)
(6.12)
RS R = 13.3 logQ + 46.5'• (standard deviation = 7.0)
(6.13)
o.<£r
sii
31
I
<
ROCK MASS QUALITY Q
»
10
Figure 6.6. Co rrelat ion between Geomechani cs Classification and Q-system.
A comparison of the stand-up time and the maximum unsupported span, as shown in Figure 6.7 reveals that the Geomechanics Classification is more conservative than the Q-system, which is a reflection of the different tunneling practice in Scandinavia based on generally excellent rock and long experience in tunneling. A comparison of the support recommendations by six different classification systems is given in Table 6.14. This study was made (Bieniawski, 1976) during the construction of a-railroad-tunnel described by Bieniawski'and“Maschek,i975:Thetunnel, 5.5 m wide and 3.8 km long, was characterized by highly variable rock ' conditions - from very poor to very good. In addition, a one-year tunnel-monitoring program featuring 16 measuring stations facil itated a comparison bet ween the classification ratings of rock conditions with the amount of rock movement, the rate of face advance, and the support used. This project thus afforded an ideal opportunity for comparing the various classification systems. More recently, Moreno Tallon (1982) made a detailed compariso nof the rock mass classification schemes in a tunnel in Spain. Although the above comparisons are interesting and useful, it is believed that one should not necessarily rely on any one classification system but should conduct a ■sensitivity analysis and cross-check the findings from one classification with another. This would enable a better ‘feel’ for the rock mass.
RECENT DEVELOPMENTS
«
VERY GOOD ROCK Q> 100
STAND UP TIME, hours
tzy
T a b l e 6 . 1 4. C o m p a r i s o n o f r o c k m a s s c l a s si f ic a t io n s a p p l i e d a t Geo mec hanics Classification (Bieniawsfci, 1973) ! Locality
Class
I H .6
Very good rock R M R = 83 II Good rock RMR = 67
H 2
II I Fair rock R M R = 52
H 3
IV Poor rock R M R = 29
H 5
Very poor rock RMR =15
Support O c c a s i o n a l s p o t b o l t in g
a railroad tunnel (width 5.5m)
1 3 0
Q-system (Barton* 1974) R S R c l a s s i f ic a t i o n ( W i c k h a m , 1 97 4) Class
Support
Good rock Q = 33.0
S p o t b o l t in g o n l y
Class
Support
RSR = 68
Bolts at 2r
Locally, grouted bolts Good rock Systematic grouted bolts ( 20 m m d i a .) s p a c e d 2 - 2 .5 m , RSR = iO M ediu m ribs at 2 m ^ “ 5 <2 0 m m d ia . ) s p a c e d 1 m - 2 m • (length 2.5 m plus mesh; length 2.8 m , shotcrete 50 mm thick . ; if req. Systematic grouted bolts ; Fair rock S y s t e m a t i c g r o u t e d b o l ts (spaced 1.5-2 m, length 3 m Q = 8 .5 R S R = ; 7 R i b s 6 H 2 0 a t 1 .7 m spa ced 1.5 m, le ng th 2.8 m,-. pl u s me sh a n d 100 m m th ic k a n d m e s h J shotcrete i' . Systematic grouted bolts Poor rock Shotcrete only: 75-100 mm spaced 1- 1.5 m, length 3 m, R S R = 5 2 Ribs 6H 20 at 1.2 r Q —1.5 thick or bolts at 1m, mesh plus 10 0-150 mm 2 0 - 3 0 m m s h o t c r e t e . a nd m e s h shotcrete (ribs at I.5m) Systematic grouted bolts spaced 0.7-1 m. length 3.5 m, E Xt D oo e ly t , S h o t c r e t e o n l ^ 7 5 - 1 0 0 m m RSR = 25 N/A p o o r ro ck th ic k o r te ns io ne d bo lts 150-200mm shotcrete and Q = 0.09. at 1 m plus 50-75 mm mesh plus me dium steel : . shotcr ete and mesh ribs at 0.7m. Closed . invert
RQD classification (Deere, 1970) H 6
Excellent RQD >90
H 4
Good RQD; 75-90
Austrian classification (Pacher, 1974)' Occasional bolts only
B o l t s 2 5 m m d i a ., 2 m - 3 m l o n g s p a c e d 1 .5 - 1. 8 m a n d s o m e mesh or 50-75 shotcrete or light ribs
I Stable II Over br ea ki ng
Bolts 26 mm dia., 1.5 m lon g spaced 1.5 m in roo f plus wire mesh Bolts 2-3 m long spaced 2—2.5m , sho tcrete 5 0 - 1 0 0 m m w i th m e s h
F r e n c h c , assification (Louis, 1974) 50*mm sho tcrete o r 3 m long bolts at 3.1 m 100 mm shotcre te with mesh and 3 m bolts at 2.8 m
E m p i r i c a l m e t h o d s o f d e s i g n
128 Empirical methods oj design
Recent developments
etermine the ratings of the six classification parameters from Table 6.13 Step and calculate the Q value from equation (6.6). Step 4: Select the excavation support ratio (ESR) Step 5: Determine the support measures for the Q value and the tunnel span/E SR ratio from a paper by Barton et al. (1974). Step 6: Estimate the possible maximum unsupported span from equation (6.7). Step 7: F or comparison purposes, determine the support pressure from equation (6.8) or (6.9). A correlation has been provided between the RMR and the Q-yalue (Bieniawski, 1976). A total of 117 case histories were analyzed involving 68 Scandinavian cases, 28 'SoufiTAfrican cases,lindTl otheT3ocumente3’case histories fromlFe dinted States, Canada, Australia, and Europe. The results are plotted in Figure 6.6 from which it will be seen that the followi ng relationship is applicable: . RM R = 9 In Q 4- 44
(6.10)
Rutledge (1978) determined in New Zealand the following correlations between the t hree classi fication systems: • ' R M R = 13.5logQ + 43
(standard deviation =9.4)
(6.11)
RSR = 0.77R M R + 12.4 (standard deviation = 8.9)
(6.12)
RSR = 13.3 logQ +46 .5: (standard deviation = 7.0)
(6.13)
A comparison of the stand-up time and the maximum unsupported span, as shown in Figure 6.7 reveals that the Geomechanics Classification is more conservative than the Q-system, which is a reflection of the different tunneling practice in Scandinavia based on generally excellent rock and long experience in tunneling. A comparison of the support recommendations by six different classification systems is given in Table 6.14. This study was made (Bieniawsk i, 1976) during the construction o f a-railroad-tunnel described by Bieniaws ki"and Maschek, 1975: The tunnel, 5.5 m wide and 3.8 km long, was characterized by highly variable rock conditions - from very poor to very good. In addition, a one-year tunnel-monitoring program featuring 16 measuring stations facilitated a comparison between the classification ratings of rock conditions with the amount of rock movement, the rate of face advance, and the support used. This project thus afforded an ideal opportunity for comparing the various classification systems. More recently, Moreno Tallon (1982) made a detailed comparison of the rock mass classification schemes in a tunnel in Spain. Although the above comparisons are interesting and useful, it is believed that one should not necessarily rely on any one classification system but should conduct a ■sensitivity analysis and cross-check the findings from one classification with another. This would enable a better ‘feel’ for the rock mass.
RECENT DEVELOPMENTS
ROCK MASS QUALITY Q Figure 6.6. Corre lation between Geomec hanics Classific ation and Q-system.
12y
134 Empirical methods o f design Reference
... I
E,nR o i Proc 2 2 L 3n d G' R ° f E m Pi ri “ ' D « S " M et ho ds f or T un ne ls in p „„ ' ' ‘h RapUt E x c a m Tunneling Con/., AI ME , New York, 1979 Vol 1 dd 6R3 Z m
*
£
8
r
m
° f b'° Ck Cavi"g
a C0mplex e™ ,onmtal Mining Magazine, Vol. 140,
'5
Olivier, H. J. A new engineering-geological rock durability classification. Engineering Geobgy, Vol. 14,1979, pp. 255-279. Pacher, F., Rabcevvicz, L. and Golser, J. Zum der seitigen Stand der Gebirgsklassifizierung in Stollen-und Tunnelbau. Proceedings, X X II Geomechanics Colloquium, Salzb urg, 1974, pp. 51—58.
Frar ™ w t
A" ,0bJSerVat‘0nal appr0ach “>«“ «* «¡0 » and con.ro! of rock tunnel lining.Shotcrete for
Ground Support, Amer. Conc rete Inst. Pu bL SP-54, 1977 pn 556-596
Gh ofin d,a ' coal-measures!
pp. 422-427.
** n
"
* * *
^
bM ^ ~
P' °n R° cii Mec^ MIT, Cambridge, Mass., 1981, i^ u ^ .astaution .o ^ inl„g_and
H°t „d o n ,n?98 i 5 W27pET '
InStitUlioi> ° [ M-ing and Metallurgy,
t Methods), Pergamon, New York, 1981,211 p amC'm M m ' Tes,lng'andMonitoring (IS RM Suggested
pp. 293-308
u i Ks
Si ,Cah° n SySlem f° r exravatl0n ln natura l materials. S. Afr. C ivil Engr., July 1982,
: 0s
s
s
^ W a l e rW lt a W i W » “ * ^
^
o f M in in g E ngine ers, N e w a r k ' ’ W e 'p p . 4“ UU 197t ppD37H-50laSS diStinCti0"
-
r
^
&—
° f R aP 'd W a‘ef P raSU re F t a “ ali™ "^
A lin ed
r""ne%
r° Ck maSSeS-C° a L G M a" d BaSe M b™ W S -W “ , Vo l 23, No. 6,
fur den Stollenb.u, Geologie unrt BauWesen, Vol. 24, No . 1, 1958, ■
'
M en n
M
n ^ a ^ Z
b ir i i
constmction L pp. 241-146!'
rr
AediCtbn i°r UndCr6r0“nd EXCaVa,i0nS'
ncan r
* *
ution°rMmingEn^ as' New-Y°rk’ i97*
*» «* » ■ W
115-132.
nics classification schemes in tunnel °7 T ° l 8e“ tunneling 82 Conference, institution of Mining and Metallurgy, London, 1982,
°f ^
** * * *
Protody akonov ,M. M. fCtessifikacija Gorotworu (originally in Russian), translated into French, Tunnels at Ouvrages Souterr ains, VoL 1, No. 1, 1974, pp. 31-34. Rose, D. Revising Terzaghis tunnel rock load coefficients. Proc,'23rd U.S. Symposium on Rock Mechanics, AIME, New York, 1982, pp. 953-960. Rutledge, J. C. and Preston, R. L. Experience with Engineering Classifications of Rock for the Pre diction of Tunnel Support, Proceedings, Internation al Tunneling Symposium, Toky o, 1978,.pp. A-3-l;7. Schneider, B. Gro und Classification for Tunnel Excavation. Tunnels and Tunneling, July 1980, pp. 59-62. Selmer^Oisen, RT~Snd BrocH^~E Gen eraf Design Pr^cedUre iofTUnderpoun d^Openiffg s'in^NoTwa^r Proceedings, Fir st International Conference on Storage in ExC'dvated.Rock Caverns, ITA, Stockholm, 1977, pp. 219-226.' ' ; Serafim, J. L. and Pereira, J. P. Considerations of the Geomechanics Classification of Bieniawski.Proc. Int. Symp. on Engng Geol. and Underground Constr ., LNEC, Lisbon, Portugal, 1983. Steffen, 0. K . HL Researc h and development needs in data collection fo r rock engineering.Explorationfo r Rock Engineering, ed. Z,T. Bieniawski, A. A. Balkema, Rotterdam, 1976, Vol. 2, pp. 93-104. Stini, I. Tunnelbaugeologie. Springer-Verlag, Vienna, 1950, 336 p. Terzaghi, K. Rock Defects and Loads on Tunnel Support Rock Tunneling, with Steel Supports, eds. R. V. Proc tor and T. White , Commercial Shearing Co., Youn gstown, Ohi o, 1946, pp. 15-99. Unal, E. Design Guidelinfs and Roof Control Standards for Coal Mine Roofs. Ph.D. Thesis, The Pennsylvania S iate University, 1983. Unrug, K. a nd Szwilski, A. 2. Influence of strata control parameters on longwall mining desirig.Proc. 21s/ U S. Symposium on Rock Mechanics, Univ. of Missouri, Rolla, Mo., 1980, pp. 720-728.. Wickham, G. E., Tiedemaiin, H. R. and Skinner, E. H. Support Dete rmination Based on Geologic Predictions. Proceedings, Rapid Excavation and Tunneling Conference, AI M E, New York, 1972, pp. 43-64. Wickham, G. E., Tiiedemann, H. R. and Skinner, E. H. Ground Support Predict ion Model - R SR Concept. Proceedings, Ra pid Excavation and Tunneling Conference, A IM E, New Yo rk, 1974, pp. 691-707.
•
2 Emp irical methods o f design
I
Barton, N. Recent Experience with the Q-system for Tunnel Support. Proceedings Syw pa^ tkcp lorat,o n
classification enables the designer to gain a better understanding of the influence of the various geologic parameters in the overall rock mass behavior and, hence, gain a better appreciation of all the factors involved in the engineering problem. This leads to better engineering judgment. Consequently, it does not really matter that there is no general agreement on a single rock classification system; it is better to try two or more systems and, through a parametric study, obtain a better ‘feel’ for the rock mass. 2. Once a few rock classification systems have been applied to a given project, it may be found that a simplified classification, particularly suited for that project, will evolve. Examples of this approach are the Dinorwic Scheme in Wales and the Washington
-- ---- Me tr o~ ir th e- Ur ii te d "Sta te s:
~
~
3. Quit e apart from the engineering benefits such as design data, rock classifications have been particularly successful in ensuring better communication on the project. This leads to a high morale as well as economical and technical benefits.
for Rock Engineering ed. Z. T. Bieniawski, A. A. Balkema, Rotterdam, 3976, Vol. l.p pW f
11 ■
Barton, N„ Lien, R. and Lunde, J . Engineeri ng Classification of Rock Masses for the Deslgn of Tunne ^ ¡„^¡tution Support. Rock Mechanics, Vol. 6, No. 4,1974, pp. 183-236. Bieniawski, Z. T. Engineering Classification of Joint ed Roc k Masses. Transactions, South African Institution p . „ . j :.... ■ of C ivil Engineers, Vol. 15, No. 12,1973, pp. 335-344. Bieniawski,Z T Geomechanics Classification of rock masses and its application in mnadmg^Proceedmgs,
Third International Congress Rock Mechanics, International Society for Rock Mechanics Denver, Colo, 1974, Vol. n A, pp. 27-3 1 , ,, Bieniawski Z. T.Th e Point-Load Test in Geotechnical Practice. Engineering Geology, Vol . 9, 1975,pp. 11. Bieniawski, Z.T. Rock mass classifications in roc k engineering, Proceeding,^ 0 3 ,urn on Exploration for
RocLEngineedng^Z J . J t i e n j a w s k j ^ A B a t o a R ot t er da m^ , 6, pp . 97--1C6 Bieniawski, Z. T. Determining rock mass deformabilit y: experience from case histories. Int. J. RockJftch.
Min Sc i Vol. 15,1978, pp. 237-248. Bieniawski, Z. T. Tuhnel Design by Rock M ass Classifications. U.S. Army Corps of Engineers,
Waterway
Experiment Station, Technical Report, GL-79-19, Sep tember 1979, 133p. _ rrmnmi Bieniawski, Z T. The Geomechanics Clas sification in r ock engineering apphca ions. Proc 4ifi Int. Congress
Use of borehole data Beharioi ol Rock T —
A trend has emerged to select engineering geological parameters on the basis of ■borehol e dat a alone whic h should be sufficient f or rock mass classification purposes without the need for tests in adits or pilot tunnels. As a result of the availability of more advanc ed coring techniques such as directional dr illi ng and oriented core sampling as well as both borehole and core logging procedures, rock mass classifications can be conducted on the basis of input data from boreholes alone (Cameron-Clarke and Budavari, 1981).
Assessing the strength of rock masses As discussed in Chapter 5, Hoek and Brown (1980) recently proposed a method for the -prediction of rock mass-strength involving, rock-mass classifications (see Table 5.8).-
* ^
C »™ «».
Classification Proc 12th Int. Congr. on Large Dams, IC OL D, Mexico City, 1976, pp. Bieniawski, Z.T , Rafia, F. and Newman, D. A. Ground »« tro ! mvestigatio ns for a s s e s s m e , t of roof conditions incoal mines. Proc. 2 1st U.S. Symposium on Rock Me clmics, Rolla, Mo., AI M E, 1980, p. Ere“
' T L and Howard, T. Stability Problems Caused by Seams and Faults. Proceedings, Rapid
Excavation and Tunneling Conference, American Institution of Mining Engineers, New York, Cameron-Clarke, I. S. and Budav ari, S. Correla tion of rock mass classification parameters obtained from borecore and in situ observations. Engineering Geology, Vol. 17, 198., pp. 19 53. Cecil O. S. Correlation of Rockbolts-Shotcrete S upport and rock Quality Par ameters ,n Scandinavian
_ Tunnpk Ph DThesis-University- of-Illinois,JJ.rhana., J970, 414p.------- ------ Coates, D. F. Classification oi Rock for Rock Mechanics, International Journal of Rock M echanics an -
To enable application of the Hoek-Brown criterion to coal mining, Bieniawski and Baue r (1981) prepared a list of appropriate m and s values for coal.
C o S 'f f a id
o t i D^U.PRock!Tunnel Supports and Field Measurements. Proceedings Rap^d
Excavation and Tunneling Conference, American Institution of Mining Engineers, New York, 1972,
Application in mining Rece ntly , maj or advances were made in the use of rock mass classifications in coal mining (U nal , 1983, Ghose and Ra ju, 1981, Bien iawski et al., 1980) and in hard rock (met al) mi ning (Cummings et al., 1982, Keffii orski et al., 1983). Deta iled examples of thes e. developments are given in Chapter 10. In longwa ll mining, the rock mass classification approach has been utilized for assessment of roof spans and rock ■ cavab ility (Unr ug and Szwilski, 1980, Kidybinski, 1979).
REFERENCES Baczyns ki, N . Rock Mass C haracterization and ’ Its Appli cation to Assessment of Unsupported
C o rd in & Ef Hendron, A. J. and Deere, D. U. Rock Engineering for Underground Caverns. J S o„ U n ro un d Rock Chambers, American Society of Civil Engineers, Pheomx, Ar.ona, C u i a t ^ d o r s k i , F. S. a nd Bie niaws ki, Z. T. C a v i n g M i n e Rock Mass C ^ c a ^ p p o r t Estimation U S Bur eau of Mines Co ntra ct No. J0100103, Engineers Internationa l, 982, 195 p. Deere, D.U. Technical Description or Rock Cores for Engineer^ Purposes, Rock Mechanics Engineering Geology, Vo L 1, No. 1, 1964, pp. 17-22.
p.
D e e r e, D . U . G e o l o g i c a l C o n s i d e r a t i o n s . Rock Mechanics in Engineering Pra ctic e,^ . R . G . S t a g e
7ienkiewicz. John Wile y & Sons , Lond on, 1968, pp. 1-20.
...M\ q r; O. C.
CHAPTER
7
Observational m ethods of design
Experimental science does not receive truth from supedarscknce^she is the mistress and other sciences are the servants.
Roger Bacon
Designing underground mining and tunneling excavations by observational methods involves interpretations of monitoring data during construction. Essentially, therefore, an observational method of design is a ‘design as you go’ method but in some cases a whole philosophy has been attached to an observational method making it distinct from other approaches. An example is the ‘New Austrian Tunneling Method’ (NATM) (Rabcewicz, 1964) which has received considerable attention in the field of tunneli ng and has some very promising results to its credit. Mo st recently, the N ATM has been applied to a coal mining project in Germa ny (Albers et al., 1982, Spaun and Jagsch, 1983). The convergcnce-confinement method has'also emerged within the last three years. Both these methods rely on a number of principles for monotoring the behavior of underground excavations during construction. Whether or not any observational method is distinct in its own right, it is important to understand the broad concepts involved in monitoring rock 'structures during construction. Accordingly one should consider observational methods of design under three topics: rock , monitoring techniques, the New Austrian Tunneling Method, and the convergence-confinement method.
ROCK MONITORING TECHNIQUES Monitoring the behavior of underground excavations during construction is re cognized today as an important, and often essential, aid in the design and construction of excavations. Systematic in situ monitoring of the performance ofboth the rock mass and the support was found to be one of the most promising developments in underground construction in recent years. Dunnicliff and Schmidt (1974), analyzing the.value of in situ monitoring of tunnels, make the following observation: To be effective and useful, monitoring of tunnel construction must be carefully