1.1. 1.1.
Smal Smalll scal scalee civi civill engi engine neer erin ing g syst system emss
Small scale civil engineering structures could be small by size, cost and technology. 1.1.1. 1.1.1. Enginee Engineering ring syste systems ms and the their ir select selection ion
Main civil engineering structures used in Nepal are dealt here. I)
Retaining walls
Retaining Retaining walls help to support the mountainsi mountainside de slopes, slopes, or support support the road or slope segments segments from the valley side. hey are designed to stop an active earth pressure. Suitable sites! "ny slope where there is a problem of deep#seated $% &'' mm) instability, or where the steepness of the slope ma(es benching be nching impractical. he comparison of various retaining walls is shown in the table below! all type
Ma*. safe height
ry masonry
/m
3omposite masonry
4m
Mortared masonry
0'm
7abion wall
0'm
Reinforced earth
4m
Reinforced concrete
0'm
idth to height ratio "dvantages-imitations $!+) 0!0 to '.1!0 ell drained, fle*ible, relatively low cost2 low strength threshold '.5&!0 to '.&!0 6etter drained than mortar mortared ed masonr masonry y but with reduced strength '.5&!0 to '.&!0 Relatively easy to cons constr truc uctt on terr terrai ain2 n2 cannot tolerate settlement, poor drainage '.&!0 8le*ible without rupt ruptur uriing, ng, toler olerat ates es poor foundation, well drained, drained, relatively relatively low cost for strength epending upon Reinforcing Reinforcing e*pensive e*pensive design or diff diffic icul ultt to obtai obtain, n, dif difficul icultt to achi achiev evee tension epending upon +igh strength2 design relatively costly, re9uires advanced technical s(ills to build,
poor drainage
:ractical considerations for retaining walls!
•
"s far as possible, dry masonry should be preferred. 3areful design and supervision of foundations should be carried out. eep holes of a minimum width of 5&mm, sloping downwards, should be given every
•
one meter along and up the wall. " drainage blan(et of aggregate with a porous membrane of filter fabric $geote*tile or
•
hessian) should be placed over the weep holes. oe pro;ections should be provided to dissipate energy of water and hence to avoid
• •
• • •
erosion at toe. 6ac( batter should be used for minimizing e*cavation and construction cost. ilting ilting and benching are good ways for increasing stability against sliding. 6ac(filling is critical2 and should follow considered design specifications.
6ioengineering techni9ues should be used in con;uction with retaining walls for! < < <
:rot :rotec ecti tion on of bac( bac(ffill :rotec :rotectio tion n from from scour scour and and undercu undercutti tting ng of the the founda foundatio tions ns and and sides sides 8le*ib 8le*ible le e*tens e*tension ion to to the wall wall by plantin planting g large large bamboos bamboos,, shrubs shrubs or trees trees above above the wall. oe walls! oe walls are a type of retaining walls constructed at the toe of a slope or segment of slope. II)
Revetment walls
Revetment walls are constructed to protect the base of a slope from undermining or other damage, such as grazing by animals. hey give only protection, not support, and are not used on large, unstable slopes, where substantial retaining structures may be re9uired. Suitab Suitable le Sites! Sites! "long "long the base base of inheren inherently tly stable stable cut slopes slopes where where seepage seepage erosio erosion n can destabilize the base of large slopes2 along the foot of abandoned spoil tips which have reached their angle of repose2 along the foot of large fill sites. :ractical considerations for revetment walls! he bac( face should be vertical2 the front face should have a =!0 gradient. •
If there is a ris( of damage by biotic interference, a capping beam of cement#bound masonry could be provided at the top of the wall.
•
eep holes of a minimum width of 5&mm, sloping downwards, should be
•
given every one metre along and up the wall. 6ac(filling is critical2 and should follow considered design specifications.
Its integration with bioengineering is similar to that of retaining walls. 6reast walls! 6reast walls differ from toe walls in the fact that they provide armour function as opposed to toe walls which provide support function. hey are considered to be a type of revetment walls. III)
:rop and de dentition wa walls
>:rop wall? refers to support walls and dentition. @n very steep cut slopes, prop walls are used to support bloc(s of harder roc( where they are underlain by softer roc( bands. here differential weathering occurs, large segments of slope can become destabilized by a soft roc( band eroding away underneath it. his presents two options! either remove all the material above, or support it with a prop wall. :rop walls do not usually offer total support to the full weight of all slope material above. Rather, they stop the erosion of softer bands below harder bands supported on them. Suitable Sites! Ased only on steep cut slopes hey can be integrated with bioengineering for the protection from scour and undercutting of the foundations and sides. IB)
3hec( dams
3hec( dams are simple structures to prevent the downcutting of runoff water in gullies. hey prevent deepening, widening and head aggravating of gully by providing periodic steps of fully strengthene strengthened d material. material. 3hec( dams are designed designed to accept an active pressure pressure if it applied in the future, while permitting a safe discharge of water and debris via a spillway. Suitable sites! "ny loose or active gully. In general, anywhere on a slope where there is a danger of scour from running water. :ractical considerations! • •
• • •
" waterway or spillway notch should be provided. he dam should be (eyed right into the gully sides, ensuring that water will run down the notch without scouring. -aunching apron should be given below the dam. eep e ep holes with granular filter material should be given. hese dams should be located at the places of nic( points and other re9uired places as per the following!
here, C D spacing between two chec( dams, +E D effective dam height as measured from the gully bottom to the spillway crest, S D slope of the gully floor and F is a constant such that, F D '.= when tanS G '.H and F D '.& when tanS %D '.H Integratio Integration n with bioengineering bioengineering aids in protection protection of bac(fill bac(fill and gully floor above chec( dam, along with scour prevention. B)
Surface an and sub sub##surface drainage age
Surface drains are installed in the surface of a slope to remove surface water 9uic(ly and effi effici cien entl tly y. Surf Surfac ace# e#wa wate terr drai drains ns ofte often n use use a combi combina nati tion on of bio# bio#en engi gine neer erin ing g and and civi civill engineering structures. 3ascades are a type of surface drains used on slopes steeper than /&. Sub#surface drains are installed in the slope to remove ground water 9uic(ly and efficiently. hey are usuall usually y restri restricte cted d to civil civil engineer engineering ing struct structure ures, s, and do not normal normally ly use bio# engineering measures. +owever, bio#engineering techni9ues can be used to strengthen the slope around the drain. B I)
Stone pitching
" slope can be armoured with stone pitching which forms a strong covering. It is freely drained and will will withst withstand and consid considera erable ble water water velocit velocities ies.. +owever +owever,, it is relati relatively vely e*pens e*pensive ive in comparison with bio#engineering measures such as brush layering. Suitable sites! "ny slope up to =&. his techni9ue is particularly useful on slopes with a heavy seepage problem, in flood#prone areas or where vegetation is difficult to establish, such as in urban areas. It is also useful on gully floors between chec( dams and for scour protection by rivers. Stone pitching pitching can be strengthened strengthened by planting planting grass slips or live cuttings cuttings of shrubs in the gaps between stones. BII BII)
ire bol bolsster ter cyli cylind nder erss
ire bolster cylinders $in cross#section, a tube of ='' mm diameter filled with stone) are laid in shallow shallow trenches trenches across the slope. hey prevent surface scour and gullying gullying $by reinforcin reinforcing g and fulfilling an intermittent armouring function), and provide shallow support. 6olsters can be laid in two ways! along the contour2 or in a herringbone herringbone pattern pattern $G#G#G#G# $G#G#G#G#G#) G#) to double as a surface surface drainage system.
Suitable sites! @n most long, e*posed slopes between =& and &' where there is a danger of scour or gully gullying ing on the surfac surface. e. 3ontou 3ontourr bolste bolsters rs are used used on well well draine drained d materi materials als22 slante slanted d $herringbone pattern) bolsters are used on poorly drained material where there is a ris( of slumping. 3ontour bolsters are normally spaced at Hm centres for slope G =' and at 0.&m centres for slope between =' to /&. +erringbone bolsters are placed at 0.&m centres. 6etween wire bolster cylinders, shrub and small tree seedlings are planted at 0m centres throughout the slope. BII BIII) ire nett nettiing ire netting $gabion wire mesh) is spread over the surface of a roc(y slope to reduce the shedding of roc( debris and slow the degradation of the surface. IC)
River training wor(s
he ob;ective of river training wor(s is to! hold river in the e*isting channel2 relocate main flow channel2 and hold vertical position of river bed. ypes! •
• •
Revetment Revetment walls! hey could be gabion with stones, stone riprap, timber piles, bamboo piles, concrete slabs, old tires, sand bags, or combination of two or more of above materials. Spurs 3ut#offs 1.1.2. Interaction Interaction between between vegetative vegetative and and civil civil engineering engineering systems
Slope treatment at a particular site may be proposed to be conducted with! civil engineering syst system emss alon alone2 e2 or vege vegeta tati tive ve syst system emss alon alone2 e2 or combi combinat natio ion n of both both syst system ems. s. 8rom 8rom a bioengineering perspective, vegetative systems in con;unction with small#scale civil engineering wor(s is deemed best. Relative strength over time for civil engineering, and vegetative systems is clear from the following graphs. hese graphs are mere diagrammatic representations for comparison of the performance of each type of system2 and do not depict their actual strength. he relative strength of civil engineering structures throughout their life period decreases from a certain ma*imum value obtained right after completion of construction. In e*act contrast, the relative strength of vegetative systems is very low at the time of plantation or sowing but, with respect to time, it increases to a ma*imum value2 and is maintained so for a long period.
8actors to be considered in selecting combinations! co mbinations! • • •
-ife span of civil engineering systems ime period needed for vegetative systems to develop re9uired strength 8unction to be performed! 6oth systems used in a site must perform
similar functions2 or must complement each other. Mutual protection • " few e*amples on the relationship between civil engineering and vegetative systems! < < < <
oe oe wall wall belo below w bamboo! bamboo! all prot protect ectss the the bamboo. bamboo. :lants :lants around around end end of the toe toe wall wall!! :lant :lantss prote protect ct the the wall. wall. rees rees above above toe toe wall! wall! rees rees enhanc enhancee the perfor performan mance ce of struc structur ture. e. 8ence 8ence with with young plant plants! s! Initia Initially lly,, fence fence protects protects the the young young plants. plants. "fte "fter r the life of fence, plants replace the structure.
