Maccaferri_Muro 74 m Altura - India

7/28/2019 Maccaferri_Muro 74 m Altura - India

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www.nce.co.uk | NEW CIVIL ENGINEER 1

GEOTECHNICAL:SIKKIM AIRPORT

In the mist-smokedmountain valleys of theHimalayas a series of giantstone walls is rising high on

the steep slopes, reminiscent ofancient fortresses and Mughalemperors. High sheer curved

ramparts are more like the fan-tastical images of Buddhist tem-ples from a Hollywood movie.

Real Buddhist temples dotthis area, the autonomousIndian state of Sikkim, closeto Nepal, Bhutan, and theChinese border. But these wallsare for something much more21st century. They are a majorfeature of a new airport, the firstin the province and a crucialdevelopment for the inaccessibleterritory of plunging hillside,rice terraces and isolated townsand villages, connected bypotholed contour-hugging roadsand rickety truss bridges.

This inhospitable terrain hasprevented full developmentof the state and particularlytourism, despite major attrac-

tions like the scenic mountains,rich natural history and temples.The International Centre forOrchids is headquartered hereand a reputed 1,800 species ofbutterfly can be seen.

Strictly speaking the huge

rising structures are not wallsas such. They are gabion-basketfacings for massive reinforcedearth embankments, which helpcreate a flat platform for therunway and the aircraft apronand terminal.

Above the gabions, the upperparts of the steep sided slopesare less steep and are faced witha vegetated mesh. Maccaferriis supplying the gabions andmesh and has worked closelyon the design details with mainconsultant Mott MacDonaldIndia.

Such a dramatic use of rein-forced earth is not only unusualbut probably unprecedented.Most such structures rise notmore than perhaps 15m with afew examples in the world

Extremely high reinforced earthembankments with near verticalfacings are breaking records onthe Sikkim airport project in India.Adrian Greeman reports.

INDIAN

SLOPETRICKS

Tall order: Working in the steep slopes,mists and rains of the Himalayas


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reaching 40m for the entireembankment.

But the necessities of theproject demand somethingexceptional. The constraints

are severe, explains SharadKumar, the schemes generalmanager from client the AirportsAuthority of India.

They include steep slopes,mists and rains, nearby moun-tains on the flight path, stronglyweathered ground conditions,and a major seismic risk. Thearea experienced a strong 6.8Richer earthquake only thisSeptember.

The airport is sited on the sideof one of the local mountains,which rise in this lower Hima-

layan area to around 2.2km, andfor reasons of weather and windgusting must be within a certainheight envelope of between1.35km and 1.5km.

The selected site, near thetown of Pakyong and approxi-mately 33km from the capitalGangtok, will allow for a northto south runway to be builtalong the mountainside whichslopes at more than 50 fromwest to east in places. Becauseits 1.7km length will serve inter-national AT72, 66 seat aircraft,

the runway gradient has to beless than 0.7%.

Making space for the strip isdifficult and made even moreso by the need to completelyminimise land take.

Land is in short supply inthese regions, says Kumar.Additionally, he adds, ecologicaland landtake reasons led thelocal state to rule that fill shouldneither be exported or imported.

All this meant that acompletely balanced cut and filldesign was needed within the

tightest possible footprint, withland take kept at 56ha.

The only way to do that was bybuilding high faced structures,says Mott MacDonald seniorengineer Rahul Vaidya. Gentlerbatters would have extendedthe structures halfway down thefairly well populated valley sides.

The rear of the site will be cutback with cuts to 110m high,and embankments created lowerdown up to 74m high.

This work was a significantdesign challenge not leastbecause the geology is a mixtureof sedimentary or metamorphicrock, with some granites, schist,gneiss and all highly weatheredto depth.

The final structure has to copewith seismic loads and one of

the worlds highest rainfalls afour month monsoon here isonly part of the overall 2.5mannual precipitation. This yearhas already seen 3.9m of rain.

And the peak intensity of

the rainfall is also very high,adds Kumar. Not only that butthere are eleven local streams,calledjhoras, nine of which passthrough the runway area with allthat implies for pore pressuresand ground stability.

Rigid reinforced concreteretaining structures werelooked at but ruled out, saysVaidya. They were unsuitablein the seismic conditions andobtaining sufficient concrete inthe region would also have beenproblematic.

Instead, a more flexibledesign option of reinforcedsoil embankment was chosento support the fill area. For thefacing the options includedsystems with rigid concrete

panels; wraparound methodsand welded mesh walls. In theend, gabions were selected forthe steepest sections of wall anda wraparound mesh facing fromMaccaferri called Terramesh forthe less vertical sections.

The flexible solution is goodfor effective drainage and earth-quake resistance, says AshishGharpure, director of MaccaferriIndia.

Maccaferris Parameshgeosynthetic grid was selectedfor the main earth reinforce-ment. It is a very strong gridand allows for a design with anunusual spacing of up to 2mbetween reinforcement layersin some places. This makesit quicker to install and more

Constraints includesteep slopes, mistsand rains, nearbymountains on theflight path, stronglyweathered groundconditions, and amajor seismic risk

SIKKIMAIRPORT

PLAN

Cutbacks to form the airfieldplatform are up to 111m high.

A vegetated mat facing is usedfor erosion stabilisation

The steep mountainsides arecovered with rice terraces

where farming is possible.

North-South runway 1.7kmlong will operate only in one

direction due to adjacent highhills on the flight path


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www.nce.co.uk | 01.12.11 NEW CIVIL ENGINEER 3

economic. Most of the gridlayers are 1.6m apart.

The grids load bearingcapacity of up to 800kN/malso allows for flexibility in thedesign, minimising costs, saysMaccaferri. Main design wasto BS 8006 for the static designof slopes and walls, and to theAmerican Federal HighwaysAdministration standards forseismic design not covered in theBritish standard.

The system is being usedto form composite embank-ments, faced with large 1m deepgabion blocks in a near verticalconfiguration at the bottom andthen with a gentler 65 vegeta-tion stabilised slope above. Thesteep facings, at 6 from the

vertical are mostly in sectionswith intermediate step-backs,although at one point above astream valley there is a sheer face40m high, says Gharpure.

For the sloped facings

Terramesh is being used. Thisis a steel mesh reinforcementinterleaved with the main gridreinforcement as a secondaryreinforcement and then wrappedaround at the face. Wherespace is available this is GreenTerramesh which has coir, acoconut matting, included onthe facing. Behind it goes sometopsoil and seeding to generatevegetation cover.

Vegetated matting is the mainstabiliser for the up to 11m highcuttings behind the runway,using coir blankets made fromcoconut matting and polypro-pylene netting, held down withU-pins like giant staples untilplant growth is well established.The base of the cuttings is

stabilised with a gabion toe up to3m high.

Kumar explains that the threemost critical elements of thedesign have been drainage,drainage and drainage.

To cater for high rainfall,substantial catchwater capacityand slope drains are neededon the cuttings, with the flowcollected by longitudinal drainchannels next to the runway.

Even more important iskeeping the embankmentsdrained. To ensure this each fillarea has parallel perforated pipesdrains installed at 10m centres inthe base.

And then at the back of thereinforced section we have aspecial feature, the chimney

Pakyong town isclose by. A new

road is to bebuilt 30km to the

capital city.

Near vertical facedreinforced soilembankments helpminimise land take

A major facility ofthe Indian Council ofAgricultural Research issituated in these hillswhich are rich in orchidsand other plant species

An apron with space for two planes may bewidened for four later. The airport platformmust also find space for a terminal,carpark, control tower and emergencyfacilities to international standards.

SIKKIM AIRPORT

Indian Council of

Agricultural Research

GABION TOE WALL

0 200m

Sikkim

West Bengal

NEPAL

BHUTAN

TIBET

INDIA

SIKKIM AIRPORTPROJECT

Gangtok

Bagdogra Airport

0 40km

RUNWAY

STRIP

AIRPORTBOUNDARY

INDIA

CHINA

NEPAL

BHUTAN

SIKKIM

BANGLADESH

The system isbeing used toform compositeembankments, facedwith large 1m deepgabion blocksin a near verticalconfiguration


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drain, says Kumar. This isa essentially a dry-stone wall,250mm wide and wrapped inTerram 1000 geotextile, to actas a filter and prevent cloggingfrom the ingress of soil fines.The wall is built with 100mm to250mm flat hand-shaped and

hand placed stones. It rises thefull height of the embankment.

On top of that however hasbeen the need to deal with thejhorasrunning through the site.Mott MacDonalds solutionhas been to draw the elevenstreams together into four mainroutes through the site in largeconcrete box culverts. Concretewas selected over a steel optionbecause of its stiffness, particu-larly in a seismic event.

The culverts which also collectthe drainage from the slopes andrunway, emerge in the faces ofthe near vertical gabion walls.Here stepped structures havebeen built up using gabions,to break the flow of the waterand dissipate the energy in it,which can be significant in a full

monsoon storm.Dealing withjhorashas been

doubly important because ofa significant population livingfurther down the valley slopes.Widening work has been neededin downstream channels toaccommodate the increased flow

in the composite channels.But the streams are also

important as the main watersource for the local population.Below one of the culverts a newreservoir has been designed, says

Vaidya. From this, piped supplyconnections are being made.

If design has been complex, sotoo has the construction opera-tion.

Firstly the remote area is notthe easiest to work in. Bringingin the substantial amount of

plant had its own difficulties.Around 76 excavators of which18 are large machines, concretepaving machines, compactors,generators, a crushing plant andconcrete mixers had to be trans-ported to the site. Each had tocome along the 80km of windingand sometimes worn or potholedroads that run from the plains.

For Dehli-based maincontractor Punj Lloyd, thatusually meant breakingmachinery down into three orfour pieces to get it across thebridges which usually are limitedto around 7t to 8t maximumload.

On site work is limited bythe heavy rainfall particularlyduring the spring and summermonsoon period. Finding skilled

labour is also a problem, espe-cially to operate truck,excavators and other machinery.

The embankment work

requires significant skill. Acritical aspect is placing andproperly compacting the fillwhich is done in 300mm layers.Moisture content has to bewithin the right range, so thematerial must be turned to dryit. A compaction density of 98%is required.

Compaction is especiallyimportant with such embank-ment heights, says Kumar. Andsettlement could be significant ifit occurred.

The contractor has an onsite

laboratory to carry out a range ofsoil testing.Work is sequenced with much

of the machine work to cuttingand moving fill done underfloodlights at night. Bull-dozing the layers is done in themorning. An element of the fillwork is the removal of bouldersand stones from the placed fillmaterial.

There is a substantial work-force used locally, working in40 day cycles. They are terracefarmers mostly and need to

return home to work the fields,says Kumar. Local workers areused for much of the gabionfilling, and soil and seed place-ment for the Terramesh.

The hand filling of thegabions is part of achieving avery clean and neat appearanceon the near vertical facings. Itis also important that the localstone is placed with the strikeplane horizontal, says Vaidya,as the rock has good compres-sive strength that way but muchless along the plane.

So far this effort has takenthe structure a bit beyondhalfway. Progress has beenslowed this year in particular bythe exceptionally heavy rainsand some other factors, saysKumar. The 6.8 earthquake inSeptember has not affected theairport however, he says.

There is the concreting ofthe runway and apron area tocome as well as construction ofthe terminal and control towerbuildings. The main contract, forsome Rupees 2,560M (31M),was due to have taken two yearsfrom the start of work in January2009 but will not be completeuntil well into next year. Thework to complete the airportbuildings will continue untilDecember next year.

Plant had to betransported to thesite. Each had tocome along the80km of windingand sometimes wornor potholed roadsthat run from theIndian plains

BASIC SCHEME OF RETAINING STRUCTURES

248m

108m

304m

Boundary (west) Centreline of runway Boundary (east)

Slope protection

Cutting

Gabion wall

Formation level of runway Filling Retainingstructure

Longitudinal drain(along runway apron sides)

Foundationsoil

2m highgabion toe wall

Vegetation

Erosion control blanket

DETAIL OF GABION WALL

CROSS SECTION OF SITE

DETAIL OF RETAINING STRUCTURE

Originalgroundprofile

Backfill soil

Gravel filter

Compacted fill

High strengthflexible geogrid

Coirmat

Secondaryreinforcement,gabion facia

ANOTHER OF VARIOUS

CROSS-SECTION

Transverseperforated pvcpipe drain,wrapped withgeotextile

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Maccaferri_Muro 74 m Altura - India