Geogrid reinforced piled road over peat

A. J. Barry’, Bambang Trigunarsyah*, T. Symes’ & J. S. Younger3

’ TBV (Malaysia) Sdn Bhd, 20/f 1GB Plaza, No 6 Jalan Kampar, 50400 Kuala Lumpar, Malaysia2 LASMO Oil (Malacca Strait) Limited, Landmark Centre, Jl Jenderal Sudirman, Jakarta, Indonesia3TBV Consult, Summitmas Tower 6F, Jl Jend Sudirman Kav 61-62, Jakarta 12069, Indonesia

Abstract. The construction of roads over up to 11 m of peat in East Sumatra has required extensiveengineering investigations. Environmental constraints in a sensitive undeveloped forest area requiredthe use of a combination of locally available materials combined with geogrids to produce asatisfactory solution.

Previous difficulties with road construction in this terrain are discussed, together with the design ofthe geogrid reinforced raft and the field trials which resulted in substantial changes to the design andconstruction procedures to provide a practical and economic solution.

LASMO Oil are developing oil and gas fields on Padangand Tebing Tinggi Islands, which lie offshore EastSumatra in Indonesia and which have up to I1 m of peatoverlying soft clay. The extent of the peat swamp area inSumatra and the location of the LASMO explorationarea are shown in Fig. 1.

The LASMO development has included the construc-tion of roads and hardstands for camps, access towellheads and other operational facilities for oilexploration. As a result of settlement and flooding ofsome of the roads constructed during the initial period,LASMO commissioned a detailed engineering study ofthe engineering properties of the ground to identifymore satisfactory construction methods.

Ground conditions

Padang Island is covered by up to 11 m of recent, highlycompressible peat, the properties of which have beendescribed by Barry et al. (1992a). The thicker peatdeposits are associated with pole forest, which coversmuch of the centre part of the island, as shown on Fig. 2.This peat is underlain by a soft, becoming firm withdepth, recent clay and with sand lenses or layers atdepths greater than 35 m below sea level. The upperboundary of the clay is approximately at sea level; in theupper 3 m the clay is transitional with 15-20% organiccontent, below which the organic content decreases toabout 3%. The properties of the clay have beendescribed by Barry et al. (1992h) and the properties ofa number of Indonesian clays have been described byYounger (1990). Groundwater in the peat variesseasonally from about 0.3-l m below ground level,though seasonal and perennial swamp area are common.

The sequence of peat overlying soft clay is typical ofthis region and it is necessary to travel over a hundredkilometres westwards into the mainland before groundconditions improve, with fine sands and silts beingfound. Sources of good quality aggregate are rare, andthe most economic sources were from islands nearSingapore, about 600 km to the east where granitequarries are operated.

History of problems and environmentalconstraints

The first road constructed on Padang Island was to crosseast to west, from coast to coast, a distance of about24 km (Fig. 2). The early stages of constructionproceeded satisfactorily from the coast, using a 0.5 m

0 200

The Site

Fig. 1. Location of study site.

From EDDLESTON, M., WALTHALL, S., CRIPPS, J. C. & CULSHAW, M. G. (eds) 1995, Engineering Geobgy of’ Construction.Geological Society Engineering Geology Special Publication No. 10, pp 205-210

206 A. J. BARRY ET AL.

LEGEND

- Road

- Setttedl?oad

Pola Forest

Mixed Foreat

,

P A D A N G -ISLAND \‘

0 5 10

uCl~h¶d

a Vlllaga

Fig. 2. Location of trial areas.

thick crushed stone pavement. However, as the roadprogressed inland ground conditions worsened and thestone pavement was underlain first by a geotextile, thenby two layers of timber as a corduroy raft, and finally byup to four layers of corduroy.

Subsequently, many areas of road settled excessivelyand flooded during the rainy season, requiring them tobe closed. Emergency repairs consisted of adding morecorduroy and stone, and subsequent investigationsproved up to 3.5 m of stone on repaired sections ofroad which were only just above natural ground leveland thus still susceptible to flooding.

These problems derived from the very high compres-sibility of the inland peat; this was confirmed at an early

stage of the detailed investigation when some simplebearing tests were carried out. A 40 gallon drum (0.57 mdiameter) was founded at ground level after only aminimum of levelling. After loading with water failureoccurred at a ground pressure of 21 kN/m’.

In a second test the drum was found at the water-table, at a depth of 450 mm below ground level. Thoughsignificantly less settlements occurred before failure, thefailure load was similar to that of the first test. The twotests are compared in Fig. 3; there is significant timedependent behaviour, but at low stresses, say up to15 kN/m2, the equivalent modulus value is estimated as200 kN/m2 below the water-table and 100 kN/m2 abovethe water-table. For the roads previously built with

GEOGRID REINFORCED ROAD OVER PEAT 207

0 5

Prew~re ( kNlm2 1

to 15 20 251

-c Load 0.45mbelow surface

1 --o-- Load at surfaceI

Loaded Area 0.57m diameter

hour

daY

Settlement ( mm 1

Fig. 3. Results of bearing test on peat.

0.5 m pavement and four layers of corduroy, this wouldproduce about 0.5 m settlement on 7 m depth of peat,which was the typical thickness over the failed sections.This agrees broadly with the behaviour of the failedroads, but total settlements of the roads were higher, asa result of significant time dependent behaviour.

In view of the possible environmental consequences ofengineering works LASMO had carried out extensivestudies of the forest peat ecology and had identified theneed to maintain the natural high ground water levels.Therefore deep drainage, which has been adoptedelsewhere, was not an acceptable solution. The environ-mental considerations have been described by Barry etal. (1992~).

Design solution

A number of options were considered, including the useof various types of lightweight fill and a piled geogridreinforced raft. A major factor in the decision to adoptthe latter solution was the uncertainty about the need forfurther roads. Significant capital investment would havebeen needed to set up production facilities for light-weight fill and this could not have been justified by thisproject alone.

The design solution also had to be considered inrelation to the materials available; the was no engineer-ing fill available either on the islands or on the mainlandcoastal region. Timber was available locally fromlicensed concessions in the peat forest. The timber, aspecies called bintangor, is very straight and a consistentsize of 150-200 mm diameter; however, it can rotcompletely in about two years when placed in theground above the water-table.

Once the concept of the design solution was selected,the further options to be reviewed were the spacing ofthe piles, the practicality of splicing lengths of pile,controlling the rot of timber piles above the water-table,the use and size of pile caps and the number of layers ofgeogrid in the pavement.

Raft design

Trials with timber piles pushed through the peat andacting in friction in the underlying soft clay had beencarried out and their load-carrying capacity was there-fore known. The results of these trials are described byBarry et al. (1992b). As the roads have an oil-boundcrushed stone pavement and routine maintenanceprocedures are well established, it was felt that the raftdesign should be critically reviewed and factors of safetykept to a minimum to achieve an economic solution.

Jones et al. (1990) have shown, by finite elementanalysis, that there is a significant reduction in thecalculated stress in the geogrid if some allowance ismade for the support of the soft foundation soil.Furthermore, Netlon, the manufacturers of Tensar,had reported the successful construction of embank-ments on soft ground in Malaysia (Dobie, pers. comm.)using bakau timber piles at 0.75 m spacings, wheretheory had shown that the geogrid would not performadequately; this may be because of the support providedby the soft clay foundation. However, the results of thefield trial on the peat, described in the following, hadshown that in the present situation little benefit would beobtained in this respect as the peat is much morecompressible than even very soft clay.

The required height of the f inished road wascontrolled by the need to raise the road above floodlevel, to have a sufficient pavement thickness for thetraffic and to minimize load on the formation and piles.An initial embankment height of 0.5 m, consisting ofgrid reinforced stone pavement, was adopted for thedesign, which led to pile spacings of the order of l-1.5 m. The possibility that this configuration would notallowing arching action to develop in the stonepavement, or that the arching would be destroyed bytraffic vibration, was a concern. However, as the crushedstone in existing pavements was tightly locked evenwhen placed over layers of corduroy which moved whentrafficked, full arching was assumed in the design.

For the field trials a Tensar SSl geogrid was chosen asit was immediately available. A short-term workingstrain of 3.5% was selected, equivalent to a workingstress of 18 kN/m2 or 60% of the quality controlstrength. This is higher than the recommended valuefor the 10 year life required and, in addition, no furtherderating was included for construction damage. Thisapproach was adopted qualitatively to take account ofthe likely composite action of the geogrid, acting intension, and the stone mattress, acting in compression,

208 A. J. BARRY ET AL.

which is not allowed for in the conventional designprocedure. The conventional design procedure usesMarston’s formula for positive projecting subsurfaceconduits (Spangler & Handy 1973) to calculate theproportion of the load carried by the piles and thegeogrid; the stresses in the geogrid are then calculatedassuming a parabolic deflected shape (Leonard 1988).The stress in the Tensar for various pile spacings andpile cap diameters calculated in this manner is shown inFig. 4.

This design caters only for the dead load of theembankment and assumes that transient live loads arecapable of being carried without significant distress. Aslonger term live loads would overstress piles and thegeogrid, the design was based on a requirement thatthere would be no long-term parking of heavy vehicleson these roads. Pavement design was not consideredusing concepts of life related to standard axles, as mostof the traffic would be during construction. Afterconstruction the traffic would consist of a smallnumber of light vehicles for workers and inspectorsto travel to the well locations, with a workover rigweighing up to 80 t requiring access to the wellhead ona few occasions during the estimated 10 year life of thewell.

Field trials

A trial location was selected where further roadconstruction was due to start shortly, where the depthof peat was 8 m; borings showed the properties of thepeat and underlying clay to be typical of thosethroughout the deep peat areas on the island andtherefore representative of the likely problems to beencountered with road construction. A length of 200 mof proposed road was divided into eight sections, forwhich different designs were selected. These includedone section with no piles as a control; piles spaced at 1

and 1.3 m, a section with 0.6 m diameter pile caps andsections with one and two layers of geogrid. The pileswere designed to carry the calculated dead load with afactor of 1.3 on the ultimate capacity measured fromfield trials.

The timber piles were delivered in 6 m lengths,suitable for handling by one or two labourers. A pilesplice was designed, using 1 m lengths of easily available150 mm diameter steel pipe as a collar, spiked to thepiles. However, on-site an alternative design wasadopted using 25 mm steel reinforcing bars insertedinto holes drilled into the axis of the pile. For the trialthe pile caps were steel fabrications, but it was intendedthat if pile caps were adopted they would be of concretefor normal construction, which would also overcomeproblems of timber rot in the unsaturated zone of thesoil. The tops of all piles used in the trial were treatedon-site against rot using a bituminous compound.

The trial was designed to identify the most economicof the designs which would perform satisfactorily.However, at an early stage in the trial it becameapparent that short-term loading during constructionwas a critical factor. As this required the design to beamended the problems encountered are described insome detail, as follows.

Water was standing at just above ground level at thetime of the trial. A hydraulic excavator travelling on acorduroy mat alongside the proposed road was used topush the piles into the ground. Labourers carriedforward the 6 m lengths of pile for installation andtheir repeated passage compressed the peat around theinstalled piles, so that in some places the piles werefinally up to 100 mm above the ground. As the corduroymat for the excavator had been moved forward it wasnot practicable to push the piles in further; also, it wasknown that the setup of the piles was considerable andthis would have made pushing the piles further in verydifficult.

Pile Cap Spacing (m)0.6 7

1.3m c/c 1 Sm c/c0.5 -

Fig. 4. Geogrid design stresses.

GEOGRID REINFORCED ROAD OVER PEAT 209

After the first layer of geogrid was placed over theoriginal ground and the pile heads or pile caps, stonewas tipped by trucks and spread by a D4 bulldozer,which has a total weight of about 12 t. It was found thatduring this operation the piles punched through thegeogrid and trucks attempting to drive over the thinlayer of uncompacted stone broke through it. Althoughimprovements were made, by tipping onto completedsections of road and bulldozing material forward, thedifficulties persisted, there being no lighter plantavailable. High loadings due to the stone stockpilewere exacerbated by the bulldozer climbing the stock-pile.

through. This then gave sufficient support to place asecond layer of geogrid and a layer of stone, which wasthen supported on the piles. This was a very uneconomicsolution, as the cost of the imported stone was a largepart of the total cost.

An alternative approach was therefore adopted forone section (section 4). One layer of corduroy was putdown on the ground between the piles before the geogridwas laid. This allowed construction traffic loadings to becarried without damage to the incomplete road and theconstruction proceeded as previously intended. The roadwas opened to traffic immediately after the completionof construction.

As a result of these difficulties a number of sections A comparison of the behaviour of section 4 andwere built as an unpiled 0.5 m thick geogrid reinforced section 5, which was constructed without piles as amattress below the pile heads which had punched control section, is shown in Fig. 5. There is some

JWX July Aug Se@

0 PILES

TWEEN PILES

SECTION 4 ( PILED 1Settlement of Lower Layer of Gewid

22oomo-l ( IE; !Y I5 4 0 0 I

SECTKIN 4 ( PiLED 1Settlement oi Upper Layer of Geogrid

800’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’SECTIONS(uIpKED)Settlement of Upper and Lower Layers of Gewrid

Fig. 5. Results of field trials

210 A. J. BARRY ET AL.

settlement of piles due to construction loading, possiblymovement of the relatively crude splices of the piles andthe overstress resulting from the additional weight ofstone. The first layer geogrid movements include somebedding down as they are placed on a very uneven andcompressible peat surface. The trial was subjected to thepassage of a workover rig and support trucks, eachweighing 60 t, soon after construction, and later by alarger rig weighing 80 t. On older, unpiled roads thepavement deflected and water rose near the toe of theembankments when these trucks passed. On the trialsection there was no visible distress and no directlyassociated settlement was observed in the monitoring.

The thickness of stone in the unpiled section wasmeasured by subsequent trial pits to be 0.85 m; this ispartly because settlement during construction was madeup by adding further stone so that the road could beopened to traffic.

Conclusions

The designs initially adopted for the field trials requiredclose supervision, skilled plant operatives and carefulselection of the most suitable plant. The addition of asingle layer of corduroy as a working platform allowedmuch of these restraints to be dispensed with andprovided a more practical solution for constructingroads in this remote location. This confirmed the valueof field trials incorporating the actual limitations whichwill be encountered during subsequent constructionworks.

The layer of corduroy is likely to rot relatively quicklyand there will be load transfer to the piles. Some furtherset t lement can be expected as a resul t , but thecompressibility of the rotted timber will not be higherthan that of the peat. The section 4 design as amendedby the f ield tr ial was therefore adopted as therecommended method for further road construction

and has been completed successfully in the subsequentsections of road. The cost of this road at 1991 prices isUS$350000/km; comparison with the cost of roadspreviously constructed would need to take account ofthe extensive emergency maintenance and the actual andpotential costs of roads being closed in an unplannedmanner.

The complexity of the interaction between the piles,the corduroy raft, the stone pavement and the geogrid,and the time-dependent behaviour of the geogrid, makesit difficult to compare the actual behaviour withpredictions. Nonetheless it appears that a geogridreinforced pavement of this nature has the potential tocarry greater loads than predicted by conventionaldesign.

Acknowledgements. The authors are grateful to Pertamina forpermission to publish this paper.

References

BARRY , A. J., BRADY , M. A. & YOUNGER , J. S. 19920. Roadson peat in East Sumatra. Summation in Print: Environ-mental Geotechnics, Geotechnical Engineering. Journal ofSoutheasl Asian Geotechnical Society, 23, 145-162.

-, BAMBANG T RIGUNARSYAH , SY M E S, T. & YO U N G E R , J.S. 19926. Piling for roads over peat in Sumatra. In :Geotropika 92, International Conference on GeotechnicalEngineering, Kuala Lumpur.

JO N E S, C. J. F. P., LA W S O N , C. R. & AY R E S, D. J. 1990.Geotextile reinforced piled embankments. In: 4th Interna-lional Conference on Geotextiles. Geomembranes andRelated Products, The Hague, Vol. 3. Balkema, Rotter-dam, 1017-1018.

LEONARD , J. W. 1988. Tension Structures, Behaviour andAnalysis. McGraw Hill, New York.

SPANGLER , M. G. & HANDY , R. L. 1973. Soil Engineering.Intext Educational Publishers, New York.

YOUNGER, J. S. 1990. Geotechnical characteristics of soft claysin Indonesia. In: Proceedings of AIT Symposium, Bangkok.