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1. INTRODUCTION

Raft and pile foundations are the two alternativefoundations normally adopted by geotechnical engineers tosupport heavy structures like storage tanks and tall buildings.Raft foundation is adopted in situations where the structureis designed for higher order of total and differentialsettlements. In the case of restriction on settlement pilefoundations are adopted. In real situations the combinationof these two types of foundation system compensates thelimitations of one on the other, which has resulted on effectivefoundation known as piled raft. The concept of piled raft isnot new and has been described by several authors includingZeevaert, 1957. The piled raft foundation is becomingessential in situation where large tanks used for storing oilsand hazardous liquids and gases designed with stringentconditions especially on total and differential settlements.Further this problem is apparent in case of loose to mediumsand deposits wherein permissible total and differentialsettlements are lesser than clay despite the settlement isimmediate. Therefore, understanding of interactionmechanism between the soil, raft and piles is essential todesign the piled raft foundation.

Various methods of analyses of piled raft foundationhave been developed over last three decades and an overviewof the literature indicates that the research have progressedin three different directions namely numerical modelling,field monitoring on real size piled raft and study on laboratorymodels. Numerical models are based on finite elementanalyses or boundary element analyses taking various modelsfor pile, raft and the soil. Appropriate Ep/Es ratios have beentaken and parametric study has been done to study thesettlement and load sharing behaviour. Studies conductedon instrumented prototype piled raft rely mostly on the Es

values estimated through back working of the parametersfrom the load tests. However the influence of pile installationand construction sequence has not been well accounted sofor.

Tests on simulated small scale models in the lab appearto be a good procedure to understand the settlement andload sharing behaviour of the piled raft. Weisner and Brown(1980) studied the behaviour of model piled raft system byconducting tests on over consolidated clay and validated theirfindings through finite element analyses. Cooke (1986)carried model tests on freestanding groups and piled rafts instiff clay and reported that longer the length of pile lesser isthe settlement of piled raft system. Horikoshi and Randolph(1996) presented the results of centrifuge model tests onpiled raft on kaolin clay. All these works are on the preparedoverconsolidated clay beds. Work on piled raft supportedon sand has gained momentum very recently. Kim et al.,(2002) conducted model tests on piled raft embedded in sandand developed a genetic algorithm based analysis foroptimizing pile locations. Turek and Katzenbach (2003)reported the results of model tests conducted on loose anddense sand bed. Most of the laboratory studies reportedcovers only the effect of spacing of piles on load sharingbetween raft and pile. Therefore an elaborate parametricstudy for circular rafts is essential to establish the load-settlement and the load sharing behaviour, so that a designprocedure can be evolved. This paper presents the results ofa series of tests conducted in the Geotechnical engineeringlaboratory of Anna University, Chennai.

2. MODEL TESTS

Tests were performed on models of piled raft and plainraft founded in sand beds of different densities with andwithout raft contact on soil. All the tests were carried out in

STUDY ON THE BEHAVIOUR OF CIRCULAR PILED RAFT IN SAND

V. Balakumar, Research ScholarK. Ilamparuthi, Professor & HeadV. Kalaiarasi, Graduate StudentDivision of Soil Mechanics and Foundation Engineering, Anna University, Chennai – 600 025

ABSTRACT : In order to understand the behaviour of circular piled raft system installed in sand of various compactnessa series of tests were conducted on laboratory models. The parameters related to soil, pile and raft have been varied and theirinfluence on the load sharing behaviour of the bearing elements (pile and raft) described by the piled raft coefficient havebeen brought out through 1g model tests conducted in the laboratory. Reduction in settlement as well as influence of settlementon the load sharing behaviour has also been brought out.

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a steel tank of dimension 1000mm x 600mm x 600mm. Thesand used in this study is free from fines with coarse, mediumand fine sand contents of 3%, 85% and 12% respectively.The Cu and Cc values are 2.63 and 1.22 respectively and isclassified as uniformly graded sand (SP). The averagedensities of sand bed at which experiments conducted are16.2kN/m3, 15.5kN/m3 and 14.8kN/m3 for the dense (φ=410),medium (φ=370) and loose (φ=340) conditions respectively.The models used in this study are fabricated from Perspex.Perspex sheets of 6mm, 8mm and 10mm thickness have beenchosen for the raft and solid rods of 6mm, 8mm and 10mmdiameter have been used as piles.

2.1 Experimental Procedure

One of the most important aspects in such a model studythat influences the results is the preparation of the bed. Sandraining technique has been adopted with calibrated heightof fall and controlled compaction. For dense and mediumdense conditions, preweighed sand for each layer was placedand compacted by supplying calibrated energy. In the caseof loose sand, the known weight of sand was poured into thetank from precalibrated height. The sand bed was preparedfrom the bottom of the tank in layers. A specially madetemplate was placed on top of the sand bed and piles weredriven through an outer sleeve fitted to the template. Pileswere left projecting and the raft was fixed on them. Thefoundation was vertically loaded using a hydraulic jack fittedto a loading frame and the load required for the penetrationof piles was monitored by a proving ring of 20kN capacity.The settlement gauges placed on opposite corners havingtravel of 50mm and a least count of 0.01mm were reset. Theload was applied in very small increments and thecorresponding settlements were recorded.

3. RESULTS AND DISCUSSION

Tests were conducted on circular raft of 200mmdiameter with various pile lengths of 120mm, 160mm and200mm. The parameters investigated are length of pile,diameter of pile, configuration of piles and density of sandbed. Influence of these parameters on settlement reductionratio and piled raft coefficient are discussed below.

3.1 Load-Settlement Behaviour

Figure 1 shows the load-settlement curve for a freestanding pile group and the pile group with raft in contactwith the soil in medium dense sand. The load carryingcapacity of pile group, in the case of raft in contact with thesoil is higher than free standing pile group except for thesettlements less than 3mm. At this settlement free standingpiles reached their limiting resistance, there after pilesexhibited uncontrollable settlement under limiting load. Inthe case of raft in contact with the soil the resistance of thepiles increased steadily with settlement and reached thelimiting load of free standing pile at a settlement of 2mm.

This behaviour is due to the increase in the normal stress onpile, as a result of the transfer of load from the raft to thesoil.

Fig 1 Load-settlement behaviour of pile group (N=21,L=200mm d=10mm, φ=370)

Figure 2 shows the typical load-settlement curve for pilegroup, plain raft and piled raft. The load-settlement curveindicates that in the initial stages more load is transferred tothe piles and the raft takes lesser load. As the settlementincreases beyond 2mm the raft starts taking more load thanthe piles. It indicates that the pile has mobilized almost itsfull resistance within a settlement of 2mm. However, the piledraft show increase in load with the settlement. Similarobservations were made in tests on piled rafts tested in looseand dense sand as well as piles of various lengths anddiameters.

Fig 2 Load-settlement behaviour (N=21, L=200mm,d=10mm, D=200mm, t=8mm and φ=370)

3.2 Settlement Reduction Ratio

In order to understand the influence of variousparameters of piled raft on the reduction of settlement aparameter called settlement reduction ratio Sr [=(δr-δpr)/ dr]was used. Where δr and δpr are the settlements of raft andpiled raft respectively for the given load.

3.2.1 Effect of Length of Pile on Sr

Figure 3 shows the effect of slenderness ratio of pile onthe settlement reduction ratio in medium dense sand. The Sr

presented is for a settlement corresponding to 5mm, 15mmand 20mm of plain raft. The Sr ratio increases with increasein L/d ratio irrespective of the magnitude of piled raftsettlement. The Sr ratio is higher for lower settlementirrespective of L/d ratio.

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Fig 3 L/d ratio Vs Sr

3.2.2 Effect of Pile Diameter on Sr

Three different diameters of piles have been used tostudy the effect on settlement reduction. The reduction insettlements is presented for the settlement of 2mm, 10mmand 20mm of plain raft. Figure 4 shows the effect of variationin diameters of pile on the settlement reduction ratio in thecase of piles of 160mm long embedded in medium densesand. Higher the diameter higher is the settlement reduction.The maximum settlement reduction ratio is 0.75 for the raftsettlement of 2mm and for the diameter of pile of 10mm.This indicates higher the area ratio (area of piles/ raft area)of piled raft higher is the reduction in settlement.

Fig 4 Effect of pile diameter on Sr

3.2.3 Effect of Bed Density on Sr

The variation of Sr with the density is presented in Figure5 for 5mm, 10mm, 15mm and 20mm settlements of plainraft. Sr value increases with the increase in density. Theincrease in Sr is almost linear with density for all thesettlements compared except 5mm. Further the variation inSr ratio for settlements more than 5mm is marginal for agiven density. This observation also confirms that reductionin settlement is effective only when the load on pile is lesser.The maximum reduction is around 70% in dense sand for asettlement of 5mm.

3.2.4 Effect of Pile Radial Angle on Sr

Figure 6 shows the effect of configuration of piles spacedat a radial angle of 200, 300, 360 and 450 on the settlementreduction ratio for 5mm, 10mm and 20mm of plain raft. Thenumbers of piles in each configuration are 37, 25, 21 and 17for α=200, 300, 360 and 450. The settlement reduction ratiodecreases almost linearly with radial angle and is independent

of order of settlements compared. The reduction in settlementis least for 450 which is attributed to lesser in number ofpiles.

Fig 5 Variation of Sr with density

Fig 6 Radial angle (α) Vs Sr

3.3 PILED RAFT COEFFICIENT

The distribution of the total load between the two bearingelements of piled raft (pile and raft) is described by the piledraft coefficient αpr which is the ratio between the sum of thepile loads (SRpile) and the total load (Rtot) on the piled raft.

3.3.1 Effect of Length of Pile on αααααpr

Figure 7 shows the variation of αpr with settlement ofpiled raft in medium dense sand for L/d of 12, 16 and 20.The αpr increases with increase in lengths of pile which isindependent of magnitude of settlement. Lesser the settlementand higher the L/d ratio (longer the pile) higher is the αpr

value. However the variation in apr value for settlements of10mm and 20mm are negligible. This indicates that the loadshared by the piles remain almost same at higher settlements.

3.3.2 Effect of Pile Diameter on αααααpr

Figure 8 shows the effect of diameters pile on piled raftcoefficient. The αpr value decreases with increase insettlement but it increases with increase in pile diameter.The minimum αpr obtained is 0.1 for pile diameter of 6mm.This indicates that the load shared by the 6mm diameter pileat settlement of 20mm is 10% of the total load. However theload shared by the 10mm diameter pile is 25% for the sameorder of settlement.

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Fig 7 L/d Vs αpr

Fig 8 Diameter Vs αpr

3.3.3 Effect of Bed Density on αααααpr

The variation of apr with the density is presented inFigure 9 for 5mm, 10mm, 15mm and 20mm settlements ofplain raft. The apr decreases with increase in density for thesettlement of 5mm and 10mm. At higher settlements the aprvalue is higher for medium dense condition than loose anddense sand.

Fig 9 Density Vs αpr

3.3.4 Effect of Pile Radial Angle on αααααpr

Figure 10 shows the effect of configuration of pilesspaced at a radial angle of 200, 300, 360 and 450 on the piledraft coefficient. When the radial angle is small the area ratioof piles is higher therefore, the apr is also higher irrespectiveof the order of settlement indicating that the piles take moreload. As the settlement increases the load taken by the pilereduces and beyond a certain limit the variation in the radialangle does not appreciably alter the apr value. This indicatesthat at higher level of settlement increase in the number ofpiles does not influence the load sharing of the piles verymuch.

Fig 10 Radial angle (a) Vs apr

4. CONCLUSION

1. The parametric study clearly indicates that the pile groupin the piled raft has a higher group capacity than thefree standing group confirming that the confining stressincreases around the piles.

2. In the initial stages the settlement and load sharingbehaviour is influenced equally by length and diameter.However at higher level of settlement the behaviour isinfluenced more by the length than by the diameter.

3. The performance of piles as settlement reducer is morepronounced in the case of loose sand than in mediumdense and dense sand. This indicates the improvementof the soil properties due to pile installation, though theimprovement has not been quantified.

4. Increase in the number of piles does not have appreciableinfluence on the performance of piled raft at higher ofsettlements.

REFERENCES

Cooke, R.W. (1986) Piled raft foundations on stiff clay acontribution to design philosophy. Geo-technique, 2, 169-203.

Horikoshi, K., and Randolph. M.F. (1998). A contributionto optimum design of piled rafts, Geo-technique, 48 (3), 301-317.

Kim, H.T., Yoo, H.K., and Kang, I.K. (2002) GeneticAlgorithm-Based Optimum Design of Piled Raft Foundationswith model tests. Jl. Of South East Asian GeotechnicalSociety, 1-9

Turek, J, and Katzenbach, (2003) Small-Scale model testswith combined pile-raft foundations', Proc. 4th Int. Geo-tech.Seminar on Deep Foundations on Bored and Auger Piles,Ghent, 409-413.

Weisner, T.J. and Brown, P.T. (1980). Laboratory Tests onModel Piled Raft Foundations, Jl. of the Geotechnical EnggDivision, 106(GT7), 767-780.

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