8/8/2019 Piled Raft Case Study in Malaysia

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A piled raft case study in MalaysiaPatrick Wong

Coffey Geotechnics Pty Ltd, Sydney, Australia

Keywords: piled raft, pile group settlement interaction, pile load testing

ABSTRACT

This paper presents a piled raft case study in Penang, Malaysia. The building footprint is about 90mby 76m with a 4m deep basement. The central office tower is 21 storeys in height and occupies anarea of 41m by 35m, with a 2 storey retail podium over the remaining area.

The site is underlain by deep marine sediments (>120m), and the original foundation designrequired the use of large diameter bored piles to carry column loads that are in excess of 20MN. Aninnovative and more economical foundation solution, comprising a piled raft was adopted. Itcomprised a combination of 500mm and 600mm diameter prestressed driven spun piles supporting araft having a thickness of 0.5m in the podium area and 1.7m in the tower area. The solutionadopted resulted in significant cost and time savings compared to the original design.

The piled raft design approach is described and settlement predictions compared with fieldperformance.

1 SITE CONDITIONS AND PILE DESIGN PARAMETERS

The site is located on Lot 131, Jalan Sultan Ahmad Shah, Georgetown in Penang, Malaysia. It coversan area of approximately 0.785 hectares, and is situated on the northern coastline of Georgetown,adjacent to the Strait of Malacca.

A review of the 1:500,000 Geological Map of the Peninsular of Malaysia reveals that the regionalgeology of the city of Georgetown consists of unconsolidated Quaternary deposits. These deposits

constitute marine and continental deposits of clays, silts, sands and gravels to greater than 120mdepth.

Based on a programme of investigation comprising boreholes, cone penetration tests and laboratorytesting, the adopted geotechnical model for the piled raft design is summarised in Table 1. Thedesign parameters were originally based on empirical correlations of skin friction and end bearingresistance with SPT and CPT results, and were later refined based on a pile load testing programdiscussed in Section 2.

Table 1: Adopted Geotechnical Model and Pile Design Parameters

Adopted Pile Design ParametersUnit DepthInterval

(m)

TypicalSPT

Value

TypicalCPT Cone

Tip

Resistance(MPa)

ElasticModulus

(MPa)

Ult.Skin

Friction(kPa)

Ult. EndBearing

(kPa)

Soft clay/silt (treatedby lime piles)

0.0 - 8.0 0 - 2 50 Not tested 85 to 200(85)

140(125)

7500(6000)

Below 120 200 (200)

Note: Values in brackets were original parameters adopted for Class A Prediction (i.e. Predictionmade of pile capacity and stiffness prior to load testing, using the method described in Poulos 1980)

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The parameters for the upper soft clay/silt layer took into account the effect of groundimprovement carried out by chemical lime piling to facilitate design and construction of thebasement excavation, improve trafficability by construction machinery at bulk excavation level, andto improve the stiffness of the material immediately below the foundation raft. A description ofthe chemical lime piling ground treatment carried out at this site is presented in Wong (2004). Inbrief, the undrained shear strength of the upper soft layer was increased from an average of

about16kPa to between 27kPa and 36kPa.

2 PILE LOAD TESTING PROGRAMME

To enable refinement of the original geotechnical model for final design of the piled raft for theproject, an extensive program of pile load testing was carried out. In particular, the ultimate loadcapacity and stiffness of 600mm diameter spun piles (pile type preferred by the client) were ofcritical interest. The pile load testing program comprised:

Static load tests one using Constant Rate of Penetration (CRP) and two using MaintainedLoad Test (MLT) methods.

7 dynamic load tests using CAPWAP signal matching techniques

The test piles had a wall thickness of 80mm, and were driven to penetrations of between

27m and 57m.

Pile settlement during test loading was measured using 4 dial gauges (A, B, C and D) located aroundthe pile perimeter at 90o apart, and a central survey point (Reference 1) located on the pressurejack. The central point was measured by survey against a remote bench mark and is considered tobe more accurate compared to the dial gauge readings which were affected by movement of theground and reference beams during testing. The results of the pile load testing are summarised inTables 2 and 3. An example of the static pile load testing on test pile TP8 is shown in Figure 1.

Figure 1 Static Pile Load Test (TP8)

The following observations and conclusions were made from the test results:

The end bearing resistance did not appear to have been fully mobilised in both the staticload and dynamic load tests.

The dynamic load tests provided relatively close match with the static load test results.

The Class A prediction provided relatively good prediction of the pile ultimate load,although the parameters were probably conservative in view of the fact that the test pileswere not fully mobilised as indicated above.

The shorter pile TP2 had an initial axial stiffness that was practically the same as thesignificantly longer test pile TP4. The structural stiffness of the relatively slender piles was

found to be important with respect to limiting settlement. During final design, the wall

0

1000

2000

3000

4000

5000

6000

0 10 20 30 40 50 60

Settlement (mm)

Load(kN)

TP8(2) - Gauge A TP8(2) - Gauge B TP8(2) - Gauge C

TP8(2) - Gauge D Reference 1

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thickness of the piles beneath the tower was increased to 100mm, and it was decided to fillthe centre of the hollow spun piles with mass concrete to increase their structural stiffness.

Although the actual failure load was assessed to be greater than the predicted ultimateload, it was decided to limit the operational ultimate load of the tower piles to 6000kN, atwhich the settlement corresponds to about 10% of the pile diameter.

Table 2: Summary of Dynamic Pile Load Test Results (600mm dia. Spun Piles)TestPile

Pene-tration

(m)

DateInstalled

DateTested

MobilisedSkin

Friction(1)(kN)

MobilisedEnd

Bearing(1)(kN)

MobilisedTotal

Resistance(kN)

Settlementat

MaximumLoad (mm)

TP2 27 16-9-00 25-9-00 2030 670 2700 15

TP4 57 13-9-00 25-9-00 4870 1860 6730 40

12-9-00 15-9-00 3330 1150 4480 26TP5 55

12-9-00 25-9-00 4970 1210 6180 31

44 12-9-00 15-9-00 2740 950 3690 20TP6

48 15-9-00 25-9-00 3590 1050 4640 26

TP7 55 15-9-00 25-9-00 4190 1660 5850 32

TP8 55 16-9-00 25-9-00 4180 1570 5740 32(1) Mobilised skin friction and end bearing resistance were based on signal matching CAPWAP analysis

Table 3: Static Load Test Results And Comparison With Dynamic Load Tests

Pile Ultimate Load (kN) Settlement at Ultimate or

Maximum Load (mm)

Test Pile Class A

Prediction

Static Load

Test

Dynamic Load

Test

Static Load Test Dynamic Load

Test

TP2 (MLT) 3329 > 2700 2700 20 15

TP4 (CRP) 6533 > 6500 6730 > 77(2) 40

TP8 (MLT) 6062 > 5540(1) 5740 > 40(2) 32

Notes:1. Maximum load could not be sustained due to equipment problem2. Pile continues to creep slowly at maximum applied load

3 PILE GROUP SETTLEMENT INTERACTION FACTORS

In addition to dial gauges mounted on the test pile, 6 survey reference points were also establishedat various distances from the pile. Reference 1 was located on the pressure jack (i.e. zero distancefrom the pile) and 5 other references located at distances of 2m, 5m, 10m, 15m, 20m and 25m fromthe pile. This was an attempt to assess pile group settlement interaction effects, although the

survey points established would at best only measure the ground settlement influence due to pilesettlement.

Figure 2 shows the measured ground settlement profiles at various distances away from test pileTP4. The pile to pile settlement interaction was expected to be less than the ground to pilesettlement interaction and therefore the adopted values were those assessed using program DEFPIG(Poulos, 1991), which are also shown in Figure 2.

However, DEFPIG calculates the interaction of two adjacent piles in a group, and does not take intoaccount the presence of intermediate piles that may provide shielding effects which tend to reducethe settlement interaction. Furthermore, when the distance between two piles increases, theoperating soil modulus between the two piles becomes higher due to small strain effects.Therefore, for large pile groups, the use of theoretical pile interaction factors from single piles has

been known to over-predict the pile group settlement as discussed in Poulos (1993). This aspectwill be further discussed later in Section 5 of this paper.

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4 PILED RAFT DESIGN

4.1 Piled Raft Design Concept

For piled raft design, it is a well recognised and accepted practice that the piles do not need to bedesigned to have conventional geotechnical factors of safety. The piles could be regarded assettlement reducers and as long as the entire foundation system has a satisfactory factor ofsafety, and the system performs satisfactorily with respect to serviceability criteria, some of thepiles can be designed to yield under ultimate load conditions.

If a conventional pile foundation design were to be adopted, over 400 piles would have beenrequired for this project. Using the piled raft design concept, only 293 piles were required. Thedesign involved a number of iterations to optimise the raft thickness and piling layout. The finalsolution adopted comprised a raft having a thickness of 0.5m in the Podium area and a thickness of1.7m beneath the Tower over an area of about 49m by 43m that extends 4m beyond the edge of theactual tower footprint. The extension of the thicker raft beyond the edge of the Tower was toenable piles to be strategically located at the corners of the Tower where load concentration wasfound to occur based on the analysis results (see Section 4.2). 293 nos. prestressed spun piles wereadopted in the final design as summarised in Table 4 below:

TABLE 4: DETAILS OF PROPOSED SPUN PILES

Design Allowable

Structural Capacity(kN)Location PileDiameter(mm)

Adopted

UltimateGeotechnicalCapacity

(kN)

Numbers

Wall

thickness(mm)

Design

LengthbelowBase ofRaft (m)

Compression Tension

500 2,100 87 80 23.5 2,000 760PodiumRaft 600 2,700 34 80 23.5 2,800 910

TowerColumns

600 6,000 76 110 to 17m80 thereafter 53

3,700 930

Core 600 filledwithmass

concrete

6,000 96 110 to 29m80 thereafter 53

3,700Concrete

infill

ignored

930

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50

spacing to diameter ratio, s/d

SettlementInteractorFactor

TP4 Load Test(Measured

Ground/PileSettlement Ratio)

Pile to PileInteraction AssessedUsing DEFPIG

Figure 2 Pile Group Settlement Interaction Factors

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4.2 Design Approach and Analysis Results

Initially, hand calculations were carried out to satisfy the Strength Limit State requirements toapproximate the number of piles required, such that the overall design capacity of the piled raft

gRug = gRug (raft) + gRug (piles) is greater that the design action effect, R s*. Geotechnical strength

reduction factors, g, of 0.71, 0.67, and 0.74 were adopted for the raft, podium piles, and tower

piles respectively, based on the extent of site investigation data and pile load testing.

For the detailed design, numerical analyses of the proposed piled raft system were carried out usingcomputer program GARP (Poulos, 1994) to assist the structural designer in the following:

SLS - assess whether the predicted settlement and differential settlements are acceptable,and how these may impact on the structural design and/or construction sequence.

ULS - to obtain the design action effects for design of the structural elements of thefoundation system (i.e. raft and piles in that they must have adequate structural capacityagainst the predicted loads/bending moments).

For both of these conditions, g values of 1.0 are applicable. It should be pointed out that for the

calculation of structural design action effects, the use ofg values of less than 1.0 could result inlower structural action effects being calculated, which could result in an unsafe structural design.

For the ULS analysis, 11 load cases were analysed with different partial load factors to modelvarious load combinations including wind loads and uplift from buoyancy effects duringconstruction. Bending moments and shear forces were computed to enable structural design to becarried out, and pile loads were computed for checks to be made regarding the structural capacityof the piles. With respect to pile loads under the ULS cases, the following findings were ofparticular interest:

The analysis indicated a stress concentration at the corner of the core, due to the rigidityof the central core. The 1.7m thick raft beneath the core was artificially increased to 25m

in the analysis to account for the structural stiffness of the building core. This analysis hasenabled the piles to be more efficiently located around the edge and corners of the core.

Under the individual tower columns, maximum working and ultimate pile loads werecalculated to be 3,400kN and 5,653kN. The maximum working and ultimate pile loads forthe piles beneath the core (mainly at the corners) were calculated to be 3,722kN and5,987kN respectively under over-turning wind loading. These are slightly above theallowable structural capacity and close to the ultimate geotechnical capacity of the 53mlong 600mm diameter spun piles (see Table 4). Under conventional pile design conditions,this would not have been allowed. However, as the entire foundation system has satisfiedthe design requirements, overloading of some of the piles was considered to be acceptable.

For the SLS case, the computed maximum settlement and differential settlements were as follows:

Settlement beneath core 130mmLocal Raft Rotation 1:257Differential settlements between the Tower column and the core 1:360Differential settlement between Tower columns 1:430

Although some of these computed values were relatively high, the design was considered to beacceptable on the acknowledgement that the pile group settlement prediction was probablyconservative due to likely over-estimation of pile group interaction effects in this large piled raftdue to pile shielding and small strain effects away from the loaded pile. As a precaution, thestructural engineer incorporated a construction joint between the Tower and Podium areas toenable rotation to occur to reduce the impact of potential differential settlement. As it turned out,actual settlements and differential settlements were significantly lower than those compared as

discussed in Section 5.

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5 FIELD PERFORMANCE

Following construction of the raft (completed May 2001), 54 survey points were established at thetop of the raft for subsequent settlement monitoring. 23 of the survey points were located withinthe Tower area and the remainder located within the Podium area. The tower structure wascompleted in November 2001 and monitoring continued until March 2003. The following settlement

monitoring results were obtained:

Settlement of the Podium ranged from 0mm to 2mm.

Settlement of the Tower ranged from 15mm to 28mm with an average of about 21mm.

There was practically no increase in settlement between completion of the Tower inNovember 2001 and March 2003.

The maximum settlement observed was only 21.5% of the predicted settlement, and this wassurprising even though it was recognised during the design stage that the estimated settlement waslikely to be over-predicted due to the conservative estimates made on pile settlement interactionfactors. Besides the difficulties associated with estimating pile settlement interaction factors forlarge pile groups, a possible cause to the relatively small settlement observed may be that the liveload component of the structure has not yet been fully effected during the monitoring period. If

the live load component (which was about 26.3% of the total working load) were removed, then thepredicted maximum settlement would have been about 96mm only. This is still over 3 times theobserved maximum settlement.

Another possible cause in over-estimating the pile settlement interaction factors for this project isthe fact that the piles used are relatively slender, and about 10mm to 15mm of the settlement onthe heavily loaded piles was due to elastic compression of the pile shaft. It is probable that theelastic compression of the pile shaft should not have been included in the assessment of pile groupsettlement interaction.6 CONCLUSIONS

Although the settlement of the piled raft was significantly over-predicted, the projectdemonstrated the successful use of the innovative piled raft solution. The following benefits werederived:

More economical, driven spun piles were used instead of the original intended largediameter bored piles.

Fewer piles were possible compared to conventional pile foundation design.

Satisfactory over-all performance was achieved even though some of the piles could beloaded close to their ultimate geotechnical capacity under ultimate load conditions.

Design confidence in this project was achieved via a detailed geotechnical investigationsand a prototype pile load testing programme, with settlement monitoring during and afterconstruction of the building.

The significant over-prediction of settlement observed supports the general view that pile

settlement interaction factors may be over-estimated for large pile groups due to small strain andpile shielding effects. The author also speculates that for slender piles, perhaps the elasticcompression component should be ignored in assessing pile settlement interaction factors. Theseaspects warrant further research.

REFERENCESPoulos, H.G. (1980) User Guide to DEFPIG Deformation Analysis of Pile Groups, School of CivilEngineering, University of Sydney.

Poulos, H.G. (1993) Settlement prediction for bored pile groups, Deep Foundations on Bored andAuger Piles, Van Impe (ed.) 1993, Balkema, Rotterdam, 103 - 117.

Poulos, H.G. (1994) Alternative Design Strategies for Piled Raft Foundations, 3rd Int. Conf. DeepFoundations, Singapore, 239 244.

Wong P. K. (2004) Ground Improvement Case Studies Chemical Lime Piles and DynamicReplacement, Australian Geomechanics Jnl. Vol. 39, No. 2 June 2004, 47-60.