0038 Patrick Wong - A Piled Raft Case Study in Malaysia

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					                          A piled raft case study in Malaysia
                                          Patrick 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 90m
by 76m with a 4m deep basement. The central office tower is 21 storeys in height and occupies an
area 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 design
required the use of large diameter bored piles to carry column loads that are in excess of 20MN. An
innovative and more economical foundation solution, comprising a piled raft was adopted. It
comprised a combination of 500mm and 600mm diameter prestressed driven spun piles supporting a
raft having a thickness of 0.5m in the podium area and 1.7m in the tower area. The solution
adopted resulted in significant cost and time savings compared to the original design.

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

1   SITE CONDITIONS AND PILE DESIGN PARAMETERS

The site is located on Lot 131, Jalan Sultan Ahmad Shah, Georgetown in Penang, Malaysia. It covers
an 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 regional
geology 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 120m
depth.

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

Table 1:         Adopted Geotechnical Model and Pile Design Parameters
 Unit                        Depth         Typical      Typical      Adopted Pile Design Parameters
                            Interval         SPT      CPT Cone        Elastic      Ult.     Ult. End
                               (m)          Value         Tip        Modulus       Skin     Bearing
                                                     Resistance       (MPa)      Friction     (kPa)
                                                         (MPa)                    (kPa)
 Soft clay/silt (treated    0.0 - 8.0        0-2          <0.5      3.5 (1.8)    20 (17)        -
 by lime piles)
 Firm clay/silt            8.0 - 13.5        2-5        0.7 – 1       15 (8)     35 (30)        -
 Stiff clay/silt          13.5 - 24.5       5 – 10     1.5 – 1.8     20 (14)     45 (40)        -
 Sandy gravel/gravelly    24.5 - 29.0      15 – 35   Not tested      100 (50)    60 (50)      6000
 sand                                                                                        (6000)
 Stiff sandy clay and     29.0 - 54.0      20 – 35     1.5 – 3.5     30 (30)     55 (40)      2200
 silt                                                                                        (2200)
 Very stiff to hard clay   54.0 – 120     30 - >50 Not tested       85 to 200      140        7500
 and silt                                                              (85)       (125)      (6000)
                           Below 120                                200 (200)
Note: Values in brackets were original parameters adopted for Class A Prediction (i.e. Prediction
made of pile capacity and stiffness prior to load testing, using the method described in Poulos 1980)
The parameters for the upper soft clay/silt layer took into account the effect of ground
improvement carried out by chemical lime piling to facilitate design and construction of the
basement excavation, improve trafficability by construction machinery at bulk excavation level, and
to improve the stiffness of the material immediately below the foundation raft. A description of
the chemical lime piling ground treatment carried out at this site is presented in Wong (2004). In
brief, 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 the
project, an extensive program of pile load testing was carried out. In particular, the ultimate load
capacity and stiffness of 600mm diameter spun piles (pile type preferred by the client) were of
critical interest. The pile load testing program comprised:

    •   Static load tests – one using Constant Rate of Penetration (CRP) and two using Maintained
        Load 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 around
the pile perimeter at 90o apart, and a central survey point (Reference 1) located on the pressure
jack. The central point was measured by survey against a remote bench mark and is considered to
be more accurate compared to the dial gauge readings which were affected by movement of the
ground and reference beams during testing. The results of the pile load testing are summarised in
Tables 2 and 3. An example of the static pile load testing on test pile TP8 is shown in Figure 1.


                              6000
                              5000
                  Load (kN)




                              4000
                              3000
                              2000
                              1000
                                 0
                                     0          10          20         30            40      50         60
                                                                 Settlement (mm)

                                         TP8(2) - Gauge A         TP8(2) - Gauge B        TP8(2) - Gauge C
                                         TP8(2) - Gauge D         Reference 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 static
        load 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 piles
        were not fully mobilised as indicated above.
    •   The shorter pile TP2 had an initial axial stiffness that was practically the same as the
        significantly 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
        thickness of the piles beneath the tower was increased to 100mm, and it was decided to fill
        the 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 ultimate
        load, it was decided to limit the operational ultimate load of the tower piles to 6000kN, at
        which the settlement corresponds to about 10% of the pile diameter.

Table 2:       Summary of Dynamic Pile Load Test Results (600mm dia. Spun Piles)
 Test     Pene-      Date         Date       Mobilised Mobilised      Mobilised                 Settlement
  Pile   tration   Installed     Tested         Skin         End        Total                        at
           (m)                               Friction(1) Bearing(1)   Resistance                 Maximum
                                                (kN)         (kN)        (kN)                   Load (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
  TP5       55      12-9-00      15-9-00        3330         1150       4480                        26
                    12-9-00      25-9-00        4970         1210       6180                        31
  TP6       44      12-9-00      15-9-00        2740          950       3690                        20
            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        Static Load      Dynamic Load       Static Load Test     Dynamic Load
                 Prediction           Test             Test                                    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 problem
    2. 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 established
at various distances from the pile. Reference 1 was located on the pressure jack (i.e. zero distance
from the pile) and 5 other references located at distances of 2m, 5m, 10m, 15m, 20m and 25m from
the 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 pile
settlement.

Figure 2 shows the measured ground settlement profiles at various distances away from test pile
TP4. The pile to pile settlement interaction was expected to be less than the ground to pile
settlement 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 into
account the presence of intermediate piles that may provide shielding effects which tend to reduce
the settlement interaction. Furthermore, when the distance between two piles increases, the
operating 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 aspect
will be further discussed later in Section 5 of this paper.
                                              1
                                             0.9

              Settlement Interactor Factor
                                             0.8                                                    TP4 Load Test
                                             0.7                                                    (Measured
                                             0.6                                                    Ground/Pile
                                                                                                    Settlement Ratio)
                                             0.5
                                             0.4
                                                                                                    Pile to Pile
                                                                                                    Interaction Assessed
                                             0.3                                                    Using DEFPIG
                                             0.2
                                             0.1
                                              0
                                                   0     10      20       30       40     50
                                                        spacing to diameter ratio, s/d



                                                   Figure 2 – Pile Group Settlement Interaction Factors


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 be
designed to have conventional geotechnical factors of safety. The piles could be regarded as
“settlement reducers” and as long as the entire foundation system has a satisfactory factor of
safety, and the system performs satisfactorily with respect to serviceability criteria, some of the
piles can be designed to “yield” under ultimate load conditions.

If a conventional pile foundation design were to be adopted, over 400 piles would have been
required for this project. Using the piled raft design concept, only 293 piles were required. The
design involved a number of iterations to optimise the raft thickness and piling layout. The final
solution adopted comprised a raft having a thickness of 0.5m in the Podium area and a thickness of
1.7m beneath the Tower over an area of about 49m by 43m that extends 4m beyond the edge of the
actual tower footprint. The extension of the thicker raft beyond the edge of the Tower was to
enable piles to be strategically located at the corners of the Tower where load concentration was
found to occur based on the analysis results (see Section 4.2). 293 nos. prestressed spun piles were
adopted in the final design as summarised in Table 4 below:

TABLE 4:         DETAILS OF PROPOSED SPUN PILES
                          Adopted                  Wall                                               Design         Design Allowable
Location       Pile      Ultimate               thickness                                             Length        Structural Capacity
            Diameter Geotechnical Numbers         (mm)                                                below                (kN)
              (mm)       Capacity                                                                    Base of     Compression     Tension
                            (kN)                                                                     Raft (m)

Podium             500                                   2,100            87             80            23.5             2,000      760
Raft               600                                   2,700            34             80            23.5             2,800      910
Tower              600                                   6,000            76         110 to 17m                         3,700      930
Columns                                                                             80 thereafter       53
Core        600 filled                                   6,000            96         110 to 29m                      3,700         930
              with                                                                  80 thereafter       53         Concrete
              mass                                                                                                    infill
            concrete                                                                                                ignored
4.2         Design Approach and Analysis Results

Initially, hand calculations were carried out to satisfy the Strength Limit State requirements to
approximate the number of piles required, such that the overall design capacity of the piled raft
φgRug = φgRug (raft) + φgRug (piles) is greater that the design action effect, Rs*. Geotechnical strength
reduction factors, φg, 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 using
computer 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 the
          foundation system (i.e. raft and piles in that they must have adequate structural capacity
          against the predicted loads/bending moments).

For both of these conditions, φg values of 1.0 are applicable. It should be pointed out that for the
calculation of structural design action effects, the use of φg values of less than 1.0 could result in
lower 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 model
various load combinations including wind loads and uplift from buoyancy effects during
construction. Bending moments and shear forces were computed to enable structural design to be
carried out, and pile loads were computed for checks to be made regarding the structural capacity
of the piles. With respect to pile loads under the ULS cases, the following findings were of
particular interest:

      •   The analysis indicated a stress concentration at the corner of the core, due to the “rigidity”
          of 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 has
          enabled 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 were
          calculated to be 3,400kN and 5,653kN. The maximum working and ultimate pile loads for
          the piles beneath the core (mainly at the corners) were calculated to be 3,722kN and
          5,987kN respectively under over-turning wind loading. These are slightly above the
          allowable structural capacity and close to the ultimate geotechnical capacity of the 53m
          long 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 satisfied
          the 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                                                  130mm
          Local Raft Rotation                                                      1:257
          Differential settlements between the Tower column and the core           1:360
          Differential settlement between Tower columns                            1:430

Although some of these computed values were relatively high, the design was considered to be
acceptable on the acknowledgement that the pile group settlement prediction was probably
conservative due to likely over-estimation of pile group interaction effects in this large piled raft
due to pile shielding and small strain effects away from the loaded pile. As a precaution, the
structural engineer incorporated a construction joint between the Tower and Podium areas to
enable 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.
5   FIELD PERFORMANCE

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

The maximum settlement observed was only 21.5% of the predicted settlement, and this was
surprising even though it was recognised during the design stage that the estimated settlement was
likely to be over-predicted due to the conservative estimates made on pile settlement interaction
factors. Besides the difficulties associated with estimating pile settlement interaction factors for
large pile groups, a possible cause to the relatively small settlement observed may be that the live
load 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 the
predicted maximum settlement would have been about 96mm only. This is still over 3 times the
observed maximum settlement.

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

6   CONCLUSIONS

Although the settlement of the piled raft was significantly over-predicted, the project
demonstrated the successful use of the innovative piled raft solution. The following benefits were
derived:
    • More economical, driven spun piles were used instead of the original intended large
        diameter 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 be
        loaded close to their ultimate geotechnical capacity under ultimate load conditions.
    • Design confidence in this project was achieved via a detailed geotechnical investigations
        and a prototype pile load testing programme, with settlement monitoring during and after
        construction 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 and
pile shielding effects. The author also speculates that for slender piles, perhaps the elastic
compression component should be ignored in assessing pile settlement interaction factors. These
aspects warrant further research.

REFERENCES
Poulos, H.G. (1980) User Guide to DEFPIG – Deformation Analysis of Pile Groups, School of Civil
Engineering, University of Sydney.
Poulos, H.G. (1993) Settlement prediction for bored pile groups, Deep Foundations on Bored and
Auger Piles, Van Impe (ed.) 1993, Balkema, Rotterdam, 103 - 117.
Poulos, H.G. (1994) Alternative Design Strategies for Piled Raft Foundations, 3rd Int. Conf. Deep
Foundations, Singapore, 239 – 244.
Wong P. K. (2004) Ground Improvement Case Studies – Chemical Lime Piles and Dynamic
Replacement, Australian Geomechanics Jnl. Vol. 39, No. 2 June 2004, 47-60.

				
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