Piled Raft And Compensated Piled Raft Foundations

Piled Raft And Compensated Piled Raft Foundations For Soft Soil Sites

Harry G. Poulos1 Fellow ASCE Abstract This paper re-visits the compensated piled raft foundation system, and outlines a simplified approach to the analysis of both conventional piled rafts and compensated piled raft foundations for the support of structures on very soft clays. Two cases are considered: the first where only applied load acts on the foundation, and the second where both applied load and externally imposed ground settlements act. It is demonstrated that the use of compensation, via excavation of the soil and embedment of the raft, can lead to significant reductions in settlement compared to normal (uncompensated) piled rafts. Importantly, when ground settlements occur, the use of compensated piled rafts can lead to significantly reduced differential settlements between the structure and the ground, compared to the case where the structure is supported by end bearing piles. Indeed, the latter may be counter-productive and lead to the not uncommon situation where the building stands well above the surrounding ground. A simplified design approach is proposed, and it is demonstrated that this simplified approach can lead to a computed behavior which is consistent with past experience in Mexico City. Introduction Piled rafts have proved to be an economical alternative to conventional pile foundations in circumstances in which the soil below the raft can provide significant bearing capacity and stiffness to supplement that of the piles (Randolph, 1994; Katzenbach et al, 1998, Poulos, 2001). There is however a reluctance on the part of many foundation designers to consider the use of piled raft foundations in soft clays, for at least two reasons: i. the soft clay often provides only a modest bearing capacity and stiffness for the raft, with the piles having to carry the vast majority of load. ii. if the soft clay is likely to undergo settlement, for example due to reclamation filling or dewatering, the soil may settle away from the base of the raft, again leaving the piles to carry the load. Despite these reservations, piled rafts have been used successfully in the past, most notably in Mexico City, where Zeevaert (1957, 1973) pioneered the use of rafts and compensated rafts with friction piles. 1

Senior Principal, Coffey Geosciences, 8/12 Mars Rd., Lane Cove West, NSW Australia, 2066. Email: [email protected]

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Given the proven usefulness of this type of foundation system, and the increasing need to build structures on very soft clays, it seems appropriate to re-visit compensated piled raft foundations. This paper discusses the performance of piled rafts in soft clays, and examines the following issues: i. the behaviour of piled rafts on soft clays, without excavation compensation or ground settlements; ii. the behaviour of compensated piled raft foundations on soft clays; iii. the effects of ground settlement on the performance of piled rafts and compensated piled rafts. Characteristics of Behavior of Piled Rafts on Soft Clays – No Ground Movements For the purposes of illustrating the key features of piled raft behavior, the simplified problem in Figure 1 has been analyzed. In this figure, L=pile length, d=pile diameter, Ep = pile Young’s modulus, h=soil layer depth, Es’ = drained Young’s modulus of soil, ν’ = drained Poisson’s ratio, cu=undrained shear strength of soil, fs=ultimate shaft friction, fb=ultimate end bearing capacity of pile, and fap=ultimate bearing capacity of raft. The problem involves a relatively stiff square raft or footing, supported by 9 timber piles with an average diameter of 300 mm, in a very soft clay layer underlain by medium dense sand. To simplify the analysis process, constant (average) values of drained Young’s modulus and undrained shear strength have been adopted for the soil layer. The analysis was done by using the FORTRAN computer program PRAWN (Piled Raft With Negative Friction; Poulos, 1993). This uses a boundary element representation of both the piles and the raft (which is assumed to be relatively rigid). The soil was treated initially as an elastic continuum, but limiting values are assigned to the contact pressure beneath the raft, the shaft friction along the piles, and the tip resistance of the piles. In this way, non-linear behaviour of a piled raft foundation can be simulated. Vertical ground movements along the length of the piles can also be specified, so that the effects of negative skin friction or soil heave can be incorporated. Figure 2 shows the computed load settlement curves, for various pile lengths, ranging from zero (i.e. raft only) to 20m (piles are end bearing on the sand). As would be expected, the settlement at any load level decreases as the pile length increases. When the piles are end bearing on the medium dense sand, the settlement is reduced substantially compared to the case of floating piles. The influence of pile length is illustrated more clearly in Figure 3, in which the relative reduction in settlement of the raft, the load sharing, and the overall factor of safety, are plotted against pile length, for a particular applied loading (22.2 kPa, equivalent to a total load of 0.8 MN). Clearly, the addition of the piles increased both the stiffness and overall factor of safety of the foundation, but the actual settlement remains quite large (from Figure 2) unless the piles are end-bearing. In that case, the raft carries almost no load.

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Characteristics of Behaviour of Piled Rafts on Soft Clays – With Ground Movements For the problem illustrated in Fig.1, the program PRAWN has been used to study the characteristics of piled raft behavior when ground settlements due to an external source also occur. Such situations are common in certain urban areas (e.g., Bangkok, Mexico City, Houston) because of the pumping of groundwater for water supply. Fig. 4 plots the computed settlement of the foundation, as a function of ground surface settlement, for various pile lengths. For the floating pile cases considered, the settlement of the foundation continues to increase (almost linearly) as the ground settlement increases. In this case, the increase in settlement of the foundation is essentially equal to the increase in the ground surface settlement. However, for the 20 m long end-bearing piles, the foundation settlement reached a maximum value of about 16 mm for a ground settlement of about 100mm, and then remained constant thereafter. At first, the adoption of end-bearing piles may appear to be the best solution, as attested by the vast majority of solutions found in practice. However, it is instructive to plot the difference in settlement between the foundation (assumed rigid here) and the surrounding soil, as shown in Fig. 5. This now puts a very different perspective on the foundation performance, because the end-bearing pile solution gives a differential settlement which increases with increasing ground movement, whereas the raft with floating piles experiences almost constant differential settlement, regardless of the ground surface settlement. This differential settlement decreases as the pile length increases, and floating piles founded just above the medium dense sandy layer will experience only a modest differential settlement. Thus, from the viewpoint of differential settlement, the optimum solution is to use piles which are founded just above a bearing stratum (1 to 2 m, to allow for settlement of the tip). Such a solution has the potential to provide reasonable stiffness and load capacity, without giving rise to difficulties associated with large and continually increasing differential settlements. Characteristics of Behavior of Compensated Piled Rafts Compensated piled rafts involve the excavation of soil, before or after piles are installed, in order to reduce the net increase in load applied by the foundation to the underlying soft soil. The removal of soil reduces the vertical effective stress in the soil, thus putting it in an overconsolidated state and reducing its compressibility. The subsequent loadings of the foundation will therefore tend to cause less settlement than if no excavation of the soil had been carried out. The key issues to be addressed in the design of compensated piled rafts are as follows: 1. The maximum depth to which an excavation can be carried out. 2.

The effect of the overconsolidation caused by the excavation on the stiffness and ultimate load capacity of the raft.

3.

The effect of the overconsolidation on the stiffness and ultimate load capacity of the piles.

Each of these issues is addressed below. Maximum Depth of Excavation If the sides of an excavation are adequately supported and if no piles are present, the maximum depth of excavation is dictated by bottom heave considerations. Expressions for the factor of

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safety of the excavation are given several references, for example Clough et al (1989). In the case of a deep layer of soft clay whose undrained shear strength increases linearly with depth, the factor of safety decreases as the depth of the excavation increases, as expected. However, the factor of safety also increases with increasing excavation width, in contrast to the more commonly described case of a constant undrained shear strength with depth. When piles are installed prior to the excavation being carried out, an expression can be derived for the equivalent cohesion for a pile-reinforced soil, and this equivalent cohesion can be used in the conventional equations for base heave stability, in place of the undrained cohesion. Assuming that the clay layer is deep, the Davis and Booker (1973) expression may be used for the foundation bearing capacity of the footing on the equivalent soil mass with a “crust”. The following expression can be derived for the equivalent cohesion for vertical loading of the reinforced soil:

cev = where Nc Pvu F Ar P B t

1  Pvu pB  −  N c  F . Ar t 

(1)

= bearing capacity factor (5.14 for strip foundations, 6 for circular foundations) = ultimate vertical bearing capacity of the piled foundation = correction factor, which can be approximated as 1.0 for many cases = plan area of footing or raft = rate of increase of soil shear strength with depth = footing or raft width = shape factor (4 for strip, 6 for circular foundation).

Pvu is computed from normal pile capacity theory, and can be taken as the lesser of the sum of the individual pile capacities and the capacity of a block containing the piles and the soil between them. As an example of the beneficial effect of piles on the stability of an excavation, Figure 6 shows the computed factor of safety against bottom heave for a 20 m square excavation, when 300 mm square precast concrete piles, at spacings of 1.0 and 1.5 m, are present. As would be expected, the factor of safety tends to increase with increasing pile length, but only for lengths in excess of about 5 to 10 m. Also, the factor of safety increases as the pile spacing is reduced. Effect of Excavation on the Stiffness and Ultimate Load Capacity of the Raft As a first approximation, it would appear reasonable to make the following assumptions with respect to raft behaviour to allow for the possible effects of excavation: 1. The modulus of the soil used to compute the raft stiffness is the unload/reload value until the average contact pressure below the raft reaches the “preconsolidation” pressure, i.e. the footing pressure required to cause virgin (first-time) loading of the footing to occur. For average contact pressures in excess of this “preconsolidation pressure”, the first loading modulus value is used. 2.

The ultimate bearing capacity of the raft is unaffected by the excavation process, other than for the effect of embedment, which will tend to increase its capacity.

The above modifications have been incorporated into a modified version of the piled raft program PRAWN, entitled PRAWNCPR.

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Effect of Excavation on the Stiffness and Ultimate Load Capacity of the Piles The excavation process will tend to cause ground movements which may influence the stiffness of piles which have already been installed. Excavation-induced ground movements have been incorporated into PRAWNCPR, but the possible effects of excavation on the soil modulus around the piles have been ignored, since the process of pile installation generally causes a significant "preloading" of the soil around and below the pile shaft. Moreover, the simplifying assumption is made that the ultimate axial capacity of the piles is also unaffected by excavation. Typical Analysis Results For a Compensated Piled Raft The simple problem shown in Fig.1 has been analyzed, in order to examine the influence of excavation (and hence compensation) on the foundation behavior. The following assumptions have been made: 1. The unload/reload Young's modulus of the soil is 5 times the first load modulus. 2.

The piles are installed prior to the excavation.

3.

The ground surface heave due to excavation is computed from a simple elastic analysis and the ground heave is assumed to decrease linearly with depth to zero at a depth of 20 m below the ground surface.

The following stages are modeled: 1. The ground heave (estimated to be 38 mm at the ground surface) occurring prior to foundation construction and loading. 2.

The application of the design load (800 kN, equivalent to an average applied pressure of 22.2 kPa) to the foundation.

3.

The gradual development of ground settlement (e.g., due to creep or previous dewatering) up to a surface settlement of 300 mm.

Fig. 7 shows the computed settlement of the foundation, after initial excavation of the soil to a depth of 1.5 m, as a function of the net ground movement after the initial heave caused by excavation. Also shown is the corresponding relationship for the case of no excavation. Fig. 8 shows the corresponding relationship for the proportion of the applied load carried by the piles. The following observations can be made: 1. The compensated foundation (with excavation) experiences less settlement than the normal piled raft foundation. 2.

For the compensated foundation, the net settlement of the foundation is almost exactly equal to the net ground settlement. Thus, in this particular case, there is virtually no differential settlement between the foundation and the surrounding soil.

3.

For the compensated foundation, the piles take a significantly smaller proportion of the load than for the uncompensated foundation, i.e., the raft is more effective for the compensated foundation.

4.

As the ground settlement increases, an increasing proportion of load is transferred to the piles.

In summary, the simple example described herein illustrates the potential for a compensated piled raft foundation to reduce both the absolute settlement and the differential settlement between the

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foundation and the surrounding soft soil. It therefore provides a means of developing a foundation that works and settles "with the ground", rather than one which "fights the ground". A Practical Approach To The Design of Compensated Piled Rafts (a)

Without External Ground Movements

The following steps outline a practical approach for computing the load settlement curve to failure for a compensated piled raft, without the effects of vertical ground movement. This approach is an extension of the simplified “PDR” method described by Poulos (2002), and produces a tri-linear load-settlement curve. The steps in the approach are as follows: 1. 2. 3.

4. 5.

6.

7.

Compute the maximum depth of excavation, which can be employed, and select a depth He which is less than or equal to this value. Compute the reduction in vertical pressure, pe due to the excavation to a depth He. Assess the applied average pressure, pec, that needs to be applied to the raft (prior to excavation) to cause virgin loading of the raft. pec may be estimated approximately as the average difference between the preconsolidation pressure and the in-situ effective vertical pressure within the depth of influence of the raft (typically, a depth of about 1.5 times the raft breadth). In a soft normally consolidated soil, pec would be approximately zero, ignoring any quasi-preconsolidation due to prior creep settlements. Assess the soil modulus within the depth of influence of the raft for reloading, E2. This will typically be 3 to 5 times greater than the modulus for the soil for the virgin loading state (E1). Compute the incremental stiffness of the raft foundation (Kr) for two cases: a. For the soil in the virgin loading state (Kr = Krn) b. For the soil in the reloading state (Kr = Kro). The raft stiffness can be computed from a variety of settlement calculation approaches, for example, the elastic method described by Mayne and Poulos (1999). For preliminary purposes, the required geotechnical parameters can be estimated either from in-situ testing, field measurements on test piles and footings, or via correlations such as those summarized by Kulhawy and Mayne (1990). Compute the stiffness of the pile group, Kp, for the number of piles being considered. Because of the simplifying assumption made, this value will be the same, whether the soil below the raft is in a virgin loading state or a reloading state. The pile group stiffness can also be computed from elastic theory (Randolph, 1994; Poulos, 1989). Compute the incremental stiffness of the piled raft and the load sharing between the raft and the piles, using the equations of Randolph (1994), as set out below. a. Incremental stiffness of piled raft foundation, Kpr, Kpr = [Kp + Kc (1-2αcp)]/[1-αcp2 Kc/Kp]

(2)

b. The proportion of load carried by the raft: Pc/Pt = Kc(1-αcp)/[Kp+Kc (1-2αcp)] = X

(3)

where Kp = stiffness of pile group Kc = stiffness of raft αcp = raft-pile interaction factor Pc = load carried by raft

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Pt = total applied load on foundation The factor αcp can be computed from an expression given by Randolph (1994), but can often be taken to be between 0.6 and 0.8, with the larger value being relevant for larger numbers of piles. There will be two sets of values for the piled raft stiffness Kpr and the load sharing factor X: i. The values for the soil below the raft in the virgin loading state, Kprn and Xn, using the initial loading soil modulus E1; ii. The values for the soil below the raft in the reloading state, Kpro and Xo, using the unload/reload soil modulus E2. 8.

Apply the applied load P on the piled raft foundation in a series of relatively small increments. For each increment, while the soil below the raft remains in the reloading state, use Kpro to compute the incremental settlement ∆Si : ∆Si = ∆Pi / Kpro

9.

(4)

where ∆Pi = increment in applied load in increment i. Compute the average pressure acting on the base of the raft, pri, as follows: pri = pri-1 + Xo. ∆pi

(5)

where pri-1 = raft pressure for previous increment Xo = proportion of raft load for the reloading state ∆pi = average pressure increment = ∆Pi/raft area. 10. 11.

12.

Check for the state of the soil below the raft. While pr < pe + pec, the incremental stiffness of the raft is Kpro and the load sharing factor is Xo. When pa> pe + pec, then the remaining increments use the values for the virgin loading state, Kprn and Xn. Check for the pile capacity being fully mobilized, i.e, if Ppi > Ppu, where Ppi = load carried by piles at increment i, and Ppu = ultimate capacity of piles. If this condition is met, then, for subsequent increments, only the raft will be able to carry the additional loads. In computing the incremental settlement in this case, the relevant raft stiffness is used, i.e. Kro is the soil below the raft is in the reload state, and Krn if it is in the virgin loading state. Steps 8-11 are repeated until the ultimate capacity of the piled raft system is reached.

With the above approximate approach, a quadri-linear or tri-linear load-settlement curve is obtained, as shown in Figs. 9a-c. There will be three possible cases: a. Case 1, in which the soil becomes normally consolidated before the pile capacity is fully mobilized (Fig. 9a). In this case, up to the load at Point A, the soil remains in the reloading state, and the piled raft stiffness and load sharing are Kpro and Xo respectively. At Point A, the stiffness and load sharing of the piled raft system change to Kprn and Xn, to reflect the reduction in raft stiffness from Kro to Krn. At point B, the capacity of the piles is fully mobilized, and beyond that point, the incremental settlement is governed by the raft stiffness for the virgin loading state, Krn. This situation holds until the ultimate capacity of the piled raft system is reached. b. Case 2, in which the pile capacity is fully mobilized while the soil below the raft remains in the reloading state (Fig. 9b). In this case, the Point B, represents full

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mobilization of the pile capacity, and Point A represents the transition from the reloading to the virgin loading state below the raft. The incremental settlements from Point A to Point C (where the ultimate capacity is reached) are governed by the raft stiffness Krn. c. Case 3, in which the soil conditions below the raft remain in the reloading state when the ultimate capacity of the piled raft is reached (Fig. 9c), In this case, the pile capacity is fully mobilized at Point B, and the incremental settlements from Point B to Point C are governed by the raft stiffness Kro. In this case, the loadsettlement curve is tri-linear. The above computational process can be readily evaluated via a spreadsheet. (b)

Incorporating the Effects of External Ground Movements.

The following approach is suggested as a very crude first approximation to assess the required length of the piles in order to obtain equal settlement of the compensated piled raft foundation and the ground surface adjacent to the foundation. For simplicity, it is assumed that the external settlement profile decreases linearly with depth, from S0 at the ground surface to zero at a depth of zh, the depth of the soft clay layer. The average settlement of the piled raft without the effect of ground movements (S1) is given via the approach described above, taking into account the effects of overconsolidation on the soil modulus and also the possibility that the capacity of the piles may become fully utilized at a load less than the design load. The settlement of the foundation due to the external ground movement, S2, can be approximated as follows: S2 = S0 (1-ze/zh)

(6)

where ze = depth of excavation zh = total thickness of settling soil layer. Based on the solutions obtained from the computer analysis, it appears that the settlement of the piled raft foundation will be approximately equal to that of the soil at the base of the raft. On the basis of this assumption, the total settlement of the foundation is then S1 + S2. Application to La Azteca Building Case The case of the La Azteca building was described by Zeevaert (1957). The building exerted a total average loading of about 118kPa, and was located on a deep highly compressible clay deposit which was also subjected to ground surface subsidence arising from groundwater extraction. The building was founded on a compensated piled raft foundation, consisting of an excavation 6m deep with a raft supported by 83 concrete piles, 400 mm in diameter, driven to a depth of 24m (i.e. the piles were about 18m long below the raft). Fig. 10 shows details of the foundation, the soil profile, the settlement computed by Zeevaert, and the measured settlements. The settlement without piles computed by Zeevaert (from a onedimensional analysis) was substantial, but the addition of the piles was predicted to reduce the settlement to less than half of the value without piles. The measured settlements were about 20% less than the calculated settlements, but nevertheless confirmed the predictions reasonably well.

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The approximate analysis described in the present paper was applied to this case, excluding the effects of ground settlements, which were not detailed by Zeevaert in his paper. The following approach was adopted: 1. The one dimensional compressibility data presented by Zeevaert was used to obtain values of Young’s modulus of the soil at various depths, for the case of the soft clays in a normally consolidated state. A drained Poisson’s ratio of 0.4 was assumed. The modulus values thus obtained were typically very low, of the order of 0.5 – 1.0 MPa, and lower than would have been anticipated on the basis of the measured shear strength of the clay. 2. The bearing capacity of the raft was estimated from the shear strength data provided by Zeevaert, and was found to be about 200 kPa. This represented a factor of safety of about 1.7 on the average applied loading of 118 kPa. 3. The settlement of an uncompensated raft was computed using these modulus values together with conventional elastic theory. A very large settlement, in excess of 2.3 m, was obtained for the final settlement. 4. The settlement of a compensated raft was computed, assuming a 6m depth of excavation, and assuming that the soil modulus values for the overconsolidated state were 10 times those for the normally consolidated state (based on the oedometer data presented by Zeevaert). The additional raft pressure to recommence virgin loading conditions, pec, was taken to be zero. A settlement of the order of 988mm was thus computed. 5. From the pile load tests reported by Zeevaert, values of the single pile capacity and stiffness were obtained, these being about 735 kN and 25 MN/m respectively. 6. For the 83 piles used in the foundation, the group stiffness was computed by using the approximation of Poulos (1989) and applying a factor of 9.1 (the square root of the number of piles, i.e. 830.5) to the single pile stiffness. A group stiffness of about 230 MN/m was calculated. 7. The average settlement of the foundation for an uncompensated piled raft was computed, using the equations developed by Randolph (1994) for the piled raft stiffness. A settlement of about 1.08 m was obtained. The analysis indicated that, in this case, the raft would carry only about 4% of the load under elastic conditions, and that the capacity of the piles would be mobilized fully under the design load of about 78MN. 8. The effects of carrying out a 6m deep excavation (as was actually used) was simulated by reducing the thickness of the soil profile accordingly, and again assuming that, for the raft, the soil Young’s modulus for the over-consolidated state was 10 times that for the normally consolidated state (based on the laboratory oedometer data published by Zeevaert). The stiffness of the raft was thus increased significantly, leading also to a significant increase in the stiffness of the piled raft foundation, to about 300 MN/m. The raft, at the design load, was found to carry about 40% of the total load, and the computed settlement under that load was reduced to about 280 mm. The analysis results are summarized in Table 1. It can be seen that the settlement of the compensated piled raft is about 26% of the settlement of the piled raft without compensation, 29% of the settlement of the compensated raft alone, and only about 12% of the value for the uncompensated raft. Zeevaert’s calculations gave larger settlements than those computed above, being about 1000 mm for the compensated raft alone, and about 370 mm for the compensated piled raft. This represented a reduction in settlement of about 63% in using the compensated piled raft rather than the compensated raft alone. This compares reasonably well to the 71% reduction in settlement computed from the present approach. It is also interesting to note that the measured settlements about 2 years after the commencement of construction were about 20% less than those predicted

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by Zeevaert. At that stage, the measured settlement was about 205 mm and the computed settlement from Zeevaert was 250 mm, i.e. about 68% of the final predicted settlement. Assuming a similar rate of settlement, the prediction made by the current approach for the settlement after 2 years would be about 192 mm, in fair agreement with, but somewhat less than, the measured 205mm. Clearly, the combined use of piles and compensation via excavation, leads to a foundation that provides a superior performance to that of an uncompensated piled raft or a compensated raft alone. Table 1 Summary of Computed Average Settlements Case Computed Average Final Ratio of Settlement to Settlement mm Settlement of Compensated Raft Raft alone, no compensation 2342 2.37 Raft alone, with compensation 988 1.0 Piled raft, no compensation 1084 1.10 Piled raft, with compensation 283 0.29

Concluding Remarks This paper has reviewed the applicability of piled rafts and compensated pile rafts for foundation construction on very soft clays. While the traditional approach to this situation is to install end bearing piles to rock, it may not always provide the best option. In particular, when ground settlements are occurring due to (for example) pumping for groundwater or prior filling, the use of end bearing piles may result in very large differential settlements between the pile-supported structure and the surrounding ground. Under these circumstances, it is preferable to adopt an option which involves the use of floating piles. The paper explores the general characteristics of behavior of piled rafts and compensated piled rafts for the case of vertical loading, and also under the action of externally imposed ground movements. The paper then sets out practical design approaches for a compensated piled raft foundation. This type of foundation has been used successfully in Mexico City, and it combines the relief of overburden stress as a result of excavation, with the additional capacity and stiffness that can be provided by combining piles with a mat or raft foundation. A relatively simple approach outlined in this paper appears to have the potential to estimate the settlement performance of such a compensated piled raft foundation system. Acknowledgements The author gratefully acknowledges the great value of several discussions with the late Michael W. O’Neill in relation to piled rafts and their applicability to a wider range of situations than is commonly acknowledged in practice. The author also thanks John C. Small for his useful comments.

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References Clough, G.W., Smith, E.M. & Sweeney, B.P. (1989). “Movement control of excavation support systems by iterative design”. Found. Eng. Current Principles and Practices, Ed. F.H. Kulhawy, ASCE, 2: 869-884. Davis, E.H. and Booker, J.R. (1973). “The effect of increasing strength with depth on the bearing capacity of clays”. Geotechnique, 23: 551-563. Katzenbach, R., Arslan, U., Moorman, C. & Reul, O. (1998). “Piled raft foundation: interaction between piles and raft”. Darmstadt Geotechnics, Darmstadt University of Technology, No.4, 8292. Katzenbach, R., Arslan, U. and Moorman, C. (2000). “Piled raft foundations in Germany”. Ch. 13 of Design Applications of Raft Foundations, ed. J.A. Hemsley, Thomas Telford, 323-391. Kulhawy, F.H. and Mayne, P.W. (1990). “Manual on estimating soil properties for foundation design. Report No. EPRI EL-6800, Cornell University. Mayne, P.W. and Poulos, H.G. (1999). Approximate displacement influence factors for elastic shallow foundations”. Jnl. Geotech & Geoenv. Eng., ASCE, 125(6): 453-460. Poulos, H.G. (1989). “Pile behaviour – theory and application”. Geotechnique, 39(3): 365-415. Poulos, H.G. (1990). DEFPIG Users Manual. Centre for Geotechnical Research, Univ. of Sydney, Australia. Poulos, H.G. (1993). “Piled rafts on swelling or consolidating soils”. Jnl. Geot. Eng., ASCE, 119(2): 374-380. Poulos, H.G. (1994a). “An approximate analysis of pile-raft interaction”. IJNAMG, 18: 73-92. Poulos, H.G. (1994b). “Settlement prediction for driven piles and pile groups”. Spec. Tech. Pub. 40, ASCE, 2: 1629-1649. Poulos, H.G. (1999). “Common procedures for foundation settlement analysis – are they adequate?” Keynote Lecture, Proc. 8th Aust. New Zealand Conf. Geomechanics, Hobart, 1: 1-25. Poulos, H.G. (2002). “Simplified design procedure for piled raft foundations”. Deep Foundations 2002, Ed. M.W. O’Neill and F.C. Townsend. ASCE Spec. Geot. Pub. No. 116, 1:441-458. Poulos, H.G. and Davis, E.H. (1980). Pile foundation analysis and design. John Wiley, New York. Randolph, M.F. (1994). “Design methods for pile groups and piled rafts”. Proc. 13th Int. Conf. SMFE, New Delhi, 5: 61-82. Zeevaert, L. (1957). “Compensated friction pile foundation to reduce settlement of buildings on the highly compressible volcanic clay of Mexico City”. Proc. 4th Int. Conf. SMFE, London, 2: 8186.

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Zeevaert, L. (1973). Foundation engineering for difficult subsoil conditions. Van Nostrand Reinhold, New York.

Figure 1. Pile raft problem considered

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Figure 2. Computed load-settlement curves for 9 piles – no ground movements

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Figure 3. Computed behavior of piled raft – no ground movements

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Figure 4. Effect of ground movement on piled raft foundation settlement

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Figure 5. Differential settlement between foundation and ground surface

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Figure 6. Influence of pile length on factor of safety against basal heave

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(mm)

Figure 7. Settlement of foundation versus ground movement

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Figure 8. Proportion of load carried by piles, with and without compensation

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Figure 9. Simplified load-settlement curves for compensated piled raft foundation

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Figure 10. Details of La Azteca building on compensated piled raft (Zeevaert, 1957).

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