Effectiveness of Piled Raft Foundation for Seismic Performance of Tall Buildings
Dr Atul K Desai, Head, Applied Mechanics Department, S V National Institute of Technology, Surat.
The last two years have seen the start of many mega construction projects in India, which include several skyscrapers having more than 40 floors and several more than 150m tall. Their foundation types may be very much different depending on the structural loading and subsurface conditions. In recent years, there have been an increasing number of structures using piled rafts as the foundation to reduce the overall and differential settlements. For cases where a piled raft is subjected to a non-uniform loading, the use of combined pile-raft can improve the performance of the foundation.
Computer aided finite element coding is done by sap2000 v14 for analysis of different foundation system with inclusion of varying soil parameters with respect to different soil layers. For piled rafts embedded in layered soil, the modulus of each layer of soil is used in the computation and accurate solutions are obtained without the use of an averaging technique.
IntroductionFoundation engineers have long recognized that the use of piles in conjunction with a raft foundation is one way of reducing the total settlement of the raft. This technique is most effective when the raft is quite adequate with respect to bearing capacity and relatively few piles may be required to reduce the settlement to within the accep- table limits. Due to the complexity of the system, designers have tended to neglect the contribution by the raft and base design calculation on the behavior of the pile group.
In the past few years, there has been an increasing recognition that the use of piles to reduce raft settlements and differential settlements can lead to considerable economy without compromising the safety and performance of the foundation. Such a foundation makes use of both the raft and the piles, and is referred to here as a pile-enhanced raft or a piled raft. Technical Committee TC18 of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) has focussed its efforts since 1994 towards piled raft foundations. Despite this recent activity, the concept of piled raft foundations is by no means new, and has been described by several authors, including Zhang and Small, Davis and Poulos (1972), Reul and Randolph, among many others.
Fundamental Concept of Soil Structure InteractionSoil-structure interaction (SSI) is the prediction of the response of soil to the loading of a structure as a function of deflection and the corresponding response of the structure. The soil may be fine-grained, coarse grained, or rock. The structure may be a mat, a footing, a caisson, or a pile. The loading may be seismic, dynamic, short-term, sustained, or cyclic.
Analysis of the piled raft is one of the most challenging problems in soil structure interaction. A piled raft foundation composes conventional piles and a rigid raft as shown in Fig. 1. Considering each of these foundation elements separately leads to the conclusion that interaction is inevitable. The mat alone is certainly affected by the presence of the piles because the foundation is much stiffer than with the soil alone. The piles alone are affected by the earth pressure from the raft because the increased lateral stresses on the piles affect the capacity in side resistance. The problem can be solved by use of the finite-element method where appropriate shell elements or solid elements can be used for modeling the raft. Beam elements can be used for modeling piles. The soil around the piled raft system can be conveniently modeled as solid elements or spring elements. The modern FEM software also provides advanced contact elements for modeling the interface between the pile/raft and soil.
3D Finite Element Model
|Figure 1: Soil-structure interaction of a piled raft structure||Figure 2: 3D Model of Non Soil-Structure Interaction Vs Soil StructureInteraction|
A complete three-dimensional analysis of a piled raft foundation system can be carried out by finite element analysis (e.g. Katzenbach et al, 1998). In principle, the use of such a program removes the need for the approximate assumptions inherent in all of the above analyses. Some problems still remain, however, in relation to the modelling of the pile-soil interfaces, and whether interface element should be used. If they are, then approximations are usually involved in the assignment of joint stiffness properties. Apart from this difficulty, the main problem is the time involved in obtaining a solution, in that a non-linear analysis of a piled raft foundation can take several days, even on a modern computer running at high frequencies. Such analyses are therefore more suited to obtaining benchmark solutions against which to compare simpler analysis methods, rather than as routine design tools.
Parametric Study (Analysis)
|Figure 3: Acceleration vs Time Response for Bhuj Earthquake|
|Figure 4: Velocity vs Time Response for Bhuj Earthquake|
For the whole study, twelve models of building are created. Foundation along with surrounding soil is considered for analysis. The layering effect of soil is also considered. Each layer consisting 10m of depth namely stiff clay, medium sand, dense sand. The values of modulus of sub-grade reaction taken as per J.E. Bowels. The building is 156m high with the slenderness ratio of 6.64. For analysis, the building along with foundation was modelled as frame and shell element consisting of 9049 and 3305 elements. Spring elements are used to represent the effect of soil for analysis. The Discretization of shell element was done at the rate of 1.2m x 1.2m with 0.3m of sub mesh. The Bhuj earthquake data is simulated in computer program SAP2000 v14 to run the time history analysis. Time history analysis is used for this study. Bhuj earthquake data is taken for analysis. The applied horizontal acceleration force is in x-direction only. Bhuj earthquake data was recorded at Ahmadabad station; the Latitude & Longitude of 23o 02’ N, 72o 38’ E. There were 26706 Acceleration data points (in m/s/s) recorded at 0.005 sec of interval. Those data were recorded on 26th January at 08:46:42.9 Indian Standard Time with the Mb = 7.0, Ms = 7.6.
The plot of acceleration history, velocity history and displacement history for Bhuj earthquake are shown here.
Comparative AnalysisPeak Ground Acceleration has been used over years to provide a convenient anchor point for the design spectra of various specified regulatory agencies. This peak acceleration can be obtained from the acceleration response of a time history plot. It is therefore necessary to know the acceleration plot of a time history along with the acceleration plot resulting from it at different points in the structure. This is important because the acceleration records of a particular earthquake may give its behaviour at that particular site however the behaviour of structure under the influence of such an earthquake is a localized phenomenon and is different for different points in the structure. In more, this response may vary depending on nature of earthquake excitation and the type of soil, which is under the influence of such an earthquake.
|Figure 5: Displacement vs Time Response for Bhuj Earthquake|
Foundation flexibility has significant effect on the response of tall slender structures. The nonlinearity in soil mass can result in increasing or decreasing displacement response depending on characteristic of ground motion and structure. This effect of soil on the displacement is more pronounced for softer soils.
CASE-I Soil-Structure Interaction (SSI) Vs Non Soil-Structure Interaction (NSSI)Soil conditions have a great deal to do damage with structures during earthquakes. Neglecting the effect of Soil-Structure Interaction (SSI) is valid for certain class of structures and soil conditions, such as light structures in relatively stiff soil. Unfortunately, the assumption does not always hold true. In fact, the SSI can have a detrimental effect on the structural response, and neglecting SSI in the analysis may lead to unsafe design for both the superstructure and the foundation. In this Study, importance of SSI on tall building is considered. The specific objective of the study is to identify the circumstances under which it is necessary to include the SSI effects in the design of Structure.
CASE-II Piled Raft Foundation Vs Raft FoundationIn this case, comparison is made between Piled raft foundation and raft foundation. Static as well as Dynamic load cases are applied as per IS: 1893 – 2002. Vertical load and lateral load are considered in static load case. Settlements are calculated across the middle of foundation system along the x-direction. The aim of this study is to check the settlement of foundation system along with the behaviour of superstructure with these foundations system.
|Figure 6: Time Period of SSI and Non SSI Structure|
Results and Discussions
Two structures with fixed base assumption and foundation system along with soil were analysed by time-history method and modal analysis method. The responses were calculated in terms of time period and acceleration response at different height of super structure.
Acceleration ResponseIn this study, first six mode of time were calculated for ssi and non-ssi structure. Results are quite different for both structures. When considering ssi effect, time period increases from 3.37% to 5.03% for first 3 modes. Which means structure becomes more flexible when foundation system and soil is adopted for analysis.
|Figure 7: Acceleration Response at Top of Building forBhujEarthquak||Figure 8: Acceleration Response at 0.5h (Half Height) of Buildingfor Bhuj Earthquake|
|Figure 9: Settlement of Foundation for Vertical Load Case1.5(DL+LL)||Figure 10: Settlement of Foundation for lateral Load Case1.2(DL+LL+EQX)|
It is seen from the above graph that the values obtained by assuming fixed base without ssi is more than that obtained by assuming flexible base with ssi. In some case, ssi value is more at one or two point but looking at throughout time period these values are less compare to fixed base non-ssi structure. Thus the consideration of ssi effect reduces the peak acceleration to super structure. Maximum acceleration values are in decreasing manner from top to half height of superstructure. The reason for this to happen is that long duration earthquake with high PGA have more energy flux and it takes long time for the structure to dissipate energy. The energy gets dissipated after getting transferred up to full length of structure hence the top portion has maximum acceleration. The difference of response in both cases is also noteworthy. Here Acceleration values were calculated at every second of time and plotted for 10 to 20 sec out of 135 sec time duration of Bhuj Earthquake.
Case-II Displacement Response (Static)In Figure 9, Results show that maximum settlement observed in raft foundation is 18.5mm while in piled raft foundation value is 0.4mm for load case 1.5(DL+LL). For load case 1.2(DL+LL+EQX), the values are 16mm and 0.3mm for raft foundation and piled raft foundation (Figure 10). Value is decreased for raft foundation which is 13.3mm for load case (DL+LL+WL) while value remains constant for piled raft foundation. Here noticeable thing is the nature of settlement in raft foundation with vertical and lateral load case. Referring to above graph, linear line shows the nature of differential settlement in raft foundation for later load case. Consideration of wind load also shows differential settlement in raft foundation (Figure 11). The amount of differential settlement is nearly 4mm. Tilting of raft foundation is also seen for lateral load case. For vertical load case, there is no amount of differential settlement seen in raft foundation. Coming to piled raft foundation, result shows excellent performance for above three load combination applied to this foundation. Linear line also suggests that Piled raft foundation control the tilting of foundation.
Case-II Displacement Response (Dynamic)
|Figure 12: Settlement Response of Foundation System (At Edge)for Bhuj Earthquake||Figure 13: Settlement at Centre of Foundation System for BhujEarthquake|
|Figure 14.Horizontal Deflection at Top of Building withDifferent Foundation System for Bhuj Earthquake||Figure 11: Settlement of Foundation for lateral LoadCase (DL+LL+WL)|
Figure 12 shows two different values for two different foundation systems. Values are in positive and negative form. Positive value indicates upward direction in Z- axis while negative value indicates downward direction in same above axis. Result shows that values are positive for raft foundation which indicates uplift in foundation system. Amount of uplift varies from 0.5mm to 0.7mm. For piled raft foundation values are in negative form which indicates no uplift. Again the performance of this foundation system is excellent and settlement value varies from 0.1mm to 0.3mm. When these settlement values calculated at centre of foundation (Figure 13) system tells different story. Raft foundation settles at the amount of 10mm in downward direction which means no uplift while piled raft foundation settles at the amount of 0.15mm in same direction. Here the noticeable thing is raft foundation settles 67 times more than the piled raft foundation throughout earthquake time period. Figure 14 shows horizontal deflection at top of superstructure. Again piled raft foundation gives good results compare to raft foundation. It is clear from above graph that piled raft foundation controls top deflection of super structure more efficiently than the raft foundation.
ConclusionConsidering Soil-Structure interaction (SSI) makes a structure more flexible and thus, increasing the natural period of the structure compared to the corresponding rigidly supported structure. This effect is also repeated in Time History analysis in which calculated acceleration at 0.5h and 1h (top of the structure) is less compared to rigidly supported structure. Thus considering the SSI reduces the acceleration. Acceleration at top portion of the structure is more compared to lower portion of structure in both SSI and NON-SSI.
While comparing piled raft foundation with conventional raft foundation for static load case, settlement ranges from 13.5mm to 19.5mm in raft foundation and 0.3mm to 0.4mm in piled raft foundation. Differential settlement is found to be 4mm in raft foundation while Piled raft foundation shows no sign of differential settlement. Calculated values suggested that there is a tilting of foundation in raft foundation for lateral static load case while for same load case no tilting of foundation is observed in piled raft foundation. For dynamic load case, Uplift is observed in raft foundation though the amount of uplift is very small. There is no uplift observed in piled raft foundation. Horizontal deflection at top of the super structure is more in raft foundation than the piled raft foundation for dynamic case. Thus piled raft foundation controls the top deflection more efficiently than raft foundation.
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