6.1 Summary

6.1.1 Pile behaviour

Investigations into the behaviour of pile foundations in liquefied soil observed in both case histories from previous earthquakes and from experimental studies using large scale shake tables and seismic centrifuges revealed the key features influencing pile response. It was found that the loads on piles during earthquakes arise from both soil displacement and inertial loads from the superstructure. The occurrence of liquefaction results in a degradation of soil stiffness and an increase in soil displacement. It was found that in liquefied soil two distinct phases occur in the pile response: a cyclic phase and a lateral spreading phase. For both cases the pile response depends on the pile stiffness, the stiffness and strength of the liquefied soil throughout the shaking, the level of fixity at both the head and tip of the pile, the interaction effects in pile groups and the presence of a non-liquefied crust layer. In general, the most severe damage occurred at the pile head and at interfaces between liquefied and non-liquefied soil.

6.1.2 Simplified analysis methods

Existing simplified analysis methods were reviewed; the most appropriate were seismic displacement methods. One such method was described in detail and applied to a case study of a bridge foundation. On this case study a parametric study was performed where the key parameters in the analysis, namely the stiffness and strength of the liquefied soil, the magnitude of the lateral ground displacement, the ultimate pressure from the crust load and the magnitude of inertial load, were varied to identify key features of the response. It was found that for stiff piles the choice of liquefied soil properties has a large effect on the response. Also, for stiff piles undergoing lateral spreading, it was found that a threshold ground displacement exists. Once this threshold value has been reached any further increases in ground displacement have no further effect on the pile response. Increasing loads at the pile head due to a combination of a non-liquefied crust layer and inertial loads leads to a transition from stiff to flexible pile behaviour.

6.1.3 Advanced analysis methods

concept, was described in detail and applied to the same case study. The analysis results gave detailed information on the free field ground response, including the effects of soil density and the interaction between shaking intensity, excess pore water pressure development and ground deformation. The performance of the piles was rigorously evaluated by taking into account the highly complex nature of the loads and soil – pile interaction.

6.1.4 Two layer finite element modelling

In order to improve the capabilities of 2-D finite element modelling, a new two-layer finite element analysis technique was presented. The two-layer method was applied to two cases: first the effects of deep-soil-mixing walls on liquefaction remediation were evaluated and then the simulation of pile groups in laterally spreading soil was considered. In both cases the new two-layer model was able to model features of the response that conventional one-layer models cannot. The simulation of the deep-soil-mixing walls showed a decrease in pore pressures and ground deformation inside a DSM cell, this decrease depended on the size of the cell and the analytical trends agreed with experimental results and more rigorous 3-D simulations. The simulation of the pile groups showed that the flow of soil past a pile group can be simulated using the two-layer model; conventional one-layer models are unable to do so. This enabled the two-layer model to predict different pile deformations depending on the location of a pile within a pile group.

6.2 Conclusions

The characteristics of damage to piles in liquefied soil during strong earthquakes have been identified through investigation of experimental tests on full-size shake tables and seismic centrifuges and back-calculations from well-documented case studies. The key damage features and factors influencing the pile response have been summarised.

Regarding seismic assessment of pile foundations in liquefied soil, two levels of assessment have been studied; a simplified design-orientated method and an advanced time history method. In the simplified method the effects of varying key parameters have been evaluated and summarised, this provides guidance to designers on how to choose these key parameters. Use of the described beam-spring analysis method is recommended in practice as it captures the key features of the response but is based upon readily available site investigation data. It is recommended that for the identified key parameters a range of values should be used in design. The description of the advanced effective stress analysis provides guidance its

simplified analysis. It can be concluded that the effective stress analysis is appropriate for special or important structures and can be used to give an accurate, performance based assessment of pile response in liquefied soil.

Finally, a two-dimensional, two-layer finite element modelling technique was developed. This technique was successful in predicting three dimensional aspects of deep-soil-mixing walls and pile groups in liquefied soil. For cases where such aspects are important to the overall response of the foundation, this method is an alternative to the exhaustive demands of full 3- D analysis.

In document Behaviour of piles in liquefiable deposits during strong earthquakes (Page 139-141)