Abstract

In the literature the analysis of piles-soil-cap interaction received little attention from the geotechnical engineering community. This is mainly due to the complexity of the problem and the difficulties involved in experimental and analytical modelling. This thesis presents highly sophisticated analytical and numerical models to investigate the problem stated and incorporating the pore water pressure dissipation, which takes place during the consolidation process for the plane-strain, axi-symmetrical and three dimensional cases. Furthermore, the theories developed estimate the nonlinear load-settlement relationship of pile-soil-raft interactive foundation, the proportions of loads carried by the raft and piles, the increasing process of the ultimate bearing capacity of piles and the effective stress changes which take place in the soil mass. Evaluation of piles-soil-caps interaction during pore water pressure dissipation and the consolidation process may positively impact on the foundation settlement and the load sharing mechanism. The interaction is a nonlinear operation which involves the piles in the group, soil surrounding the piles, piles' cap (raft), and excess pore-water pressure (EPWP) in the soil. In the literature, due to the complexity of the problem stated, the role of the pore water pressure was ignored and accordingly, the raft will share the foundation load when the piles reach the ultimate load. Under this condition, the sharing ratio of the soil-piles load does not change during consolidation and further overestimates the contribution of the raft to the total load. This thesis presents a nonlinear method of analysis to evaluate the load sharing ratio during the consolidation process and accordingly as a result of the pore water pressure dissipation. The proposed analysis establishes the load-sharing ratio as a function of the load level and load location on the raft. The initial pore water pressure distribution after pile driving was also investigated. It was noted that the pore pressure generated during driving is not only due to cavity expansion but also due to an increase in mean total stress caused by the skin friction along the pile's shaft and on the pile tip. Furthermore, the pore pressure generated by the residual forces is relatively small and can be neglected. The analysis of strength-stress relationship shows that the excess pore pressure generated during pile driving increases almost linearly with depth, which confirms field measurements. Furthermore, fractures in soil during pile driving make the excess pore pressure fall to a stable level equivalent to the effective overburden pressure. This becomes a major factor, which should be considered in the estimation of the excess pore pressure generated within the pile group. Analytical models are developed to simulate the cases of pore pressure dissipation for plan-strain, axi-symmetrical and rectangle-area problems with only horizontal permeating, and 3-D dissipation problem for uniform soil. Moreover, the numerical inversion of Laplace transform to find solution of pore-pressure dissipation in layered soil is presented. The changing process of the ultimate bearing capacity of pile foundations due to the interaction process is presented. The proposed theories are practical and easy to use. Furthermore, charts for the consolidation level for a pile group and pile length are also given in this thesis. The simplified and convenient interaction analysis methods established in this thesis were validated using the results obtained by a sophisticated numerical model. This method is capable to estimate the load-settlement curves of pile-soil-raft nonlinear interactions and accordingly, the variations of load sharing proportions. Key words. piles-soil-raft interaction, pore-water pressure, initial distribution models, nonlinear analyses, consolidation process, numerical methods, effective stress analysis