Interface Shear Behavior Between Clayey Soil Material And Concrete Foundation Pile Subjected To Basement-supplement Retrofit

Pile is one of the most widely used configurations for founding structures. Recently, urban underground space development is challenged by one issue that how to properly interpret the bearing behavior of constructed building pile subject to further excavation and adding new basement. This thesis investigates the whole-process behavior of interface shear between pile and soil. According to the sequence of construction, a few characteristic stages that the existing pile experienced were observed, including pile installation, service condition and excavation for adding basement. The mechanisms and influences of pile aging, excavation unloading and post-installation residual stress were described and analyzed. By means of numerical simulation, laboratory test and theoretical modeling, the behavior of existing foundation pile experiencing excavation unloading was explored. The major contributions and findings are summarized as follows.(1) The further-excavating reconstruction of the underground garage of Zhejiang Hotel was taken as the observed case. The vertically loaded single pile was simulated by establishing a plane model in the finite element software ABAQUS. The derived pile capacity was compared with other reported simulations to verify the correctness of the model. Then the founding stratum of bed rock is replaced by clayey soil, so as to simulate a vertically compressed friction pile in relatively soft material. The pile capacities governed by the Mohr-Coulomb model and the modified Cambridge model were obtained and closely compared. The results show that the growth of shaft friction with increasing pile-head load is greater for pile surrounded by better soil; friction softening may occur in case of weak-strong alternating soil layers; the modified Cambridge model predicts larger pile settlement than the Mohr-Coulomb model.(2) The interface shear behavior between concrete pile and silty clay was simulated through large-scale direct shear tests. The interface shear forces were obtained alo ng with the visual appearance of sheared soil particles at different moments of setup period. The effect of time on interface shear was thus interpreted in terms of prediction equations. The results show that the interface shear force increases with increasing setup duration; the trend of increase is close to logarithmic and may be divided into three phases by regarding 1d and 109 d as the boundaries; the interface normal pressure is positively related to the initial value but independent of the following increment of the shear force; as the shear tests proceed, the particles of silty clay are displaced, revolved, more and more stripped.(3) By analyzing the mechanism of excavation effect with respect to the pile-soil relative displacement, the distribution of pile shaft friction after excavation was theoretically derived. Then the finite element software ABAQUS was used to simulate the process of further excavation. The simulation accounted for the effect of excavation depth and width on pile shaft friction as well as the effect of friction-distributing pattern on pile capacity. The results show that tension is induced by excavation in a constructed pile where positive and negative frictions are distributed along the upper and lower portions; a simplified 5-segment distribution pattern of shaft friction is derived on the basis of linear-increment assumption; increases of excavation width and especially excavation depth put adverse effect on the pile capacity; although the excavation-induced shaft friction has little effect on the ultimate pile capacity, removal of such friction will underestimate the limit-state pile settlement.(4) By concerning the supplementary pile jacked into the excavation face, a theoretical model was established to calculate the post-jacking residual stress in the pile. The press-in and rebound process of installing the supplementary preformed pile was simulated by the finite element software ABAQUS. The unloading residual pile stress and shaft friction were derived and the effects of excavation width and the property of end-bearing soil were carried out. The results show that a rock-socketed pile compared to a floating pile has greater residual stress and shaft friction and deeper neutral-point position; wider excavation face and pile-supplement extent as well as greater ratio of pile-end resistance before unloading correspond to smaller residual stress and shaft friction and shallower neutral-point position.