Behavior Of Pile Foundation During Large-scale Deep Excavation

Soil unloading and rebound induced by large-scale deep excavation may affect the performance and integrity of existing pile foundation inside the excavation zone. Several instances of pile damage caused by large-scale soil unloading and rebound been reported in literature. However, the behaviors of existing pile beneath the excavation zone due to excavation-induced basal heave have not been well studied and understood. In this study, non-linear finite element is employed to investigate the behavior of single pile and pile groups inside the excavation zone during deep excavation. The study mainly focuses on the effect of basal heave on the behavior of piles.It is found that in numerical simulation of deep excavation, finite element model with considering diaphragm-soil interface show closer agreement with the measure data than diaphragm-soil nodes coupled model. Therefor the diaphragm-soil interface model should be use in behavior analysis of pile subjected to excavation-induced soil movement.Single pile analysis shows during deep excavation embedded pile is dragged upward by basal soil. The upper part and lower part of pile are subjected to positive and negative skin friction, respectively, which lead to considerable axial tension stress along pile shaft or even pile failure. The tension stress tends to increase with the increasing of excavation depth at early stages and then decrease with the increasing of excavation depth when excavation depth is larger. Parametric studies show the major parameters that will affect the pile behavior include pile length, limit friction of pile-soil interface, soil elastic modulus and et al, while diameter of excavation zone and friction coefficient of pile-soil interface of deep stratum have little influence on the the pile behavior. Comparison shows the soil consolidation induced by dewatering during excavation will lead to a slight drop in the tension stress.In pile groups analysis, the convergence difficulty of large-scale three-dimensional contact interface analysis is solved. Analysis of pile groupd shows piles closer to the center of excavation zone are subjected to a larger tension force, although the difference is alleviated by the increasing of excavation depth.Axialsymmetric finite element analysis is conducted to evaluate the behavior of bottom-enlarged piles in Shanghai Expo Underground Substation Project. Results show that tension force induced by excavation exerts a more negative influence on uplift piles than other piles. Therefore this influence should be taken into consideration during pile design. Analysis shows the bottom-enlarged uplift piles are subjected to a larger tension force during excavation than ordinary uplift piles. The tension force of pile under working load can be deduced from combination of the tension force induced by excavation and the tension force induced by working load.