Calculation of neutral surface depth and pile-soil stress ratio of rigid pile composite foundation considering influence of negative friction

The computing method for determining the neutral surface depth and pile-soil stress ratio of rigid pile composite foundation is studied under skin friction of pile using a linear model. A cushion-pile-soil deformation compatibility equation is derived based on the settlement deformation relationship among different constituent parts of rigid pile composite foundation. The distribution of the skin friction is simplified from a test curve to a linear model by taking into account the influence of negative friction on working properties of the pile and soil. Additionally, considering the top of the pile piercing into the cushion and the bottom of the pile into the substratum, and using the cushion-pile-soil deformation compatibility equation, a model is created to describe the neutral surface depth and pile-soil stress ratios on the top surface and neutral surface of the pile. Using the proposed method, the effects of various factors on the neutral surface depth and pile-soil stress ratio are analyzed. The results show that the neutral surface depth ratio of the rigid pile composite foundation is within 0.15~0.35, and that the pile-soil stress ratio on the neutral surface of the pile increases by 10%~40% of the top plane of the pile. Finally, the proposed method is applied to an engineering field test and a rigid pile composite foundation model test, and the calculated values are consistent with the experimental ones.