Study On Pile-soil Stress Ratio And Neutral Surface Depth Of Rigid Pile Composite Foundation Considering Influence Of Negative Friction

The rigid pile composite foundation develops rapidly in recent years as a foundation treatment technique. The upper load acts on piles and soil through the cushion, and the piles and soil can deform coordinately and bear load together. It is inevitable to form a negative friction zone in the upper part of the pile, which has a great impact on the stress and deformation of the piles and soil. The pile-soil stress ratio is an important index to reflect the load distribution of the piles and soil, and also an important parameter to calculate the carrying capacity and settlement. The pile-soil stress ratio should be studied considering the influence of the negative friction. Not only the pile-soil stress ratio on the top surface of the pile should be studied, and also the neutral surface depth and pile-soil stress ratio on the neutral surface of the pile should be studied. This can truly calculate the biggest pile-soil stress ratio and the most unfavorable condition of the pile, which has important engineering research value.This paper based on the load transfer method, considering the influence of negative friction on working properties of the piles and soil, established a model that described the interaction between piles and soil. Additionally, combined with the cushion-pile-soil deformation compatibility equation, a method was created that calculated the neutral surface depth and pile-soil stress ratios on the top surface and neutral surface of the pile,and the calculation method was proved to be correct by a concrete example. A static load test of the rigid pile composite foundation in a real project was simulated by using the FLAC3 D, and a numerical model was created that described the actual situation of the project through the inversion analysis method. Then the calculation method was applied to the real project, and the calculated values were consistent with the simulation values.Finally, based on the numerical simulation of the static load test of single pile composite foundation, the rigid pile group composite foundation model was established, and the change law of the neutral surface depth and pile-soil stress ratios associated with the influence factors was simulated and discussed.The results show that the neutral depth and the pile-soil stress ratio are influenced bymany factors. The neutral surface depth ratio of the rigid pile composite foundation is within 0.15-0.55, and the stress of pile in the neutral surface increased generally by 4-30%of that in the top surface. So we should not neglect the influence of the negative friction on the piles and soil in the engineering application. It is suggested that when designing the carrying capacity of the pile, the pile-soil stress ratio on the neutral surface should be the calculation index of the load distribution of piles and soil. When calculating the settlement and the carrying capacity of the soil, the pile-soil stress ratio on the top surface should be the calculation index of the load distribution of piles and soil, which is more practical and safety.