Research Of The Bearing And Settlement Reliability Of Controlling-settlement Composite Pile Foundation

Controlling settlement composite pile foundation is a special type of foundation, which is based on the principle of sharing upper load between pile group and soil by their interaction. It has been used widely in soft soil areas in China. Its design method has been included in the construction engineering code 'the design code of ground and foundation' (DGJ08-11-1999) in Shanghai region. The advantages of this type of foundation could be making full use of its bearing capacity and settlement control ability, reducing the number of pile used in foundation, thus decreasing the cost of project and reducing the influence on environment.The design method based on overall safety coefficient can not reflect the probability of failure risk radically. The design method based on reliability could be much more full-scale and scientific, which has been used by many design Codes in recent years. At present, the design method based on reliability is still not adopted in Shanghai code and need research further.Analysis the soil parameters are the first step to research the reliability of composite pile foundation precisely. A new calculate method is been given in this paper which can consider the deviation of soil itself, the uncertainty of soil system and the deviation of soil which transfer from point deviation to space deviation.Based on the research of soil parameters, the research of the reliability of composite pile foundation which has the unity ultimate expression is given next. In this paper, the reliability of bearing capacity and the reliability of settlement are calculated by JC method separately. And then based on analysis the relativity of those two kind of reliabilities, the reliability of composite pile foundation is analysis. Then analysis the sensitivity of every influence factor of respective reliability indexes in combination with examples.The research of the reliability of composite pile foundation which has the unity ultimate expression overlooks the non-uniform force between piles and the non-uniform force of soil which under the pile raft, so this research method has limitation. To improve this limitation, the reliability of composite pile foundation which has not the unity ultimate expression is given in this paper which combined the Monte-Carlo method and nonlinear finite element method.But in the process of analysis, the conflict is give birth between the calculate time of nonlinear finite element method and the times which Monte-Carlo method needed to assure the accuracy. To solve this conflict, two aspects have been improved: (1) simplified the nonlinear finite element method, mainly simplified the calculation of pile stiffness matrix; (2) adopted the improved Monte-Carlo method, reduce the sampling times which unnecessary.The system reliability method is given in this paper to analysis this kind of multi-limit states problem. This paper presented a theoretical frame of system reliability method on composite pile foundation, which divided failure mode of composite pile foundation into three parts: bearing capacity failure mode, settlement failure mode and differential settlement failure mode. Finally analysis the relation of those failure modes and analysis the respective sequence of failure and every influence factor of probability of failure in combination with examples.In the finality, the problems requiring further studies are discussed.