Research On Cushion Of Rigid-Pile Composite Foundation

With the continuous progress of cmposite foundation theory, the rigid-pile composite foundation was applied more and more widely. Cushion is the core technology of the rigid-pile composite foundation. It has received much concern of soil workers because of its multiple uses, such as guaranteeing pile-soil interaction, adjusting the load distribution ratio and the horizontal load between pile and soil, reducing stress concentration of the base bottom, and so on. The existing design of cushion is determined mostly according to the norms and experience in engineering, while the relevant theory has many assumptions, laboratory tests can not simulate the actual situation completely on account of the limitations of their size and boundary conditions, therefore, the setting of cushion of rigid-pile composite foundation has been up in the air. For the strenghs and weaknesses of existing research, the set issues of cushion thickness of rigid-pile composite foundation are studied deeply.(1) The research status of cushion of rigid-pile composite foundation is analyzed firstly, the existing problems of cushion in study are pointed out, failure modes and reasonable cushion thickness are discussed in detail, and its limitations are also pointed out.(2) The load tests of rigid-pile composite foundation in actual projects are simulated by the large-scale numerical analysis software FLAC3D, the numerical results agree with the measured well. The results show that the constitutive model and parameters are in line with the actual situation, and using FLAC3D programs to simulate the properties of rigid-pile composite foundation is feasible.(3) Under different pile-soil stiffness ratio, the relationship of pile-soil stress ratio with the variation of the thickness of cushion is studied by numerical method, the results show that the thickness of cushion is not limited to specifications required by 15-30cm, and the hardness of soil has significant impact on the set of cushion thickness, so the thickness of cushion should be distinguished based on different geological conditions. (4) With the variation of thickness of the cushion, the relationship(N-S curve) of load of pile top and settlement is studied, the results further illustrate the thickness of cushion of composite foundation can adjust the bearing capacity of single pile, and then optimize the design of composite foundation.(5) The size of pile diameter of composite foundation has a great influence to the pile-soil stress ratio, when the pile length and the cushion thickness are constant, with the pile diameter increases, the pile-soil stress ratio getting smaller and smaller; under different pile diameters, the trend of the pile-soil stress ratio with the cushion thickness are not the same. So, reasonable thickness of cushion should consider the impact of pile diameter, and the ratio of cushion thickness and pile diameter is proposed as a standard to measure the pile-soil stress ratio.(6) The implement processes of cushion thickness optimizing the design of rigid-pile composite foundation are introduced by engeering projects, and the results verify the feasibility of this method.