Numerical Analysis Of Influence Factors On Cushion In Long-Short-Pile Composite Foundation

As one of key technique of composite foundation, Cushion wasn't researched deeply by experts both at home and abroad. So cushion didn't have mature design theory and computing method. The research about role and impact factor of cushion is very important because the long-short-pile composite foundation is a new form of composite foundation. This thesis systematically researched cushion mechanism of action and effects to characteristics of the long-short-pile composite foundation. This research has great impprtant for cushion rational setting, cushion material selection and composite foundation designing. There are different forms of the long-short-pile composite foundations. This thesis mainly studies composite foundation with rigid-flexible and long-short piles:1. Literature summary of current research situation about composite foundation, the long-short-pile composite foundation and cushion technique.2. Analysis mechanism of cushion and summarized empirical formula for penetration at cap using numerical simulation and curve fitting. Explored calculation method of the long-short-pile composite foundation stress ratio of pile to soil and load sharing ratio3. Feasibility verifying tests of FLAC3D calculation program in the long-short-pile composite foundation study based on engineering example. With the FLAC3D calculation program, this research studied effects of cushion thickness, elastic modulus and angle of internal friction to the long-short-pile composite foundation settlement, stress ratio of pile to soil, load sharing ratio and penetration at cap. Our conclusions are as follows:(1) For the design of long-short-pile composite foundation of the fourth chapter, in order to meet the requirements, while bearing capacity and to minimize settlement, the best cushion thickness is 15cm.(2) Increase of cushion thickness and decrease of elastic modulus made long pile to bear load decreased. Increase of cushion thickness and decrease of elastic modulus made short pile and soil to bear load increased, penetration at cap cushion thickness and foundation settlement increased.(3) Increase of cushion thickness and decrease of elastic modulus made short pile load sharing ratio increased with load increase, It made short pile play its full role.(4) Cushion angle of internal friction has small effect on the long-short-pile composite foundation when the angle between 30 degree and 50 degree. When angle of internal friction is 20 degree, cushion can't be used because it is flow.