Numerical Analysis On The Bearing Behavior Of Long-short-pile Composite Foundation

As a new method of foundation treatment, the long-short-pile composite foundation has many advantages, such as reasonable using of the capacity of pile and soil, good resistance earthquake capacity, low cost, short construction period and so on., which has been widely used in soft soil areas, such as Hangzhou, Ningbo and so on., where a good social and economic benefit is achieved. However, compared with other foundation treatment methods, the theoretical research about the long-short-pile composite foundation lags far behind the practice. Therefore, it has important theoretical significance and engineering application value to study the engineering characteristics and bearing mechanism of long-short-pile composite foundation.Based on the domestic and foreign related research results, this paper takes the long-short-pile composite foundation as the research object and investigates the bearing behavior of the long-short-pile composite foundation using three-dimension finite differential software Flac3 D. The main work and conclusions are as follows:(1) A brief introduction is made on the research status and results obtained at home and abroad of the long-short-pile composite foundation.(2) The basic principle of the finite differential software Flac3 D, the solving process, the application range, the fundamental principle of interface and the selection of the parameters of the pile and soil, etc., are introduced.(3) Using the software Flac3 D, the bearing behavior of the composite foundation of the single long pile, the single short pile and group piles, respectively, are simulated. The numerical results show that there have obvious differences in the foundation settlement, pile settlement, axis stress of the pile and additional stress between pile and soil, etc., of single-pile and of group piles composite foundation, where the size and mechanical parameters of the foundation, cushion layer, soil layer and long/short pile are the same.(4) The bearing behavior of the long-short-pile composite foundation is simulated and analyzed under different thickness of cushion cap and cushion, modulus of cushion, length of long pile or short pile. The conclusions are obtained as follows:① The settlement of the foundation and of the pile body, and the load sharing ratio of the short pile and of the pile-soil increases with the increase of the cushion thickness of the long-short pile composite foundation, but the load sharing ratio of the long pile is decreased. with the increasing. With the increase of the modulus of the long-short-pile composite foundation, the load sharing ratio of the foundation and of the pile body is decreased, but the load sharing ratio of the long pile and of the pile-soil is increased.② The settlement of foundation and the load sharing ratio of the short pile and of the soil deceases with the increasing of the pile length of long pile, but the load sharing ratio of the long pile increases. The settlement of the foundation and of the pile, and the load sharing ratio of the long pile and of the soil are decreased with the increase of the length of the short pile, but the load sharing ratio of the short pile is increased. ③ Both the settlement of the foundation and of the pile increase with the increase of the thickness of the cushion cap, but the increase of the amount is small. There is no change in the value of the axial force of the pile top and the stress ratio of the pile to soil.