Research On Engineering Properties Of Gravel Pile Composite Foundation Under Complex Site Conditions

Gravel pile composite foundation is widely used in the southern regions where groundwater level is lower. Recently, it is also used in the loess-like silt layer of northern regions. Due to loess-like silt's specific engineering characteristics, such as big void, non-gravitycouapsible, its reinforcement mechanism has double characteristics of loess and silt. The reinforcement mechanism of the gravel pile is different from the gravel pile composite foundation in soft soil.The paper combining engineering practice of the gravel pile dealing with loess-like silt in Shi Jia Zhuang northern suburbs jail, through the indoor soil tests and microscopic experiments of soil, has discussed the gravel reinforcement mechanism of which the gravel pile is in loess-like silt by physical and mechanical properties and microstructure characteristics of soil. And load tests have confirmed pile body's load-supporting characteristics. Meanwhile, numerically simulate combining load tests and analyze engineering properties of the gravel pile. Results are as follows.1.Compared with physical and mechanic indexes of foundation soil before making piles, indexes of soil around the piles change significantly after making piles and its changes have certain regularity, such as water content and void ratio reduce significantly, compression modulus and dry density of soil somewhat increase . In addition, dry density and void ratio change significantly and show apparent gradient change along radial. And when water content of soil body reaches 15% or so, its cohesion reaches maximum. Soil's friction angle of adjacent pile decreases lowest, and farther away from the pile, it decreases greater.2.By microscopic experiments of soil, find that interface of soil particles changes after soil is dealt with and soil particles show flake. Porosity becomes small, point number on the contact surface between particles decreases, soil particles orients arrangement, it is the result of soil's strength improving and deformation reducing.3.Comparing the load tests and heavy dynamic sounding, find that the results can not represent the loading characteristics of composite foundation, but similar to that of gravel pile, when testing the gravel pile composite foundation, especially the miscellaneous containing gravel,brick and so on, using the loading tests on composite foundation of single pile.4.Numerical simulation results of composite foundation are as follows.1) The vertical compressive stress of pile body has a greater effect than that of soil around piles, compressive stress of pile body centralizes, and additional stress in the upper part of pile body is larger. Vertical displacement of composite foundation is smaller than that of natural foundation, and in the range of pile body reinforcing, piles and soil around piles integrally sink.2) To the gravel pile composite foundation with different pile lengths and diameters, the pile-soil stress ratio of composite foundation changes regularly with the increase of pile length. Under the load plate to 3 times pile diameter, pile-soil stress ratio is the highest level. Additional stress decays quickly with the increase of pile length, and when pile length is more than 3m, the displacement changes little. So after pile length is more than 3m, it has little meaning for improving the bearing capacity of composite foundation by increasing pile length. With the increase of pile diameter, functional range of additional stress arising from composite foundation inside and soil around piles increases, and additional stress level inside composite foundation is more and more high.3) The thickness of cushion stresses a significant regulation on pile-soil. When cushion thickness is more and more great, pile stress is more and more small. It is because increasing the thickness of cushion makes part of the additional stress transfer to soil around piles. With the increase of the thickness of cushion, stress of soil around piles becomes more and more large, but when the thickness of cushion is more than 0.3m, stress of soil around piles becomes small, so cushion thickness has played a major role of sharing pile-soil stress when the thickness of cushion is less than 0.3m, and cushion thickness has played a major role of bearing when the thickness of cushion is more than 0.3m.