Rigid Foundation Piling Composite Foundation Test Research

Along with the high speed development of our national economic, the urbanization progresses rapidly. The urban infrastructure constructions guided by the sustainable development, which tend towards the two directions of the high-rise construction and the development and use of the city's underground space, and the increasing demands of the geotechnology and the designing theory greatly promote the development of our composite foundation processing and the construction pit retaining technology for building foundation. The rigid-pile composite foundation is one of the technologies which are often used in the architectural engineering ground processing in our country at the present time. The commonly used rigid piles mainly include: CFG piles, Pressed-in piped piles, non-sand concrete piles and so on.The ground reinforcements of the weak subgrade aim to enhance the subgrade load bearing capacity and to reduce the settlement of subgrade. The pile length to satisfy the subgrade bearing capacity and to reduce the subsidence deformation is different, and thus composite subgrade composed of rigid piles of different length appeared in the recent years, in which the long piles control deformation and the stub piles control bearing capacity. For the time limit of a project, the efficiency, the loading equipments and abilities, the tests for the composite subgrade bearing capacity carried on in the projects and researches in our country at present concentrate on the researches about the single pile and composite subgrade bearing capacity of the single-pile-composite foundation. But reports on the tests for multi-pile cushion-caped composite subgrade load bearing capacity are very few, and especially the multi-pile composite subgrade with long-stub pile working together. The design and application of the long-stub piles composite subgrade which lack the instructive theory are leading the relevant theories; therefore, it may cause waste and the unsafe of projects. According to the need of the projects, the development of experimental study for the bearing capacity of rigid-pile composite subgrade with different length is one of the key questions which are to be solved in the fields of composite subgrade. This paper does the research work which deals with composite subgrade composed of rigid piles with different length. The test includes 2 groups of the rigid stub(4m) 4-pile composite subgrade, 3 groups of the rigid long-stub (4m/6m) 4-pile composite subgrade , 6 groups of the single piles (3 groups of 4m 's, 3 groups of 6m's) The specimens are Pressed-in piped piles. The Q-S curve about the static load of the single piles, the P-S curve, the Q-P curve and the Q-n curve about the static load of the composite subgrade and so on are obtained. In the light of the method of the static loading test, the bearing capacities of the long pile, the stub pile, and the earth between them are displayed. The properties of composite subgrade composed of rigid piles with different length under the vertical load have been studied. They have the strong usability in the project, and may provide reference for the theoretical study and guidance for the engineering design. The main conclusions of the test include:(1) The piles in the composite subgrade limit the lateral deformation of the soil between the piles and reduce the vertical deformation of the soil, and thus the bearing capacity of the soil is enhanced. The restriction of piles on the in-between soil is related to the quantity of piles and the rate of substitution, and is bigger with the increase of them. The static load experiment of 4-pile composite subgrade has reflected the restriction on the lateral deformation about the group piles to the soil between the piles to a certain extent.(2) The lateral restriction of piles, the negatively frictional effect and the function of the bedding cushion make the bearing capacity of the soil between piles higher in the composite subgrade compared with the natural ground. The coefficient of the bearing capacityβis more than 1.0.(3) Under the vertical load for composite subgrade composed of rigid piles with different length , the stress in piles, soil and the pile-soil stress ratio increase along with the increase of the load, but also the ratio of the long pile head increases slightly quicker than the one of the stub pile head., and the variation is not significant.(4) When the composite subgrade composed of rigid piles with different length reaches the characteristic load criteria, there is much difference of the coefficient of the bearing capacity between the long piles, the stub piles and the soil between the piles. While the bearing capacity of composite subgrade composed of rigid piles with different length determined by the stress compound algorithm (present National Code method) is estimated, the computed result indicates that the coefficient of the bearing capacity of the soil between the piles 6 may take more than 1(the test:β=1.068>1), and that of the single pile should be smaller than 1(the test : (?)₁ =0.71, (?)₂ =0.28).