Scale Effects On The Bearing Capacity Behaviors Of The Rigid Pile Composite Foundation

According to the advantages of reliable quality and fast construction speed, rigid pile composite foundation is used more and more widely in construction. It becomes more and more favorite especially in some important buildings such as high-rise buildings which require strict settlement control. In order to discover the working behaviors of the rigid pile composite foundation, a lot of researches have been done, which greatly pushed the development of the theory in rigid pile composite foundation. However, there are still some problems in these research areas. For example, composite foundations behave differently when the scales of the foundation change. There are three types of load tests for the composite foundation, which are single pile load test, single pile composite foundation or multi-piles composite foundation load test. But even the multi-piles composite foundation load test can’t perfectly represent the load bearing characters of the actual composite foundation. Second, the empirical coefficient method is still used to correct the settlement calculation result of rigid pile composite foundation. Studies should be carried out to make the empirical coefficient method to a theorized method. More researches should be taken to discover the mechanism of rigid pile composite foundation.In order to solve these problems, this paper carried out the following works and obtained some results:(1) The influence of foundation scale on the bearing behavior of rigid pile composite foundation were discussed based on the relationship between foundation additional stress distribution and foundation scale, and the relationship between ultimate bearing capacity of foundation and foundation scale. To consider the influence of foundation scale, a new method which introduces the tangent modulus theory were put forward.(2) In order to study the influence rules of the foundation scale, several groups of model tests, such as load tests under four different scale plates, foundation soil load test, composite foundation load tests and single pile load tests, were carried out using a self-made model test apparatus. A series of load test results were acquired.(3) According to the results of the model tests, the bearing capacity behaviors of the rigid pile composite foundation and the differences of the mechanical characters between pile and soil under different foundation scales were analyzed. Some rules were discovered as follows: With the same loading stress and pile replacement ratio, the composite foundation settlement and pile-soil stress distribution ratio increases with the increasing of loading plate scale. The load carried by the pile increase at the same time. (4) That will not be very accurate if we determine the bearing capacity of a composite foundation directly by the result of the composite foundation load test for the influence of plate scale. New suggestions were proposed to solve this problem. Besides load tests of single pile and single pile composite foundation, the load test of the soil should also be done. After that, the p-s curve of the plate composite foundation can be calculated by a new method, which calculates the bearing capacity of the ground by overlapping sigle pile and soil p-s curves. The result is verified by the composite foundation load test. Then, the p-s curve of the actual foundation soil can be calculated by the tangent modulus method, which will be added to the p-s curve of the single pile load test. The p-s curve of the actual composite foundation is then obtained by these procedures. The p-s curve obtained can be used to assess the bearing behaviors of the actual composite foundation scientifically, determine the requirement of bearing capacity of the composite foundation.(5) Based on the analyses of the pile-soil deformation coordination, load transfer mechanical behaviors and interaction mechanism, a systematical design method according to the settlement control in rigid pile composite foundation was established using the tangent modulus method. The pile-soil deformation coordination and load transfer characters are taken into account in this method and the bearing behavior of the actual foundation coincide with this method well, as were verified using an engineering project.(6) The method of rigid pile composite foundation design and the method for determining the bearing capacity of composite foundation were verified by a CFG pile composite foundation case. At the same time, it is also verified that the bearing behavior of the rigid pile composite foundation discovered by the model tests coincide with the real cases well.(7) The interaction and cooperation of pile and soil in the composite foundation with cushion were studied. The thickness and modulus of the cushion within the range of normal value changes have some influence on the pile-soil stress distribution ratio of composite foundation. However, it does not impact a lot to the total settlement of the composite foundation.