Nonlinear Settlement Analysis Of Composite Foundation With Piles Of Scattered Materials(Rigid Piles)Based On Tangent Modulus Method Of Plate Loading Test

According to the technical code for the treatment of building foundation,the compression modulus of composite foundation is calculated by multiplying the ratio of the characteristic value of bearing capacity of composite foundation and the characteristic value of bearing capacity of natural foundation by the compression modulus of natural foundation,and then the settlement of composite foundation is calculated by the method of layered summation,it is only an empirical method to calculate the compression modulus of composite foundation by the ratio of characteristic value of bearing capacity.The difference between the maximum value and the minimum value of empirical coefficient given by the code is 5 times.According to the load test curve of undisturbed soil foundation,Yang Guanghua established the tangent modulus equation of the relationship between the tangent modulus of undisturbed soil and the stress level.The tangent modulus method considered the influence of the stress level of soil on the tangent modulus,and reflected the nonlinear characteristics of foundation settlement.However,the settlement calculation of this method in the composite foundation of dynamic compaction bulk material pile,the composite foundation of dynamic compaction bulk material pile and rigid pile is still lack of practical test.This paper studies as follows:(1)In the code for design of building foundation,when the layered summation method is used for settlement calculation,the vertical additional stress coefficient in soil is obtained based on the boussinesque solution of elastic half space under the action of flexible load,but the combined bending rigidity of raft foundation and superstructure is very large in the actual project,which can be approximated as rigid foundation.The vertical additional stress of any point in the soil under the flexible load is only related to its geometric position,load size and pressing plate size,and has nothing to do with the elastic modulus and Poisson's ratio of the soil,but under the rigid load,the vertical additional stress of any point in the soil is related to the elastic modulus and Poisson's ratio of the soil,due to the complexity of the analytical solution formula,the finite element numerical method is used to calculate the vertical additional stress of soil under the rigid load.(2)In this paper,the settlement characteristics of multiple composite foundation with scattered material piers and piles are studied.First,the small pressure plate test is carried out on undisturbed soil,single pier of scattered material and single pile,and the load settlement curve of undisturbed soil,single pier and single pile is obtained.At the same time,the large pressure plate test of pier and pile composite foundation is carried out,and the load settlement curve of composite foundation is obtained.Due to the large spacing between piers and piles,only the size effect of the pressing plate of the soil mass is considered.The load settlement curve of undisturbed soil under the action of large compression plate is calculated by the tangent modulus,which solves the problem of size effect of undisturbed soil compression plate test.The load pressure of undisturbed soil,single pier of bulk material and single pile of bulk material corresponding to the same settlement amount in the large pressing plate range of composite foundation is superposed.The superposed load is divided by the area of the large pressing plate,and the load(stress)-settlement curve under the large pressing plate of composite foundation is calculated and compared with the test results of the large pressing plate of composite foundation,from the research results,the average value of settlement calculated by the center point and corner point under the action of flexible load is the most consistent with the test results,and the accuracy of settlement curve calculated by the center point and corner point under the action of rigid load is the second,but it is better than the standard algorithm.(3)Based on the analysis of the settlement of a composite foundation composed of gravel reinforcement and rigid pile,the replacement rate of gravel reinforcement in the composite foundation is relatively high,and the characteristic value of bearing capacity of natural soil has reached 160kpa.The difference between the bearing capacity of reinforcement and that of compacted soil is not significant,so the reinforcement and that of compacted soil are combined into one,which is called composite soil for short,thus,the three-dimensional composite foundation composed of rigid pile,gravel reinforcement and soil is simplified as the two-dimensional composite foundation composed of rigid pile and composite soil,and the engineering calculation is simplified.Because there is no size effect in the rigid pile,only the pressing plate size effect of the composite soil is considered.The load settlement curve of the composite soil under the raft foundation is calculated by the same method in the last section.The pressure of the rigid pile and the composite soil corresponding to the same settlement in the raft foundation range is superposed,and the superposed load is divided by the raft area,the load settlement calculation curve under the raft foundation is obtained,and compared with the measured load settlement curve of the building.From the analysis results,it can be seen that the calculated average value of the foundation corner and center point under the flexible load is consistent with the measured settlement of the building.