| The secant modulus method of undisturbed soil is a new settlement calculation method proposed by Professor Yang Guanghua.The secant modulus method of undisturbed soil based on the pressure plate test is used to calculate the settlement instead of the compression modulus.The secant modulus method considers the influence of the stress level of soil on the secant modulus.Different secant modulus is used for different depths of soil,which reflects the nonlinear characteristics of foundation settlement.This method has achieved good results in the settlement calculation of single-layer soil foundation,but due to the limited influence depth of plate test,its settlement calculation effect in multi-layer soil composite foundation is still lack of practical test.In this paper,the settlement of multi-layer soil dynamic compaction composite foundation,multi-layer dynamic compaction soil and rigid pile composite foundation are analyzed.The main research contents are as follows:(1)In the technical code for building foundation treatment,the composite soil layer compression modulus is used to calculate the settlement of the composite foundation,and then the empirical coefficient of settlement calculation is used to correct the settlement.The secant modulus method can directly obtain the secant modulus of the soil body according to the pressure plate test instead of the compression modulus for settlement calculation,without the empirical coefficient correction of settlement calculation.For the composite foundation of upper hard and lower soft multi-layer soil,because of the limited size of the plate test,it can only reflect the secant modulus of the upper soil within the influence range of the plate stress,and can not get the secant modulus of the lower soil.Due to the difficulty of the pressing plate test of the lower deep soil mass,this paper presents a calculation method of the initial secant modulus of the lower soil mass,that is,the initial secant modulus of the lower soil mass is obtained by dividing the deformation modulus of the soil mass obtained from the standard penetration test by the empirical coefficient of settlement calculation given in the technical code for building foundation treatment,and then different initial secant modulus is adopted for the upper and lower soil masses According to the layered summation method,the settlement is calculated and compared with the measured settlement of the multi-layer soil dynamic consolidation composite foundation.The results show that the settlement of composite foundation calculated by this method is close to the measured settlement,and the accuracy is higher than that calculated by the compression modulus of composite soil layer in the technical code for building foundation treatment.(2)Under the action of flexible load,the center point of additional stress is the largest and the corner point is the smallest.The corresponding additional stress coefficient can be obtained by looking up the table,which is convenient for application.However,the settlement of each point under the raft foundation calculated by the additional stress under the flexible load is quite different.However,the bending rigidity of the actual raft foundation formed by connecting with the upper structure is very large.The short-sighted is the rigid foundation,and the rigid foundation loading is similar to the equal displacement loading.The analytical solution of the vertical additional stress at any point in the lower soil is very complex.In this paper,the additional stress of the lower soil under the rigid foundation loading is solved by the finite element software ABAQUS.The results show that for the multi-layer soil dynamic compaction composite foundation project,the settlement difference between the center point and the corner point of the foundation calculated by the additional stress under the rigid load is small,which is consistent with the measured settlement.Under the action of flexible load,the settlement of raft foundation center point and corner point is quite different,but the average settlement of center point and corner point is still close to the measured settlement.(3)When calculating the settlement of composite foundation with the combination of multi-layer soil dynamic compaction foundation and rigid pile,the pile and soil are regarded as two independent system.When calculating the settlement of soil,the influence of pile is ignored.The load settlement curve of dynamic consolidation composite foundation in the range of raft foundation is obtained by using the above method.Combined with the static load test of single rigid pile,the load settlement curve of rigid pile with cushion is obtained.Then the rigid pile with cushion and the dynamic pile with cushion corresponding to the same settlement amount in the range of raft foundation are calculated The load pressure of ramming composite soil is superposed,divided by the area of raft foundation,and then the load(stress)-settlement curve of rigid pile composite foundation under raft foundation is calculated,and compared with the field measured settlement.The results show that this method still has good accuracy in the settlement calculation of rigid pile composite foundation. |
