Study On The Soil Strength Character By The Unloading Effect Of Foundation Pit Excavation

Foundation pit engineering is often encountered in civil engineering. And it is one of the most sophisticated technology. To raise the level of foundation pit engineering design depends on the proper selection of parameters of soil and reasonable calculation method of soil pressure. For a long time, there is a great progress on the calculation of earth pressure through unremitting efforts of mang experts and scholars. The specific conditions which the deep foundation pit excavation is a process of soil unloading are ignored because soil shear strength indexes of soil tests of conventional triaxial compression test is used to design the supporting structure of deep foundation pit. So the deformation of deep foundation pit supporting structure and soil pressure value than the actual monitoring have certain differences.The experimental study and finite element numerical simulation analysis method are used to analysis the effect of excavation unloading on strength properties, considering the condition of actual excavation pit excavation unloading. Then, the following studies on strength properties of soil under the excavation unloading were carried out.(1)Unloading rebound deformation law and unloading influence depth of Fuzhou silt soil are researched and the rebound deformation of a soil pit under the effect of excavation unloading is simulated on ordinary high pressure consolidation apparatus. the critical unloading ratio and the boundary of silt soil unloading ratio were analyzed. Then,the formula of rebound was deduced. The rebound amount of silt soil was estimated by the layered summation method.(2)Four groups of triaxial test accord with the characteristics of deep foundation pit excavation wall soil mass stress state were carried out according to the actual stress state of soils in foundation pit side wall during the deep foundation pit excavation process. The soil shear strength was obtained by simulation of deep foundation pit side wall under the effect of excavation unloading the actual stress path of soil. And than it was compared with the shear strength which was obtained by conventional triaxial compression test.(3)Then, modulus value of the Duncan-Chang model under the condition of unloading which was to be used for calculation was deduced.(4)Typical deep foundation pit excavation engineering was simulated and analytical calculated with the help of large finite element software MIDAS. The effect of excavation unloading soils strength properties was studied by the soil under loading and unloading conditions of ground surface subsidence, uplifts and supporting structure deformation were compared.The deep foundation pit excavation pit soil unloading rebound deformation law and unloading influence depth were grasped, the bottom soil unloading the rebound amount was estimated, the shear parameters of the soil which is closed to the actual excavation unloading lateral wall was got, the modulus value of Duncan-Chang model under the condition of unloading which was used to calculate were put forward, and foundation pit supporting structure was designed had an important theoretical and practical significance. The rationality of the theory in the specific conditions of foundation pit excavation was verified by using large-scale finite element software Midas.