Research On Deformation Mechanism Of Deep Foundation Pit In Soft Soil

Most of the urban areas in China are distributed along the coast,along the river and in the middle and lower reaches of the river,and the geological foundation is mainly soft soil.In order to better satisfy the safety and stability of itself and surrounding buildings and structures in the process of underground engineering construction under soft soil conditions,it is necessary to make an in-depth study on the deformation mechanism of underground engineering in the process of construction under such geological conditions.Based on the engineering background of Ma Tou Tan Park Station of Wuhan Metro Line 6,this paper studied the deformation mechanism of excavation surface of subway station in soft soil area by means of theoretical analysis,laboratory test,numerical simulation and field measurement.The main work is as follows:(1)The typical soft soil of Ma Tou Tan Park Station of Wuhan Metro Line 6 was selected for indoor geotechnical tests.The seepage,mechanics and microstructure properties of the soft soil were studied,and the relevant test parameters of the soil at different depths within the foundation pit area were obtained.In order to explain the macroscopic deformation characteristics of soft soil,a three-dimensional ideal model of soil particles is introduced,and the change of the thickness of the adsorbed water layer on the surface of soft soil particles is considered to be an important factor affecting the macroscopic deformation of soft soil.(2)In order to simulate the soil deformation characteristics of excavation surface in the process of excavation,the TSZ series automatic triaxial instrument was modified to meet the test requirements of unloading axial pressure to zero.Triaxial unloading tests were carried out on unaltered soil.The test results show that the negative excess pore water pressure generated by unloading increases with the increase of unloading rate,and its peak value decreases with the increase of confining pressure.(3)In order to analyze the variation of the excess pore pressure of the sample in the triaxial unloading test,a one-dimensional seepage consolidation model is introduced,combined with the basic working conditions of the excavation and unloading of the foundation pit,and the consideration of the external load is obtained.Regarding the seepage consolidation differential equation of the excess pore pressure u,the analytical solution of the excess pore pressure u(z,t)and the springback of the excavation surface S(t)is obtained.(4)Plaxis 2D/3D finite element software was used to further explore the deformation characteristics of the excavation face of the foundation pit,and to simulate the deformation characteristics of different excavation faces and a specific section at different excavation stages in the process of foundation pit construction in soft soil area.Results show that the different excavation face show different deformation characteristics,and as the foundation pit excavation,excavation face gradually by the elastic deformation to plastic deformation on the deformation and uplift of the bulge deformation state,and is subject to the influence of subway station foundation pit space opposite effect,long side direction of the foundation pit,and present "in the middle is big,small on both ends of deformation characteristics;In a particular section,with the excavation of the foundation pit,the rebound deformation in the middle of the foundation pit and near the ground wall increases obviously.