Research On Design Method Of Upper Limit Principle Of Deep Excavation Of Metro Station In Rich Water Stratum

With the increasingly prominent positive role of urban rail transit for urban development,more than 40 large a nd medium-sized cities are building or actively applying for urban rail transit.Due to the influence of drainage and rainfall,the seepage of groundwater and surface water has attracted the attention of scholars and experts,especially for deep excavations in sand and gravel strata.The permeability coefficient of sandy soil and sandy gravel is large,and they are granular,so it is usually necessary to take certain precipitation measures,which causes the formation of underground seepage.When the cohesion of soil is poor,the deformation,settlement and stability of excavations are all adversely affected by hydraulic impact.In order to ensure the safety of construction of this kind of deep excavations and complete the project construction efficiently,in view of the seepage problems and soil characteristics in the construction of deep excavations,the following main research contents are proposed:(1)The calculation method of earth pressure for retaining structure of deep excavations considering the condition of drainage and dewatering.According to the different drainage schemes,the seepage action of retaining structure is divided into three basic types: static water field,vertical seepage and non directional seepage.Based on the limit analysis method and considering the dilatancy of soils,a modified nonlinear Mohr Coulomb model is used to calculate the active earth pressure and passive earth pressure of retaining structure under different seepage modes(2)The displacement prediction method of deep excavations based on continuous displacement field.Based on the numerical simulation and investigation statistics,the deformation mechanism of deep excavation under the condition of multi support excavation is obtained.The limit analysis is used to obtain the theoretical prediction method of deep excavation considering the influence of vertical seepage.Considering the influence of supporting conditions on deep foundation pit,the displacement prediction method of deep excavation considering the rigidity of retaining pile wall structure is proposed.(3)Research on stability of deep foundation pit based on continuous displacement field.The concept of seepage force is introduced to establish a continuous deformation model for the stability analysis of deep excavation considering seepage.By establishing the displacement field of narrow foundation pit,the deformation law of deep foundation pit with sandy pebble under the influence of certain width is analyzed.The stability analysis method of narrow foundation pit is put forward,and the safety factor of the stability of excavation is analyzed.(4)Based on the displacement prediction method and stability research results,t he optimization method of envelope design and process design considering the upper limit principle is proposed.The empirical evaluation method of stiffness coefficient of deep excavation supporting structure is established.The shear strength method is introduced into the system to form the stiffness of the supporting structure considering multiple factors.