Research On Deformation Control Of Steel Support Servo System For Subway Deep Foundation Pit

Due to the continuous deepening of urbanization,more and more cities begin to build subways,and the subway station foundation pit has the characteristics of large area,deep excavation and complex surrounding environment,which requires more strict control standards for the construction of subway foundation pit.The traditional prestressed steel support generally has the problem of axial force loss.It is difficult for the enclosure structure to meet some high deformation requirements of the project,in order to solve this problem,servo steel support system came into being.The servo steel support system adjusts the steel support axial force in real time by adding the axial force sensor and jack to the steel support.In the design of subway station foundation pit,due to the differences in soil properties and surrounding environment,there are often large safety risks in the design of foundation pit envelope.In order to ensure the safety of foundation pit,some subway foundation pits cause excessive safety redundancy of the envelope structure,resulting in a waste of resources and reducing the economy of the project.Therefore,taking Hangzhou Wulinmen Line 3 subway foundation pit as the engineering background,aiming at Hangzhou clay and silty clay dominated strata,the deformation characteristics of foundation pit are analyzed by means of combining theoretical calculation,measured analysis and numerical simulation.On this basis,the optimization of foundation pit supporting structure based on the deformation control of foundation pit retaining structure is carried out.The main research contents are as follows:(1)The mechanical characteristic of servo steel support is to make up for the compression of steel support under axial pressure through the extension of jack,and the mechanical mechanism is active compression;The servo steel support system is very necessary in the area where the deformation of foundation pit is strictly controlled because the length of the support cannot be adjusted independently and the displacement of the ground connecting wall is too large.By referring to domestic and foreign literature,this paper summarized the deformation law of the subway deep foundation pit envelope and its surrounding environment as well as the relevant methods to control the deformation,collected74 engineering cases,including 37 servomechanical and 37 servo foundation pits,analyzed the deformation of the envelope structure,surface settlement and insertion ratio of the servomechanical foundation pit and the servo foundation pit,and compared them.The advantages of servo system in controlling foundation pit deformation are obtained.(2)Based on the principle of minimum potential energy and Rayley-Ritz method,the equilibrium equation of the force of the diaphragm wall considering the self-complement of the supporting axial force is proposed by the superposition method.By introducing the correction coefficient of the supporting axial force,it is shown that the horizontal displacement of the diaphragm wall calculated by this equation is consistent with the simulated value in the variation trend,which proves the reliability of this method.Moreover,the calculation of the excavation face and the parts below the excavation face are more consistent with the simulated values than the parts above the excavation face,which is caused by the discontinuity of the piecewise function.(3)By using small deflection theory,large deflection theory and Euler formula to calculate the ultimate bearing capacity of Wulinmen steel support,it is shown that the theoretical calculation will not make the steel support buckling or instability.Using German DIN18800,American AISI and GB50017-2017 to calculate the ultimate bearing capacity of steel supports,it can be found that the bearing capacity calculated by the American standard is larger than that of the national standard,indicating that China is more conservative in determining the bearing capacity of steel supports,but there is little difference between the two.On this basis,the safety factor of 0.6 is increased.Finally,the safe and stable ultimate bearing capacity of steel support which can be applied in engineering is determined.(4)The characteristics of foundation pit deformation are analyzed by using the method of measurement data and numerical simulation analysis.With the excavation of foundation pit,the deformation of envelope structure,surface settlement and building settlement gradually increase,and the deformation shape of envelope structure presents a bow shape with large middle and small two ends.With the excavation of foundation pit,the deformation trend of surface settlement is consistent with that of traditional envelope structure,showing the distribution characteristics of groove type.With the excavation of the foundation pit,the settlement of the building is basically larger,and the inclination rate of the building is also larger.(5)According to the principle of controlling single influencing factor,seven influencing factors of foundation pit deformation characteristics are analyzed by numerical simulation method.The results show that: different influencing factors have different degrees of influence on foundation pit deformation,the larger the overall stiffness of the enclosure structure,the smaller the deformation.The steel supports at different positions have different control effects on the deformation of the enclosure structure at the same coaxial force increment.The contribution value of the steel supports closer to the maximum deformation depth of the enclosure structure is the largest,and vice versa.The greater the servo axial force is,the better the deformation control effect is,but the greater the axial force is,the greater the internal force of the enclosure structure will also increase.It is necessary to control the size of the servo axial force.The deformation control effect of the steel support system is the best,and the deformation control effect is the worst when applied separately.(6)The optimization analysis of the foundation pit of Wulinmen subway in Hangzhou shows that the same control effect can be achieved by reducing the number of steel supports at the end well,reducing the number of steel supports from eight to six in each layer,and improving the size of the servo axial force.For the standard section of foundation pit,the deformation of the enclosure structure is controlled within the standard range by setting the steel support servo system reasonably.For the deep foundation pit support in this paper,the two concrete bracing(b)is the best choice;In the practical application of the project,we should consider the geological conditions of the project,the depth of the foundation pit,the enclosure structure and the number of supporting channels and other factors to choose the best support layout.