Study On The Influence Of Pipe Jacking On The Mechanical Characteristics Of Tunnel Structure In Sandy Soil Layer

The Transverse hole cover method combines the advantages of the Pile-Beam Arch(PBA)Method and the Pipe Proofing Method,which can effectively control the displacement of surrounding soil during the underground excavation construction of subway station.Its remarkable feature is that the pipe curtain is pushed into the soil above the station roof in a close row in the pilot tunnel,and then the subsequent excavation construction is carried out under the support of shed cover system.Advance pilot tunnel is the starting point of pipe jacking construction,which plays the role of providing back reaction for pipe jacking.It is an important bearing structure,and its safety and stability are very important.In this paper,taking Shenyang Metro Line 4 Shifudalu station as the engineering background,taking the leading pilot tunnel of the transverse hole cover method as the research object,the influence of pipe jacking on the mechanical characteristics of tunnel structure in sandy soil layer is studied by means of numerical simulation.The main work and research results are as follows:By using Midas GTS NX finite element analysis software,the displacement field and stress field law of pipe jacking tunnel in excavation and pipe jacking stage are compared and analyzed.The results show that the main influence range of jacking reaction is the whole back height in vertical direction and about 5 times of load width in horizontal direction.In the main influence range,the spatial distribution of horizontal displacement and earth pressure of the back soil is similar.The vertical distribution is between U-type and V-type,and the distribution form in horizontal direction is V-type.The influence of the size,position,range of action,thickness and elastic modulus of the initial support,and the reinforcement range and reinforcement degree of the back soil on the mechanical characteristics of the tunnel structure are studied in detail by the single variable method,which provides reference for the design and construction.The results show that when the soil reaches the passive limit state,the horizontal displacement of the soil will increase sharply,and then the soil pressure will continue to increase,but the increasing rate will be significantly reduced.When the load acts on the middle position,the stress and deformation of the structure are the largest,followed by the upper position,and the smallest in the lower position.The increase of initial support thickness and elastic modulus can significantly improve the bearing capacity of the structure,but when the thickness or elastic modulus reaches a certain value,there is no obvious effect when the thickness and elastic modulus continue to increase,that is,there is a critical initial support thickness and elastic modulus;In the same way,the reinforcement of the back soil can improve the bearing capacity of the soil and effectively control the deformation of the soil,and there are obvious critical values for the range and rigidity of the reinforced soil.Based on the analysis of the distribution characteristics of the back soil reaction in the limit state,two calculation methods of the back soil reaction are proposed,which are suitable for pipe jacking in the sand layer tunnel,and the calculation formula of the allowable jacking force is given.