Study On Mechanical Response Of Tunnel And Overlying Soil In Vertical Jacking Construction Stage Of Shield Shaft

The vertical jacking method refers to a construction method in which a vertical pipe is formed by using hydraulic jacks to turn the shaft upward and pass through the overlying soil in the tunnel.This technology is widely used in domestic hydraulic shield engineering.In nonunderwater shield projects such as electric power,integrated pipe gallery,and subway air shaft,it is often necessary to build shafts to meet the functions of ventilation and smoke exhaust.The traditional shafts construction method of excavating foundation pits has disadvantages such as long construction period,high cost,and susceptibility to environmental influences.Therefore,it is necessary to extend the vertical jacking method to the shaft construction of non-underwater shield tunnels.The clarification of the deformation characteristics of the tunnel during the jacking process and the failure mechanism of the overlying soil is the key to the application of the vertical jacking technology.Therefore,it is of great significance to study the mechanical response of the tunnel and the overlying strata during the vertical jacking stage.Therefore,based on a certain proposed electric shield tunnel project in Guangzhou,this paper conducted model tests to explore the force and deformation response of the tunnel during the vertical jacking stage,the change of jacking force,and the macroscopic failure characteristics of the overlying soil.Based on the model test,this paper investigated the segment and joint deformation by using finite element simulation when the jacking force is the maximum,and adopted the discrete element method to establish the vertical jacking particle flow model consistent with the experiment to analyze the failure mechanism of the overlying soil from a mesoscopic point of view.And then,some useful conclusions were obtained.Main work and research results in this work are as follows:(1)Taking the Guangzhou typical electric power shield tunnel with an outer diameter of6 m and an overlying soil thickness of 1D and 2D/3(D is the outer diameter of the tunnel)as the prototype,a scaled shield tunnel model was designed and constructed to explore the characteristics of the tunnel’s stress and deformation and the failure mode of the overlying soil during the vertical jacking process.The results show that in the early stage of jacking,the tunnel as a whole sinks,the waist converges,and the vault and bottom sink,while in the later stage of jacking,the tunnel rises as a whole and the waist expands horizontally.The influencing scope of vertical jacking on segment circumferential stress is manifested on the opening rings and the adjacent two segment rings on both sides.The dislocation at the bottom between the open ring and the adjacent ring is the largest.The jacking force has experienced significant increase,significant decrease,and gradual increase,and the maximum occurs when the soil is compressed to the limit.In the jacking stage,the overlying soil undergoes a process of compressive shear,local failure,crack and extension,failure surface connection,crack development,soil column formation,redistribution,and crack filling.The failure surface of the overlying soil has typical nonlinear characteristics and presents cubic function distribution.The failure range of overlying soil is significantly larger than the empirical value.(2)Based on the model test and combined with the finite element simulation,the analysis of deformation of segment and joint at the maximum jacking force was completed.The results show that the calculated results of segment deformation are in good agreement with the test results,and the maximum circumferential segment opening occurs at the bottom of the two open rings,the maximum segment dislocation occurs at the bottom between the open ring and the adjacent ring.And this paper conducted parameter sensitivity analysis,and put forward reasonable control measures.(3)Based on the vertical jacking particle flow model which was compared with test results to verify its rationality,the internal mechanism of the overlying soil failure from a mesoscopic perspective was analyzed.The results show that the soil at the end of the shaft is mainly compressed during the initial stage of jacking,forming a triangular compression zone and the bonding of particles in the compression zone fail.Subsequently,there is a displacement difference between the soil on the upper part and on both sides of the shaft and the surrounding soil,and the area where the bond failure occurs expands to the surroundings and gradually develops into a shear failure surface,which is in good agreement with the test results.The end resistance first increases rapidly,and then slowly decreases,and the side friction resistance appears an cyclic process that first increases and then decreases.The parameter sensitivity analysis was carried out to obtain the deformation response of the overlying soil and change of end resistance under different overlying soil thickness and jacking speed.