Study On The Face Stability Of Horseshoe Shield Tunnel Based On Limit Analysis

It is very important to keep the stability of the tunnel face in the process of shield tunneling.With the successful application of large section horseshoe shield technology in Menghua Railway,it is of great academic significance and engineering application value to study the stability of the excavation face of the horseshoe shield tunnel.In this paper,numerical simulations and theoretical analysis are used to establish the analysis model of the excavation face stability of the horseshoe shield tunnel in single stratum and layered stratum.The influence rules and characteristics of different factors on the failure process and critical pressure of the excavation face of the horseshoe shield tunnel in case of collapse are studied.The main work and achievements are as follows.?1?The collapse of the tunnel face under different buried depth and stratum conditions is simulated by FLAC^3D.The displacement curve of the tunnel face and the simulation solution of the critical pressure are obtained.The results of the numerical simulation show that when the support force decreases to a certain value,the soil mass of the tunnel face will be into the state of failure and the displacement of the tunnel face will increase sharply.According to the contour maps of stratum displacement,plastic zone and plastic shear strain increment,the development process of the tunnel face collapse is analyzed to obtain the mode and the influence range of failure area under the conditions of single stratum and layered stratum.?2?Based on the results of numerical simulation,using the spatial discretization technique and the upper-bound methods,the mechanism for the collapse of horseshoe shield tunnel in single stratum is proposed.The rationality of the analytical model is verified by comparing the analytical model and the numerical model from two aspects of the solution of critical collapse pressure and the scope of the failure area.The influence of tunnel depth,stratum parameters and section shape on the critical pressure are studied based on the proposed theoretical model.The results show that the critical collapse pressure of the horseshoe shield tunnel will not always increase with the increase of the buried depth,but tends to a constant because of the soil arching effect in the stratum.The stronger the stratum is,the better the self-stability of the stratum is,and the less the support force required for the tunnel face is.Compared with circular or rectangular section tunnel,the excavation area of horseshoe section tunnel and the support force required to maintain the stability of the tunnel face are both the minimum under the same function requirements.?3?The mechanism for the collapse of horseshoe shield tunnel in layered stratum is proposed by using the spatial discretization technique and the upper-bound methods based on the results of numerical simulation,which improves the applicability of the analytical model.The rationality of the analysis mechanism is verified by numerical simulations.Based on the extended theoretical model,the influence of the location of the stratum interface and the variation of the stratum strength parameters on the critical pressure is studied in dual layers condition.Meanwhile,in view of the condition that there is weak interlayer in front of the tunnel face,the influence of the position,thickness and strength of the interlayer on the stability of the tunnel face are studied.The results show that it needs the largest support force when the weak interlayer is within the lower half of the tunnel face,which is the most unfavorable to its stability control.In addition,the greater the thickness and the smaller the strength of the weak interlayer,the more unfavorable it is to maintain the stability of the tunnel face.