Construction Mechanics Of Forked Tunnels With Small Spacing And Variable Section During Enlarging Excavation And Opposite Construction

Forked tunnel with small spacing consist of the main tunnel sections、big arch section、small distance and so on,it has the characteristics of large span,multiple arch and tunnel with small spacing simultaneously.The force conversion of the supporting structure is complex.At present,the theoretical derivation、numerical simulation and practical application of forked tunnel with small spacing under the condition of broken surrounding rock are still in the exploration stage,lacking of corresponding technical specifications and construction experience.Therefore,it is important to study the construction process of forked tunnel with small spacing,the safety of tunnel excavation is analyzed based on this,the corresponding construction optimization scheme is given,which provides necessary experience for the corresponding project.Based on the tunnel of S1 and S2 lines of Guiyang East Railway Station,a three-dimensional numerical calculation model is is carried out to analyze the stability of surrounding rock and the stress characteristics of the supporting structure along with different excavation methods and the key construction process.The main research contents are as follows:(1)By using numerical analysis,The deformation and stress characteristics of the supporting structure are studied along with the section of the tunnel changes from small to large and the transverse expansion + reverse expansion,the stress difference on the supporting structure at the variable section before and after disassembly is compared,the influence of geological strength index and supporting parameters is analyzed.(2)Through the numerical simulation of crossover tunnel construction,the deformation of surrounding rock and the Ground subsidence of the expressway with construction are analyzed.the deformation and stress distribution of two middle rock pillars with different widths in the crossing line are studied,the optimal reinforcement scheme of middle rock pillars are obtained by comparing and selecting.(3)A theoretical analysis model for the stability of face in two-way excavation is established based on limit equilibrium method,and determine the applicability of the model,the tunnel during opposite construction with different length of reserved rock pillar and no support section is studied,the influence of soil parameters(cohesion,internal friction angle)、height-width ratio and buried depth ratio and other parameters on the stability of the face of face during opposite excavation is analyzed,the correctness of the model is verified by(4)comparing the theoretical solution with the simulated solution.(5)From the perspective of construction process mechanics,A deeper stability analysis is carried out on the construction procedure of the opposite excavation of the large-span tunnel using the double-side-drift method,furthermore,the effects of different reserved rock pillar lengths,different support parameters and different spacing of tunnel face on the instability of tunnel vault surrounding rock and tunnel face are studied.