Study On The Shape Of Collapse Surface Of Large Span Tunnel Under Complex Conditions And Engineering Application

In the past decades,the country has planned and built a large number of underground projects in order to accelerate economic development and facilitate people’s travel.As an important traffic structure of underground engineering,tunnel has higher requirements for above-ground environment treatment and underground structure reinforcement.Due to the complexity of the geological conditions in the tunnel（such as karst development,the existence of cavities,groundwater development,fissure development,earthquake,etc.）,it is not uncommon for tunnel construction to collapse,which requires consideration of the complexity of geological conditions in the prior survey and design,and construction support,in order to reduce or even eliminate the risk of tunnel collapse.In this paper,based on previous research,the collapse model of a shallow buried tunnel under the coupling of surface load and pore water pressure is constructed and the analytical solution of the collapse curve is obtained,based on the existence of cavities,groundwater development,fracture development and earthquake in the Lianhua Tunnel in Pingxiang City,Jiangxi Province.The collapse curves of a tunnel under the coupling of seismic and seepage forces and the presence of a hidden cavity under the elevated arch are studied.This paper aims to provide a reference for similar tunnel projects in medium-weathered enclosing rocks through relevant studies.The specific research results are as follows.（1）Using the upper limit analysis method and energy conservation theorem,a two-dimensional analysis of the tunnel collapse surface under the double coupling effect of surface load and pore water pressure was carried out based on the nonlinear Hoke-Brown criterion,and finally the formula of the tunnel collapse curve under the coupling effect was derived,and it was found that the collapse curve f（x）is similar to a parabola with the opening downward and truncated a small part of the upper end,and the key to influence the curvature of f（x）is in the structural surface A and B values of the surrounding rock,heavinessγ,pore water pressure r_u,uniaxial compressive strengthσ_ci,and the pore water pressure r_uand surface loadσ_shave a greater influence on the collapse range of f（x）.（2）Sensitivity analysis was conducted on the change of tunnel collapse surface shape when the cavity under different positions was located above the tunnel.The test scheme was optimized by orthogonal test design and then modeled and calculated by finite element software,and the parameter sensitivity analysis was carried out by combining the cloud map results.The study shows that:the sensitivity of the hidden cavity parameter changes to the collapse surface can be reflected from the area enclosed by the contour and the tunnel;the diameter of the cavity is the first factor affecting the size of the collapse surface when the eccentric cavity under different positions is located above the tunnel.The next factor is the angle of the cavity,while the distance from the tunnel has less influence compared to the former two.（3）Based on the methods,theorems and criteria in study（1）,a two-dimensional analysis of the collapse surface under the coupling of seepage and seismic forces and when the cavity is located directly below the elevation arch of the tunnel was carried out,and the collapse curve formula was derived,and the influence of the above parameters on the shape and extent of the collapse surface was analyzed by varying the size of the surrounding rock parameters.The study shows that the collapse curve is similar to a parabola with an upward opening and a small part truncated at the bottom.The collapse surface increases with the increase of pore water pressure coefficient r_p,and decreases with the increase of seismic force coefficient kv,and the effect of k_von the collapse surface is much smaller.（4）Using the Lianhua Tunnel in Jiangxi as the engineering background,the collapse mode of the tunnel was determined by analyzing the field measurement data as well as the field survey of the site after the collapse.According to the formula derived above,the collapse curve was plotted by substituting the actual surrounding rock parameters of the project,and was verified by comparing with the actual field measurements,and the research results were extended to other projects.