Safety Analysis Of New Built Tunnel Undercrossing Existing Structures And Its Engineering Application

With the increasing construction projects,such as highways,subways,and railways in China,the cross impacts of transportation lines are becoming more and more serious.In order to fully utilize underground space and reduce the engineering costs,tunnels are often applied to undercross the existing structures when newly built or renovated highways or railways,resulting in an increasing number of undercross tunnelling projects.During the construction of underpass tunnels,uneven settlement of existing structures may occur,and in severe cases,the existing structures may be damaged so these structures cannot be used normally.To ensure that the impact of underpass tunnels on existing structures during construction is within a controllable range,it is necessary to study the safety of existing structures and newly built tunnels,so that some useful findings can be provided for later reinforcement and support.This study combines engineering requirements and systematically analyzes the deformation and stress characteristics of surrounding rock of underpass tunnels and existing structures through theoretical analysis,numerical simulation,and indoor experiments;The vertical and horizontal safety distances are derived,and the uncertainty of parameters are considered,reliability analysis on tunnels and structures are conducted;At the same time,insitu testing is applied to analyze the stability of the structures.The main research content and conclusions include:(1)By using the principle of similarity,determine the size of the indoor model test and build a model box with 2m length ×0.3m Width ×2m height;Based on conventional indoor tests,the lining material ratio was determined.The influence of burial depth on the deformation of tunnel surrounding rock and ground settlement of roadbed was summarized through model experiments;(2)The influence of tunnel construction at different angles(0°,30°,45°,60°,and 90°)on the settlement of existing tracks was analyzed through numerical simulation.It was found that the settlement of the existing track caused by tunnel excavation has a significant spatiotemporal effect.The increased in settlement of the existing track caused by underpass construction can be clearly divided into three stages: the early deformation stage,the construction deformation stage,and the subsequent deformation stage;(3)Based on practical engineering,the derivation of collapse curve analysis for underpass tunnels under different structures was studied.Based on the nonlinear Hoke Brown criterion,combined with the upper bound theorem and energy conservation criterion,an analytical solution for the tunnel collapse curve under seepage only and seismic and seepage coupling effects is derived,and the minimum vertical safe distance of the tunnel under different conditions is given;(4)Based on the rule of limit equilibrium method and the Mohr Coulomb criterion,the horizontal influence distance of the tunnel under linear conditions was derived;The deformation and stress of rock and soil mass are mostly nonlinear,the expression of half excavation width of shallow tunnel is derived through Mohr envelope based on the nonlinear criterion,and this value is used as the collapse criterion of shallow tunnel;Considering the uncertainty of parameters,the upper limit of failure probability for the minimum horizontal safety distance of shallow buried tunnels was derived from the perspective of probability methods and verified through engineering.(5)By introducing RSM and vector projection method,a set of tunnel surrounding rock stability analysis methods based on vector projection influence stress surface method has been formed;Furthermore,a new response surface method combined with LASSO regression method is proposed to solve the problem of multi-collinearity of random variables,obtain accurate regression coefficients,and improve the calculation accuracy of reliability;(6)By using the new response surface method and the Chebyshev inequality method,the failure probability of tunnel surrounding rock was calculated for three different scenarios of underpass railways,highways,and residential areas based on the vertical safety distance,horizontal influence range,and reserved deformation,respectively.The results show that,the new proposed method is basically consistent with the Monte Carlo method method,The probability of failure of the surrounding rock calculated based on the reserved deformation is generally higher than that calculated by the above horizontal and vertical safety distance criteria,this is mainly because that the reserved deformation value is more conservative.It is recommended to use the reserved deformation method for safety analysis of underpass tunnel systems in practical engineering.