Risk Assessment Of Surrounding Rock Deformation In Large Section Railway Tunnel Construction Under Soft Stratum Condition

With the rapid development of high-speed railway,long tunnels and tunnel groups under complex geological conditions have appeared one after another.In the process of construction,more and more tunnels have to pass through the weak surrounding rock zone with the characteristics of breakage,poor cementation,rheology,strong weathering and high stress,often resulting in large-scale failure accidents.In view of this,this study takes a large section railway tunnel project under construction in soft stratum as an example to establish a set of risk assessment index system and implement hierarchical control according to the risks that may be encountered in the actual construction process.Firstly,based on the engineering geology and hydrogeology conditions of the tunnel,combined with the application process,the deformation mechanism of surrounding rock which may be caused by the application process is analyzed,and the safety system engineering method is comprehensively used to identify the risk of surrounding rock,and a total of 35 risk factors of 7 categories affecting the surrounding rock deformation of the tunnel are found.Then,principal component analysis was used to screen the risk factors of tunnel surrounding rock deformation,and 18 risk factors whose cumulative contribution rate was less than 0.7 were removed,and the evaluation index system of surrounding rock deformation during railway tunnel construction was established.Among them,the criterion layer is hydrogeological conditions,design factors,construction factors,management factors,and the index layer is the surrounding rock grade,excavation method,surrounding rock disturbance,monitoring measurement and other 17 risk assessment indicators.Thirdly,combined with the tunnel surrounding rock deformation risk index system,the Duffin equation is constructed to analyze the tunnel surrounding rock deformation risk possibility,and the quantified value of surrounding rock deformation possibility is obtained through Monte Carlo simulation.The results show that the tunnel is in a stable state when it is constructed under the designed conditions.When the actual site conditions deviate from the design,the possibility probability of surrounding rock deformation in 10 places within the research interval is obtained through Monte Carlo simulation.Fourthly,the average loss of unit length tunnel surrounding rock deformation is calculated by using the deformation amount of surrounding rock per unit length tunnel section and the average loss cost,and the calculation method of direct economic loss to measure the loss of surrounding rock deformation risk consequence is proposed,and the quantified value of surrounding rock deformation consequence in 10 places within the research interval is obtained.Finally,based on the acceptable risk standard determined in the Technical Code for Risk Management of Railway Tunnel Engineering(Q/CR 9247),combined with the formula of expected cost ratio of tunnel surrounding rock deformation,the risk classification standard of tunnel surrounding rock deformation is established,and the risk assessment of surrounding rock deformation is carried out for the tunnel project under construction.The evaluation results show that there are 10 surrounding rock deformation in the study area,8 of which are moderate risk and 2 of which are high riskThe quantitative evaluation results of surrounding rock deformation risk in large section tunnel construction under soft stratum based on duffin equation and Monte Carlo proposed in this study are basically consistent with the calculation results given by the standards specified by the International Tunnel and Underground Space Association,thus verifying the rationality and reliability of the risk assessment method.