Risk Analysis And Control Of Collapse In Tunnel Construction In Flysch Formations

Flysch is a special type of semi-deep and deep-sea sedimentary rocks,generally composed of sandstone,mudstone and other interlayers.Flysch formations have the bad characteristics of extremely developed bedding planes,high degree of breakage,and easy softening in the event of water.Improper disposal during the construction of flysch tunnels can easily cause collapse disasters,resulting in significant economic losses and casualties.It is of great practical significance to carry out research on the risk assessment model for the collapse of flysch tunnel construction.Therefore,this paper first uses numerical simulation to study the deformation and stress characteristics of different bedding plane parameters,layer thickness and dip angle of the surrounding rock of flysch tunnel.On this basis,through a statistical analysis of a large number of actual examples of layered soft rock tunnel collapse,the main factors affecting the collapse are obtained.Then,using analytic hierarchy process,fuzzy comprehensive evaluation method and index system method,a risk assessment model for the collapse of flysch tunnel is established.The main work content and research results are as follows:(1)Through statistical analysis of actual engineering cases of a large number of layered soft rock tunnel collapses,it is concluded that there are three main types of influencing factors affecting layered soft rock tunnel collapse: geological factors,design factors and construction factors.And a qualitative analysis is made on the influence of various factors on the tunnel collapse.On this basis,combined with the characteristics of flysch,it is determined that the main factors affecting the collapse of the flysch tunnel are the grade of surrounding rock,groundwater,fault fracture zone,bias pressure,rock softening coefficient,attitude of rocks,tunnel burial depth,tunnel span,support timing lag and improper construction control.(2)FLAC3D was used to simulate different bedding plane parameters,rock layer thickness and rock layer inclination angle test programs,and the deformation law of the surrounding rock of flysch tunnel under different bedding plane parameters,rock layer thickness and inclination angle was obtained.The results show that,the parameters of the bedding plane have a significant effect on the deformation of the tunnel,and the friction angle in the bedding plane has a greater influence on the distribution of the plastic zone of the surrounding rock of the tunnel.As a whole,the greater the thickness of the flysch rock layer,the smaller the tunnel deformation and the better the stability of the surrounding rock;with the increase of the dip angle of the flysch rock layer,the overall deformation of the tunnel surrounding rock increases first and then decreases.Affected by the dip angle of the rock layer,in addition to the dip angles of 0 ° and 90 °,the deformation of the surrounding rock along the side of the rock layer at the symmetrical position of the tunnel is significantly greater than that of the side of the inverse rock layer.(3)Using analytic hierarchy process,the weights of various factors affecting the collapse of the flysch tunnel were calculated,and the quantitative analysis of the influence of each factor on the tunnel collapse was conducted.On this basis,a fuzzy comprehensive evaluation method was used to establish a risk assessment model for the collapse of a flysch tunnel,and the rationality of the assessment model was verified through actual engineering cases.(4)Based on the quantitative and qualitative analysis of the influencing factors of the flysch tunnel collapse,the index system method is used to establish the flysch tunnel collapse assessment model before construction and the flysch tunnel collapse risk dynamic assessment during construction.The pre-construction collapse risk assessment model consists of the collapse risk assessment index system and the collapse risk severity assessment model;the collapse risk assessment model during construction is based on the pre-construction collapse risk assessment,combining the actual geological conditions on site,monitoring data,and on-site construction technology.The established risk assessment model is applied to actual engineering cases,and the results obtained are consistent with the actual construction on site.