Quantitative Risk Analysis Of Large Deformation And Instability Of Layered Soft Rock Mass During Tunnel Excavation

Layered rock mass is a kind of common engineering rock mass,its mechanical parameters has significant spatial variability.Under excavation,the Layered rock mass tunnel envelope is prone to large deformation and instability damage,which in turn threatens the safety of construction personnel and equipment.In order to effectively evaluate the risk of large deformation and instability of layered soft rock tunnel surrounding rock,a series of studies were carried out,including monitoring of tunnel surrounding rock displacement and relaxation depth,characterisation of rock elastic modulus conversion and spatial variability,dispersion of non-Gaussian parameters with airfield and calculation of surrounding rock instability damage probability,using the Zhengwan high speed railway Baokang tunnel as the research background,and integrating field monitoring,statistical analysis and numerical simulation.The quantitative risk analysis of large deformation and instability of the surrounding rock provides technical support to ensure the safe and efficient construction of layered soft rock tunnels,with the main research contents as follows.(1)Monitoring of surface displacement,arch pressure and loosening circle of the surrounding rock in layered soft rock tunnels was carried out.The results show that the surrounding rock deteriorates rapidly and its deformation and damage have significant asymmetric characteristics of large left and small right,large top and small bottom.Based on the displacement monitoring data and the bilinear all-over nodal model as the ontological model,the sensitivity analysis and intelligent inversion of rock mechanical parameters were carried out to calculate and examine the distribution of deformation and damage in layered soft rock tunnels under excavation and unloading,laying a computational foundation for quantitative risk analysis by means of numerical simulation as the core.(2)The conversion from elastic wave velocity to elastic modulus of layered rock is achieved by means of empirical equations,and the optimal probability density distribution function of the elastic modulus is fitted by means of probabilistic methods.Based on geostatistical theory,the spatial variability of the elastic modulus is investigated,and the variation function and autocorrelation function of the elastic modulus along the strike and tendency of the rock formation are determined.The theoretical analysis was carried out to solve for the equivalent mean and standard deviation of the elastic modulus,the eigenvalues and eigenfunctions of the autocorrelation function,the standard normal random variables and the number of truncation terms,and to construct a random field for the elastic modulus parameters based on the Karhunen-Loève series expansion method.By means of cell traversal,the coupling of the elastic modulus random field to the FLAC3 D grid is achieved.The distribution characteristics of the random parameters in the stochastic finite difference grid model are tested to effectively characterize the spatial variability of the elastic modulus of the layered rock mass.(3)A non-invasive stochastic analysis was used to calculate the convergent displacements of the surrounding rock in the arch-waist area of the tunnel under the random distribution of elastic modulus.The limit state equation for the permissible deformation of the surrounding rock under the initial support of the tunnel was determined using the ultimate relative displacement as the index.Based on the Monte Carlo method,the deformation damage probability of the surrounding rock is calculated,and the risk analysis of the amount of deformation instability of the surrounding rock considering the spatial variability of the elastic modulus of the layered rock is realised.The quantitative risk analysis method is used to compare and analyse the quantitative instability risk of surrounding rock under system anchors,steel arches and overburden grouting,respectively,to determine the construction method that can effectively inhibit the instability of large deformation of surrounding rock,which provides technical support for the safe and efficient construction of the Baokang Tunnel.