Research On Deformation And Stress Characteristics Of Surrounding Rock And Calculation Method Of Surrounding Rock Pressure In Loess Tunnel Based On DEM

China has built a large number of tunnels in the vast loess area.During the construction process of loess tunnels,diseases such as surface cracking,support intrusion,and cave collapse occur frequently,and have not yet been fundamentally resolved.The reason for this is that loess has special properties such as vertical joint development and loose structure,which makes it difficult to accurately obtain its physical and mechanical parameters and has large dispersion.This also leads to significant differences between loess tunnels and rock tunnels in deformation laws,support structure stress characteristics,and other aspects.This article relies on the shallow buried section of the Hejiazhuang Tunnel on the Zhengxi High Speed Railway and the deep buried section of the Mizhi 1 Tunnel on the Yusui Line.Based on on-site measured tunnel support deformation data,numerical simulation and theoretical analysis are used,and a combination of microscopic and macroscopic analysis methods is used to study the deformation and stress evolution characteristics of the surrounding rock during the entire construction process of the loess tunnel,as well as the calculation method of the surrounding rock pressure of the loess tunnel.The main research results are as follows:(1)Based on the indoor direct shear test and its discrete element simulation,the surrounding rock micro parameters are inverted,the traditional model generation method in PFC is optimized,and the discrete element models of Hejiazhuang Tunnel and Mizhi No.1Tunnel are established;Based on the on-site measured support deformation data of Hejiazhuang Tunnel and Mizhi No.1 Tunnel,a discrete element simulation method(DLSM)for deformation support is proposed.The time correspondence between the measured and simulated deformation of surrounding rock is established,which more accurately reflects the actual deformation of initial support and induces corresponding deformation of surrounding rock.The interaction mechanism between surrounding rock and support structure in discrete element simulation is optimized.(2)Based on discrete element simulation,the surface displacement characteristics,internal displacement characteristics of surrounding rock,and distribution characteristics of surrounding rock force chain during the entire construction process of shallow and deep buried loess tunnels were studied and compared.The phenomenon of stress redistribution in surrounding rock and the generation and evolution mechanism of arch effect were revealed at a micro level;Through quantitative analysis of the development status of surrounding rock fractures,it was found that the bottom of the sliding surface of the loess tunnel had a horizontal outward expansion,and the fracture angle of the surrounding rock of the loess tunnel was significantly greater.The failure modes of shallow and deep buried loess tunnels were given;Based on the spatiotemporal distribution characteristics of surrounding rock stress and porosity,the evolution laws of the inner and outer boundaries and their shapes of pressure arches in shallow and deep buried loess tunnels were given,and it was found that the height of the surrounding rock compaction zone and pressure arch zone in loess tunnels coincided.(3)According to the failure mode and mechanical transfer mechanism of surrounding rock after excavation of loess tunnel,the surrounding rock is divided into undisturbed area,pressure arch area,bearing area and loose area.According to the composite pattern of each area,the loess tunnel is divided into Class I shallow buried,Class II shallow buried and deep buried loess tunnels,and the corresponding calculation model of surrounding rock pressure-composite arch structure model is established respectively;Based on the failure characteristics of loess tunnels,the formulas for calculating the failure angle and the width of the loose zone of the surrounding rock of loess tunnels are proposed;Based on the measured support deformation and numerical simulation results,a calculation method for the height of the loose zone and the thickness of the pressure arch zone of the loess tunnel is provided;Based on the quadratic parabolic strength criterion of loess and elastic-plastic analysis of plane stress problems,the mechanical transfer mechanism between the soil masses in various regions after excavation of loess tunnels was revealed.The calculation methods for the surrounding rock pressure of Class I shallow buried,Class II shallow buried,and deep buried loess tunnels were provided respectively;Taking the Hejiazhuang Tunnel as an example,the results obtained by this method are greater than those obtained by the standard method,and less than those obtained by Terzaghi method,the Beer Baumann method and the full soil column method.This method better approximates the measured values while ensuring safety.