Study On Failure Mechanism And Deformation Control Of Tunnels In Soft And Hard Interbedded Rock Mass With High Geo-Stress

Tunnel building is moving in the direction of becoming ‘deep,lengthy,enormous,difficult,and dangerous’ in the western region due to the influence of variables including topography,hydrogeology,and rock stratum conditions.The surrounding rock of soft and hard interbedded tunnels under high ground stress is significantly shattered,the total strength is lowered,and there is anisotropy due to the effect of interlayer joint surfaces.Asymmetric significant deformation is quite likely to happen during tunnel excavation,endangering the security of ongoing work and subsequent operations.As a result,based on the background of the Jianshan Tunnel in the Zhonglan Section of the Yin-lan High Speed Railway,this thesis has conducted in-depth research on the strength variation rules and failure modes of soft and hard interbedded rock masses under different layer thicknesses and inclinations,as well as the deformation mechanism and control technology of soft and hard interbedded tunnel surrounding rock,using methods such as data investigation,on-site testing,indoor tests,and computer simulation.(1)The strength characteristics and failure modes of soft and hard interbedded rock masses under different dip angles are analyzed,and it is obtained that the mechanical properties of soft and hard interbedded rock masses are determined by the soft layer,soft and hard bedding plane,and hard layer at low,medium,and high dip angles,respectively.The corresponding failure modes are split failure,shear failure,and composite failure.(2)Soft hard interlayered model specimens with various layer thicknesses and bedding numbers of soft hard,soft hard soft,and hard soft hard were created based on the similarity principle,orthogonal testing,and range analysis.It was discovered during indoor triaxial shear and uniaxial compression testing that an increase in bedding numbers decreased the model’s total strength;the failure modes of the hard layer and the soft layer,respectively,are tensile failure and plastic dilatation and swelling.At the soft-hard interface,misalignment and slide are likely to happen,which causes model instability.(3)An enhanced Cosserat medium model is obtained based on Cosserat medium theory by introducing the elastic-plastic parameters Cosserat shears modulus Gc,softening modulus Ei,and internal length parameters lp.The homogenization constitutive equation of soft and hard interbedded rock mass at arbitrary dip angles is derived using the coordinate transformation principle;based on the indoor test results,the strength curve of sand mud soft hard interbedded rock mass varying with the inclination angle is proposed.(4)The discrete element software 3DEC is used to compare and analyze the displacement of surrounding rock,the stress of support and the crack propagation of the same layer thickness,different dip angles and different layer thickness under various conditions.The results show that the unsymmetrical displacement reaches the peak value at the mid-dip angle(30 °～60 °),and it is easy to form the small eccentric tension and large eccentric compression area of the support structure.With the increase of the thickness of the hard layer,the displacement of surrounding rock,the stress of the support and the crack propagation have improved to some extent.When the three-step reserved core soil construction is adopted and combined with the asymmetric arrangement of long and short anchor bolts,the effect can be significantly improved.The maximum asymmetric displacement and crack length can be controlled within 100 mm and 30 mm respectively.(5)Based on the similarity theory,a set of soft and hard interbedded rock tunnel model test system suitable for high ground stress is designed.The system can simulate the effect of high ground stress in three directions,and impose boundary constraints on the stratum.The improvement effect and feasibility of asymmetric support with long and short bolts are verified by analyzing the deformation of surrounding rock and the stress of support under different support forms of medium-dip interbedded rock.(6)The on-site risk analysis of the Jianshan Tunnel indicates that deformation control measures should be implemented and monitoring should be strengthened;this thesis proposes on-site deformation control measures for tunnels from the perspectives of excavation methods and the construction of long and short anchor bolts,and verifies the implementation effect of the control measures through analysis of monitoring data on the deformation of surrounding rock in the on-site test section and the monitoring data on the deformation of surrounding rock in the on-site test section and the monitoring data on the deformation.