Deformation Mechanism Of Surrounding Rock In Permafrost Tunnel Under Melting

After the excavation of the permafrost tunnel,the surrounding rocks are exposed to the air,and the temperature between the surrounding rocks and the air will force heat exchange between them,which will cause the surrounding rocks of the permafrost tunnel to melt and form a melting circle.In addition to the natural environment,blasting operations and shotcrete will also further increase the temperature of air in the cave,aggravate the melting of the surrounding rocks.It is due to the presence of ice in parts of the permafrost surrounding rock,and ice is highly susceptible to temperature.Once the surrounding rocks temperature reach to the critical temperature of the solid and liquid phases-zero degrees,it will begin to melt,and the physics and mechanical properties of surrounding rocks in permafrost will change drastically,seriously threatening the stability of the tunnel structure.This paper focuses on the deformation and failure of surrounding rocks under the melting effect of the permafrost tunnel,Starting with a large number of references and relevant documents,through the literature survey,summarizing the case of surrounding rocks deformation and failure of permafrost tunnel at home or abroad,the types and influencing factors of deformation and failure of surrounding rock in permafrost tunnel under the melting effect are studied.Based on the type and influence factors of the deformation and failure,elastic-plastic analysis model for surrounding rock of permafrost tunnel under melting is established.Combined with the theory of convergence confinement method,the influencing factors of surrounding rocks mass of permafrost tunnel under the action of melting are analyzed.The theoretical analysis is supplemented and verified with numerical simulation and practical engineering,in order to fully grasp the deformation and failure mechanism of tunnel surrounding rocks under melting,and provide theoretical guidance for tumiel design and construction in permafrost regions.The main research results are as follows.（1）The statistical analysis shows that the main deformation and failure modes of surrounding rocks of permafrost tunnel under melting mainly include partial block collapse,collapse and primary support deformation.The factors that affect the deformation and failure of surrounding rocks are established as follows:surrounding rocks conditions,depth of melting circle and supporting structure.（2）According to the deformation and failure modes of surrounding rocks and the influencing factors,combined with the actual engineering of Fenghuoshan Tunnel on the Qinghai-Tibet Railway,the elastic-plastic analysis model of the permafrost tunnel under melting is established and solved,and analyzes the influences about the surrounding rocks deformation in surrounding rock conditions,the depth of melting circle,the supporting structure by use of the convergence-constraint theory.The analysis shows,with the same depth of the melting circle and support conditions,the displacement of the surrounding rocks around the tunnel will increase with the deterioration of the surrounding rock conditions.Under the same displacement and melting depth of the surrounding rock,the higher strength of the supporting structure and the earlier working time will cause the smaller displacement in surrounding rocks,the same as the condition of surrounding rocks and supporting,the displacement of surrounding rocks around the tunnel will increase with the increase of melting circle’s depth.（3）Taking Fenghuoshan tunnel as a simulation object,numerical simulation was carried out by FLAC^3D to simulate the influence of surrounding rock conditions,the depth of melting circle and the supporting structure on surrounding rock deformation of permafrost tunnel.The corresponding relationship between displacement and surrounding rock was obtained,and the results of theoretical analysis are verified,too.（4）Combined with the surrounding rocks parameters and design of Fenghuoshan tunnel project,the analytical solution is solved through theoretical analysis,The results of theoretical analysis and the numerical simulation results which obtained by FLAC^3D are compared with the actual monitoring data of the engineering.The rationality of the established analytical model and theoretical assumptions was verified.This theoretical method can provide a theoretical reference for the construction and design of the permafrost tunnel in practical engineering.