Failure Characteristics and Influencing Factors of Tunnel Surrounding Rock in Gently Dipping Sandshale Stratum
The gently dipping sandshale stratum is often a nearly horizontal soft-hard interbedded rock mass,with significant anisotropy in strength and deformation,and the stability mechanism of tunnel surrounding rock is complex.Taking a deep-buried section of Yijun Tunnel on Xi'an-Yan'an HSR as the study object,based on on-site geological survey,this paper establishes a numerical model of horizontal sandshale interbedding by RFPA,studies the failure characteristics of surrounding rock under unsupported conditions,analyzes the influence law of layer thickness ratio,strength ratio and lateral pressure coefficient of soft and hard rock on tunnel surrounding rock failure,and verifies through on-site deformation monitoring.The results show that:(1)The anisotropic characteristics of gently dipping sandshale rock mass are significant.After tunnel excavation,tensile failure along the bedding plane occurs at the arch crown,and the failure range of surrounding rock gradually decreases with the decrease of layer thickness ratio.(2)The smaller the strength ratio,the more significant the anisotropy of the rock mass.Shear failure is prone to occur at the junction between soft and hard layers of arch crown,and the range of failure is larger.(3)When the lateral pressure coefficient is 0.25,the surrounding rock presents X-type failure and tensile failure occurs at the arch crown.With the increase of the lateral pressure coefficient,the failure range of the surrounding rock decreases and presents a crescent-shaped distribution.When the lateral pressure coefficient is 1,the surrounding rock presents circular failure.(4)The increase of confining pressure,layer thickness ratio and strength ratio can weaken the anisotropic characteristics of gently dipping sandshale rock mass,reduce the acoustic emission number,and decrease the damage degree of rock mass.
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