Study on softening characteristics of mudstones in shallow buried strata in Xinjiang and countermeasures for tunnel support

The Xinjiang water conveyance tunnel is a shallow buried mudstone stratum. After the water absorption, the strength of the surrounding rock softens and large deformation occurs. The support is difficult and the risk is high, which has a great impact on the progress of the project. In this paper, the mudstone of a water conveyance tunnel in Xinjiang is taken as the research object. Based on the XRD test analysis of the mudstone in the tunnel, the uniaxial compression and SEM analysis of the mudstone under different water content conditions are carried out, and the internal fissures of the rock sample are revealed. The variation of the pore structure clarifies the strength softening characteristics. The results show that the clay mineral content of the tunnel is as high as 50 %, and the proportion of montmorillonite is close to 90 %, which is easy to absorb water and soften. As the water content increases, the interior of the rock sample will undergo the process of micro-crack development, development and penetration, accompanied by the loss of pore water and internal minerals. By analyzing its saturated water absorption curve, the water absorption process is divided into three stages: rapid water absorption, decelerating water absorption and uniform water absorption stage. The Boltzmann prediction function is used to fit the rock sample strength variation curve after water absorption, and its strength can be changed. It is divided into three states: non-destructive strength, rapid softening and softening and rest state. The critical point for obtaining the softening of water absorption strength of this type of mudstone is 6 % water content; the comprehensive analysis shows that the water softening law is: the clay minerals in mudstone, mainly montmorillonite, have strong water absorption capacity. With the increase of water absorption, the free water content in the mudstone pores is significantly increased, the skeleton strength is gradually reduced, and an unstable state is formed, which is easily deformed and destroyed when subjected to external force. Based on this, the principles of stability control of such soft rock engineering are proposed, which is to determine the soft rock type and deformation mechanics mechanism, formulate targeted stability control strategies, and adopt the support of mechanical properties such as constant resistance, large deformation and high prestress material. The research results are of great significance for understanding the strength and softening characteristics of the mudstone in the water conveyance tunnel, the deformation and failure mechanism of the tunnel surrounding rock and the support design.