Instability Properties and Deformation Control Methods of Rocks Surrounding Xigeda Strata

Xigeda strata are a type of fluvio-lacustrine deposit with half-diagenesis in southwest China, characterized by bad water stability; the deposits become mud and collapse when they come into contact with water; therefore, disasters are possible during the process of tunnel excavation. Large deformations of the surrounding rocks and collapse are the ill effects during constructions of traffic tunnels. Thus, relying on the Tong Zilin, Ya Kou, and Yan Bian tunnel engineering in the Xigeda formation in the Pan-xi region, and based on the field sampling and indoor laboratory tests of the mechanical properties of the surrounding rocks, finite element models of the shallow and deep Xigeda strata tunnels under different water content were established simultaneously to study the instability properties of the surrounding rocks of Xigeda strata for different water content and to explore the deformation control methods accordingly. The results show that Xigeda formation is sensitive to water, and the light-gray shale embedded with sandstone is most affected by water in its physical and mechanical properties. The deformation of the surrounding rocks increases when the water content ranges from 0% to 25%, but the deformation degree is small. When the water content ranges from 20% to 25%, the degree of deformation increases significantly, and there is increased possibility of instability. Under the conditions of large buried depth and high water content, accumulated deformation of the tunnel invert uplifting and tunnel face extruding increase sharply, followed by accumulated deformation of the vault. The tunnel face extruding deformation mainly occurs in the junction between the upper step and middle step. According to the instability properties of the surrounding rocks of Xigeda strata, construction methods of the surrounding rock with the rock either being shale embedded with sandstone or sandstone embedded with shale are examined for conditions where water is either present or absent.