Engineering properties of collapsible loess tunnel

The collapsibility of loess and, the arid and semi-arid climate, complicated landform and topography, and particular geological environment in loess zone potentially have adverse effects on the structure of loess tunnel. Based on the engineering properties of collapsible loess tunnel in the process of construction and operation, this paper presented classification standards for the geotechnical environment, water-immersion environment, and integrated environment of loess tunnel, according to different landform, topography, stratigraphic structure, erosion degree, subsurface water, and surface water source conditions. Furthermore, computed analysis method for the collapse deformation of loess tunnel foundation was proposed. According to differential settlement of loess foundation acting on lining structure, control standard of subgrade settlement during train movement, and influence of collapse deformation of building foundation on structure, classification standard for the foundation of collapsible loess tunnel was put forward. Moreover, considering the application limitation of present “Code for Building Construction in Collapsible Loess Zone” and the engineering properties of collapsible loess tunnel, assessment method for the collapse deformation of loess tunnel foundation and computed method for confining pressure were presented, respectively, corresponding to the stability of surrounding rock during construction and the influence of collapse deformation on lining structure. Collapsible compression pressure can be determined by the coefficient of vertical compression pressure, and the collapse deformation of tunnel subsoil thus can be calculated with the experimental coefficient of collapsibility. Introducing structural index in the Terzaghi equation of surrounding rock pressure, relationship between surrounding rock pressure and structural index was analyzed.