Abstract: The causes of tunnel collapse are complicated. In order to study the causes of a collapse in the tunnel excavation in saturated completely weathered granite soils, this article analyzes the loss of fine particles from the tunnel excavated area. Combined with the triaxial test and the permeability test results, this paper analyzes the structural damage of the completely weathered granite and factors that causing the tunnel collapse. The results are shown as follows. Firstly, as the tunnel is excavated, seepage causes the gradual loss of fine particles at the excavated surface and changes the composition of the completely weathered granite soil structure, so that both the dry density decreases and the void ratio increase. While the coefficient of permeability increases significantly, this leads to intensified piping. The structural damage in the granite soils caused by the seepage is the physical factor of the collapse. Secondly, While the seepage causes the structural damage, the disintegration of the completely weathered granite soils gradually increases, the surrounding rock is softened by the water, the cohesion, the internal friction angle and the shear resistance, the strength decreases accordingly, further reducing the self-stability of the surrounding soil and rock mass, which is the mechanical factor for the collapse of blocks during the excavation process. The structural damage of the completely weathered granite soils caused by the seepage induces tunnel collapse. Controlling the seepage conditions of the groundwater in the stratum is the key to minimize the deterioration of the completely weathered granite mechanical properties due to the seepage and can significantly improve the stability of the tunnel.

Key words: Completely Weathered Granite, Collapse, Soil Loss, Soften Effect