Abstract: In this article, gypsiferous surrounding rock exposed in Liangshuijing tunnel area of the Yichang-Badong expressway is taken as the research object by uniaxial compression test and scanning electron microscope test to explore the softening behavior and strength degradation mechanism of gypsum rock under different moisture contents and drying and wetting cycles. The research results indicate that the gypsum rock’s strength and deformation index are very sensitive to moisture content. It is soft rock with low compressive strength and has obvious softening characteristic in water. The mechanical parameters including uniaxial compressive strength, elastic modulus, and deformation modulus reduce with the increase of moisture content in rock, displaying a good function relationship; whereas Poisson’s ratio approximately increases with the increase of moisture content. The drying and wetting cycles have an obvious effect on the degradation of gypsum rock’s mechanical properties. The uniaxial compressive strength, elastic modulus, and deformation modulus reduce with the increased times of drying and wetting cycles, showing a good logarithmic relationship; while Poisson’s ratio increases slightly with the increased times of drying and wetting cycles. The microscanning images show that water is obviously destructive to the structure of gypsum rock. Corresponding prevention and control measures should be taken during the tunnel construction to prevent the adverse effects of bad characteristics of gypsum rock.

Key words: tunnel’s surrounding rock, gypsum rock, moisture content, drying and wetting cycle, uniaxial compression, scanning electron microscope