Wetting deterioration of soft sandstone - microscopic insights

Some sandstone may become weaker and softer as it is immersed in water. It has been reported that the excavated surface of a tunnel, being exposed to a very humid working environment for just several days, becomes very soft such that squeezing of the tunnel is subsequently encountered. This paper focuses on the wetting deterioration, including strength and deformability, of the juvenile diagenetic sandstone sampled from Mushan and Tongken Formations in Taiwan. The wetting deterioration of sandstone is explored by a series of compressive strength experiments on both dry and wet specimens. To identify the mechanism causing the wetting deterioration, petrography, XRD and SEM examination were conducted to study the microstructure of sandstone. Experimental results indicate that 40 ~ 60 % strength reduction and about 50% reduction of the deformation modulus occurred when the sandstones were fully wetted. Both sandstones possess similar petrographic features: high matrix content (about 30%) and grain-to-grain microstructure with about 60% of contacts being tangent type contact. The matrix mainly comprise of clay minerals, including illite, kaolinite and chlorite. Among the clay minerals, chlorite is the most soluble in water and easiest to be leached out. Furthermore, when sandstone is wetted, the type of fracture transfers from the trans-granular fracture of dry state to inter-granular fracture. These phenomena imply the underlying mechanism of wetting deterioration as the following process: (1) The matrix is softened as sandstone is wetted; and (2) The grains tend to roll more freely and distribute the applied load within the matrix without breaking many other grains such that the fracture pattern is transferred from trans-granular type to inter-granular type. ? 2018 ISRM International Symposium 2000, IS 2000. All rights reserved.