Influence of petrographic parameters on geotechnical properties of tertiary sandstones from Taiwan

Abstract

While tunneling through Tertiary sandstones in Taiwan, tunnel squeezing occurred in some of the sandstones (e.g., Mucha Tunnel and Chungho Tunnel in northern Taiwan), but not in all of them. This phenomenon indicates that the geotechnical properties of Tertiary sandstones differ from one sandstone to another, and further characterization is of interest. Laboratory experiments were conducted to explore the geotechnical characteristics, including strength, deformational behavior and wetting softening phenomena, of more than 13 Tertiary sandstones from northern Taiwan. It was found that these sandstones differ from hard rocks in having significant shear dilation and wetting softening behavior, which is defined as the reduction of both strength and stiffness of dry sandstone due to wetting. Such distinct behavior can occur even when the sandstones have medium to moderate strength. Based on the degree of wetting softening behavior as well as the deformational behavior, two types of sandstones, types A and B, were identified. Relative to type A sandstone, the type B sandstone is characterized by more deformation, shear dilation and more significant wetting softening. In addition, how the microscopic parameters (or petrographic parameters) affect the macroscopic mechanical behavior, including the deformational behavior, uniaxial compressive strength (UCS) and strength reduction ( R) due to wetting, was studied by petrographic analysis. Among the petrographic and physical parameters evaluated, the grain area ratio (GAR) and porosity are found to be the key parameters. As a result, empirical relations of UCS and strength reduction ( R) due to wetting, expressed in terms of porosity and grain area ratio, respectively, are proposed. Comparisons were made to sandstones worldwide from published data, and it is found that these two empirical relations are also applicable to other sandstones. Finally, a classification method, based on UCS and R on the one hand and the petrographic parameters (grain area ratio and porosity) on the other, is proposed for Tertiary sandstones to indicate the geotechnical characteristics of sandstones. This proposed classification method can also be expressed in terms of grain–matrix–porosity content of sandstones. It is also found that porosity has more influence on the UCS than grain and matrix content does.