Microscopic characteristics of problematic tertiary sandstone as revealed by grain-wide local deformation

摘要

In Taiwan, tertiary sandstones have a diagenetic age of no more than 36 million years, and such a relatively short rock-forming period is insufficient to enable them as hard rocks. Therefore, these tertiary sandstones are often characterized as medium to weak rocks, with uniaxial strength ranges from 10 to 80MPa [1,2]. In the past, several squeezing and collapse cases have been reported while tunnels were constructed within these rock strata [3]. In order to identify the deformation behavior of such sandstones, Jeng et al. [1] studied a total of 13 kinds of sandstones obtained from eight geological formations of Taiwan. From the comparison on the petrographic features with the mechanical properties of these tertiary sandstones, including the uniaxial compressive strength (UCS) and the reduction of strength due to wetting (R = VCS_(wet)/UCS_(dry)), it was found that these tertiary sandstones can be classified into two groups: Type A and Type B (with R >0.5 and R < = 0.5, respectively). The deformation characteristics of each group of sandstones are proposed: Type A have R>0.5 and hence have relatively less tendency of wetting softening both in strength and stiffness. Type B have R < = 0.5, and are characterized by greater deformation and more significant wetting softening.