Shear behavior of waste rock materials in drained and undrained ring shear tests

The denudation processes in abandoned mines pose environmental and social problems. Insufficient remediation and cleanup modify the physicochemical properties near the mine sites. In addition, large amounts of sediment and debris can flow downward due to heavy rainfall. Here, the shear characteristics of mine deposits are presented rather than their chemical contaminants, which include heavy metals. Drained and undrained ring shear tests were performed on waste rock materials that were collected from pyrophyllite mine deposits in the mountainous region of Korea. These samples were classified as gravelly sandy soils (i.e., with an effective grain size of D₁₀ = 0.3 mm and a uniformity coefficient of c_u = 5). The following three tests were performed: (i) shear stress measurement with shearing time for a given shear velocity (i.e., 0.1 mm/sec), (ii) shear stress as a function of shear velocity, and (iii) shear stress as a function of normal stress. The fully saturated samples were subjected to different normal stresses (i.e., 25 and 100 kPa) and shear velocities (i.e., 0.01, 0.1, 1, and 100 mm/sec). The waste materials exhibited a strain-softening behavior regardless of the drainage and shear velocity condition. In addition, the shear stress was strongly influenced by the shear velocity and increased with increasing shear velocity and normal stress in the post-failure stage. Using grain size distribution analyses, we show that significant grain crushing occurs in the shear zone during shearing. Under relatively high shear velocity conditions (i.e., >;100 mm/sec), the grain crushing effect is more significant and results in rapid mass movements.