FRICTIONAL BEHAVIOR OF SANDSTONE FRACTURES UNDER PRE-PEAK CYCLIC LOADING CONDITIONS

Abstract

The objective of this study is to determine the effect of cyclic loading under pre-peak condition on shearing behavior of fractures in Phra Wihan sandstone. The fractures are artificially made in the laboratory by tension inducing method. Specimens are sheared cyclically under forward and fully backward loading paths with constant normal stresses of 1, 2, 3 and 4 MPa. Two cyclic loading conditions are performed on separate sets of specimens: constant stress amplitude and constant displacement amplitude. A total of 50 shear cycles are applied for both conditions. The results indicate that the pre-peak cyclic loading can slightly reduce shear strengths and stiffness of the sandstone fractures. The cyclic loading under both conditions can degrade the second order asperities of the fractures. The energy required to shear the fractures increases with loading cycles for constant stress amplitude testing, but decreases for constant displacement amplitude testing. The monotonic shear tests conducted after cyclic loading show the fracture peak shear strengths that are slightly lower than those of the identical fractures without cyclic loading.