SHEAR STRENGTH REDUCTION OF ROCKJOINTS DUE TO CYCLIC LOADING

摘要

During strong earthquakes, relative large cyclic displacements may be occurred between the differentwalls of rock joints. These cyclic displacement degrade the first and second order asperities along the jointsurface and reduce shear strength of rock joint.In this experimental study, the variation of shear strength of rock joints during large cyclic displacement isinvestigated to simulate the effects of strong earthquakes on rock joints. Artificial Jointed samples havebeen prepared adopting a developed moulding method using special mortar and tested in direct shearapparatus under cyclic loading conditions. The tests have been performed in different levels of normalstresses to investigate the shearing behaviour of rock joints located at different depths from groundsurface. In each test, several displacement cycles have been applied on the samples and the variation ofshear strength; asperity degradation and wearing have been studied during each cycle.Based on the results of this research, it was concluded that shear strength of rock joints is in close relationwith number of loading cycles and rate of asperity degradation in all levels of applied normal stresses. Inaddition it was concluded that the level of normal stress will control the displacement and rupturemechanism of jointed rock. In high level of normal stresses during shear displacement the asperities willbe cut from their bases while when applying lower levels of normal stress, the predominant displacementmechanism is sliding.Finally, based on the evaluations of experimental results, a mathematical model has been developed forprediction of shear strength in large cyclic loading conditions.