Amini Hosseini, K. - Assist. Professor, Getechnical Eng. Research Center of IIEES, Tehran, I.R. Iran,
Boulon, M. - Professor, Laboratoire ۳S, Universite Joseph Fourier, Grenoble, France
Jafari, M. K. - Assoc. Professor, Geotechnical Eng., Research Center of IIEES, Tehran, I.R. Iran
Pellet, F. - Assoc. Professor, Laboratoire ۳S, Universite Joseph Fourier, Grenoble, France

Dynamic loading due to strong earthquakes could make relative large displacement between the walls of rock joints and could have considerable effects on shear strength. Through a laboratory study, the variation of shear strength of rock joints during large cyclic displacement is nvestigated to simulate the effects of strong earthquakes. Artificial Jointed samples have been prepared using a developed moulding method with special mortar and tested in direct shear apparatus under cyclic loading conditions. The tests have been performed in different levels of normal stresses to investigate the shearing behaviour of rock joints located at different depths from ground surface. In each test, displacement cycles have been applied on the samples and variation of shear strength;asperity degradation and wearing have been studied during each cycle. The shear strength of joints is in close relation with number of loading cycles in all levels of applied normal stresses. Finally,based on the experimental results, a mathematical model has been developed for evaluation of shear strength in large cyclic loading conditions