Vahab Sarfarazi - Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran.
Hadi Haeri - Department of Mining Engineering, Higher Education Complex of Zarand, Shahid Bahonar University of Kerman, Kerman, Iran.
Mohammad Fatehi Marji - Department of Mine Exploitation Engineering, Faculty of Mining and Metallurgy, Institute of Engineering, Yazd University, Yazd, Iran.
Gholamreza Saeedi - Department of Mining Engineering, Shahid Bahonar University of Kerman, Kerman, Iran.
Amir Namdarmanesh - Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran.

In this paper, the effect of variations in the number and area of the rock bridges on the non-persistent discontinuities is investigated. In this regard, blocks containing rock bridges and joints with dimensions of ۱۵ cm * ۱۵ cm * ۱۵ cm are prepared from plaster. The available rock bridges that have occupied ۰.۲, ۰.۴, and ۰.۶  of the shear surface show latitudinal extension along the shear surface. There are variations in the number and extension of the rock bridges in the fixed area. For each of the samples, tests are performed on three blocks of the same material, by putting it under various direct normal stresses. Normal stresses were ۳.۳۳, ۵.۵۵, ۷.۷۷ kg/cm۲. Also the obtained shear strength by laboratory tests was compared with the outputs of Jenning's criterion and Guo and Qi's criterion to determine the accuracy of these criteria for predicting the shear strength of non-persistent joints. The results show that the tensile crack started in the rock bridge under normal stress of ۳.۳۳ kg/cm۲. Mixed-mode tensile shear cracks were propagated in the rock bridge under a normal stress of ۵.۵۵ kg/cm۲, while a pure shear crack developed in the rock bridge under a normal stress of ۷.۷۷ kg/cm۲. With the increase of normal stress, the number of microfractures increased. The variance in the number of rock bridges in the fixed area of the rock bridge does not affect the friction angle along the shear surface. Furthermore, the cohesion along the shear surface shows a small decrease with the increasing number of rock bridges. Also by the increase in the area of rock bridges, the friction angle along the shear surface remains constant, while at the same time, there is an almost linear increase in cohesion. Guo and Qi's criterion predicts the shear strength of the non-persistent joint exactly close to the shear strength of the physical samples.

rock bridge, shear failure, Shear Properties