Shear Behaviour of Rock Joints

Rocks in natural state always consist of joints and presence of intact rock is very difficult because some weathering action is always taking place in nature. Compared to intact rocks the weathered rocks has very less crushing and shearing strength, thus this requires the study of strength parameter of rock masses. In jointed rocks, the shear displacement is accompanied with vertical movement or dilation and this property plays an important role and should be known to design engineers. For experimental work it is very difficult to use actual rock present in field therefore artificial samples of rocks are prepared. The material used for preparations of sample should be easily and cheaply available in market for example mortar, POP etc. can be used but we have considered POP. The joints can be prepared at any angle but in this analysis the joints are taken unequal with asperity geometry of 15-30 and 15-45. The geometry of 15-30 represent that the facing angle is 30. The sample are prepared with POP at the moulding water content of 50 % by weight then direct shear test is conducted at 14 days of air dried curing and test is done in conventional direct shear test machine. The direct shear test can be conducted in CNL or CNS condition. In CNL condition, the normal stress is kept constant but in actual the normal stress does not remain constant, in field CNS condition occurs. In CNS condition stiffness remains constant, the result obtained from CNS condition gives higher value of shear stress but this condition of testing is not adopted because the equipment developed in the past for CNS boundary condition was either having difficulty in changing the boundary conditions or it was not useful for wide variety of rock joints. The orientation of rock during testing plays an important role in resisting shear stress. Direct shear stress carrying capacity for 15-45 asperity sample as compared with 15-30 asperity sample will more and dilation will also more in case of 15-45 asperity sample as compared with 15-30 asperity sample. In case of 15-45 asperity sample and 15-30 asperity sample, the peak shear displacement increases up to 0.4MPa normal stress but it drastically reduced at 0.5 MPa because at this value the normal stress tries to prevent the horizontal displacement and finally the result obtained experimentally are compared with well known equation of Barton (1973), Bandis (1982) and Pattons (1966) and Shrivastava and Rao (2012) criteria.