Correct evaluation of shear and deformation behaviour of rock joints under cyclic shear loads is very important for safe and economical design of civil and mining structures in rock mass. The importance of it, increases many folds when these joints are filled by infill material. When deformation of rock mass normal to shearing plane is resisted by the surrounding rock mass or rock anchor or bolt provided to increase stability to the rock mass. The shearing of these joints no longer takes place under constant normal load (CNL) conditions but it takes place under varying normal load condition, whereas normal stress on the shearing plane increases during the shearing process, this condition is referred as constant normal stiffness condition (CNS). Hence, in order to assess the effect of change in normal stress on shearing plane, two types of planar and non planar synthetic infilled joints are prepared. The asperity angle of planar and non planar joints are kept as 0°-0° and 15°-15° respectively. These joints are prepared with plaster of Paris and tested under CNS boundary condition. Sample size of joints is kept at 298*298*125 mm, and in between the joints; infill material of thickness 5mm and 10mm is placed to create infilled rock joints. These joints were tested under CNS boundary condition at different initial normal stresses (Pi) by keeping frequency and amplitude constant under cyclic shear load, using servo controlled large scale direct shear apparatus. Test results indicate that there is no change in normal stress on the shearing plane and it remains same at Pi for planar infilled joint. But for non planar joints it changes with changing in the infill thickness and number of shear cycle. lnfilled material thickness and initial normal stress play important role on cyclic shear behavior of non planar infilled joints at CNS boundary condition. It is also observed that there is no effect of CNS boundary condition on shear behaviour of infilled joints at high normal stress and infill thickness.

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