Cyclic Frictional Responses of Planar Joints Under Cyclic Normal Load Conditions: Laboratory Tests and Numerical Simulations

Abstract

An accurate quantification of the frictional behaviour of joints under cyclic normal load conditions during the cyclic shear process is important to characterize the joint and fault interactions during earthquakes and rock bursts. We conducted experimental studies and numerical simulations to investigate the cyclic frictional responses of planar joints subjected to cyclic changes of normal loads. Experiments were conducted on artificial rock-like planar joints using a large shear box device (GS-1000), with different vertical and horizontal impact frequencies, vertical impact load amplitudes, horizontal shear displacement amplitudes, and normal load levels. The average normal displacement of the upper block increased with decreasing normal load and decreased with increasing normal load during each cycle. The normal displacement decreased gradually with increasing number of shear cycles due to damage to the micro-asperities at the contact surface. Shear force and the apparent coefficient of friction (k = FShear/FNormal) changed cyclically with a change in shear direction, where k followed a square wave curve with the same peak value at the stable shear stage. The cyclic normal load amplitudes, horizontal shear displacement amplitudes, cyclic normal load frequencies, cyclic horizontal shear frequencies, and static normal force levels had little influence on the peak values of k. Numerical simulations proved that the spatial movement pattern of the loading plate and upper block of the specimen rotated clockwise or anti-clockwise at different shear displacements. Due to the rotation of the upper block, shear and normal stresses distributed at the contact surface were inhomogeneous, which generated a stress gradient along the interface. Consequently, the samples were damaged at the two edges due to the high local stresses. Finally, a mathematical equation is proposed, which can be used for predicting the shear strength of planar joints under cyclic changes of shear velocity and normal load.