Mechanism of anomalous low friction phenomenon in deep block rock mass

Deep rock mass has the unique “self-stressed” block-hierarchical structure, anomalous low friction (ALF) was one of the typical nonlinear geo-mechanical and dynamic responses in deep block rock mass, which occurred as the result of movements of large-scale geo-blocks under the impact of external pulses (such as a deep confined explosion, earthquakes, rock bursts and etc.). ALF phenomenon obtained its name to describe the curious phenomenon that the friction between interacting geo-blocks quasi-periodically disappears at some discrete points in time along the direction orthogonal to the direction of the external pulse. With the objective to confirm the existence of the ALF phenomenon and study the geo-mechanical conditions for its occurrence experimentally and theoretically, laboratory tests on granite and cement mortar block models were carried out on a multipurpose testing system developed independently. The ALF phenomenon was realized under two loading schemes, i.e., blocks model and a working block were acted upon jointly by the action of a vertical impact and a horizontal static force, as well as the joint action of both vertical and horizontal impacts with differently delayed time intervals. We obtained the rules on variation of horizontal displacements of working blocks when the ALF phenomenon was realized in two tests. The discrete time delay intervals, corresponding to local maxima and minima of the horizontal displacement amplitudes and residual horizontal displacements of the working block, satisfied canonical sequences multiplied by . Some of these time intervals satisfied the quantitative expression . At last, 1D dynamic theoretical model was established, the analytical results agreed better with the test data, while the quantitative expression drawn from test data was not validated well in theoretical analyses.