Frictionless shear at great depth and other paradoxes of hard rocks

Abstract

Shearing with very low friction is regarded as responsible for the high energy release from deep-seated earthquakes and rock bursts in deep underground mines, but in spite of considerable attention to the problem no consensus of opinion has been reached regarding the physical explanation for the low friction. Alternative hypotheses include melting, lubrication, excess pore pressure, velocity effects and vibration at the interface. Here, however, we demonstrate that under high confining stresses shear fractures developing in pristine hard rocks can exhibit dramatically low shear resistance over a certain displacement range, before the residual frictional strength is mobilised, due to the intrinsic nature of the fault structure. This feature is due to a special fault structure formed during the fault development. The discovered phenomenon can explain a number of anomalies observed in the field and laboratory conditions, but in particular the very high energy released from deep-seated earthquakes and rockbursts.

Analytical Determination of Energy Release in a Coal Mass

“…The coal burst source is the mechanism that triggers or induces the damage mechanism visible on the excavation surface. The coal burst source is generally associated with a seismic event that can be performed at a wide range of local magnitudes [1][2][3][4][5][6]. Indeed, mining-induced seismicity can reach moderate values of ground velocity and acceleration, and in some cases on the surface may lead to the generation of low-intensity earthquakes [7].…”

“…This sudden shear displacement can result in the generation of vibrations that cause coal ejections when they are situated near the excavation boundaries [5]. Tarasov and Randolph [6] devoted special attention to comprehensively elaborating the individual and inconsistent behaviours of hard rock at the significant depth, which are directly in line with the rock fracture condition in deep excavations. Tarasov and Randolph [6] broadly indicated that the shear failure procedures, under the significantly low frictional condition between the engaged surfaces, can be classified as the main reason for the release of energy.…”

“…Tarasov and Randolph [6] devoted special attention to comprehensively elaborating the individual and inconsistent behaviours of hard rock at the significant depth, which are directly in line with the rock fracture condition in deep excavations. Tarasov and Randolph [6] broadly indicated that the shear failure procedures, under the significantly low frictional condition between the engaged surfaces, can be classified as the main reason for the release of energy. Based on the suggested frictionless mechanism, the level of the brittleness of the confined rock/coal masses might be increased under high stress conditions.…”

Numerically and Analytically Forecasting the Coal Burst Using Energy Based Approach Methods

“…The coal burst source is the mechanism that triggers or induces the damage mechanism visible on the excavation surface. The coal burst source is generally associated with a seismic event that can be performed at a wide range of local magnitudes, normally ranging from undetectable up to 5 [6]. Indeed, mining-induced seismicity can reach moderate values of ground velocity and acceleration, and in some cases its effects on the surface can be compared with low-intensity earthquakes [7].…”

“…At the place of the source of the energy, where it is located in a plane of weakness inside the coal mass, the released energy provokes shear displacement along the plane, which in revolve generate vibrations that persuade coal ejections when they are situated near the excavation boundaries [7]. Tarasov and Randolph [6] have explained a number of special and inconsistent behaviours of hard rock at the significant depth that are directly related to rock failure mechanisms in deep excavations. They determined that the procedures of the shear failure, with respect to the significant low friction, can be classified as the main reason to release energy.…”