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.
The paper discusses the variation of rock brittleness with confining pressure σ 3 for rocks of different hardness, failed under triaxial compression with σ 1 > σ 2 = σ 3 . Experimental results presented in the paper show that hard rocks unlike relatively soft rocks increase their brittleness with rising confining pressure σ 3 . The harder the rock the greater is the effect of embrittlement. Most hard rocks become hundreds of times more brittle compared with uniaxial compression approaching absolute brittleness. The shear rupture energy at these conditions becomes vanishingly small. Estimations made for Westerly granite show that the maximum brittleness corresponds to σ 3 ≈ 300 MPa.A special shear rupture mechanism is proposed to explain this phenomenon. In accordance with this mechanism the embrittlement results from reduction of friction within the rupture zone with rising confining pressure. The efficiency of this mechanism is a function of rock hardness and confining pressure. In most hard rocks failed within a certain range of confining pressure this mechanism can create transient negative shear resistance -referred to as 'negative friction' -which makes rocks superbrittle and failure abnormally violent. Estimations show that superbrittle rock behaviour takes place at confining pressures corresponding to the seismogenic zone of the earth crust. The new concept advances our knowledge about hard rock properties at great depths and our understanding of mechanisms governing the nucleation of deep seated dynamic events (earthquakes and shear rupture rockbursts).
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