Fan-structure shear rupture mechanism as a source of shear rupture rockbursts

It has been observed in field and laboratory conditions that failure of intact hard rocks at highly confined compression can be accompanied by abnormal violence. Under both of these conditions the failure process is associated with shear rupture development. David Ortlepp, who acquired more than 40 years of experience in the study of shear rupture rockbursts in deep and ultra-deep South African mines, emphasized this phenomenon (Ortlepp, 1997; Ortlepp et al., 2005): ‘All rockbursts, by definition, involve sudden and often violent displacement of rock. Occasionally however, larger incidents cause damage of such intense violence that it seems that our knowledge of the mechanism of damage is completely inadequate.’ Special field studies (Gay and Ortlepp, 1979; McGarr et al., 1979) have revealed that shear ruptures causing abnormally violent rockbursts are created in intact rock mass. An important f f eature is that they nucleate in zones o highly confined compression that are some distance away from excavation (on the excavation surface the minor stress is equal to zero). It was shown that these mine tremors and earthquakes share the apparent paradox of failure at low shear stresses, while laboratory measurements indicate high material strengths (McGarr et al., 1979). Recent laboratory studies of post-peak failure of hard rocks (characterized by uniaxial compressive strength above 250 MPa) at highly confined compression (σ1 > σ2 = σ3 when σ3 > 50 MPa) support Ortlepp’s idea about inadequate understanding of the failure mechanism at these loading conditions (Tarasov, 2008, 2010; Tarasov and Randolph, 2008, 2011). Some observed abnormalities that cannot be explained on the basis of conventional approach are presented in Figures 1 and 2. Figure 1 shows two sets of generic stressstrain curves for different levels of confining pressure σ3. Figure 1a represents the conventional (well-studied) rock behaviour associated with increasing post-peak ductility with rising σ3. For clarity, the variation of the post-peak curves is indicated by dotted lines. Figure 1b represents the unconventional type of rock behaviour. Here, increasing σ3 can lead to a contradictory variation of post-peak properties. In fact, rock behaviour can be changed from Class I to extreme Class II and then to Class I again. Class I is characterized by a negative post-peak modulus M = dσ/d∈, and Class II by positive (Wawersik and Fairhurst, 1970). At extreme Class II behaviour, values of postpeak modulus M and elastic modulus E = dσ/d∈ can be very close, indicating extremely small post-peak rupture energy (compare Fan-structure shear rupture mechanism as a source of shear rupture rockbursts