Progressive failure of hanging wall and footwall Kiirunavaara Mine, Sweden

Abstract

This paper investigates the mechanism of crack coalescence and peak strength of rock-like materials containing parallel frictional cracks under uniaxial compression loading. The specimens used in this study are the sandstone-like material proposed by Wong, Chau 1997 and are containing different number of parallel frictional cracks ranging from two to forty-two parallel cracks. Various values of inclination of preexisting cracks α, bridge angle β (inclination between the inner tips of the two preexisting cracks) and the frictional coefficient μ are used for parametric studies. Two “rules of failure” for the speciments containing three cracks are proposed, and it is being tested with our results. In particular, we found that only crack coalescence between two cracks will occurs (not three) and the pair of cracks with a lower value of peak strength (as predicted by the results of Wong, Chau 1997) will dominate the process of coalescence; and that the mixed and tensile modes of coalescence are the preferential modes and the preferred orientation for crack coalescence is 75°. In contrast to the observation by Horri, Nemat-Nasser 1985 for specimens made of resin, shear zone can also be formed in our specimens containing 18 and 42 cracks under uniaxial compression. In addition, we found that the peak strength of the specimens does not depend on the initial crack density but depends on the actual number of cracks involved in the coalescence. This result provides a plausible explanation for our previous observation (Wong et al. 1996) that the peak strength in Yuen Long marbles does not decrease with the increase of the initial crack density as a threshold value is exceeded.