Fracture Mechanisms and Whole-Process Early Warning for Perilous Rock Collapse

Classical fracture mechanics assumes a homogeneous medium, whereas a perilous rock body (a cliff block cut by dominant fissures, elsewhere called an unstable rock block) is a heterogeneous jointed rock mass; this contradiction, and the lack of an established route to rock mass scale fracture toughness, is the central obstacle to applying fracture mechanics to geological disasters. This paper discusses the fracture mechanisms of perilous rock collapse and their application to whole-process early warning. Models for the three types of perilous rock, sliding, toppling, and falling, are established and solved to give the mode I and mode II stress intensity factors, a mixed mode failure criterion, and a real-time stability coefficient Fs(t) = KIC/Ke. This paper proposes an approach for determining the fracture toughness of perilous rock using equivalent rock characterized by the rock mass rating (RMR) and the Hoek–Brown parameters; for the homogeneous model material, the route returns 1.88 MPa·m0.5 against a measured KIC of 2.11 MPa·m0.5, agreeing within 11%, and the characteristic length it yields bounds the rock mass quality for which the linear elastic criterion applies. In demonstration tests on four identical specimens, whose individual records deviate from their mean by 2.31–4.74%, the mean record traversed the four warning states in order; at the loading rate of the tests, the stable stage lasted 192 min against 2 min and 1 min for the quasi-stable and limited stability stages, and failure came with almost no visible prior deformation, so that in these tests, the fracture information remained observable through 98.5% of the loading history in which deformation gave no visible signal.