A Criterion for Brittle Failure of Rocks Using the Theory of Critical Distances

This paper presents a new analytical criterion for brittle failure of rocks and heavily overconsolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as the critical distance. This fracture criterion is known as the Point Method, and is part of the Theory of Critical Distances, which is utilized in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, ó0, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, óc and ót. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (óc/ót=3-50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low confining stresses.The work presented was initiated during a research project on “Structural integrity assessments of notch-type defects", for the Spanish Ministry of Science and Innovation (Ref.: MAT2010-15721)

Stability Analysis of a Rock Column in Seismic Conditions

Abstract The stability of a rock column located on a high conglomerate cliff in Roverino, near Ventimiglia, in northwestern Italy, has been analysed by continuum and discontinuum modelling, in both static and seismic conditions. The rock column, which is 40 m high and nearly 36,000 m3 in volume, exposes the residential area below the cliff, housing more than 4,000 people, to a high risk level. As the area is located in a seismic region with estimated peak acceleration between 0.24 and 0.28 g, the stability analyses were carried out in both static and seismic conditions. Continuum and discontinuum modelling of the rock mass was carried out by using the finite difference methods and the distinct element methods. It is shown that both methods are effective in describing the modes of instability of the rock column in static conditions. On the contrary, a convincing description of the rock column response induced by earthquake excitation is achieved bydiscontinuum modelling