The presence of so-called extension fractures in brittle rock, in a compressive environment, is commonly observed in deep level mining and also in many laboratory applications. At present, these fractures are not considered in typical constitutive models for rock mass behavior. Laboratory tests on relatively large quartzite discs demonstrated that the formation of these extension fractures is preceded by a ubiquitous damage process. This damage mechanism and its effect on fracture formation is investigated.
La presence de soi-disant ruptures en extension des roches fragiles dans un environment compressif, est generalement observee dans les mines ultra profondes et aussi dans beaucoup d'applications en laboratoire. Jusqu'à present, ce type de rupture n'a jamais ete considere dans une quelconque loi constitutive regissant pour des roches massifs. Des essais en laboratoire des grands disques en quartzite ont demonstree que la formation de ces ruptures en tension est precedee par un processus de dommage ubiquiteux. Ce mechanisme de dommage ainsi que ses effets sur la formation de ces ruptures est analyse.
Die Anwesenheit von sogenennte Extensionsbruchen im zerbrechlichen Gesteinten unter Druckspannung ist allgemein wahrgenommen in tief liegenden Minen und auch in vielen Labor Experimenten. Gegenwartig sind diese Bruchen nicht in typischen Modellen fuer Fels Verhalten beriicksichtigt. Labor Experimente mit relative großen Quartz Scheiben demonstrieren das Material Schadigung die Formation von Bruchen vorausgeht. Dieser Schadigungs Mechanismus und seiner Wirkung auf Bruch Formation ist untersucht.
Extension fractures are defined here as fractures which are aligned with the direction of the major principal compressive stress. They must therefore have formed in response to excessive tensile stresses, as shear deformations are typically absent. Presently no adequate explanation appears to be available. for the formation of large scale extension fractures in an environment which is subjected to compressive stresses.
Sub-critical micro fracturing can be related to the micro structure of the rock mass. Local stress concentrations and differential properties can, lead to local tensile failure, even if global stresses are compressive, Such local tensile failure does however not automatically lead to the formation of large, scale extension fractures and associated global failure. Coalescence of micro fractures is often assumed to result in the formation of macro fractures. However, this is a process which is not well understood and certainly not well represented by common constitutive and numerical models. Many attempts have been made to represent this coalescence process (Fairhurst and Cook, 1966; Horrii and Neman-Nasser, 1986; Kemeny and Cook, 1987; Zheng et al., 1989; Shen, 1995), but these, have not resulted in an adequate model for practical applications.
Current constitutive models, representing the failure of a homogeneous rock mass in a compressive stress environment, are typically based on shear failure criteria. In principle it is possible to reproduce localized shear failure by the formation of shear bands or shear fractures. These features do, however, not appear to represent the observed extension fractures. They typically assume a substantial inclination with respect to the principal stress direction.
