Time-dependent compressive failure around an opening

• Published in: International journal of rock mechanics and mining sciences (Oxford, England : 1997) Vol. 34; no. 3-4; pp. 310.e1 - 310.e15
• Main Author: Tharp, T.M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-04-1997
• Subjects: Anisotropy; Brittle Failure; Compressive Strength; Constitutive Relations; Fracture Mechanics; Mechanical Properties; Numerical Analysis; Time Dependency; Compressive Strength; Constitutive Relations; Time Dependency; Brittle Failure; Anisotropy; Fracture Mechanics; Numerical Analysis; Mechanical Properties
• ISSN: 1365-1609; 1873-4545
• DOI: 10.1016/S1365-1609(97)00064-6

Compressive failure of rock at a free face is a time-dependent process that proceeds outward from the free face. The model for general compressive failure is the sliding crack model of Kemeny, Cook 1987, including crack interaction and subcritical propagation of wing cracks. For a rock mass with an array of propagating cracks near a free surface, moduli are different in all three orthogonal directions. Elastic constants are derived for the resulting orthorhombic symmetry. This model of identical, equally spaced interacting cracks maximizes the reduction in stiffness with crack propagation. The resulting stress reduction around an opening prevents failure even for high levels of stress. It fails to predict the commonly observed slabbing or exfoliation failures. For σ3/σ1 small, failure is typically more localized than assumed in the above model. This situation is modeled as propagation of a single long crack with periodic out-of-plane jogs with frictional sliding. Such a crack can propagate in the σ1 direction at nearly constant stress. Proximity of a free face increases KI, however, for long cracks the effect of the free face is small. A small compressive σ3 greatly reduces KI, and with the σ3 gradient found near an opening, subcritical propagation velocity will drop precipitously away from the opening. This is consistent with failure by progressive exfoliation of thin sheets.