Rock Failure In Compression Research Articles

Review of unloading tests of dynamic rock failure in compression

Dynamic rock failure at the walls of an opening, so-called rock burst or strain burst is one of the major types of rock failure capable of causing human and financial loss. Initially the rock at the place of the future excavation is in a polyaxial stress state. When excavating an opening, the rock elements at the excavation boundary are overloaded in the tangential (with respect to excavation wall) direction and unloaded in the radial direction. This situation can be reproduced by the modified true triaxial test in which only one surface of prismatic rock sample is unloaded. This paper reviews the tests under true triaxial unloading condition and the corresponding failure modes and types (static or dynamic). In these tests the violent (dynamic) ejection of rock fragments and arc-shape fractures near the free face are often observed. However these tests are affected by friction between the sample and the loading platens. We analyse the end friction effect and show that unlike the conventional end friction effect produced in uniaxial and biaxial loading tests, the true triaxial unloading test induces additional shear stress needed to prevent sliding of the rock sample from the loading platens. This additional shear stress is shown to considerably affect the geometry of fractures.

Rock failure in compression : Nemat-Nasser, S; Horii, H Int J Engng SciV22, N8–N10, 1984, P999–1011 (paper to the proceedings on media with microstructure and wave propagation, Houghton, Michigan, 24–25 Jan 1983)

Rock failure in compression : Nemat-Nasser, S; Horii, H Int J Engng SciV22, N8–N10, 1984, P999–1011 (paper to the proceedings on media with microstructure and wave propagation, Houghton, Michigan, 24–25 Jan 1983)

Rock failure in compression

Mechanisms of rock failure in compression — axial splitting and shear failure — are studied, based on a microscopic consideration. Analytical models are constructed and model experiments on plates of a brittle polymer are performed. It is shown that unstable growth of tension cracks which propagate from the tips of pre-existing cracks and curve towards the maximum compressive direction, is the fundamental mechanism that produces axial splitting of a uniaxially compressed rock specimen, whereas shear failure of a triaxially compressed specimen is a result of sudden growth of tension cracks at tips of a suitably arranged interacting set of microcracks. The simultaneous out-of-plane unstable growth of a suitably oriented row of cracks is analyzed and, on the basis of this model, the variations of the “ultimate strength” and the orientation of the overall fault plane with the confining pressure are estimated. The brittle-ductile transition is discussed with the aid of a model which includes both tension crack extension and plastic zone development from the pre-existing cracks.

Post-failure behavior of a granite and diabase

Post-Failure Behavior of a Granite and Diabase Compression experiments have been performed on Westerly granite and Frederick diabase at a mean strain rate of 0 (10−5) sec.−1. In such tests Westerly granite and Frederick diabase exhibit Class II failure behavior between zero and 22000 psi and between 500 psi and 2200 psi, confining pressure respectively. In order to control brittle failure in these rocks, therefore, energy must be extracted from the samples. Complete stress-strain curves for Westerly granite can be divided into a number of characteristic regions. Each of these regions can be described in terms of distinct fracture patterns. In Westerly granite in uniaxial compression and at less than 2 900 psi confining pressure, the fracture patterns consist of local cracks predominantly parallel to the direction of greatest compression, slabs and narrow intensely crushed shear zones. At greater 2 900 psi confining pressure the fracture patterns consist of local cracks predominantly inclined to the direction of greatest compression and of macroscopic shear fractures. At high confining pressure, local cracks tend to propagate in their initial plane. The fracture patterns in Frederick diabase are sensibly the same as those in Westerly granite at 22000 psi confining pressure. The ultimate strength of Westerly granite above 22000 psi confining pressure and in Frederick diabase is controlled by faulting. At lower confining pressure faulting in granite occurs only in the post-failure region beyond the peak of the stress-strain curves. Preliminary experiments suggest that creep in Westerly granite under high and constant stress produces the same fracture patterns that occur in the post-failure region in continuous-deformation tests. In uniaxial compression creep terminates in macroscopic fracture near the intersection of the constant stress-strain path of a creep sample with the post-failure branch of the stress-strain curve of a quasi-statically loaded specimen. A comparison of all fracture patterns suggests that the fracture mechanisms in rock depend upon confining pressure, mineral composition and grain size of the rocks tested. It is unlikely, therefore, that rock failure in compression can in general be described by a single failure criterion.