Fracture mechanisms and instability of openings in compression

When drilling circular openings (e.g., tunnels or boreholes) in brittle roc k, the in-situ stress conditions are often such that failure is initiated a t or near the wall of the opening. In this work, a mechanism of open hole i nstability is considered based on growth of pre-existing micro-fractures in the direction of greatest compression. The major factor enabling the pre-e xisting 3-D cracks to propagate extensively is the presence of the intermed iate principal compression near the opening wall (in the direction of the o pening axis). The unstable growth of wing cracks leads to separating thin r ock plates (flakes) from the bulk of the rock mass followed by their buckli ng, separation, and exposure of the fresh surface. Then this process of roc k surface spalling repeats itself eventually changing the shape of the hole . As the opening develops, its shape becomes elongated which, in turn, can affect this mechanism primarily through continuous changes in the stress co ncentrations around the opening. The sole cause of the unstable phase of cr ack propagation is the crack-boundary interaction. The opening develops if the unstable crack growth proceeds at least up to the buckling size. Otherw ise, the opening shape gets stabilized. This approach also allows for deter mining the final stable cross-section, as well as its relationship to the a pplied boundary stresses. The extent of failure is primarily determined by the initial parameters of micro-crack distribution. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.