T.R. Stacey

A simple extension strain criterion for fracture of brittle rock

A very simple, empirical criterion for fracture initiation in brittle rock is described. The criterion is particularly applicable in areas of low confining stress. Such areas occur around underground excavations, and the criterion is therefore suitable for predicting the extent of fracturing around these excavations for support design purposes. Two examples of the application of the criterion are described. These are firstly to sidewall slabbing in a haulage and secondly to spalling from the face of a bored tunnel.

A Reconsideration of the Extension Strain Criterion for Fracture and Failure of Rock

The complex behaviours of rocks and rock masses have presented paradoxes to the rock engineer, including the fracturing of seemingly strong rock under low stress conditions, which often occurs near excavation boundaries. The extension strain criterion was presented as a fracture initiation criterion under these conditions (Stacey in Int J Rock Mech Min Sci 18:469–474, 1981). This criterion has been used successfully by some and criticised by others. In this paper, we review the literature on the extension strain criterion and present a case for the correct interpretation of the criterion and the conditions suitable for its use. We argue that the extension strain criterion can also be used to provide an indication of damage level under conditions of relatively low confining stress. We also present an augmentation of the criterion, the ultimate extension strain, which is applicable under extensional loading conditions when σ2 is similar in magnitude to σ1.