A review of recent research in rock creep is presented. Two ranges of creep behavior are defined; pre-failure creep and post-failure creep. In the post-failure range, creep deformations will occur under decreasing applied stresses. The "unloading line" of the surrounding rock and the "failure line" of the creeping rock will place bounds upon how creep deformations will occur as the stress drops. Changes in regional stresses as brought about by mining, will serve to reposition the reference point for creep, and will lead to either sudden creep rate increases or decreases. The approach taken aids in explaining creep data taken from the Burgin Mine near Eureka, Utah.
In the past there has not been a complete understanding of how creep can affect rock behavior underground. This becomes especially obvious when we attempt to analyze extensometer and convergence gage measurements taken in active mines. A correlation between mining activity and these measurements taken over time always exists, but the full understanding of how they are influenced by mining is always a problem. The purpose of this paper is to review our present knowledge of creep, specially post-failure creep, and to present some ideas about how to inter interpret strain measurements.
A full scale, unconfined compression test for a rock sample serves as a basis for classifying the creep behavior of a particular rock. In Fig. 1 is shown a test of this type performed at a constant rate of strain¹. The two familiar regions of behavior are evident, the pre-failure region and the post failure region. In the latter, an increment of stress increase is not possible without sample failure, and in fact to preserve stability every increase in strain requires a decrease in stress.