Strength of Utah Coal Evaluated Using Laboratory Tests with an Unloading Path

Abstract

ABSTRACT: Conventional triaxial tests evaluate rock strength using a load path where the minimum principal stress is held constant and the maximum principal stress is increased monotonically until the peak strength is achieved. Although brittle rock mechanical behavior is relatively well-understood under this loading path, in many practical rock mechanics applications, damage and failure develop as a result of loss of confinement (decreasing minimum principal stress). Since the inelastic behavior associated with damage that develops prior to rock failure is not path-independent, it is possible that rock damage and strength characteristics may differ depending on the loading path considered. To investigate this potential path-dependence, a series of tests were performed on Utah Coal where, following attainment of a target confining stress and deviatoric stress, the confining stress was decreased at a constant rate until failure while maintaining the deviatoric stress. These results were then compared to previously published data for Utah Coal obtained using conventional laboratory testing procedures. The previously published data for Utah Coal showed highly confinement-dependent strength behavior, where the peak strength increased by as much as up to 50% of the mean unconfined compressive strength (UCS) with confining stresses as low as 1% of the UCS applied. The peak strength and crack damage (CD) envelopes were found to be highly anisotropic, but the crack initiation (CI) envelope was relatively insensitive to the angle between the loading direction and the primary cleat orientation. In this study, although the CI stress, CD stress, and peak strength envelopes are all generally similar for both the loading and unloading tests, some of the test results for samples with cleating inclined at 30° to the loading direction suggest the CD stress envelope may be slightly lower under unloading conditions than under loading conditions.