Date of Award
Master of Science (MS)
Failure envelopes used to characterize the shear strength of geological materials are generally obtained through conventional triaxial testing of multiple specimens and most of the specimens are heterogonous and expensive to obtain. Therefore, the multistage triaxial test was introduced to make use of a single specimen for material strength characterization, compared to the conventional triaxial test that requires at least three specimens. In the multistage triaxial test, the challenge to obtaining meaningful data is identifying when to terminate each loading stage. The goal is to be as close to the peak strength of the material but below the stress threshold that causes damage (referred to as the yield point), and consequently underestimation of the strength during the subsequent stages. In brittle rock, failure can occur suddenly, making it difficult to terminate each stage before yield and failure. In a document adopted by the the International Society of Rock Mechanics (ISRM), Kovari et al. (1983) suggest that axial load should be increased until the axial stress-strain curve shows a horizontal tangent, accepting that some damage will be done. More recently, alternatives have been suggested that terminate each stage of the multistage test based on volumetric strain measurements: the point of dilation (where the specimen volume has become larger than its original volume, due to the formation of microcracks) or the point of inflection (where the specimen stops contracting and starts expanding (due to the initiation of microcracks).
In this study, stress-strain curves available from 72 conventional triaxial tests on Utah coal from a previous study sponsored by the NIOSH Spokane Mining Research Division were used to identify the stresses associated with the volumetric dilation and inflection points, to investigate whether either of these would provide an adequate failure termination point for multistage tests performed on this material. In many of the tests, the volumetric dilation stress was higher than the yield stress, so the inflection point was identified as a better choice for the failure termination point. Three successful multistage tests were performed on specimens of the Utah coal using volumetric inflection as the termination point. The shear strength parameters obtained from these multistage triaxial tests are in very good agreement with those obtained from the conventional triaxial tests (average friction angle within 1°, average cohesion within 6%).
Orilogi, Emmanuel, "Evaluating Volumetric Strain as a Predictor of Yield and Peak Strength for the Multistage Triaxial Test: A Case Study with Utah Coal Specimens" (2019). Graduate Theses & Non-Theses. 216.