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Development of a poly-axial platen for testing true-triaxial behavior of rocks

Garg, P and Pandit, B and Mishra, B and Babu, GLS (2021) Development of a poly-axial platen for testing true-triaxial behavior of rocks. In: Canadian Geotechnical Journal, 58 (12). pp. 1839-1854.

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Official URL: https://doi.org/10.1139/cgj-2020-0247


Mining at greater depths can lead to stress-induced failure, especially in areas of high horizontal in situ stress. The induced stresses around the opening are known to be in a poly-axial stress state where, s1 = s2 = s3 with special case of s3 = 0 and s1, s2 = 0 at its boundary, where s1, s2, s3 are major, intermediate, minor principal stress, respectively. The conventional triaxial testing does not represent the actual in situ strength of the rock in regions of high horizontal stress, as it ignores the influence of intermediate principal stress (s2 ). The typical poly-axial testing (biaxial and true-triaxial tests) of intact rock mostly requires sophisticated and expensive loading systems. This study investigated the mechanical behavior of intact rock under a poly-axial stress state using a simple and cost-effective design. The apparatus consists of a biaxial frame and a confining device. The biaxial frame has two platens that apply equal stress in both directions (s1 = s2 ) on a 50.8 mm cubical specimen when placed inside the uniaxial loading device. The confining device performed separate biaxial tests under constant intermediate principal stress (s2 = constant) and true-triaxial tests when used along with the biaxial frame. This study then compared the failure modes and peak strength of Berea sandstone specimens with other biaxial� triaxial devices to validate the design of the poly-axial apparatus. We also performed uniaxial tests on both standard cylindrical samples and prismatic specimens of different slenderness ratios. These tests provided a complete understanding of the failure mode transition from standard uniaxial compressive tests to triaxial stress conditions on cubical specimens. Additionally, this study determined best-fitted strength envelopes for biaxial and triaxial stress state. Based on regression analysis, we found a quadratic polynomial to be a good fit to biaxial strength envelope. For the true-triaxial strength envelope, we found the three-dimensional (3D) failure criterion to be a good fit with R2 of 0.964. © 2021 The Author(s).

Item Type: Journal Article
Publication: Canadian Geotechnical Journal
Publisher: Canadian Science Publishing
Additional Information: The copyright for this article belongs to Authour
Keywords: Cost effectiveness; Failure (mechanical); Failure modes; Regression analysis; Sandstone; Stress analysis, Axial stress; Berea sandstone; Biaxial frame; Biaxial platen; Intermediate principal stress; Poly-axial testing; Strength envelope; Stress paths; Stress state; True triaxial tests, Testing, failure mechanism; in situ stress; induced response; rock mechanics; sandstone; testing method; triaxial test
Department/Centre: Division of Mechanical Sciences > Civil Engineering
Date Deposited: 04 Jan 2022 05:37
Last Modified: 04 Jan 2022 05:37
URI: http://eprints.iisc.ac.in/id/eprint/70857

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