Koosmen, K, Serati, M & Craig, B 2023, 'Multistage triaxial testing of intact rock: volumetric strain-based methods applied to rock slope design', in PM Dight (ed.), SSIM 2023: Third International Slope Stability in Mining Conference, Australian Centre for Geomechanics, Perth, pp. 343-358, https://doi.org/10.36487/ACG_repo/2335_20
(https://papers.acg.uwa.edu.au/p/2335_20_Koosmen/)
Abstract: Triaxial testing of intact rock is commonly undertaken to provide input parameters for rock slope design. Multistage triaxial tests offer economies in terms of time, sample quantities and costs, and for these reasons have found commonplace in engineering practice. Volumetric strain-based methods are sometimes employed for multistage testing to minimise damage accumulation in early test stages. This is done by (1) terminating early test stages at volumetric strain reversal, (2) straining the final stage until failure occurs, then (3) inferring a peak stress for the earlier stages based on a correction factor derived from the final stage.
This paper provides a review of multistage triaxial testing along with fundamental rock mechanics theories that underpin the volumetric strain-based multistage testing method. Results from single and multistage tests are then used to demonstrate possible errors that may result if multistage tests are conducted over a range of stresses where compressive failure of intact rock is controlled by different failure mechanisms. Finally, some examples and discussion are provided to demonstrate the possible impact that these errors may have on rock slope stability assessments.

Keywords: intact rock strength, triaxial testing, multistage methods, rock fracture mechanics