Assessment of the creep behavior of siltstone for the Snowy 2.0 hydropower station using multistage uniaxial creep tests

Abstract

Multistage uniaxial creep tests with durations up to four months were performed to investigate the time-dependent behavior of the siltstone rock formations into which the Snowy 2.0 hydropower station will be excavated. Short term experimental tests are used to determine the stress levels of the creep tests and to assess the effect of creep on the short-term parameters of the siltstone. The samples have been collected from boreholes at a depth of about 650-850 meters beneath the surface where the power station complex will be constructed. The samples are extracted from the Ravine Beds unit formation and the main lithology is composed of siltstones with interlaminated to interbedded sandstones. The relative percentage of siltstone/sandstone within the rock mass ranges from 80/20% to 50/50% and on average is 70/30%. The siltstone features a medium strength matrix, a very stiff matrix, and a very low porosity. During the creep tests, the results show consistently negligible secondary creep deformations after the full development of the primary stage, and this finding was found to be valid regardless of the initial water content of the specimens. Additionally, a tertiary creep is observed on some of the tests under failure stress levels, where the tertiary creep of the siltstone is described with sudden brittle creep deformations. Ultimately, empirical and rheological creep models are used to interpret and fit the creep deformations. The results of the models are compared to creep deformations measured in the laboratory.