The temperature effect on swelling of shales under cyclic wetting and drying

Abstract

Swelling rock such as shale can generate uplift force below foundations often resulting in structure deformation. The expansive upward force can be reduced, however, if the rock undergoes cycles of hydration and dehydration. An effort was attempted to measure the changes of swelling potentials (i.e. pressure and strain) of shales while the samples underwent two cycles of wetting-and-drying treatments at different temperatures between −10 and 23°C. Two sets of twenty-five shale samples collected from one coal mine in interior Alaska and two mines in southwestern Pennsylvania were tested under confined and free swelling conditions. The maximum swelling pressure and strain developed within the shales were related to three factors: shale moisture susceptibility, testing temperature, and initial tempering air humidity. As a result of the first cycle swelling tests, 76% of the samples failed (i.e. cracked) in the confined test and 56% of samples similarly failed in the free swelling tests. Samples that did not fail were brought back to their initial state of water saturation. The second cycle swelling tests were next performed on those samples. This attempt allowed measurements of a reduction of swelling pressure and strain of the shale samples. Swelling pressure and strain of the samples decreased considerably in the second cycle of tests. The reduction in swelling potential was likely caused by the growth of microcracks in the samples during the first cycle of swelling test. The swelling pressure and strain measured in the second cycle test were noted to have strong correlation with the swelling potentials measured in the first cycle.