Environmental degradation of claystones

Abstract

The paper presents the results of a comprehensive experimental programme carried out to study the effects of relative humidity cycling on the degradation of argillaceous rocks. Lilla claystone, a lowporosity Tertiary rock, was used for this purpose. Four aspects were analysed: (a) the influence of the number of relative humidity cycles; (b) the amplitude of relative humidity cycles; (c) the stress level; and (d) the effects of using liquid water or vapour during wetting paths. The application of relative humidity cycles induced a progressive degradation of the rock in terms of accumulative irreversible volumetric swelling, irreversible reduction in rock stiffness, and tensile strength. The irreversible expansion increased with the amplitude of the relative humidity change. However, it reduced with increase of the confining pressure. This irreversible behaviour accelerated when liquid water was used during the wetting paths. Microstructural analysis has shown that the degradation pattern of Lilla claystone was associated mainly with fissuring, as a consequence of non-uniform deformations of the clayey matrix. This phenomenon leads to the opening of fissures at the weaker interfaces of the clayey matrix with detrital, non-active minerals. A damage law derived in terms of the accumulated volumetric irreversible strain has been proposed to represent the progressive loss in volumetric and shear stiffness as well as the tensile strength.

The response of Opalinus Clay when exposed to cyclic relative humidity variations

“…The applied RH cycles caused an irreversible increase in the specimens' volumes as swelling always exceeded the amount of shrinkage. Pineda et al (2014) found that higher peak-to-peak amplitudes in RH cycles (cycles between 20 and 99 % were compared to cycles between 50 and 99 %) led to larger volumetric swelling. This effect was less pronounced for specimens that were tested under higher confinements.…”

“…Few studies exist in which the influence of cycles in RH on the drying-swelling characteristics of clay shales has been investigated (e.g. Grice, 1968;Van Eeckhout, 1976;Olivier, 1979;Pham et al, 2007;Farulla et al, 2010;Cardoso et al, 2011;Yang et al, 2012;Pineda et al, 2014). Grice (1968) noted that specimens of Utica shale that were immersed in water disintegrated completely after oven drying.…”

“…a net shrinkage) have been measured for RH cycles at 8.5 MPa axial load. Pineda et al (2014) experimentally investigated the influence of RH cycles on the degradation of Lilla claystone in a long-term RH cycling experiment using ultrasonic wave velocity measurements and Brazilian tensile strength tests. The applied RH cycles caused an irreversible increase in the specimens' volumes as swelling always exceeded the amount of shrinkage.…”

Slope stability in slightly fissured claystones and marls

“…The preceding discussion indicates that two main effects control weathering: the atmospheric cycles (evaporation-rainfall induced wetting) and the confinement stress. An investigation of the damage induced by cycles of suction change at different isotropic confining pressures was reported by Pineda et al (2013). Core samples of Lilla claystone, a similar low plasticity dense claystone to Castrovido, were subjected to stress-suction cycles paths involving different confining stresses and a varying intensity of suction change (Pineda et al, 2013).…”

“…An investigation of the damage induced by cycles of suction change at different isotropic confining pressures was reported by Pineda et al (2013). Core samples of Lilla claystone, a similar low plasticity dense claystone to Castrovido, were subjected to stress-suction cycles paths involving different confining stresses and a varying intensity of suction change (Pineda et al, 2013). Sample stiffness was measured during testing by P and S-wave transmission.…”