Bentonite homogenisation during the closure of void spaces

Harrington, J.F.; Daniels, K.A.; Wiseall, A.C.; Sellin, P.. 2020 Bentonite homogenisation during the closure of void spaces. International Journal of Rock Mechanics and Mining Sciences, 136, 104535.

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In a geological repository, the disposal of radioactive waste will result in the creation of engineering voids. Bentonite is commonly proposed as a sealing material as a result of its high swelling capacity. As the bentonite expands, the non-uniform development of porewater pressure and its coupling to total stress within the bentonite, may impair homogenisation. In this study we present results from five laboratory tests performed on sodium- and calcium-based bentonites to examine their swelling potential and capacity to homogenise over extreme bentonite-to-void ratios. Results demonstrate that even under these extreme ratios, the bentonite is able to swell and ultimately fill each void, creating a small swelling pressure. The swelling pressure development is spatially complex and time-consuming, and does not appear to be influenced by friction. Instead, it is characterised by plastic yielding of the clay with 70%–80% of the volume change associated with clay expansion adjacent to the void. This leads to heterogeneity illustrated by the presence of persistent differential stresses and the non-uniform distribution of moisture contents. Increases in the moisture content were measured but did not always correlate with the development of swelling pressure. This disequilibrium of the system is likely a reflection of the test durations and the slow evolution in the rates of change in swelling and porewater pressure beyond 130 days. Given the length of the experimental tests presented here, the time required to achieve full homogenisation of the clay is likely to be many years, if it occurs at all. Gravity segregation was also present in horizontal tests, further impairing clay homogenisation. However, as presented in this paper, it is possible to define functional relationships describing the bentonite swelling potential across engineering voids of differing size. This information will assist in establishing a safety case for bentonite usage in geological radioactive waste disposal.

Item Type: Publication - Article
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ISSN: 13651609
Date made live: 17 Nov 2020 10:04 +0 (UTC)

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