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Behaviour of compacted bentonite at elevated temperatures

Stratos, Panagiotis 2017. Behaviour of compacted bentonite at elevated temperatures. PhD Thesis, Cardiff University.
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Abstract

Compacted bentonites are proposed to be used as the barrier and backfilling materials in underground disposal of high level nuclear waste. The in situ boundary conditions in this case dictate exposure of compacted unsaturated bentonites to elevated temperature from the waste containers and hydration upon fluid uptake from the surrounding host rock. In the past, small- and medium-scale laboratory tests and in situ tests in prototype repositories have been considered by several research organizations. Most of the earlier studies have focussed in studying the thermo-hydraulic-mechanical-chemical behaviour of compacted bentonites when subjected to temperatures below 100 °C. In this thesis laboratory test results of several non-isothermal and non-isothermal hydraulic tests on compacted MX80 bentonite are presented. Compacted bentonite specimens were subjected to temperatures of either 85 or 150 °C at one end, whereas the temperature at the opposite end was maintained at 25 °C. A hydraulic pressure of 600 kPa was applied during the non-isothermal hydraulic tests. The temperature and the relative humidity were monitored along predetermined depths of the specimens. The axial stress was monitored at the opposite end of the high temperature source. The influences of thermal and thermo-hydraulic gradients on the water content, volume-mass properties, suction, chemical composition and mineralogy were studied in detail. The test results showed that thermal gradients caused redistribution of water content (soil suction), whereas thermo-hydraulic gradients caused both redistribution and an increase in the water content within compacted bentonites, leading to development of axial stresses at the opposite end of the heat source. An increase in the water content and degree of saturation towards the opposite end of heat source indicated development of a low permeability zone at the interface between the restraint and unsaturated bentonites. Under the applied thermal and thermo-hydraulic gradients, the dry density increased near the high temperature source end, whereas it decreased at the opposite end. A variation in the dry density is attributed to the combined influence of shrinkage of bentonite at and near the heat source, relaxation of confinement at the opposite end of heat source and macro and micro structural changes within compacted bentonites. Mineralogical transformation (montmorillonite to illite) did not occur under the applications of both thermal and thermo-hydraulic gradients. A variation of the water content however caused shifting of c-axis spacing indicating hydration and dehydration of elementary layers of montmorillonite.

Item Type: Thesis (PhD)
Date Type: Submission
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: thermo-hydro-mechanical tests; compacted bentonite; thermo-hydraulic tests; thermo-hydro-mechanical-chemical behaviour of compacted bentonites.
Date of First Compliant Deposit: 21 May 2019
Last Modified: 21 May 2019 09:09
URI: http://orca-mwe.cf.ac.uk/id/eprint/122541

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