Laboratory results from hydro-mechanical tests on a water unsaturated bentonite

Abstract

Highly compacted bentonite has been proposed as the buffer material for embedding waste canisters in the Swedish concept for the disposal of nuclear waste. Although the bentonite is expected to become saturated in the repository, it is of importance to understand the process of saturation as it develops with increasing swelling pressure and decreasing suction. A laboratory programme has been carried out with the purpose to improve both the general understanding and the available models of the hydro-mechanical behaviour of unsaturated, swelling bentonite. The water saturation process is often modelled as driven by a gradient in suction (or negative pore water pressure). This study investigated the impact of factors such as void ratio, degree of saturation and external load on suction. The majority of the tests were designed to determine the increase in water content during free swelling or during constant volume conditions, where the swelling pressure derived by the prevented swelling has been measured. The water content was controlled by imposing a specified relative humidity for each test. Tests with constant water content and externally applied load were also performed. Relative humidity was measured in these tests. The clay used in the tests was MX-80, a commercial bentonite from American Colloid Co., with a liquid limit w L of about 500%. This paper illustrates the impact of confinement and influence of initial water content on the final swelling pressure and water content at an imposed relative humidity. Details of the testing techniques have also been given. In an associated paper a proposed hydro-mechanical model for the saturation process in bentonite, based on these results, has been presented [Dueck, A., Börgesson, L., 2007. Model suggested for an important part of the hydro-mechanical behaviour of a water unsaturated bentonite. Engineering Geology 92, 160–169].