Thermo-Hydro-Mechanical properties of MX-80. Results from advanced laboratory tests

General Highly compacted bentonite is proposed as the buffer material in the Swedish concept for disposal of nuclear waste. The saturated homogenized bentonite is expected to fully act as a buffer material between the waste canister and the surrounding bedrock. Material models describing the thermo-hydro-mechanical (THM) behaviour of the buffer material have been created with the purpose to simulate and predict the behaviour in a repository both before and after water saturation. The material models of water saturated and water unsaturated buffer material are complicated and contain a number of parameters that need to be determined. The present report is a compilation of results concerning thermo-hydro-mechanical laboratory tests on saturated and unsaturated buffer material. The main purpose of the report is to supply modelling groups with available results for improving models and determine parameters that can be used for the THM modelling of the behaviour of the buffer.Retention curves The relation between water content and relative humidity has been determined in a number of test series for some specific conditions, e.g. different initial water contents. Two methods have been used; the sorption balance method and a method with jars as desiccators. The majority of the results were derived from tests where RH was controlled and the response of the bentonite samples was measured. The results are given as water content versus relative humidity in diagrams and in tabular form. Volume change The volume change of water unsaturated bentonite specimens has been investigated by compression tests and swelling/shrinkage tests for some specific stress and moisture paths. The constant relative humidity was generated by the vapour equilibrium technique combined with an air circulation system. Measured stresses, deformation and relative humidity are presented versus time in diagrams and the final values at different stages are also presented in tabular form. Moisture transport The moisture transport in bentonite specimens has been investigated in tests where specimens with different initial states were exposed to different gradients of RH and where the corresponding gradients of water content, at transient conditions, were determined. The constant relative humidity was generated by the vapour equilibrium technique combined with an air circulation system. The results are presented, in diagrams and in tabular form, as gradients of relative humidity and corresponding gradients of water content. The axial stresses are also presented. Moisture redistribution Redistribution of moisture was investigated by studying unsaturated specimens exposed to temperature gradients under constant volume conditions and without supply of additional water. The results are given in diagrams as gradients in water content, density, void ratio and degree of saturation. The gradients are given both as absolute values and normalized values. For modelling purpose the resulting gradients in water content and density are also given as changes from the initial state. Triaxial tests Two triaxial tests were run on water saturated bentonite specimens. Since the influence of stress history was studied, the results in terms of effective average stress and deviator stress are compared to results from previous published tests.