  Clays and claystones are considered in some countries (including Belgium,
France and Switzerland) as a potential host rock for high activity long lived
radioactive waste disposal at great depth. One of the aspects to deal with in
performance assessment is related to the effects on the host rock of the
temperature elevation due to the placement of exothermic wastes. The potential
effects of the thermal impact on the excavated damaged zone in the close field
are another important issue that was the goal of the TIMODAZ European research
project. In this paper, some principles of waste disposal in clayey host rocks
at great depth are first presented and a series of experimental investigations
carried out on specific equipment specially developed to face the problem are
presented. Both drained and undrained tests have been developed to investigate
the drained thermal volume changes of clays and claystone and the thermal
pressurization occurring around the galleries. This importance of proper
initial saturation (under in-situ stresses) and of satisfactory drainage
conditions (in spite of the significantly low permeability of claystones) is
emphasized, leading to the development of a new hollow cylinder apparatus. It
is observed that claystones cannot be considered as overconsolidated clays
given that they can exhibit, as the Callovo-Oxfordian claystone does, a
thermoplastic contraction. Mechanical and thermal hardening are however
observed, extending to claystones the knowledge already gained on clays. A new
method of determining in the laboratory the thermal pressurization coefficient
is described and the data obtained allow completing existing data in the field.
Finally, the hollow cylinder apparatus makes it possible to demonstrate that
the good self-sealing properties of clays and claystones can be extended to
temperature effects, an important conclusion in terms of performance
assessment.
