IN 1974 SCK/CEN LAUNCHED A R&D PROGRAMME CONCERNING THE POSSIBILITIES FOR DISPOSAL OF HIGH-LEVEL SOLIDIFIED AND ALPHA-BEARING RADIOACTIVE WASTES IN A CONTINENTAL STRATIFORM CLAY FORMATION (BOOM CLAY) SITUATED BELOW ITS OWN SITE. SEVERAL SPECIFIC INVESTIGATIONS STILL NEED TO BE FURTHER UNDERTAKEN IN ORDER TO CHARACTERISE MORE ACCURATELY THE ARGILLACEOUS FORMATION IN VIEW OF ASSESSING ITS APPROPRIATENESS FOR HOSTING RADIOACTIVE WASTE AS WELL FROM ENGINEERING POINT OF VIEW AS FOR LONG TERM SAFETY AND PERFORMANCE EVALUATIONS. IN SUPPORT OF THESE ALSO FURTHER MODELLING EFFORTS ARE REQUIRED IN ORDER TO IMPROVE AND CONFIRM OUR PREDICTION CAPABILITY.
THIS RESEARCH IS FOCUSSED UPON IN SITU INVESTIGATIONS RELATED TO THE (THERMO-)MECHANICAL BEHAVIOUR OF CLAY.
SEVERAL FIELD TESTS ARE DEVELOPED AND PERFORMED JOINTLY WITH ANDRA.
This research item focuses on in situ investigations related to the (thermo-) mechanical behaviour of clay. Three main items are covered:
stress measurements using hydraulic stress monitoring stations;
detection of microfractures in the clay host, mainly using geophysical seismic techniques;
long term mechanical behaviour of clay.
This report describes the investigations carried out and results recorded by SCK/CEN for the two first items.
The stress monitoring stations appear to be more reliable in measuring relative pressure variations with time rather than absolute values of stress, even after studying and improving the characteristics of the surrounding grout.
The seismic techniques used appear to be sensitive and accurate enough for detecting induced fracturation in the clay host, even for the low temperature gradients assumed; this is in agreement with laboratory measurements on clay samples intended to quantify the influence of both temperature and consolidation on the velocity.
BACCHUS (backfilling control experiment for high level wastes in underground storage) is a large scale experiment which considers a compacted clay based material around a heater implanted in the host clay in order to investigate the thermal behaviour of the Boom clay, as well as the thermal and hydraulic transfers through a highly compacted material. Beside the experiment itself and its original design, material characterization and instrumentation survey were important aspects in which considerable experience was gained. In this respect, specific sensors (thermal shock and time domain reflectometry probes) adapted to the particular experimental conditions were developed. In situ investigations ran from November 1988 (implementation in clay) to August 1990 (end of the 5 months heating phase).
1. STRESS MEASUREMENTS IN NON-FROZEN CLAY
2. FRACTURING OF CLAY.
3. LONG-TERM DILATOMETRIC TESTS.