Saturation development in the bentonite barrier of the Mock-Up-CZ geotechnical experiment

Abstract

This paper reports on both the methodology and the results of the measurement of saturation development during the operation of the Mock-Up-CZ experiment. This experiment simulates the engineered barrier system hydration process of the Swedish KBS-3V vertical disposal system. The Mock-Up-CZ model consists of a barrier of bentonite blocks and a heater which simulates the radioactive waste. The potential inflow of granitic water from the natural barrier is simulated by employing a flooding system installed on the inner side of the experimental tank. The whole experiment is enclosed in a cylindrical pressure tank the construction of which allowed for the high pressure caused by the considerable swelling of the bentonite. Six measurement profiles along the vertical profile of the tank allow the continuous measurement of the various on-going processes. A number of sensors were placed inside the bentonite barrier (40 thermometers, 50 hydraulic pressure cells and 37 humidity sensors). The experiment commenced on 7th May 2002. In the first phase, the bentonite barrier was loaded only with the heat created by the heater for a period of six months. During the second phase, which commenced on 4th November 2002, in addition to the continuing thermal activity, the barrier was saturated with synthetic granitic water. This phase will most likely be concluded at the end of 2005. It has been found that of all the geotechnical measurements currently being performed, the measurement of changes in water content is the most challenging. The results obtained from employing any of the usually accepted methods for the measurement of indirect water content are often somewhat unreliable. Despite the problematic nature of these methods, 37 humidity sensors were installed in the barrier. Moreover, the construction of the experimental tank provided for sampling locations with access points allowing core sampling by means of drilling. These samples allow the determination of the water content profile of the barrier using a simple gravimetric method. The advantage of this method is that it provides an accurate and detailed description of changes in the barrier’s water content in a cost-efficient way. After core drilling, the openings thus created are immediately re-sealed with bentonite plugs thereby allowing for the repeated exploitation of sampling points. By combining the results of indirect water content measurement using sensors and the results obtained from the laboratory water content analysis of core samples, accurate information on barrier saturation has been provided on a continuous basis since the launch of the experiment. Approximately 90% saturation (in the vicinity of the measurement points) had been reached by October 2003, the saturation process measurement results corresponding to developments in swelling pressure. Surprisingly, considerable differences were detected in swelling pressure values. It is expected that an explanation for this problem will be found during the eventual dismantling of the experiment. The fact that identical water content values have been recorded (October 2004) independent of distance from the heater would seem to prove that full saturation is approaching. Interestingly, temperature has not been found to affect the degree of saturation.