Experimental study on the creep behavior of buffer materials in the deposition hole of a repository

With the extra long term consideration for safety in a geological disposal repository, the creep behavior of the buffer material in a deposition hole can have an effect on the long-term stability of the engineered barrier. In this study, the hydro-mechanical properties and creep modeling parameters were determined experimentally on MX-80 bentonite. These include swelling pressure, water conductivity, and shear stress-strain behavior obtained from constant-stress direct shear test. The experimental works involved testing the bentonite in 3 different modes, namely, unsaturated, saturation under constant volume, and saturation after pre-swell. The pre-swelling process was designed to simulate an expansion of the buffer material due to mass losses of bentonite upon resaturation in a deposition hole.

          The experimental results show that in the constant volume mode and pre-swell mode, the swelling curve each has its own characteristics and, as the density of buffer decreases, the swelling pressure continues to decrease but in different trends. The shear strength obtained in the 3 modes is different while the values for the latter 2 modes are close. In the constant stress shear test, the higher the shear stress level, the faster the increase in shear strain rate. The shear strain rate is found to be higher in the pre-swell mode. Thus, for the long term safety of the deposition hole, the mass loss of buffer material should be limited to prevent degraded performance against creep deformation.

          Finally, numerical simulation was implemented to predict the creep deformation in a deposition hole for extended periods of time, in order to provide a reference for the design of the deposition hole and disposal tunnel. It is estimated that, at the junction of the buffer material and the backfill material, the vertical displacement increases continuously and reaches its maximum at 100,000 years.