Earthquake induced rock shear through a deposition hole. Effect on the canister and the buffer

Existing fractures crossing a deposition hole may be activated and sheared by an earthquake. The effect of such a rock shear has been investigated in a project that includes both laboratory tests and finite element calculations. The buffer material in a deposition hole acts as a cushion between the canister and the rock, which reduces the effect of a rock shear substantially. Lower density of the buffer yields softer material and reduced effect on the canister. However, at the high density that is suggested for a repository the stiffness of the buffer is rather high. The stiffness is also a function of the rate of shear, which means that there may be a substantial damage on the canister at very high shear rates. In order to investigate the stiffness and shear strength of the buffer material a number of laboratory test series has been performed with shearing of water saturated bentonite samples at different densities and shear rates. From those tests a material model of the buffer that takes into account the density and shear rate has been formulated. Shear rates up to 6 m/s have been tested. The rock shear has been modelled with finite element calculations with the code ABAQUS. A three-dimensional finite element mesh of the buffer and the canister has been created and a number of calculations with simulation of a rock shear have been performed. The rock shear has been assumed to take place perpendicular to the canister axis in either the centre of the deposition hole or at the quarter point. The shear calculations have been driven to a total shear of 20 cm. Four buffer densities between 1950 and 2100 kg/m3 at water saturation and shear rates between 0.0001 and 1000 mm/s have been modelled. The influence of buffer density, shear plane location, shear rate and magnitude of the shear displacement are analysed and discussed. The results show that the influence of especially the density of the buffer and the location of the shear plane are very strong but also that the shear rate and the magnitude of the shear displacement have a significant effect. At the two lower densities an eccentric shear plane is more dangerous but at the two higher densities a centric shear is worst. At the conservative combination of a shear rate of 1 m/s, a shear displacement of 20 cm and the density 2100 kg/m3 the cast iron insert is strongly affected with maximum plastic strain of 19% but at the reference case with the buffer density 2000 kg/m3 and the shear displacement 10 cm the plastic strain is reduced to 1.6%