Shear resistance of backfill components' interfaces in nuclear waste deposition tunnels

The purpose of the study was to investigate the shear strength behaviour of the interface of tunnel backfilling components in nuclear waste deposition tunnels. The studied disposal concept in this project is the KBS-3V, where the holes are drilled vertically into the bedrock from the deposition tunnel floor. The waste deposition tunnels will be excavated into an approximate depth of 420 m. The determined shear strength parameters will be used in the numerical modelling of the buffer/ backfill interactions. The analysis of the results can also be used in the reliability evaluation of the modelling results. In this case, the tested backfill materials were Friedland clay as block material, bentonite pellets, mixtures of granulated bentonite and crushed rock as foundation bed material and granite stones. Investigated granite stones were classified to three different types depending on their surface roughness. The followed test method was direct shear box test using large scale equipment. The side dimension of the square test box was 300 mm. A large testing program of 26 shear box tests was carried out in the laboratory of Soil Mechanics and Foundation Engineering. Seven different interfaces were tested: block / block, block / pellets, block / granulated bentonite, block / foundation bed material, smooth stone / pellets, intermediate roughness stone / pellets and rough stone / pellets. Calculations of some material properties (bulk density, dry density, water content, grain distribution) were also implemented. On the basis of the shear box tests made, it seems that the interface between granite stone and bentonite pellets have the highest value for friction angle. The interface between blocks got the lowest friction angle. The obtained values for the strength parameters in the interfaces block / granulated bentonite and block / foundation material are quite similar. Moreover, not important differences can be observed in the smooth and intermediate roughness granite stone