Permeability Of Block Joint Between Compacted Bentonite-sand Mixture As HLW Buffer/Backfill Material

At present, several foreign countries have commenced the full-scale compaction test for geological disposal of high level radioactive waste. Generally, installation of clay blocks with sector shape at waste pollution area is a feasible engineering method. The block joints, as hydraulic defect, influences the impermeability performance and weakens the strength of geological disposal barrier. The self-healing and anti-seepage performance of block joint has become a new concern in engineering design.In this paper, GMZ bentonite and sand are mixed and compacted integrity round cake and the split round cakes with joint, having a dry density of 1.7g/cm3 and a sand ratio of 30%in static compaction method. The block joint is installed with a width of 2mm, and back filled with different materials such as bentonite powder, bentonite in plastic limit, and the mixture of bentonite grain with grain size of 0.25-0.5mm and dry density of 1.8g/cm3 and bentonite in powder as the percentage of 30%.50%,70% and 90%. Compared with integrity sample, the hydraulic conductivity, termal conduction property and microstructure of samples with different joint treatment types are examined using a flexible wall permeameter to study the self-healing and anti-seepage performance of block joint.The research shows that, with respect to integrity sample, the hydraulic conductivity of samples with different joint treatment types varies between K=2.51-4.94×10-10 cm/s, and, the permeability of the type of grain ratio with 70% is lower, because its grains arrange tightly and the pore distributes densely after healing. The average thermal conductivities of different joint treatment types are measured at λ,=1.43-1.63W/(m·k). The thermal conduction property of the type of grain ratio with 70% is better. Except bentonite in powder and in plastic limit, there is little change in the variance of thermal conductivities of other joint treatment types, and the values are lesser. Thus, the thermal conduction properties of these types are regarded as stability. In conclusion, the type of grain ratio with 70% is selected as the best joint treatment type.The research on self-healing mechanism of block joint shows that, when bentonite material is mixed with water to swell, there is thin film encapsulating the surface of sand grains to fill in the pore between sand gradually. On the other hand, bentonite material in joint and bentonite block are mixed with water to swell. The big pore between bentonite grains are filled to become many small pores gradually. Under lager confining pressure, the structure between joint and bentonite block are recombinated. Intrinsical structural plane of joint is disappeared, and the density of joint is closed to the nearby block gradually, namely the joint is healed.