Permeability Of Sand-bentonite Mixtures Block With Joint With Rigid-wall Permeameter

Regarding the deep barriers to high-level radioactive waste disposal,the artificial barrier used is a mixture of solid sand and bentonite that are compacted with high pressure.Two on-site construction methods are in-situ compaction and block masonry.This artificial barrier plays an important role in a variety of ways,impeding the penetration of groundwater in the surrounding rock into the repository,and the migration of the nuclide in the repository,hindering the out-diffusion of high-level radioactive waste in the repository.In the high-pressure solid block masonry method;there is a construction joint between the blocks.The joints are weak hydraulic parts of the buffer.There is no definitive solution as to how to deal with the inevitable construction joints between the blocks.Through lab simulation tests,the overall permeability of sand-bentonite mixture joints under rigid wall conditions was studied.Appropriate joint materials are preferred to provide a certain reference for the treatment of construction joints.This paper tends to study the joint treatment form of sand and bentonite mixture compacted blocks.Through permeation test,heat conduction test,scanning electron microscope test,mercury injection test,etc.,the permeation performance of different types of joint treatment and thermal conductivity after saturation penetration were studied.Scanning electron microscopy?SEM?test and mercury injection test were used to analyze pore distribution and hydration after saturated permeability of specimens in different joint treatment forms from a microscopic point of view,analyzing the reasons for the differences in permeability and thermal conductivity.The main research contents are as follows:Using the Gaomiaozi bentonite and quartz sand as the experimental material,the round cake-like and semi-circular pie-shaped small blocks with a diameter of 10.20 cm,a height of 2.00 cm,a dry density of 1.80 g/cm³ and a water content of 12.2%were prepared.In contrast to the complete block sample,the other joints are bentonite powder joints,70%bentonite and bentonite particle mixture jointsand paste-like bentonite joints.The differences in permeation performance and heat conduction performance were analyzed by penetration test and heat conduction test.Research indicates:?1?In the early period of infiltration,the influence of joints on the overall infiltration condition is not obvious.The difference in infiltration rate is not very large,and it decreases with the increase of water content;in the middle of infiltration,the bottom of the sample is close to saturation,and the difference in indirect suture causes different infiltration rate.Infiltration rate from small to large:complete sample,powder sample,paste sample,particle sample;in the later period of infiltration,the difference of infiltration rate is more obvious,and the infiltration rate remains stable.?2?In the non-saturated infiltration stage,the time when the water discharge rate reaches the steady state is:complete sample and paste sample about 130h;powder sample about 630h,the effluent rate of the particle sample increased rapidly in the first 630h,and began to decline slowly after 630h.?3?The saturation permeability coefficient calculated from the data of the post-infiltration period is:complete sample is 1.76×10^-10cm/s,and powder sample is 2.05×10^-10cm/s,paste sample is 2.30×10^-10cm/s,particle sample is 3.78×10^-10cm/s.The request all meet the permeability of buffer backfill materials.?4?The test of the thermal conductivity of the sample after infiltration showed that the particle sample the thermal conductivity of the joint is significantly greater than the other two sets of joints large thermal conductivity,and closes to the thermal conductivity of the two sides of the joints of the joint.Scanning electron microscope test and mercury intrusion test were performed to observe the joint of the sample from a microscopic point of view.The test results showed:the pore structure in the joint area of the powder sample and the paste sample was not obvious,and the hydration healing effect was good of both.There were pores between the aggregates in the joint area of the particl sample and did not completely heal;from the pore size distribution curve,four groups of tests were found that there are inter-aggregate pores and inter-laminar pores in the aggregate.