Experimental study and numerical simulation of coupled thermal-hydro-mechanical behavior of a combined bentonite block-joint sample

Buffer material is the key component of the engineered barrier in a high-level radioactive waste (HLW) repository. To evaluate its retardation capability to block groundwater seepage and radionuclide out-migration, it is necessary to understand the performance of densely compacted bentonite blocks and joints combination under coupled thermal-hydro-mechanical (THM) condition. For this end, a combined bentonite block-joint sample (CBJ) is employed as a representative volume element of the engineered barrier and an apparatus is elaborately developed for testing its coupled THM behavior. The CBJ sample is prepared with two densely compacted blocks composed of the Gaomiaozi (GMZ) bentonite and one joint filled with bentonite-sand mixture. With the application of the apparatus, the coupled THM responses of the CBJ sample was tested with a heating temperature of 80 degrees C and an injected water pressure of 0.1 MPa for 266 days. Based on the test results, the characteristics of heat transport, water seepage, development of swelling pressure, and the squeezing deformation at block-joint interface within the CBJ sample were obtained, additionally, a new index called the self-sealing degree was defined for evaluating its self-sealing capability. For comparison, the governing equations for the unsaturated buffer material under coupled THM condition was derived and integrated into a finite element program. The laboratory test was numerically simulated and compared with the measured data. In general, the numerical simulations are in reasonable agreement with the test results. This study shows that the developed apparatus can be used in the indoor physical simulation test of buffer materials for the deep geological disposal of HLW, and that the understanding of responses of the CBJ to THM coupling action is helpful to the engineering design and optimization of buffer materials.