Investigation on successive gas breakthroughs behavior of saturated GMZ bentonite under rigid boundary conditions

Understanding impacts of gas breakthrough processes on the sealing ability of the bentonite buffer/backfill materials is crucial for the safety evaluation of the nuclear waste geological repository. In this work, the residual capillary pressure method was utilized to conduct successive gas breakthrough tests on compacted Gaomiaozi bentonite specimens. During each gas breakthrough test, the upstream gas pressure was increased in a step-by-step way until the gas breakthrough was recorded. Water permeability tests were performed before each gas breakthrough to ensure that the bentonite specimen was fully saturated. Finally, after experienced five successive gas breakthrough processes, the bentonite specimen was submitted for the mercury intrusion porosimetry (MIP) test. For comparison, four other parallel specimens that experienced one to four gas breakthroughs, respectively, were also submitted for the MIP tests to examine the possible damage of the bentonite matrix resulted from gas breakthrough processes. Results show that, during the first three successive gas breakthrough tests, the gas breakthrough pressure and the snap-off pressure decreased rapidly from 4.46 to 3.66 MPa and from 0.51 to 0.26 MPa, respectively. An exponential decrease in the maximum effective gas permeability from 3.75 x 10-18 to 3.17 x 10-19 m2 with gas breakthroughs experienced could also be observed. On contrary, both of the saturated water permeability and the pore size distribution show little difference as compared to its initial value. These results indicate that gas breakthrough process could induce a degradation of gas-tightness capacity of the bentonite specimen. Meanwhile, the gas injection pressure was reset to zero before conducting the water permeability test. Reducing the gas pressure will induce a contraction in the radius of the gas pathway, even leading to complete closure. Consequently, the water permeability and the pore size distribution remained almost unchanged.