Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. When the compacted bentonite blocks are emplaced, technological voids related to different interfaces involving the buffer material are created, and their hydro-mechanical behavour is of primary importance for the safety of disposal. In this study, the hydraulic resistance of the interface between compacted MX80 bentonite and Boom Clay is investigated in the laboratory using an injection cell. The results obtained show that when water is injected, the technological gap is quickly reduced due to the bentonite swelling. When water pressure reaches the hydraulic resistance of the interface, hydraulic fracturing takes place with a drastic pressure decrease. After fracturing, water injection continues and bentonite continues to swell. A higher subsequent pressure is needed to produce a new hydraulic fracturing. After a certain time, the hydraulic resistance becomes high enough so that no further fracturing occurs, suggesting that the technological gap is sealed.