[en] Sealing of two moderately dipping 300 mm diameter holes that had been bored from a niche adjacent to the ramp in the Aespoe URL in conjunction with earlier geohydrological investigations (TRUE Project, Winberg et al. 2000) was successfully completed in early 2010. The work was preceded by a pilot test on the ground surface for investigating if a simple version of sealing by pressing down clay pellets into clay mud is feasible. The borehole was simulated by a steel tube that was filled with smectite mud in which a cage filled with clay pellets was moved down. The experience from this experiment was that the mud used was somewhat too stiff and that strong vibration significantly reduced its viscosity but not sufficiently much. The predicted average density at water saturation was 1,780 kg/m³ (dry density 1,240 kg/m³). The finally selected sealing method comprised casting of concrete plugs where the inflow of water was significant (up to 30 l/min), and to install precompacted blocks of smectite-rich clay where the rock was 'dry'. The finally matured clay plugs will have a density of around 2,000 kg/m³ (dry density 1,580 kg/m³) and a hydraulic conductivity of less than E-12 m/s, which is estimated to be at least one order of magnitude lower than the bulk conductivity of the surrounding rock mass (Pusch 2008). This provides excellent sealing of all parts of the holes deeper than about 1-2 m below the tunnel floor. Groundwater flow in the rock around the clay plugs can cause some minor erosion but this effect is deemed negligible considering the coagulating effect on released clay particles by the brackish Aespoe water. Inspection of the closed upper borehole ends showed accumulation of some very dilute mineral suspension after a few days. A careful analysis of the mineral content showed that it did not contain smectite particles, hence certifying that erosion of the clay plugs in the holes had not taken place. The particles were concluded to represent fracture minerals