[en] The deposition of canisters with highly radioactive wastes in tunnels at 500 m depth involves application of in siter compacted bentonite/quartz buffer material in tunnels and shafts. Such material is sometimes also specified for the deposition holes as well. One possible drawback of this procedure could be that the water which enters the buffer mass from the surrounding rock may cause erosion either by washing out tiny clay paritcles from the mass or by creating ''piping'' in it. Two test series were made in the laboratory to investigate the clay transporting power of percolating water. The tests show that piping and temporarily rapid water flow may take place. It requires, however, that rather extreme and improbably high gradients are produced in the tunnel fill. The tests also show that a steady homogeneous water flow does not transport clay particles through the grain matrix even under high gradients. The fact that a tunnel fill with 20 % Na bentonite does not obey Darcy's law, means that it will be practically impervious when only regional gradients are finally in operation. (E.R.)