Deep geological disposal of dangerous waste like mercury, solidified organic pesticides and radioactive rest products, requires suitable engineered barriers. Use of deep abandoned mines is a rational and economic alternative to construction of a repository in virgin rock but requires knowledge of the structural constitution of the rock for assessment of groundwater flow and rock mechanical conditions. Such information is much more detailed through the activities of mining companies than from exploration of virgin rock. Organizations responsible for disposal of radioactive waste in crystalline rock count on the host rock as a barrier to migration of released radionuclides to the biosphere but present investigations reduce its role to provide mechanical support of the waste packages while effective hindrance of migration of contaminants can be offered by engineered barriers, i.e. waste containers and clay embedding them. For certain waste in granular form, like Hg batteries, mixing with expandable clay and layerwise placement and compaction is deemed possible as described in the paper. The primary role of the clay is to make the clay-mixed waste very tight and ductile. By constructing liners of highly compacted clay blocks along the periphery of the disposal rooms the rock is given sufficient support and effective isolation of the waste. If the liners have a thickness of 0.5-1 m and placed in dry form the time for complete water saturation of the clay/waste mixture can be several thousands of years. Not until then migration of hazardous waste elements to the rock can start. A major role in the hydrological performance of the host rock is that of the excavation-disturbed zone (EDZ). It short-circuits the natural system of flow paths and causes quick transport of released contaminants to downstream wells.