Controlling of Disturbances due to Groundwater Inflow into ONKALO and the Deep Repository

The main goals of the report are identifying possible disturbances due to ingress of groundwater into ONKALO and the repository and identifying principles of the technical solutions for controlling groundwater leakages.

Inflow of groundwater into underground facilities and the subsequent pumping of the water to the ground surface during the construction and operation phase bring along flow of groundwater in the surrounding rock, movement and mixing of different water types, and drawdown of groundwater table. Flow of groundwater into the excavations causes migration of superficial water into the bedrock. From the point of view of the postclosure performance of the repository, the most significant impacts are related to the inflows into the deep parts of the underground facilities and to consumption of the buffering capacity of fracture fillings and rock in the vicinity of the disposal tunnels. The upconing of deep, saline water can be also related to the drawdown of groundwater table. Measured in metres, the upconing may be several times higher than the drawdown of groundwater table. During construction, rock engineering works and structural construction as well as installation of the systems works, will meet difficulties if high inflow rates are tolerated. During disposal operations, leakages may cause additional problems by making installation of buffer and backfilling works difficult or impossible.

Inflow of groundwater can be limited by a careful selection of the locations of the underground facilities and their surface connections and by forming watertight zone in the rock around tunnels and shafts. This is normally made by grouting the rock before or after excavation. More exotic techniques like freezing the groundwater in the fractures have also been applied for special purposes. Leakages can be limited also by constructing lining structures inside the tunnel at the intersections with major transmissive zones. Best available technology for reducing the groundwater leakages by grouting would include hydrogeological characterisation of the rock, design of the grouting techniques based on the characterisation results, grouting work according to specifications and verification of the result. Sealing (mainly grouting) may, on the other hand, also cause some potentially harmful disturbances. Chemical disturbances are associated to the use of cement (especially if low-alkaline cement can not be used) or organic grouting materials.

Inflowing amount of groundwater into ONKALO and the repository was estimated with analytical methods with and without sealing. Estimated total inflow into ONKALO without sealing was about 3000 l/min and after sealing 400-500 l/min. Construction of the repository showed only minor increase in the scale of 100 l/min to the total inflow rate. A balance needs to be found between avoiding disturbances due to leakages and, on the other hand, due to grouting and other preventing methods.