This paper shows an overall result for a 3-year study on the characterisation of the colloidal silica grout (CSG) under saline groundwater. The CSG is suitable for sealing narrow fractures and has been studied and applied for grouting in the deep undergrounds to satisfy the strict inflow requirements. However, this grout is known to be sensitive to the salinity of groundwater because of its gelling mechanism with an inorganic salt.

In Japan, the coastal region has been discussed to be a more suitable region for the geological disposal of high-level waste. The groundwater in this region or the fossil water observed in the deep underground may have a salinity. The purpose of this research is to enhance the existing rock grouting technology, especially for the CSG under the saline groundwater. As a first step, we performed the survey of the latest grouting technology of CSG, including a grouting workshop in Finland with Nordic authorities. This survey was aimed to identify the most significant challenges to developing the grouting methodologies for CSG under salty groundwater.

Along with the initial survey, this project focused on obtaining the various characteristics of CSG by the laboratory tests. We developed a feasible mixing methodology using an acidic pH adjuster that prevents the rapid reaction with the salt in the groundwater, and we have obtained the CSG's basic properties, and its long-term characteristics impacted by the salty mixing water or submerged by saline water up to 250 days. Moreover, we proposed a theory for the penetration of CSG under saline water, and performed the injection tests in the laboratory, with use of different CSG and the various type of surrounding groundwater, in order to verify this theory by understanding the impact of salinity to the grout penetration property. Details are shown in another paper in this symposium, see Jari et al. [2019].

Our findings in the characterisation of CSG under saline water for the basic properties and the penetration properties were recognised to attain some progress in the development of the grouting technology, which was one of the results in the discussion in the second workshop in the last year. It was also pointed out necessary to keep studying in this field for its further development, such as the development of the penetration theory by further experiments targeting the testing at the site.

The continuous study in this field by periodically exchanging the latest technologies with a hybrid approach should be necessary for further development of grouting technology. The developed grouting methodology in CSG will robustly and effectively decrease the ingress of water from the narrow fractures filled with fossil water in the disposal facilities or seawater in submarine tunnels.

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