Study on Characterization of Colloidal Silica Grout for Rock Excavation Under Saline Groundwater - 19332

A rock excavation grouting technology has been recently studied as significant technology for reducing the ingress of water into the deep repository. However, it has not been studied for applying to the coastal region, where it is discussed to be a more suitable region for the geological disposal in Japan. The latest material called colloidal silica grout (CSG) is good for sealing narrow fractures but is known to be sensitive to the salinity of groundwater because of its gelling property with salt accelerator. Although the gelling of CSG can be controlled by adding an acidic pH adjuster, the methodology for delivering the appropriate grout is not well established for such conditions of saline groundwater. Therefore, this research project was established to enhance the existing rock grouting technology for deep repositories. Initially, a grouting workshop was held with Nordic authorities in order to understand the state of the art and identify the most significant challenges to developing a generic mixing method for injecting CSG under seawater. Based on laboratory tests, we developed a feasible mixing methodology that prevents the rapid reaction with the salty water, which shows that CSG is not negatively impacted by salty mixing water or submerged by saline water. Combining the latest grouting techniques from Japan and the Nordic countries creates an optimal grouting methodology for use under seawater. Using this hybrid approach, the CSG can find wide-scale use as an agent for grouting narrow rock fractures filled with saline or fossil water in order to decrease the ingress of water in the disposal facilities. The reduction of the water ingress will enhance the safety of the repository, reduce water discharge, and diminish the long-term cost of water treatment. (authors)