Grouting and Analyses of a Fracture in Äspö HRL, Characterisation, grouting and verification of grouting results for silica sol
This report is part of the project Non-cementitious low-pH injection grout for smaller fractures that is a sub-project under the main project Injection grout for deep repositories. The project is financed by SKB (Sweden), Posiva (Finland) and NUMO (Japan). The sub-project objective is to find a grout that can be used to grout fractures with a hydraulic aperture less than 100 μm and that fulfills environmental and long-term safety demands.
This report describes a field test conducted during spring 2004 in a pillar located in the main tunnel at Äspö HRL at a depth of around 100 m below the surface. Previous studies (Fransson, 2001) show that the fracture of interest is found in a section 2.0-2.5 m in a borehole drilled parallel to the main tunnel. The transmissivity is around 5x10-8 m2/s corresponding to a hydraulic aperture of approximately 40 50 μm. The fracture has earlier been grouted with cement which resulted in a penetration length of around 0.1 m around the grouted borehole (Eriksson, 2002).
The fracture was grouted with a silica sol, produced by Eka Chemicals, Bohus under the name Eka® Gel EXP36. It consists of extremely small amorphous silica particles suspended in water. The accelerator used was a salt solution, CaCl2 (2,9%). The pH is around 10. The grout was also mixed with optical brightener to make it possible to see the grout using UV-light. The grouting design was determined using the hydraulic aperture analysed by Fransson, 2001. Three methods were used to determine the penetration length and sealing efficiency; namely detection of grout in 6 cores using UV-light, analysing part of the grouted fracture in microscope and using hydraulic tests.
The results show that five of totally six cores had traces of silica sol, which would indicate a penetration length of at least 1 m in the fracture. The microscopy analyses verify that silica sol were apparent in the fracture but not completely filled. The achieved penetration length of 1 m was compared with 2-D mathematical model. The results show that the model underestimates the penetration length with 0.25 to 0.35 m. The hydraulic tests result in a sealing efficiency of the silica sol grouting in the affected rock mass of 70%.