Silica sol for rock grouting: Laboratory testing of strength, fracture behaviour and hydraulic conductivity

A recently introduced non-cementitious grout silica sol is a refined product of colloidal silica, where the particle sizes have been reduced to between 5 and 100 nm. Laboratory tests were performed to determine the behaviour of silica sol as a permeation grout in hard rock. The tests have involved methods such as fall-cone, unconfined compression, triaxial, and oedometer tests. Samples were tested at different time intervals and in different storage environments. Results showed that the initial strength of silica sol, a few kPa, increases over time. Silica sol has a ductile behaviour during the first few days and then becomes elastic–plastic. Its hydraulic conductivity ranges from 10−10 to 10−11 m/s. When immersed in water, silica sol hardens and a thin layer of weaker strength is formed at the surface. However, this layer only extends a couple of millimetres into the sample; beyond that the silica sol is not affected, rendering breakdown by erosion a negligible risk. The conclusions are: (1) the strength obtained in silica sol after hardening is sufficient to withstand most grouting conditions; (2) when sufficiently confined, silica sol is able to withstand loading and unloading cycles; (3) a pH environment of around 11 does not appreciably change the strength of the silica sol; (4) silica sol is a material with low risk of failure under blasting vibrations; and (5) due to its low hydraulic conductivity, silica sol can be compared to low permeable clays.