Design of grouting with silica sol in hard rock – New methods for calculation of penetration length, Part I

To meet the demands laid down for recent tunnel projects in Sweden, a great deal of effort has been devoted to the grouting materials. Silica sol is a grouting material that penetrates and seals fractures where cement cannot. Studies show that in rheological terms, silica sol works differently compared with cementitious grouts and in the literature there is a lack of models to calculate the penetration lengths. In recent times there has been an increase in the use of silica sol, in major tunnel projects for example, and there is a need to understand the basics regarding its penetration in order to gain a better understanding of what can be accomplished. The fundamental difference between a gelling liquid and a cementitious grout is that initially the gelling liquid acts in the same way as a Newtonian liquid. It thus has no yield shear strength and it will continue to flow as long as there is a pressure gradient. The cementitious grout acts like a Bingham fluid, with a yield shear strength to which the penetration length can be linked. Several authors have described how the rheological models can be used to calculate the penetration length of grouts. Part I of this paper deals with how the penetration length can be computed analytically, both as 1-D channel flow and 2-D radial flow. At the end of this paper a simple rule of thumb is suggested where the penetration length is easily computed using knowledge of the hydraulic aperture and the pressure and only involving the initial viscosity of the grout and the gel induction time.In part II it is shown how the analytical models can be used to design grouting in hard rock. It also shows how the theoretical expressions are transformed into practical design criteria for grouting.