Abstract:
Permeation grouting of fractured rock involves the technique for grouting i.e pressure and time, the rock characteristics described by the fracture aperture and the grout properties governed by the shear stress and viscosity amongst other parameters. In Scandinavia the most common method for tunnel excavation is by far the drill-and blast cycle incorporating pre-grouting for every fourth to fifth round of blasting. Up to now the requirement are stated that no drilling or blasting can start before the grout has hardened enough. This is often specified by a certain time -“ waiting time” after the last borehole grouted and can range from 4 hours up to 12 hours. This study brings up a clarification of the different forces acting on the grout during a drill- and blast cycle and efforts were made to describe the grout using the shear modulus. The concept is based on that a brittle material can only withstand small deformations/strain before break occurs compared with a ductile material. Even more complicated it becomes when trying to understand how a fluid break. A conceptual model was derived describing the forces from blasting acting on the grout and what the necessary shear modulus of the cement grout is required to withstand the stress from blasting. Extensive laboratory studies were conducted to analyse the absolute shear strength of the grout and a field test to verify the blasting stresses acting on the grout. Traditional laboratory setup was used, such as the fall cone apparatus, and compared with advanced oscillation rheology measurements and triaxial shear boxes. The results indicate that the vibrations and deformations from blasting is rather small 2 meters from the blasting hole to create a break (flow) of the grout. The vibrations and resulting deformations from the actual drilling are hardly measurable. The suggestions are that drilling, and blasting can start directly after grouting provided that the water flushing from the drill is avoided. Keywords: blasting, vibration, grout, shear modulus, pre-grouting