Grouting trials in hard jointed rock-investigation, design and execution

Grouting trials were performed in crystalline rock at a depth of 450 m during construction of a 70-metre long tunnel. Hydrogeological investigations were undertaken stepwise, resulting in a successive updating of the rock description, followed by grouting design, and prognoses. The application of this coupled methodology for rock characterisation and design meant that, (a) general systematic pre-grouting could be avoided thanks to having detailed rock characterisation, and (b) an early assessment could be made about the most suitable grouting methodology. The initial description of rock, based on data obtained from a core borehole, was used to prepare a base design, and two grouting fans were anticipated. Even though further investigations resulted in design changes along a section of the tunnel, a combination of inflow measurements and pressure build-up tests gave a sound basis from which to choose the appropriate grout, and prepare for encountering more extreme conditions. Based on inflow measurements, the seating effect for Fans I and 2 was approximately 99.9 and 95 per cent respectively.