Lake Mead-Intake Tunnel No. 3 Pre-Excavation Grouting Challenges Using a High Pressure Slurry TBM

In response to the severe drought on the Colorado River basin and in order to preserve existing water capacity to the Las Vegas Valley, the Southern Nevada Water Authority is contracted a new deep-water intake (Intake No. 3) located in Lake Mead. The project (Contract No. 070F-01-C1) included three miles of tunnel with very difficult geological conditions and 650' deep shaft and marine works. This paper will present means and method, challenges and results of the three extensive grouting campaign performed during the tunnel excavation. It is remarkable that for the first time worldwide, a slurry shield TBM advanced in closed mode at 15 bar for hundreds of meters. All efforts were carried out to avoid saturation hyperbaric interventions, which were considered too risky in terms of safety, time and cost. However the contractor was fully equipped to perform an intervention at 15 bars of compressed air pressure which has never been performed on TBM tunnelling projects. In conclusion, targeted and sequenced grout campaigns with detailed custom made procedures and methodologies are effective even if conducted under severe hydro-geological and ground conditions.