Client: Metro Water Services
Location: Nashville, Tennessee
Project Value: Phase 1: $7.1m (USD) | Phase 2: $90.5m (USD)
Description: Significant upgrade to the plant’s capacity and infrastructure including extensive microtunnelling and shaft construction in challenging geological conditions.
Works entailed the construction of 10 tunnels totalling 2728 linear feet with diameters from 72 to 84 inches. Six shafts ranging from 16.5 to 106 feet in diameter and depths of 32 to 70 feet were excavated primarily in hard fractured limestone (which required rock blasting). Tunnel boring used a pressurized Herrenknecht AVN2100 TBM with a hyperbaric chamber. Extensive rock grouting and self-performed utility monitoring were conducted, especially near rail crossings.
Key challenges included tunnelling in 30 ksi limestone, managing high water tables, conducting two wet retrievals in the Cumberland River (required for the intake tunnels), and working within an active water treatment plant with many existing services. Notably, tunnelling under creeks, sewers, roads, and railways was completed with zero settlement recorded.