The $32bn question: Benchmarking California’s Delta Water Tunnel against global tunnelling risk

California’s $20.1bn (£15.3bn) Delta Conveyance Project (Delta Water Tunnel) will be the largest water infrastructure investment in the state’s history, at a planned length of 72.5km, an 11.9m external diameter and a planned schedule of 14 years of construction. Yet, evidence shows its true cost is likely $27-32bn (£20.5-24.3bn), making it one of the most expensive and extensive bored tunnels ever attempted globally...