It took 4 ‘slurry’ tunnel boring machines (TBMs) 20 months to dig the Metro Tunnel’s twin 9km rail tunnels. Slurry is a mix of clay and water that assists in tunnelling and excavation of soil material.
The used slurry was processed at specially built slurry treatment plants at the Arden and Anzac station construction sites, where it was separated from the excavated material and filtered.
Initially, the slurry was treated with lime, which required:
- large quantities of lime
- discharging sulphuric acid-treated water as waste
- sending category C spoil to landfill
- maintaining the slurry system to manage scale.
The project team then switched to treating the slurry with coagulant instead of lime, with significant environmental benefits. They devised a system that allowed real-time monitoring of residual coagulant in the used water. When no coagulant was detected, the water was reused for the next batch of bentonite.
This solution was not only cheaper, it meant:
- spoil could be reused
- filtrate water could be recycled into the slurry system
- significantly lower greenhouse gas emissions
- savings on water, consumables and spoil management.
By recycling water into the slurry system, the project saved more than 140 Olympic-sized swimming pools of water.