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Shale deposits speed US wastewater tunnel construction

Breakthrough of the tunnel boring machine (TBM) on the Black River Wastewater Tunnel, in Ohio, US, means that prevention of sewage discharge into a tributary of Lake Erie during heavy rainfall events is a step closer to being realised.

Contractor Super Excavators used a Robbins Double Shield TBM to drive the 1.7km long tunnel through shale deposits with advance rates of up to 21m per day.

Although the Cleveland shale helped fast track the excavation, it was challenging to ensure that the continuous conveyor system could keep pace. Super Excavators tunnel supervisor Gregg Rehak said that the fairly soft ground meant that cutter wear on the six month drive was minimal. “We only changed seven cutters during the bore, four of which probably could have made it to the end, but were changed for precautionary reasons,” he said.

The ground conditions did prevent issues for the lining installation and an alternative method to concrete segments was used. Super Excavators used ring beams, wire mesh, and lagging installed at 450mm intervals. However, around a quarter of the tunnel went through shale that was layered and laminated which caused the crown to start to spall before ring beams could be expanded, requiring extra chipping and rock relief.

Now the bore is complete, work is underway to remove the TBM by the end of May to allow construction of the secondary lining to start. The project is expected to be completed next year.

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