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“Rock Girl” dives deep for Hawaii tunnel drive

Construction of a new deep sewer tunnel in Hawaii, US has got underway with the launch of the tunnel boring machine (TBM).

Contracting joint venture Southland/Mole is using the Robbins 3.96m diameter Main Beam TBM – named Pohakulani , which means Rock Girl – to drive the 4.8km-long Kaneohe-Kailua Wastewater Conveyance Tunnel.

The project for the City and Council of Honolulu will improve wastewater infrastructure by eliminating overflows during rain events.

The tunnel is being driven through basalt bedrock and the deep tunnel was not the only option considered but it offered better environmental protection for the sensitive Kaneohe Bay area than a surface pipeline.

TBM was launched from a 23m deep starter tunnel and will be driven along a curved route to take the tunnel deeper and further inland and away from residential areas.

According to the joint venture, this is the first time a tunnel on this scale has been driven in the Hawaiian Islands.  The work also includes pre-grouting ahead of the TBM for groundwater control and installation of rock bolts, steel arches, wire mesh and ring beams as necessary behind the TBM.

Southland director Tim Winn has said that boring rates of up to 15m a day are expected and the tunnel will take eight to 10 months to complete.

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