TBM launch on Hawaii’s longest tunnel
9 July 2015USA – TBM launch has taken place on the Kaneohe-Kailua Wastewater Conveyance Tunnel in Honolulu, Hawaii. The 3.96m Robbins Main Bean TBM has begun excavation of the 4.6km sewer tunnel. Tunnelling through the local basalt bedrock was kicked off by contactor Southland-Mole JV from a 23m-deep launch tunnel, and as Tunnels and Tunnelling went to press, the machine had bored approximately 300m.
A spokesman for the manufacturer said: "During the tunnel design phase, it was decided that the tunnel route should travel inland and deeper underground in order to bypass one of the few residential areas along the alignment. Designers introduced an isolated curve in the tunnel alignment of 150m radius, requiring the TBM to be designed with [an unusual] back-up system. There will also be operational procedures when crews navigate the tunnel curve, requiring the machine to be operated using half strokes rather than a full TBM stroke."
The deep tunnel option was not the first design considered for the project: preliminary plans called for a smaller tunnel travelling under the bay. As Kaneohe Bay is an environmentally-sensitive area, a deep tunnel remained an attractive option. Richard Harada, of project consultant Wilson Okamoto Corporation, explains the ultimate decision: "A number of factors were considered in making the decision to build a deep tunnel including reliability, construction costs, life cycle costs, environmental impacts, constructability and qualified contractor availability."
Director of Southland Tim Winn added: "There has not been a TBM of this size in the Hawaiian Islands or a tunnel of this length. The tunnel is being driven from an active Water Treatment Plant (WTP), and space is at a premium. There are also simultaneous contracts being performed there outside the scope of our work."
