In the spring of 2015, by the idyllic shores of Oahu, a Robbins 13-ft diameter Main Beam TBM began its long journey. The TBM started its excavation on a 2.8-mile drive for a new sewer tunnel near Honolulu, Hawaii. The machine, nicknamed Pohakulani, meaning “Rock Girl” in Hawaiian, launched from a 74-ft deep starter tunnel on a mission to bore through basalt bedrock. Contractor Southland/Mole JV is building the Kaneohe-Kailua Wastewater Conveyance Tunnel for the City and Council of Honolulu, which will improve wastewater infrastructure by eliminating overflows during rain events.
The deep tunnel option was not the first design considered for the project: preliminary plans called for a smaller tunnel traveling 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 Corp., 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.”
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 500-ft radius, requiring the TBM to be designed with a unique 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 curve is not the only unusual aspect of the tunnel; in fact, a tunnel on this scale has not been built in the Hawaiian Islands before. Everything from the logistics of the tunnel operation to pre-grouting sections ahead of the TBM for groundwater control are new to the Aloha State.
Director of Southland, Tim Winn, elaborates: “There has not been a tunnel boring machine 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.”
Winn added that since there are many challenges, teamwork is the key. “Robbins Field Service has been extremely valuable during assembly and commissioning of the TBM,” he said.
As of June 2015, the TBM excavated more than 1,000 ft, and is boring at a rate of 40 to 50 ft per day in basalt rock. Rock bolts, steel arches, wire mesh and ring beams are being installed as necessary.
Upon completion, the deep tunnel will enhance water treatment capabilities and further aid in ceasing non-compliant, uncontrolled or moderately treated wastewater discharges. The Main Beam TBM is estimated to end its journey in 8 to 10 months at the Kaneohe Wastewater Pre-Treatment Facility.