Building Hawaii’s Longest Tunnel

By The Robbins Company and Hobas Pipe USA August 25, 2015

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.

Project Background

The Kaneohe-Kailua Wastewater Conveyance Tunnel project fulfills requirements of the Consent Decree between the City and County of Honolulu and the U.S. Environmental Protection Agency, increasing the reliability of the Windward Oahu sewer system. This project will install a new 3-mile long tunnel from the Kaneohe WWPTF to a new Kailua Tunnel Influent Pump Station (TIPS) at the Kailua RWWTP. Per the Consent Decree, the project is to be operational on or before June 30, 2018.

The project will be constructed under two separate construction contracts. The Kaneohe-Kailua Sewer Tunnel contract is the first and includes the tunnel, new shafts at the Kaneohe and Kailua sites, and associated near surface structures at both locations. Construction of the new TIPS with associated odor control, a new headworks facility with associated odor control, and new electrical building at the Kailua RWWTP; and additional facilities at the Kaneohe WWPTF, will be performed under a separate contract.

The Kaneohe-Kailua Tunnel contract was awarded in November 2013, with Notice to Proceed (NTP) on January 6, 2014. Final tunnel completion is anticipated in early 2017.

The $173 million project consists of 16,338 lf of tunnel (1,388 lf conventional tunnel via roadheader, 14,950 lf TBM tunnel) with a tunnel diameter of 13 ft. The final liner, fiberglass reinforced pipe, will have an ID of 10 ft. The project contains two slurry wall shafts: an 87-ft diameter, 95-ft deep launching shaft (Kailua Site), and a 30.5-ft diameter, 54-ft deep receiving shaft (Kaneohe Site). The predominant ground condition is basalt. The project includes microtunneling for surface pipelines, and construction of diversion and junction structures connecting to existing pipelines.

Digging Deep

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.”

The Robbins machine was launched from a 74-ft deep starter tunnel constructed with slurry walls.

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.

The 120-in. pipe is being manufactured by Hobas in Houston and transported by cargo ship to Honolulu. The first shipment included 4,000 ft of the 120-in. pipe. It was loaded onto the 692-ft MV Marjorie C in Galveston, Texas, for the ship’s maiden voyage.

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.

Companies involved in the project include Southland/Mole JV (contractor); Wilson Okamoto Corp. (lead designer); Jacobs Associates (tunnel designer); Bowers and Kubota Consulting (construction manager); Layne Christensen Co. (slurry wall and jet grout subcontractor); James W. Fowler Co. (microtunneling); Brierley Associates (design consultant); Hobas Pipe USA (pipe supplier); and The Robbins Co. (TBM supplier).