The Hawaiian Islands, essentially the exposed seamounts in a vast chain of oceanic volcanoes, have a long and fiery history. The volcanoes laid down basaltic bedrock that can sometimes prove to be a challenging substrate for underground construction projects. Tunnels in Hawaii have historically been smaller, of the microtunneling variety.
“I think we have the same problems with aging infrastructure as other municipalities. However, because we are an island state, some of our utilities aren’t as big as on the mainland. There is more opportunity for microtunneling. Big tunnels in Hawaii are not that common. It’s all new to us,” said an official on the project.
The big project being spoken of is Honolulu’s Kaneohe-Kailua Wastewater Conveyance and Treatment plan, which involved a deep, 4.6-km (2.8-mile) tunnel straight through the hard and often abrasive bedrock.
Weighing the Options
Plans for the tunnel were not straightforward, as local companies and the City of Honolulu did not have much experience with a full-sized, TBM-driven option. The deep tunnel was only considered later, while preliminary plans called for a smaller pipeline traveling under Kaneohe Bay. As Kaneohe Bay is an environmentally sensitive area, a deep tunnel remained an attractive option. 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 Kailua Wastewater Treatment Plant that will connect into the new pipeline was built in 1965. On the windward side of the island, there are also three main treatment plants. Currently, flows from different areas are sent to specific plants and transferred by force mains. The tunnel will replace the pump station force main system with the new gravity-fed system.
Originally the city was required by consent decree to do something with the force main system only. However, the City of Honolulu was also interested in looking at a tunnel alternative that would provide greater storage capacity for future needs. They assessed the option with community outreach and found that public opinion favored the larger tunnel. Once the tunnel option was decided upon, the city had to go back to the Environmental Protection Agency (EPA) to get approval for the change.
The deep tunnel option was approved, and the city’s next task was tunnel-specific community outreach. Since this was the first large tunnel in the Hawaiian Islands, residents had many concerns and questions. There were vibration concerns for surrounding residences, concerns about triggering rock falls, and more. Meetings were held to discuss the levels of vibrations and the much higher thresholds needed for something negative to happen. Near the Kailua plant, there was quite a bit of concern about noise and odor, as well as the drill-and-blast excavation of the launch shaft. The city responded with a large sound wall that effectively dampened noise around the shaft. Concerns were also raised about drilling and blasting, and the city responded with seismographs set up at multiple points along the tunnel alignment, while other stations monitored noise and water levels.
The Deep Tunnel Wins Out
Construction of the deep tunnel option began in 2015. The 23-m (74-ft) deep launch shaft was constructed at the Kailua Wastewater Treatment plant, while construction began on the exit shaft across the bay at the Kaneohe Wastewater Pre-Treatment facility. The contractor, the Southland/Mole Joint Venture, selected a refurbished 3.96m (13-ft) diameter refurbished Robbins Main Beam TBM to bore the tunnel. The machine, nicknamed Pohakulani, meaning “Rock Girl” in Hawaiian, is a rugged TBM originally built in 1992 for a water tunnel in South Korea. The machine was refurbished in Robbins’ Solon, Ohio, manufacturing facility and customized for harder rock conditions as well as a tight curve on the tunnel alignment with a 150-m (500-ft) radius. Back-up elements were shortened and reworked to be able to navigate the curve.
The curve was not the only unusual aspect of the tunnel; everything from the logistics of the tunnel operation to pre-grouting sections ahead of the TBM for groundwater control were new for Hawaii, although not for the tunnel contractors.
“The logistics were our biggest challenge. Everything had to come from the mainland, as we are on an island. We also had two different jobsites we needed to coordinate, one where the machine starts and the other where it’s extracted at the end,” said Don Painter, Project Manager for the Southland/Mole JV.
Training of the local crew, most of whom had not been on a TBM, also took some time. “First TBM on the island, union project, by contract we had 80% local labor and it turned out pretty well actually. On-the-job training, on the machine, we have a safety briefing every morning, and then we would train them on the machine. Even our mechanics were trained here,” Painter added.
Hard Rock Performance
The TBM was launched in spring 2015 from a starter tunnel at the bottom of the launch shaft. “Conditions wise, we definitely probed ahead at regular intervals,” said Painter. The machine hit rock as hard as 20,000 psi UCS, as well as a section with some water inflows. “We were probing ahead and hit a section with 450 gallons a minute of water. We grouted this section off right behind the cutterhead and continued to probe every 100 ft.” Bad ground, water, heat, and tunnel curves were all challenges the contractor had to overcome.
“We had to use quite a few different ground support types, and it was far more time-consuming than we anticipated. In some sections with a lot of jointing and tight materials, we had to put six to eight bolts in [per TBM stroke], as well as wire mesh and steel straps.”
In the area of the tunnel curve, operational procedures required the machine to be operated using half strokes rather than a full TBM stroke. “We got the machine around the curves, by taking wedges out of the side supports. There are several curves on this alignment, but the 500-ft radius is the tightest. We actually started the machine in a curve, of an 800-ft radius, and while it was tough to get around, we were able to do it,” said Painter.
As the TBM neared the Kaneohe plant, the ground became softer. “[The transitional section] had zero standup time in the ground and we had to use steel sets with grout in order to have a reaction surface for the grippers. We went from 62 ft per day on average to 11 ft per day,” Painter said.
Also in that section, the ground of less than 1,000 psi UCS began causing the TBM to sink. The contractor was able to keep the machine on the alignment in the soft ground by jacking the back end of the machine up and putting a positive lead under it. It was quite careful work, according to Painter.
Throughout, the contractor battled the environment as well. Average temperatures were 85 degrees Fahrenheit with 85% humidity at the surface. Temperatures at the tunnel heading often approached 95 degrees. “One of the other aspects was the salty atmosphere—electrical and mechanical components corrode easily in these conditions so we had problems. We had to replace some components, but the Robbins personnel got us back up quickly.”
Despite the challenges the TBM was able to excavate at rates up to 1,963 ft per month, 620 ft per week, and 168 ft per day. For the contractor, the design of the TBM was instrumental in maximizing advance rates. “It’s fairly easy to use. The simplicity of the design is what I like,” said Painter.
On June 23, 2016, the TBM and roadheader met. The TBM had bored 15,200 ft, or about 94% of the tunnel, with the roadheader excavating a 1,000 ft section in soft ground up to the Kaneohe plant. “Since the three-phase project will not be complete until 2018, we have not had a project completion ceremony yet. There was, however, a press conference held by Mayor Kirk Caldwell to announce the hole through,” said Robert Kroning, City and County of Honolulu, Department of Design and Construction, who also spoke at the event.
The TBM components were removed from the tunnel in August 2016, including the cutterhead, to be shipped back to the mainland. “The next steps include cleaning of the tunnel; installation of the 10-ft inside diameter pipe in the tunnel and grouting in place; construction of a tunnel influent pump station in the Kailua shaft to receive and pump the flows up to the Kailua Regional Wastewater Treatment Plant; and the construction of a new tunnel influent facility at the Kaneohe Wastewater Pre-treatment Facility. Once all is complete, we will be able to redirect flows to the new gravity sewer tunnel and decommission the existing force main that transports wastewater from the Kaneohe facility to the Kailua Regional treatment plant,” said Lori Kahikina, Director, City and County of Honolulu, Department of Environmental Services.
The project’s consent decree completion date is June 30, 2018. Upon completion, the deep tunnel will enhance water treatment capabilities and further aid in ceasing non-compliant, uncontrolled or moderately treated wastewater discharges.
This article was contributed by The Robbins Company.