City contractors have successfully bored a 3-mile tunnel beneath Oneawa Hills, a critical component of the $371 million Kaneohe-Kailua Waste Water Conveyance and Treatment Facilities Project, Mayor Kirk Caldwell announced Tuesday.

It took roughly 13 months for the tunnel-boring machine to drill the 13-foot-diameter puka that eventually will house a 10-foot pipe tasked with delivering sewage to the Kailua Regional Wastewater Treatment Plant from the Kaneohe Wastewater Pre-Treatment Facility by way of gravity.

The tunnel runs from a depth of 35 feet below ground level on the Kaneohe side and travels down to a depth of 62 feet on the Kailua side, city officials said. To complete the $200 million Phase 1, contractors will now install 885 pipe segments, each 10 feet in diameter and 20 feet long, that will carry the wastewater flow.

While the sewage will flow from Kaneohe to Kailua, the boring actually began from the shaft of the Kailua plant, moved beneath the Aikahi Gardens subdivision through Halia Street, under the Kailua side of H-3 freeway, crossed beneath Kaneohe Bay Drive and under the Bay View Golf Course to reach the Kaneohe facility.

In the $150 million Phase 2, a pump station will be installed in the Kailua shaft that will receive and move the flow up to the treatment plant. In the $21 million Phase 3, the final phase, a tunnel “influent” facility will be built at the Kaneohe facility.

The project, required by a court-mandated consent decree, began in 2013 and is expected to wrap up in 2018.

The tunnel-boring machine is nicknamed Pohakulani, Hawaiian for “heavenly rock.” Aikahi Elementary School’s robotics team came up with the name.

Like other Honolulu sewer improvements, the project is funded by sewer fees.

Caldwell, at a news conference, said this is the first time the city has used gravity tunnel technology, but it likely won’t be the last, adding that gravity tunnels are preferable to force mains.

“This is about living more sustainable and more green,” Caldwell said. “When you pump it under pressure, if there’s a force main break, a lot of sewage comes out of the ground very quickly. But if it’s gravity flow, there’s no pressure, and we’re not using any energy or electricity to pump it.”

Funding is in the 2017 budget for planning and design of a similar facility that would take sewage from Waikiki to Ala Moana, he said.

Don Painter, project manager for contractor Southland Mole, said the upfront cost of gravity sewer technology might be higher, but maintenance is cheaper than force mains. “A gravity sewer, once it’s in the ground, it’s pretty much there forever, and you don’t have to do anything to it.”

SEWER GRAVITY TUNNEL BREAKS THROUGH by Honolulu Star-Advertiser