With the completion of the transmission tunnel – the first phase and Hawaii's largest ever wastewater tunneling endeavor - the Kaneohe-Kailua Wastewater Conveyance and Treatment Facilities Project is on track for completion this summer. The second and third phases of the project are well underway; the tunnel phase, which is especially notable as Hawaii's first use of a tunnel boring machine (TBM), was completed in February.

The project, on the island of Oahu, is designed to increase storage capacity of the region's wastewater system and reduce the likelihood of overflow. The Kaneohe-Kailua Wastewater Conveyance and Treatment Facilities Project is one of several that will allow Honolulu to adhere to a 2010 consent decree to improve overall island sewage treatment, explains Quang Tran, PE,  Project Engineer with the project's tunnel contractor, Southland Contracting.

“This is an Environmental Protection Agency (EPA) federal consent decree project, based on issues with the existing sewer system. The 42-inch force main in the prior connection was deemed inadequate for the region. The city and county of Honolulu were required to build the project, which includes the tunnel plus a new pre-treatment plant on one end of tunnel, and a wastewater treatment plant on the other end. The tunnel is complete, and the entire project is expected to be commissioned in June with a local celebration for residents and project partners.

“Once the project is complete, wastewater will flow by gravity, without pumping, saving money and effort and reducing maintenance,” Tran continues. “The project will prevent wastewater overflow, especially during storms and heavy rain events.  Currently, during such events, wastewater may spill along the alignment of the force main.

“I compliment the city and county for undertaking this project; this side of the island is such a treasure – it's a paradise for tourists, but it's also environmentally sensitive.”

Gravity Tunnel Chosen After Successful Mainland Use

To meet requirements of the consent decree, the solution devised by Honolulu officials was to connect the Kaneohe facility to Kailua’s Regional Wastewater Treatment Plant with a transmission tunnel more than 3 miles long, bookended by new influent facilities. Three alternatives for the tunnel component were considered, says Tran. “One alternative was to build another force main along the existing alignment,” he reports. “The second alternative was a new force main under the bay. The third option, which was ultimately selected, was a gravity tunnel (conveying the wastewater by gravity flow rather than by the use of pumps).  These systems are commonly used in the mainland states, and have proven to be very successful.”

The second phase includes a new regional wastewater treatment plant, which will take the sewage from the gravity tunnel and lift it into the Kailua treatment plant.  The third phase encompasses a new pre-treatment facility at the beginning of the tunnel, within the Kaneohe Wastewater Pre-Treatment Facility (WWPTF). Upon completion of all phases, the existing force main that transports wastewater from Kaneohe to Kailua will be decommissioned and the flows will be redirected to the new gravity sewer tunnel.

Bowers + Kubota Consulting (B+K) is the construction manager for the project; Southland Mole JV is the tunnel contractor. Hensel Phelps serves as the general contractor on the second and third phases.

The Kaneohe-Kailua Tunnel contract was awarded in November 2013, with Notice to Proceed in January 2014. The TBM, which was specifically designed for this project and built on site, began mining from Kailua to the Kaneohe WWPTF in April 2015. The TBM was manufactured by The Robbins Co., based in Solon, Ohio.

Boring was completed in June 2016 on the tunnel project, which required the creation of a large shaft, approximately 85 feet in diameter, to launch the TBM; this shaft will be used for the new pump station.

The $193 million tunnel project consists of a 13-foot diameter rock tunnel running approximately 300 feet below the Oneawa Hills for 16,337 linear feet to connect the two existing wastewater treatment facilities. The final tunnel liner consisted of 10-foot glass fiber-reinforced resin pipe grouted in place.

The majority of the project was excavated through basaltic rock with unconfined compressive strengths up to 16,000 psi. Approximately 650 linear feet of the tunnel was excavated in soft ground and required preexcavation grouting to control groundwater inflows.

Since the finished tunnel will convey wastewater by gravity flow, it slopes from a depth of approximately 39 feet below ground level at the Kaneohe WWPTF down toward the Kailua Regional Wastewater Treatment Plant, ending at approximately 77 feet below ground level.

Logistics, Not Weather, was Biggest Challenge

Tran says that most of the challenges faced in the project were related to the necessity of obtaining equipment and materials from the mainland. “We’re 3,000 miles away from equipment support, and it was critical to build strong relationships with shipping companies and local vendors. The tunnel boring machine came from Solon, Ohio, so the logistics were challenging, as was setting up equipment of this size. Fortunately, the manufacturer provided extensive support and troubleshooting.”

Remarkably, weather did not play a major factor throughout the duration of the project, except during heavy rainstorms this past February. The storms resulted in a wastewater spill at the Kaneohe Wastewater Pre-Treatment Facility, where work on the tunnel was underway. Severe flooding left some areas of the facility under 3 feet of storm water, resulting in equipment damage.

“We knew that weather could be a problem on a tropical island, and we planned ahead for that,” Tran relates.  “We were fortunate that no hurricanes hit the island during the construction timeline.  We had built the site up well, and that prevented delays.”

Community Outreach Helped Address Residents' Concerns

The overall public reception to the project has been positive, Tran reports, although initially there were concerns over traffic, vibration, noise, and dust. He points out that investments in public information have proven beneficial.

“The city and county made an enormous effort in community outreach before the project started. Community feedback helped lead to modifications such as the alignment of the tunnel on the spine of a mountain, resulting in a curved tunnel instead of straight because of proximity to residences.

“We worked with the city and county to make sure we were transparent and that the public was thoroughly informed about the project.  One particular aspect that was very gratifying was our work with the local elementary school – we kept the students engaged in the project, and they actually came out to see the TBM at work.”

To address concerns over possible disruption to the community from project construction noise and vibration, the city and county established a comprehensive monitoring program in 2012, before the project commenced, to track ground movement, vibrations, and construction noise. Also put in place was a volunteer participation program to monitor the noise levels, a 24-hour hotline whereby community members could report an issue or ask questions, and a project website where construction updates and project information were regularly updated.

Becoming a Greener Community

In press comments, Honolulu Mayor Kirk Caldwell has summed up the benefits of the Kaneohe-Kailua Wastewater Conveyance and Treatment Facilities Project:  “It means we’re saving energy. It means we’re becoming a greener community. It’s going to move sewage – not under pressure, not using energy and wasting energy - but allowing it to be gravity flow from Kaneohe to Kailua where it’s going to be treated. When we have major storms, like the last hurricane that came through, water can be stored – when it shouldn’t be getting in the system but it does – in this tunnel, to be treated over time instead of having problems.

"Despite this being the largest sewer project in state history, this critical infrastructure project has had very little impact on our residents, while ensuring our public and environmental health and safety for generations to come.”