To increase the reliability of the Windward Oahu sewerage system, the City and County of Honolulu’s Department of Environmental Services (ENV) plans the Kaneohe-Kailua Wastewater Conveyance Tunnel, an approximately 3-mile-long gravity sewer tunnel to carry wastewater from Kaneohe’s Waste Water Pre-Treatment Facility (WWPTF) to Kailua’s Regional Wastewater Treatment Plant (WWTP). This gravity sewer tunnel will equalize flow and help prevent overflows and spills.
ENV received federal approval for the tunnel to alleviate stress on the aging sewer force main that runs under Kaneohe Bay Drive. The tunnel fulfills requirements of the Consent Decree between the City and the U.S. Environmental Protection Agency. The gravity sewer tunnel has support from State regulators and environmental groups.
Earlier planning called for installing a force main under Class AAA Kaneohe Bay, but after further review the gravity sewer tunnel was recommended. The gravity tunnel will run mostly under Oneawa Hills and is a greener alternative to the force main under Kaneohe Bay. Also, technology advancements have brought tunnel costs down. Eventually the Kaneohe Wastewater Treatment Facility (KWWPTF) will be decommissioned, saving operating, maintenance and energy costs and emissions.
By making the tunnel large enough to store peak (wet-weather) flows, above-ground sewage tanks, visual impacts and spills are avoided. Odor control systems will be installed at each end of the tunnel.
The feasibility study and design of the Kaneohe–Kailua Wastewater Conveyance Tunnel includes deep soil borings and geotechnical data. After thorough analysis of the tunnel route, the types of soil and rock composition along the route, and the latest tunnel boring technology, engineers with Wilson Okamoto Corporation prepared an Environmental Impact Statement (EIS) VOLUME#1 and VOLUME#2 for the City. Based on the findings in the EIS, and on extensive feedback from community members, environmental groups and stakeholders, the City determined the gravity sewer tunnel to be more ecologically sound and cost effective than constructing a new force main under Kaneohe Bay.
The current construction schedule of the gravity sewer tunnel will take 5 years and 6 months to complete, and involves three separate phases:
Based on valued community input, the City moved the middle portion of the gravity tunnel alignment toward the crest of Oneawa Hills. The revised route is shown in the graphic below, referred to as Alternative 7.