Kaneohe-Kailua Sewer Tunnel Wastewater Conveyance & Treatment Facility

Completed in 2019 by Southland Contracting, the Kaneohe-Kailua Sewer Tunnel Wastewater Conveyance & Treatment Facility Project was delivered to convey wastewater from the Kaneohe Wastewater Pre-Treatment Facility to Kailua’s Regional Wastewater Treatment Plant to prevent overflows and spills during heavy rainfall events. At the time of completion, this project was Hawaii’s largest ever wastewater tunneling endeavor. The project involved the construction of a 13-foot ID tunnel using a hard rock Tunnel Boring Machine (TBM), spanning approximately 300 feet below the Oneawa Hills for 16,337 linear feet, to connect the two plants. This marked Hawaii’s first use of a TBM. The tunnel was lined with a 10-foot diameter Hobas fiberglass reinforced (FRP) sewer pipe that was then grouted in place. The tunnel construction also required probing and geotechnical instrumentation, and surface and subsurface monitoring prior to TBM mining. Most of the project was excavated through Basaltic Rock under groundwater with unconfined compressive strengths up to 16,000 psi. 650 linear feet of the 16-foot inside diameter (ID) tunnel was conventionally excavated with a roadheader in soft ground thus requiring pre-excavation grouting to control groundwater inflows of 1,100+ GPM. Additionally, the team managed the excavation of 11 deep shafts, ranging in size from 27.5-87-foot ID and 30-287 feet deep. This included the installation of multiple types of support of excavation (SOE), consisting of deep/slurry diaphragm walls, secant pile walls, sheet pile walls with internal bracing, and steel rib and lagging. 30,000 cubic yards of cellular grout was also installed as annular backfill behind the tunnel’s final GFRP liner. Completed with reverse circulation drilling (RCD), the shaft intercepted the centerline of the main tunnel to be used as future intermediate personnel access shaft. Further, 1,100 gallons per minute (GPM) of groundwater inflows were pumped and treated for discharge during the mining period. A robust automatic treatment system (ATS) was installed to manage the water inflows and to provide stringent coordination and reporting required by the State of Hawaii’s Department of Health and its Environmental Protection Agency (EPA) representatives.