Windward gravity sewer tunnel nearly half complete - Hawaii News Now - KGMB and KHNL

Windward gravity sewer tunnel nearly half complete

KAILUA, OAHU (HawaiiNewsNow) -

The gravity sewer tunnel will convey wastewater flows from Kaneohe to Kailua while providing wastewater storage to help prevent overflows and spills, especially during storms.

The overall project includes multiple parts:

An approximately 3 mile-long gravity sewer tunnel from the Kaneohe WWPTF to Kailua Regional WWTP; a new shaft, diversion structure, and odor control facilities at the Kaneohe WWPTF; and a new shaft, influent pump station, tunnel influent pump, odor control facilities, and headworks facility (where raw wastewater enters the plant) at the Kailua Regional WWTP;

Intermediate (access) shaft near the Board of Water Supply (BWS) Kapaa Water Tank Site (reservoir) to provide access for emergency or future maintenance of the tunnel.

The gravity sewer tunnel will be aligned to travel under Oneawa Hills, mauka of Kaneohe Bay Drive. Tunnel construction will be primarily staged from the Kailua Regional WWTP and will involve the use of a tunnel boring machine (TBM) to drill a tunnel up to about 13 feet in diameter. An approximately 10-foot inside diameter pipe will be installed in the tunnel to handle wastewater flows. The access and retrieval points for the TBM will be at vertical shafts at either end of the tunnel. The shaft at the Kaneohe WWPTF will also be used for constructing the Kaneohe diversion structure that will intercept sewage lines; the shaft at the Kailua Regional WWTP will be used for construction of the tunnel influent pump station.

Since the finished tunnel will convey wastewater by gravity flow, it slopes from a depth of approximately 35 feet below ground level at the Kaneohe WWPTF down toward the Kailua Regional WWTP, ending at approximately 82 feet below ground level. A new Tunnel Influent Pump Station (TIPS) will be constructed at the Kailua Regional WWTP to lift the wastewater from the vertical shaft to the surface for treatment. Odor control systems will be incorporated at both vertical shafts.

The diameter of the tunnel will allow storage of wastewater during periods of high flow, such as during periods of high rainfall.

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