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Honolulu’s Sewage Tunnel Helps Preserve Kaneohe Bay
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Kaneohe Bay
A consent decree between the federal government and the City and County of Honolulu originally called for a force main to be constructed and operated beneath scenic Kaneohe Bay; a recently approved modification will dispense with that option. Wikimedia Commons/Jason Turse 

As an alternative to a force main that might have threatened a recreational bay, the City and County of Honolulu has chosen to construct a long, large-diameter storage and conveyance tunnel to prevent sewer overflows. 

July 3, 2012—After recently obtaining federal approval to modify an existing consent decree, the City and County of Honolulu is moving forward with plans to construct a large-diameter tunnel for storing and conveying wastewater. With the approximately 3 mi long tunnel, the city and county will be able to avoid implementing a controversial proposal to construct a force main beneath Kaneohe Bay, a scenic and environmentally sensitive water body that is also used for recreation.

Finalized in December 2010, the consent decree sought in part to address sewer overflows occurring at wastewater treatment facilities and collection system infrastructure owned and operated by the City and County of Honolulu. The consent decree mandated that the city and county construct a force main from the Kaneohe Wastewater Preliminary Treatment Facility to the Kailua Regional Wastewater Treatment Plant (WWTP), both of which are located on the east side of Oahu. During periods of heavy wet weather, overflows occasionally occur at the preliminary treatment facility and within certain sections of the collection system. Along with the force main, the consent decree aimed to reduce overflows by calling for the construction of two large storage tanks in Kaneohe and Kailua to collect wastewater during periods of wet weather and then convey flows for treatment as system capacity became available.

The planned route of the force main was to pass beneath Kaneohe Bay, prompting considerable concern among the public, says Lori Kahikina, P.E., the director of the Department of Design and Construction for the City and County of Honolulu. In particular, residents expressed concern that the tunneling methods that would be used to install the force main might harm the bay’s sensitive environment. Additional concerns centered on the possibility for ecological damage in the event of a catastrophic break of the force main. To allay these fears, the city and county “wanted to get the force main out of the bay as much as possible,” Kahikina says. The planned storage tanks had also generated opposition among residents living near their proposed locations.

During negotiations with the federal government leading up to the 2010 consent decree, officials with the City and County of Honolulu raised the prospect of constructing the sewage tunnel in lieu of the force main, Kahikina says. However, they were told at the time that they would have to wait until after the consent decree had been finalized before they could propose technical changes. Therefore, shortly after signing the original consent decree in late 2010, the city and county sought to update the agreement to permit the substitution of the tunnel for the force main; the tunnel would not need to pass under Kaneohe Bay. After the U.S. Environmental Protection Agency approved the change earlier this year, the consent decree had to be amended and authorized by the U.S. District Court for the District of Hawaii. Although the amendment was finalized in late March, the City and County of Honolulu did not formally announce the change until mid-May. 

Aerial view of a nearly 3 mi long, large-diameter gravity tunnel 

The recently approved alternative calls for the construction of a
nearly 3 mi long, large-diameter gravity tunnel that will store and
convey rainfall to prevent sewer overflows. Courtesy of Wilson
Okamoto Corporation/USGS 

With the change in place, the City and County of Honolulu is now in the process of designing the tunnel and its appurtenant facilities. In addition to the tunnel, the project will include the installation of a drop structure and an odor-control facility at the site of the existing preliminary treatment plant, as well as a pump station at the tunnel’s end to convey flows upward to the Kailua Regional WWTP. Together, the various facilities are expected to alleviate the existing overflow problems while eliminating the environmental concerns associated with the planned construction of the force main. “With the tunnel, we’re hoping to drastically reduce the amount of sewer overflows at both the pretreatment facility and the collection system,” Kahikina says.

The City and County of Honolulu has hired the Wilson Okamoto Corporation, of Honolulu, to design the tunnel project. Jacobs Associates, of San Francisco, is serving as a subconsultant to Wilson Okamoto for the design of the large tunnel, the first of its kind for Hawaii. Brown and Caldwell, of Walnut Creek, California, is designing the influent pump station. All of the design work is expected to be completed sometime during 2014, says Richard Harada, P.E., a project manager for Wilson Okamoto.

Because the design of the tunnel is ongoing, the exact dimensions of the structure have yet to be determined. However, the tunnel, which is expected to have a diameter between 10 and 13 ft, will be large enough to capture the excess wet-weather flows that would have been held by the storage tanks, Kahikina says.

Roughly 90 percent of the tunnel is expected to pass through hard rock, with the remaining 10 percent passing through soil. A tunnel boring machine will be used to construct the tunnel through the hard rock, but in areas with soft ground, jet grouting will be used to improve the soil before excavation is conducted by means of road-header equipment.

At the moment, the City and County of Honolulu does not know whether the tunnel and its components will cost more or less than the planned force main and storage tanks. However, the tunnel option is expected to be “more favorable” in terms of overall life-cycle cost, Kahikina says. This outcome can be attributed to such factors as the tunnel’s longer design life and the lower expenses associated with operating and maintaining a gravity tunnel versus having to pump flows through a force main.

To make room for the influent pump station to be built at the Kailua Regional WWTP, some unneeded buildings at the site will be removed beginning this December. Tunnel construction is scheduled to begin in late 2013 and end in June 2018.


 

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