Sewer lines in Hawaii typically run under roadways. But the city is considering a different approach for a system upgrade from Kaneohe to Kailua : tunneling under Kaneohe Bay or a nearby mountain ridge.

The city has identified two routes 25 feet to 40 feet below ground level as leading options to alleviate problems with the existing system that carries wastewater from Kaneohe to a Kailua treatment plant.

The lines , 3 feet to 13 feet in diameter , would stretch 3 miles and run through the earth either below Kaneohe Bay or under Oneawa Hills. The route under the bay would cost an estimated $85 million. There is no cost estimate yet on the mountain route.

The city filed an environmental impact statement preparation notice with the state last month outlining its conceptual plan focused on the two routes.

However, some observers familiar with the plan are concerned about what might go wrong, including possible leaks into the ocean, and question whether the city fully explored more conventional road alignments.

"I don’t want to write it off completely . … but it’s such a pipe dream — sorry for the bad pun — at this stage," said Robert Harris, director of the Sierra Club Hawaii Chapter.

The city says large tunnels for transportation, sewers and other infrastructure are common in many cities, and that there are already sewer pipes under Honolulu Harbor and Pearl Harbor.

"It’s not uncommon," said Jack Pobuk, capital improvement projects program coordinator for the city’s Department of Environmental Services.

Pobuk said three roadway route alignments were thoroughly evaluated and found to be substantially more expensive. Road alignments also would involve major traffic disruptions, he added.

Relying on the existing sewer main, which runs under Kaneohe Bay Drive, isn’t an option. No one wants a catastrophic failure of the aging pipe that’s already prone to sewage spills.

Problems with the system primarily involve damaged lines that allow ground water and storm water to enter and overload the sewer system, resulting in spills. During heavy rains, floodwater can also enter the system through manhole covers.

Various community groups, including the Sierra Club and the Surfrider Foundation, sued the city in 1992 because of wastewater discharges from the Windward sewage system.

The city resolved the lawsuit in 1995 by executing a consent decree with the federal Environmental Protection Agency. The decree and subsequent agreements laid out deadlines to make specific improvements over 20 years.

To date, the city has improved 12 miles of pipes in the region, and made other system upgrades.

Under the decree, a new discharge main running from the Kaneohe Wastewater Pre-Treatment Facility near Bay View Golf Park to the Kailua Regional Wastewater Treatment Plant next to Aikahi Park must be constructed and operational by 2014.

Project planning has been in the works for about three years.

Pobuk said three potential road alignments were rejected primarily because of cost.

A route along Kaneohe Bay Drive following the existing 42-inch sewer main is congested by the old line and a water main. It is possible to fit in the new line, but excavating so close to existing lines presents a hazard, and would result in a traffic nightmare and increased expense.

"You’d basically have to dig up the whole road, and we couldn’t see how that could be done and still allow traffic to get by," Pobuk said.

Another route, following Mokapu Boulevard, would have to cross Oneawa Hills, making it necessary to dig a significant tunnel or add a roughly $50 million pump station to get sewage over the ridge.

A third route along the H-3 Freeway is complicated by a state Department of Transportation prohibition on utility construction or maintenance that disrupts freeway traffic. Pobuk said the city would have to build a side road along the freeway, which makes that route cost prohibitive.

In that scenario, a 36-inch pipe would be pushed into a bore behind a steerable drill. A non-corrosive pipe would be sheathed in a steel casing for added protection.

Pushing the drill and pipe through the 3-mile route about 25 feet under the bottom of the bay might be possible, and wouldn’t disturb the marine environment, the city said. But there is a risk that it might not be possible to drill 3 miles in one direction, which would require excavation in the bay to join two segments pushed in from opposing sides of the bay.

The city could also opt to use a larger boring machine to dig a tunnel 9 feet in diameter below the bay. The tunnel would be lined with concrete and the 36-inch sewer main would run through it.

If the bay line is constructed, it would also include the addition of two enclosed storage reservoirs to prevent overflows during heavy rains. The old line would serve as a backup.

The bay line initially was the city’s preferred option. The project is in a preliminary design phase and is included in the 2011 fiscal year budget.

But recently, city engineers began looking into an alternative to the Kaneohe Bay route that would involve a larger tunnel running under the ridge line separating Kaneohe and Kailua.

The city said advancing technology of tunnel -boring machines that can line a bore with concrete segments to form a giant pipe has become less expensive.

"It looks promising," Pobuk said. A more detailed analysis, including construction and long-term cost estimates of an Oneawa Hills tunnel, is under way.

One advantage of an Oneawa Hills tunnel is that it would be dug at a slight decline so that sewage flows to Kailua via gravity. A pump would be used only at the Kailua end to send sewage into the treatment plant.

The 13-foot diameter of the finished tunnel also would make overflow reservoirs unnecessary, and the Kaneohe pre-treatment plant would be decommissioned.

Excavated rock could be used in other construction projects, the city said.

One downside to the Oneawa Hills plan is that construction would unlikely meet the 2014 deadline. EPA approval for a deadline extension and substituting a discharge main with a gravity tunnel would be needed.

Harris of the Sierra Club said he would like to see a comprehensive comparison of not just costs — but environmental consequences — for all the routes considered along roads, under the bay and beneath the ridge.

Mahealani Cypher, president of the Ko’olaupoko Hawaiian Civic Club, had a similar view, and said the city at a recent meeting with a community panel couldn’t assure her that it could adequately detect and correct leaks. The ridge line also raised concerns about effects on nearby homes and an area water reservoir.

"Both lines evoke concerns about potential environmental problems," Cypher said.

The city, in its EIS preparation notice, said no significant environmental impacts are anticipated from either route.

More detailed information on costs and environmental effects limited to the bay line and ridge line will be in a draft EIS expected to be published by the end of the year. The city could pick a preferred alternative around the same time.