The estimated cost of replacing the Elliott Bay sea wall is $300 million, less even than the conservative end of estimates that put the cost a year ago between $310 million and $390 million.

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The Seattle City Council got some good news on a big public construction project Monday: The estimated cost of replacing the Elliott Bay sea wall is $300 million, less even than the conservative end of estimates that a year ago put the cost between $310 million and $390 million.

“It feels so wrong to be excited about $300 million, and yet I am,” said City Council President Sally Clark at a briefing on the project by the Seattle Department of Transportation.

The City Council will vote Monday on moving forward with analyzing a 30-year property-tax bond measure for the November ballot. The council has until July to determine the amount and scope of the measure and decide whether to send it to voters.

Engineers say the aging sea wall, which runs the length of the downtown waterfront from South Washington Street to Broad Street, has been seriously weakened by gribbles and bore worms and could collapse in an earthquake, jeopardizing Alaskan Way, the Highway 99 viaduct, the state ferry dock, Port of Seattle operations and many waterfront businesses.

The lower cost estimate is due in part to the decision to employ a less expensive jet-grout technique to replace the rotting sea-wall timbers, rather than drilled shafts filled with concrete, said Jennifer Wieland, sea-wall project manager.

She said the jet-grout method performs better in earthquakes, but is still strong enough to last for 100 years. Drilled shafts may be used in some areas, she said.

The cost estimates don’t include rebuilding the city owned piers 62 and 63 and Waterfront Park, which have deteriorated. The piers are no longer safe for public access. The council could decide to include that work — estimated to cost between $80 million and $100 million — in the sea-wall measure to avoid tearing up the waterfront more than once.

The estimate also doesn’t include utility relocation, which is being done in conjunction with construction of a deep-bore Highway 99 tunnel and eventual demolition of the viaduct. The extent and cost of that work aren’t yet known, Wieland said.

While almost three-fourths of the estimated cost is to rebuild the sea wall and stabilize the landfill behind it, about 4 percent will go to restore shoreline and salmon habitat. A pocket beach is planned for the foot of South Washington Street, and a rebuilt sidewalk along Elliott Bay could feature glass blocks or panels to let light into the water below.

Wieland said that juvenile salmon’s eyes don’t adjust well to the shadows cast by the existing piers. The fish swim out and around the piers, making them more vulnerable to predators. Light also will allow more plants to grow, enhancing the habitat.

Bob Chandler, assistant director for strategic projects for the city Transportation Department, said the city is working with marine biologists at the University of Washington, the National Oceanic and Atmospheric Administration and regional Indian tribes on the best science to improve salmon migration and marine diversity.

The habitat restoration also could include creating more shallows, textured wall surfaces and offshore reefs. Those strategies have been used in other cities, but their effectiveness is still being studied, he said.

“We are pushing the science here. It’s a bit of a challenge to see how it works out,” Chandler said.

Lynn Thompson: 206-909-7580 or On Twitter @lthompsontimes.