In the aftermath of last year’s soil settling in the Pioneer Square neighborhood, the city will replace a quarter-mile of water main and suggest more viaduct strengthening.

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Crews will soon start digging up Western Avenue in downtown Seattle to replace a water main damaged last fall by the Highway 99 tunnel project.

Ray Hoffman, director of Seattle Public Utilities (SPU), said workers will replace a quarter-mile of 20-inch pipe in the historic Pioneer Square neighborhood.

Hoffman said it’s crucial to fix the pipe soon, to reduce flooding risk from a rupture.

Replacement is expected to begin this spring, and could take eight weeks and cost $4 million.

“This is basically a protection of public safety,” he told the City Council on Monday. “Were this line to go, it would be a big mess in Pioneer Square.”

The cast-iron water main, connected by lead fittings, has sunk as much as 1.3 inches in three measured locations, SPU reported. The suspected cause was groundwater pumping by Seattle Tunnel Partners (STP), a state contractor. STP needed to relieve water pressure that otherwise might blow like a geyser into the concrete-lined, 120-foot-deep access pit that was built to reach the front end of the damaged tunneling machine, nicknamed Bertha.

The good news is that hundreds of survey measurements by the state, contractors and city indicate the ground has stabilized since January.

In addition, Todd Trepanier, state tunnel administrator, said inspections of 60 buildings showed no structural damage.

But officials still are reckoning with late-2014 settlement. The city’s utility and transportation executives predicted Monday the soil could droop further.

According to an agreement between the city and the state, sinking of more than 0.92 inches is presumed to damage a 20-inch iron water pipe. The pact also makes it the responsibility of the Washington State Department of Transportation (WSDOT) to pay for new pipe.

City Council President Tim Burgess, a tunnel supporter, commented:

“The only dispute involves whether WSDOT sends the check now, or waits to send it until the end of the construction.”

The state hasn’t offered to pay. Talks with the city were planned Monday afternoon. “The discussions are still ongoing,” state spokesman Chad Schuster said afterward.

Besides flood prevention, Hoffman said, SPU wants to reduce the risk of Western Avenue being torn up during a traffic overflow from some future closure of the nearby Alaskan Way Viaduct.

The viaduct is scheduled to close late this year while a rejuvenated Bertha burrows under its foundations before continuing toward South Lake Union. Or an emergency closure might conceivably be triggered by a sudden tunnel-related soil loss, by vibrations from the nearby seawall replacement, or even by an earthquake.

Speaking of the viaduct, a new city-funded technical review notes the previously reported, 1-inch height loss in a few columns near Yesler Way — in weak soils where the viaduct has now lost 7 inches since the Nisqually earthquake of 2001. About $8 million has been spent by WSDOT and STP already to support highway foundations there.

The city believes even more viaduct strengthening should be considered, said Jon Layzer, deputy director of the Seattle Department of Transportation.

Trepanier said he didn’t have a comment Monday about whether to further reinforce the foundations. He indicated the state would study the city’s technical reports.

Way back in 2010, the state’s bidding instructions required tunnel diggers to cause no more than 1 inch of viaduct settlement, a contractual limit that’s been reached.

However, that figure includes a safety margin, the state says. The question is, will routine soil settling when Bertha arrives, thought to be half to three-quarters of an inch, still leave the viaduct OK for drivers?

Dave Sowers, the WSDOT’s deputy tunnel administrator for operations, said the state’s team shares the city’s interest in finding answers — while Bertha is still being repaired.