Seattle plans to do an emergency water-main replacement in Pioneer Square in August, aggravating tunnel- and seawall-related traffic obstacles.

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Western Avenue will be torn open from August to October in downtown Seattle to replace an old cast-iron water main that city officials say was damaged by the nearby Highway 99 tunnel project.

Seattle Public Utilities (SPU) described the replacement timeline in a briefing to the Seattle City Council on Monday morning.

The existing water main, installed in 1900, settled approximately 1¼ inches last fall in Pioneer Square — while groundwater was being pumped next to damaged tunnel-boring machine Bertha. The settling was beyond the threshold that SPU assumes would damage its brittle water-main connections.

The quarter-mile main repair follows Western Avenue from Yesler Way to Spring Street.

The city decided to perform what it calls an emergency replacement of the 20-inch- diameter pipe this year, rather than prolong the situation and risk a sudden rupture. Water flow in the old pipe was reduced last winter, and flows will be diverted around the area using temporary pipes.

Construction crews will try to avoid road closures during peak traffic times at the ferry terminal, said Gavin Patterson, SPU waterfront-projects manager. Water service in the new 24-inch pipe is scheduled for November. Patterson added the new pipe ought to survive upcoming tunnel work that includes going under Western Avenue.

“It’s a steel pipe that has plenty of flexibility and allows for several inches of settlement, if that occurs,” he told the council.

The city blames the sinkage, where Western meets Yesler Way, on groundwater pumping from the tunnel project. Tunnel contractors last fall removed groundwater to relieve pressure around a 120-foot-deep access vault, where a giant crane lifted Bertha’s front end to the surface for repairs in March.

The entire water-main replacement is expected to cost $8 million, SPU said Monday, an increase from its spring guess of $4 million.

A city-state agreement says the Washington State Department of Transportation must pay for a new main if tunnel-related damage is shown to exceed certain criteria, in this case 0.92 inches of pipe movement. The two sides are still negotiating over who pays how much. State soil consultants have pointed to other possible factors, such as vibration from building projects, or the historic tendency of weak fill soils in Pioneer Square to sink.

SPU reported Monday that the soil near the water main has sagged an additional one-quarter to one-half inch since January.

Repairs continue on the front end of Bertha, which is now estimated to resume digging Nov. 23, with the four-lane Highway 99 tolled tunnel to carry traffic in March 2018.

Early next year, the giant 57-foot, 4-inch-diameter boring machine is scheduled to travel under the viaduct and under the new water pipe.