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Seattle utilities officials say tunnel groundwater pumping damaged water main

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SEATTLE — Seattle Public Utility officials say a downtown cast-iron water main must be replaced after being damaged by the nearby Alaskan Way Viaduct tunnel replacement project.

The existing water main under Western Avenue, installed in 1900, settled approximately 1ΒΌ inches last fall in Pioneer Square — while groundwater was being pumped next to damaged tunnel-boring machine Bertha, utilities officials told the Seattle City Council at a Monday briefing, according to the Seattle Times (http://bit.ly/1MsYhfk ).

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

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

The city will do an emergency replacement of the 20-inch- diameter pipe this year, rather than risk a sudden rupture.

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 said the new pipe should 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 said.

The city blames the sinkage, where Western meets Yesler Way, on groundwater pumping from the Alaskan Way Viaduct replacement 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.

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. 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.

Utilities officials said 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.


Information from: The Seattle Times, http://www.seattletimes.com

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