Temporary pump station for light rail tunnel enters third month at 20th & McGraw

Dewatering well at 20th & McGraw. Image: Montlaker

A temporary pump station next to Montlake Elementary that has closed 20th Ave E since late June is beginning its third month of lowering the water table beneath the light rail tunnel under the street. Three wells were installed for constructing an emergency cross passage between the twin tunnels. When the cross passages are complete, the wells will be decommissioned and the water table can again rise above the tunnel.

Changes are coming to the pump station beginning this week. A larger pump and new sewer pipe connection will be installed to increase flow out of the ground. Sound Transit’s Roger Pence explains:

The contractor is installing a larger pump in one of the wells next week. With that pump in place, the output from the three wells is expected to be about 600 gallons per minute.

In about two weeks, they expect to remove the green water tank (Baker tank) and substantially reduce the size of the work zone. Personnel will often be onsite to monitor the operation of the wells.

A new sewer trench will connect the larger pump to the gutter pipe running down McGraw and the 60″ main under 19th Ave — allowing the large holding tank to be removed. Sound Transit is coordinating this work to ensure it doesn’t interrupt Wednesday’s first day of school at Montlake Elementary.

Several more weeks of pumping are still needed and despite the reduced footprint of equipment, the street will remain closed.