By MALI MAIN
UW News Lab
Construction on the Northgate Link Extension begins this month. Sound Transit will break ground Aug. 17 at a community celebration on the site of the future Roosevelt light rail station, at 12th Avenue Northeast between Northeast 66th and 67th avenues. During the next six months, Sound Transit will demolish the nearby QFC and surrounding buildings. Tunneling is not scheduled to begin until 2014.
Construction on the light rail between Capitol Hill and University of Washington started last summer. Along with it came the “Montlake Murmur.” Many Montlake, and some Capitol Hill, residents have complained to Sound Transit about loud noises and vibrations that seem to be coming from beneath their homes.
“You know when a car pulls up next to you at a stop light with the sub-woofer blasting and you can feel it as well as hear it?” wrote Montlake resident Kathleen Carroll in an email. “Well, imagine a gigantic sub-woofer going full blast running back and forth under your house!”
Carroll noticed a low rumbling just after construction started. Within a month, she said, the noise was “disrupting my sleep and rattling objects within my home.”
Another Montlake resident, Kari Olson, compared the sound to a ball-peen hammer the size of a VW bug being thrown down a flight of stairs.
Sound Transit spokesman Geoff Patrick said the cause was not the 670,000-pound machine tunneling through the earth, but the supply-train system designed by their contractor, Traylor/Frontier Kemper.
As the machine digs the corridor, the tunnel is lined with concrete segments brought into the tunnel on a supply train. The supply-train system is temporary, used only to carry workers, material and equipment during construction. The tracks were bolted to the concrete tunnel liner with steel ties. They were laid in segments as the tunnel was formed, sometimes leaving gaps between the rails of one track segment and the next. The faster the supply trains moved over the tracks underground, the worse the sound and vibrations became on the surface.
Patrick explained that Sound Transit’s contract with Traylor/Frontier Kemper didn’t include requirements about design of the construction track. “This is our primary lesson from the project,” he said. “We are going to specify conditions for temporary rail installation in our future Northgate Link Extension tunneling contracts.”
Patrick said Sound Transit made about $400,000 worth of improvements to the contractor’s supply-train system.
The improvements included putting rubber pads beneath the steel ties. The gaps in the rails were closed with Dutchmen, bars that join track segments to minimize the bounce and sound of the supply train. Sound Transit also ordered that the supply trains move at a slower speed.
Olson said she noticed a change after the improvements were made, but that every once in while the sounds and rumbling return. She said asking them to “please tone it down, with humor, gets results fairly quickly!”
Patrick emphasized the difference between tunnel construction and light-rail operation. When the tunnel construction is complete, the supply train tracks will be removed and the light-rail vehicle operation tracks will be installed. These tracks are not expected to cause noise or vibrations that would be detectable in homes above the light-rail tunnels.
Montlake resident Jeff Parke said he is concerned that Sound Transit has no plans to “monitor ground vibrations to make sure their prevention measures work. “ He says that the Sound Transit representatives don’t plan to “establish scientifically valid monitoring sites and collect vibration data.”
Patrick explained in an email that Sound Transit plans to test the system by running actual light-rail vehicles in the tunnels and evaluate the vibration and noise levels above ground.
Sound Transit established a 24-hour construction hotline, 888-298-2395. Residents can call this number to report any construction-related issues.
Mali Main is a journalism student in the University of Washington News Lab.Tweet