Following months of noise and vibration complaints during tunnel construction under Montlake and Capitol Hill, Sound Transit has announced its plan for controlling noise and vibration once light rail trains start running in 2015.
The noise problem was traced last fall to the contractor’s supply trains running on temporary steel tracks bolted to the concrete tunnel lining. As supply trains shuttled equipment through the tunnel, they traveled on poorly built track causing vibrations to propagate through the earth and shake houses at the surface. Sound Transit and its contractor, Traylor/Frontier-Kemper, worked quickly to mitigate the problem in the worst affected areas with rubber pads and track improvements, while admitting to residents they were caught off guard since similar issues did not arise during Beacon Hill tunnel construction.
The agency’s ‘surprise’ at vibrations from slow-moving construction trains caused residents to ask how Sound Transit planned to handle vibrations from future light rail trains running up to 55mph? Agency officials addressed the issue at a community meeting last week, outlining how they have since studied the problem and taken steps to resolve it.
Tracy Reed, U-Link Deputy Project Director, and Derek Watry, vibration consultant from the firm Wilson Ihrig & Associates, explained the differences between the contractor’s supply track and the light rail operations track (construction scheduled for March 2013 through April 2015). In short, the supply track was built on the cheap. Its low-grade rail segments had gaps up to 1.5″ wide and were often installed at different heights. Every time train wheels rolled over a gap or bump, shock waves were sent through the rail ties, concrete tunnel lining, surrounding soil and house foundations at the surface.
While the rickety supply track resembled something from the 19th Century, the light rail operations track will be engineered to modern standards. It will have continuously welded track (no segment joints) and will have rails finished to match the same profile of the train wheels used for service. The light rail trains themselves also have double suspension systems to absorb any irregularities in the track.
The most significant measure for absorbing vibrations, however, will be the use of high-compliance fasteners. These are special fasteners that encase the rail ties in rubber, isolating them from the concrete track bed and tunnel walls. Last month, Sound Transit signed a $7.1 million contract to use high-compliance fasteners along the entire U-Link line. The same fasteners are currently in use on about 40% of the existing Link Light Rail line.
Sound Transit predicts these measures will dramatically reduce noise and vibration along U-Link. Data collected last December from “disturbed” homes showed noise levels as high as 40 decibels in the Shelby-Hamlin and Boyer Basin areas — the two shallowest points along the line (the Federal Transit Administration allows 35 decibels max in residential areas). An analysis by Wilson Ihrig claimed the worst-case light rail operation scenarios would reach a maximum of 20 decibels, with normal operations near or below the levels of human perception. Here is the result of the analysis:
Many residents at the meeting expressed appreciation for the steps taken to reduce disturbance, but not everyone was happy. Several Capitol Hill residents expressed anger and frustration at construction vibration that ‘shakes their walls.’ Tracy Reed explained this was due to work constructing cross-passages between the twin tunnels and that it would be temporary. Work on 21 cross passages will continue until early 2013 and the supply track will be removed at the end of this year.
The first light rail trains will enter the tunnel in mid-2015, with service expected to start in the fall of 2016. University Link is expected carry up to 70,000 passengers a day by 2030, with trains departing every 6-minutes during peak hours, making the journey between UW and Westlake Stations in 6 minutes.