Met Council defends mitigation for LRT in front of MPR

Rubber pads are preferred over steel springs to mitigate ground-borne vibration on Cedar Street in front of Minnesota Public Radio where Central Corridor LRT trains will operate, the Metropolitan Council said in response to MPR’s lawsuit.

Acoustical testing outside Minnesota Public Radio's studios on Cedar Street.

In this picture taken in February 2009, the Metropolitan Council conducts acoustical testing at Minnesota Public Radio's studios on the Central Corridor LRT alignment on Cedar Street. The Met Council also conducted testing to study the effects of LRT vibrations on MPR.

Noise and vibration impacts have been studied and can be mitigated.

See the Final Environmental Impact Statement. (85 pages, 7 MB pdf)

The suit, filed Feb. 4, contends the Met Council is reneging on an agreement to reduce noise and vibrations from Central Corridor trains. MPR argues that the Met Council, which is the grantee of federal funds for the project, concluded that rubber is preferable to steel solely on the basis of cost, not performance. This is untrue and ignores the mitigation agreement that says achieving the required performance at the lowest overall cost is a goal.

Here are the main facts:

Rubber pad is proven/steel spring floating slab is not
Rubber pad lessens vibration below ambient levels today at MPR
Rubber pad gets the job done most cost efficiently/steel does not

Background

Installing a rubber pad floating slab on Cedar Street would fully satisfy the performance standards as agreed upon to mitigate the effects of ground-borne noise caused by LRT operation on MPR’s broadcast studios and two neighboring churches.

Rubber-supported floating slabs have a proven history of meeting transit noise and vibration mitigation, with at least 50 examples of successful, in-place installations. The performance characteristics of rubber materials are well understood and can be reliably and confidently used for the Cedar Street alignment and for the climatic conditions experienced in the area. Cost estimates show that a rubber-supported slab would be about 30 percent less expensive than a steel spring-supported slab. The Central Corridor Project Office is confident in the accuracy of its cost estimates, which indicate a steel-spring system would cost over $1 million more than a rubber-supported system. There is no engineering reason that would support the Met Council’s expenditure of additional project dollars on a solution that is not required.

Rubber’s record

Rubber-supported floating slab tracks have been used for more than 35 years in at least 50 locations worldwide, including 11 in the United States. In the United States, the LRT line in Charlotte, N.C., is the only one known to be using steel springs. The Charlotte application is inside a convention center where LRT operates on a second-level bridge above the first-floor exhibit halls, not a recording studio. This indoor application is not subject to the vagaries of Minnesota winters and therefore not comparable to Central Corridor LRT, which will run on Cedar Street in front of MPR.

The most analogous rubber pad installation is at the New Jersey Performing Arts Center (NJPAC) in Newark, N.J., which is adjacent to a street-running LRT line. While that concert hall is farther from the track than MPR will be, the Newark concert hall is a category one noise sensitive performing arts center like MPR. This LRT system has been in use since July 2006 with no adverse impacts identified. In fact, Garrison Keillor is scheduled to broadcast a live performance of a “Prairie Home Companion” on April 17, 2010, from NJPAC’s Prudential Hall next to these tracks.

Cold-climate concerns

The rubber pads that would be used in the Central Corridor LRT Project’s design would not deteriorate due to freeze-thaw cycles because the rubber material that would be used in this application does not absorb water. The project office is not aware of comparable performance data for steel springs in similar environments. The performance of steel springs within roadways subjected to salt is also not fully understood. Steel corrodes in the presence of de-icing salts and electrical currents. Rubber is inert and not affected by salt or electricity.

The predicted maximum vibration level generated in MPR’s Maud Moon Weyerhaeuser Studio by two LRT vehicles passing on a rubber-supported floating slab track when the temperature is minus 22 is lower than the existing ambient vibration level. It is also lower than the vibration level generated by existing bus operations on Cedar Street.

Ease of replacement

In the unlikely event that floating slab track has to be replaced, each approximately 30-foot long section will be designed to be removable so components can be easily replaced. Meanwhile, LRT vehicles could operate using single-track running during replacement work.