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Sound Transit: A bridge — not a berm — at B Street

Post by Doug Conarroe on Sep. 11, 2009 at 1:49 pm |
September 14, 2009 5:29 pm

Sound Transit says it will build a bridge — not an earthen berm  — over the so-called B-Street Gulch within its project site to install commuter rail tracks through Tacoma’s Dome District.

The design change for that part of the project largely was spurred by community concerns about aesthetics and wildlife,  Jim Edwards, Sound Transit’s director of capital projects,  said this week.

“In the past several months,  we became aware of the potential for a habitat trail through the gulch area,”  Edwards said.  “That caused us to go back and reevaluate things.”

Although bridges typically cost more than berms do to build, Edwards said, the move likely will not increase overall project costs because it will off-set additional costs needed for that area under previous construction plans. Overall, the D to M Street project cost estimates remain at about $160 million, he said.

Sound Transit still plans to largely build an earthen berm to elevate and extend tracks over Pacific Avenue near 25th Street, as part of 1.4-mile project to connect the D to M Street Sounder rail lines.

Opponents support a so-called “post and beam” construction option they view as a less obtrusive alternative to a berm, which they argue would visually and physically divide the neighborhood.

Several neighborhood and environmental groups recently have sent letters and emails to city officials, lobbying for design changes at the B street ravine and other key points along the project site.  Some contend the ravine, the deepest point along the project site, is a key corridor for wildlife.

The new bridge design over the ravine will be a bridgedeck set on columns at either end, providing for an open structure, Edwards said.

“The bridge structure can easily be described as accomplishing what the post and beam intended to do,” Edwards said.

It allows the ravine to remain in its natural state and provides clearance for animal passage, he said.  It also eliminates additional costs under the previous berm design, which would have required expansive digging and installation of protections to subterranean city utility lines, Edwards said.

Sound Transit previously had planned to fill in the ravine with an earthen berm, which would have been the widest such berm — 75 to 80 feet — in the D to M street project area.  Most of the proposed berms along the line range from about 20 to 40 feet wide, he said.

Although the plans have changed at the B-Street Ravine, designs for the remainder of the D to M street site remain largely the same.

“We still believe, when we looked at the options and the terrain, the best alternative for the rest of the structure is the earthen embankment,” Edwards said.

Julie Anderson, Tacoma’s Deputy Mayor and a Sound Transit Board member, said earlier this week she has yet to see any cost estimates or design plans for the proposed change.

“From what I hear, (the new bridge design) creates a sense of permeability and addresses the environmental concerns and protects utilities,” Anderson said.

UPDATE: I’ve corrected the description of the rail line.  Thanks to the readers who caught the error.

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