Oldcastle Precast, Stacy & Witbeck partner on vibration dampening light-rail

Written by Jenifer Nunez, assistant editor
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UTA

Oldcastle Precast collaborated with Stacy & Witbeck and Sound Transit in the engineering and manufacture of a 450-foot long prototype section of precast concrete "floating" rail.

The rail was made for the University Link U830 Light Rail System’s upcoming construction of a two-mile stretch of vibration-dampening railroad that will pass under the University of Washington’s Physics Department.

 

A prerequisite of the project funding was to maintain vibration levels equal to the preconstruction conditions under the University of Washington’s Physics Department. To achieve this, Oldcastle Precast and Stacy & Witbeck constructed the prototype as heavyweight, high-unit weight concrete, floating, precast slabs that rest on 7.5-inch thick elastomeric isolation bearing pads allowing the rail system to “float,” dramatically reducing the system’s harmonic effect on its surroundings.

The “floating” rail prototype is composed of 110 heavyweight precast floating slab segments; 82 standard four-foot long segments, weighing 11,000 pounds each, 14 specialized eight-foot long segments, weighing 22,000 pounds each and smaller pieces. Project tolerances were critically tight, ranging from 1/4-inch down to 1/32-inch.

The concrete density needed to be increased to 205-pounds-per-cubic-foot so hematite aggregate was used in the composition. The hematite material was chosen by Sound Transit because of its non-electro-magnetic properties and offered a low risk of negatively affecting the rail system’s electromagnetic interface cables that will run along the underside of the precast floating slabs. The system cable limited the possibility of other types of heavyweight concrete mixes.

Oldcastle Precast says initial testing of the “floating” rail, without trains, is indicating satisfactory performance, assuring Sound Transit that the project can be constructed to mitigate vibration of the rail.

The two-mile, vibration-damping railway section is part of Sound Transit’s University Link Light Rail project extending the current system 3.15 miles, all in tunnels and underground, with a double-track line beginning at the east end of the Pine Street Tunnel in downtown Seattle and includes two new stations, one at Capitol Hill and one at the University of Washington.

Service is expected to begin in 2016 which will connect the University of Washington and Capitol Hill neighborhood to downtown Seattle and the airport. University Link will serve the three largest urban centers in Washington State – downtown Seattle, Capitol Hill and the University District.

 

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