Building light rail

Surface

Surface rail runs at street level. Construction can, but doesn't always, include:

  • Site preparation such as building demolition, removal of the existing roadway or infrastructure, widening the right-of-way or removing contaminated soil
  • Temporary road construction with temporary signals and access to properties
  • Utility installation and/or relocation
  • Excavation for tracks
  • Installation of systems infrastructure such as ductbanks and drainage that runs along the track
  • Track installation
  • Station construction
  • Replacement of the street and sections of sidewalk
  • Overhead wires and traffic signal work
  • Final street restoration, landscaping and installation of permanent street markings

Elevated tracks

There are two methods of building elevated tracks, and projects often use a mix of both.

  • Cast-in-place: where support forms are built and concrete is placed in the final location
  • Pre-cast: where supports are constructed off-site and then placed using a gantry crane

A gantry crane was used to hoist the pre-cast pieces into place for the initial Central Link segments near Sea-Tac Airport, in SODO and the east portal of Beacon Hill.

Tunneling

Tunnels are typically built using one of three methods: Bored, cut and cover or sequential excavation. 

Bored tunnels
Modern tunnel construction uses sophisticated equipment called a tunnel boring machine, (TBM). The TBM, which is about the size of a football field and weighs several hundred tons, can dig through all types of soil from rock to sand. This huge machine not only digs through the earth, it builds the concrete tunnel liner and seals the pieces together. On a good day, a TBM can bore and build as much as 100 feet of tunnel. An operator steers the machine using positioning technology that is accurate to within an inch. Some TBMs can even be operated remotely. 

Cut-and-cover tunnels
Cut and cover tunnels are built from the top down, with much of the work taking place at the surface. A contractor first digs a trench along the entire length of the tunnel. When it's deep enough, a roof is built over the trench to create the tunnel. 

Sequential excavation mining
Sequential excavation mining is primarily used for smaller excavations - cross passages or stations. The work is largely done by hand, without the benefit of sophisticated machinery. The dirt is removed in a series of small, carefully planned sections and quickly supported by spraying concrete on the face of the excavation.