In the mid-'90s the Seattle area voted to build a light-rail mass transit system. The first phase running from downtown to the airport will begin operation sometime this year. The next segment will run from downtown to the University of Washington campus. The University of Washington link is currently in the design phase-- which I have been part of this winter. More specifically I'm working on the underground station that will be built next to Husky Stadium on the University of Washington campus.
You can find a summary of the UW Station design at:
UW light rail station design
The underground station, which is nearly eighty feet deep, is built using a braced slurry wall process to dig the hole. To build a slurry wall a series of 4'x12' shafts are dug into the ground around the full perimeter of the station-- about 70ft by 400ft. As each hole is being excavated, with a mechanical excavator, it is filled with a slurry of bentonite clay and water to keep the hole from collapsing. After reaching it's full depth, nearly 100 ft in this case, the hole is filled with concrete from the bottom up through a tremie pipe. As the hole fills with concrete the slurry mixture is pumped out and saved for reuse. When the hole is full of concrete a cage of steel reinforcing is dropped in.
When the slurry walls are in place around the full perimeter of the station the dirt in the center can start to be excavated. As the dirt is excavated a series of braces, each about 65ft long and weighing nearly 30 tons, are installed to support the slurry walls. My part has been to design the temporary bracing system that will support the two walls during excavation and prior to placement of the permanent concrete floors.
You can see a section through the station here:
UW station section view
Find out more about slurry walls at:
Wikipedia article on slurry walls
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