Seattle LRT system
starts with bus tunnels
CASE STUDY: An extensive light rail network in Seattle began in the late 1980s with excavation of the Seattle downtown bus tunnels. Today the tunnels accommodate both metro trains and electric bus services and mark the beginning of several underground extensions of the LRT system for Sound Transit, including the deep Beacon Hill underground station on the extension south to Tukwila; TBM drives for the U-Link and North-Link extensions; and the EastLink services that share space with highway traffic through the famous Mount Baker Ridge tunnel and run under the streets of Bellevue in the Line’s new SEM tunnel. For Sound Transit LRT, it all started with the Seattle bus tunnels.
SITE VISIT REPORT: DECEMBER 1987
A
new underground street for buses only was Seattle’s answer to urban traffic congestion. Parallel 1.3 mile (2km) long tunnels, up to 45ft under the streets of the central district, will take up to 50% of the buses off the streets and increase the present 4 mile/hr commuter traffic flows to about 12 mile/hr in peak periods.
Tunneling began in November 1986 when the Guy F Atkinson/Dillingham JV won the contract with the lowest of eight competing bids at $44.2 million, well below the $61.6 million Engineer’s Estimate. Instead of the more usual 30-to-36 month construction period for a job of this scale, Seattle Metro, the client, imposed a tight 17 month program to have the system in service by early 1990. To meet this, Atkinson needed two reliable machines, one for each tube, designed, built and delivered quickly. The geology is mostly over-consolidated sands, silt and clay with the water table rising close to the surface south of Pioneer Square Station (Fig 1). North of the Station, the water table is well below the tunnel elevation and the dry over consolidated materials can be very hard.
“Ideally, a closed, pressurized tunneling shield would be better in the water bearing ground south of Pioneer Station,” said John MacDonald, Project Manager for Atkinson, “but there are deep tie backs anchoring the many sky scrapers along the tunnel route and to deal with these we need easy access to the face. Also, as the majority of the tunnel is in dry, competent ground, we chose two open-faced machines with breasting plates to hold the upper part of the face in softer wet conditions.” Of the various machine proposals, the one from Robbins was “technically superior and most economical.” Robbins is also Seattle-based making delivery easy and quick. The machines are quite different from the more familiar open-faced shields. To provide optimum face access, the backhoe paddle-type excavator is mounted on a tripod of inter-reacting hydraulic cylinders. Robbins has built two other machines to this design, one used on the Washington DC Metro in 1974 and the other used on the Caracas Metro in 1978. The 21ft (6.4m) diameter, 140 tonne, 600hp shields have a thrust capacity of 2,400 tonne via the 20 thrust rams 7
