On March 5, the second of two 4,300ft (1,3km) long TBM drives through Beacon Hill, in Seattle, broke through, marking the end of tunneling works on the scheme (T&TNA, May 2007, p19).
The 21ft (6.4m) diameter Mitsubishi bored through the headwall of the East portal after averaging 25ft (8m)/day through glacial till for Seattle’s Link light rail tunnels. T&TNA was told that the TBM had holed through within 5mm of its target and had achieved a best performance of 50ft (16m)/day.
“Link light rail in Seattle truly is seeing the light at the end of the tunnel, or tunnels in this case,” said Sound Transit Board Chair and Seattle Mayor, Greg Nickels. The tunnels form part of the Central Link scheme, on which construction got underway in 2003. It is currently about 90% complete and on schedule to begin service between downtown Seattle and Sea-Tac International Airport in 2009.
Sound Transit’s contractor, Obayashi Corp, launched the 100m long TBM on this, its second, trip through Beacon Hill in July last year. Along its route the TBM installed over 800 pre-cast concrete segments. The tunnels connect through a NATM/SEM station 50m below the surface of Beacon Hill. Obayashi is building this facility, together with an elevated station at Mt Baker.
Test runs of the light rail trains are currently underway between Sound Transit’s Operations and Maintenance Facility and the Downtown Transit Tunnel. Further tests are scheduled at the end of the summer.
T&TNA was told that more than 6 million manhours have gone into construction since the project broke ground, with the recordable injury rate for 2007 at 4.32 against a national average of 5.9 for construction projects.
Such achievements of the workers was paid tribute by Nickels: “Later this summer we will be ready to roll Link light rail trains through the hill as we prepare for passengers in 2009. This is a major milestone for the men and women who have been a part of the construction.” n
Celebrations at the East portal of Beacon Hill Celebrations at the East portal of Beacon Hill
