In late October, T&TI was told that sequential excavation mining, described by the site team as “NATM with the lessons learnt” is currently underway on the 49m long station concourse for Seattle Sound Transit’s Beacon Hill light rail station and tunnel project in the US.
The 15.2m diameter main shaft for Seattle Sound Transit’s Beacon Hill light rail station and tunnel project in the US has already been excavated to a depth of approximately 46m. At 13m wide by 11.6m high, the concourse will serve as the connection between the shaft’s high-speed elevators and the new 1.3km long northbound and southbound running tunnels. The same method will be used for the 152m long station platforms that will be 10.3m wide by 9.1m high.
The entire project is located within glacial till soils. Two years ago, Sound Transit excavated a test shaft to investigate the soils and their behaviour. T&TI was told a direct consequence of this was to modify the station plans to encase the vertical shafts in a slurry wall. At 0.9m thick, a site source said the slurry walls have proven “very successful at avoiding flowing water conditions that otherwise would have been present at some locations”. In addition, jet-grouting was carried out to stabilise soils where geotechnical investigations had highlighted concerns.
Main contractor, Obayashi, is preparing the headwall and assembling trailing gear behind its Mitsubishi Heavy Industries (MHI) EPBM, in preparation for tunnelling to get underway this winter. At an advance rate of 12m per day, the bored tunnels are expected to be completed by late next year.
