710 Beacon Hill Station and Tunnels

Beacon Hill Station and Tunnels is a segment in the new 14-mile Central Link light rail line, a critical component in Sound Transit's long-term regional transportation network. The station, one of 12 new stations on the line, is expected to serve approximately 3,000 people a day by 2020.

The twin 4,200-ft-long tunnels, one northbound and one southbound,  were excavated using an earth pressure balance tunnel boring machine and lined with one-pass precast segmental linings measuring 18 ft 10 in. internal diameter. The design includes three cross passages to meet the National Fire Protection Association's 130 egress requirements.  Tunnel construction is in soft ground that primarily consists of firm to hard clays, but also includes water bearing sand and silt zones.

The Beacon Hill station will be mined from two shafts: one 46 ft in diameter, the other 26 ft in diameter. The shafts are 185 ft deep and act as both entrance and exit and ventilation structures. The 380-ft-long platform tunnels plus connector tunnels, concourse adit, ventilation adits and cross-passage tunnels will be constructed using the sequential excavation method (SEM) using shotcrete initial linings that take advantage of ground relaxation to reduce loading. The large diameter concourse adits measure 45 ft in diameter. Stage-grouted barrel vault pipes and grouted pipe spiles will form the presupport for the SEM tunnel. Additional tool box support items will be used where necessary. The support method will be selected onsite after ground conditions are assessed. The project includes multistage excavation sequences, including twin sidewall drifts and single sidewall drifts.

As the lead JV partner, HMM was responsible for overall project management and controls, as well as detailed design of all tunnels and portals, shafts, and mined station tunnels, including final linings, and waterproofing. The scope of work included scheduling, cost estimating, and contract drawings and specifications. HMM also provided design support during construction, including the engineering oversight of the critical SEM excavation and support activities.