Seattle Beacon Hill Station C170

Contract C170


Sound Transit, Central Puget Sound, Regional Transit Authority,
Seattle, WA,


Obayashi Corp.
BeMo Tunnelling as Consultant for SEM/NATM: Supervision, Know-how Transfer, SEM/NATM Tunnel Project Manager, SEM/NATM Tunnel Project Engineer and 3 SEM/NATM Tunnel Superintendents on site

Project location

Seattle, Washington,

Start of construction


End of construction


Project total value

3.0 million €


Highly variable glacial deposits including water bearing sands and silts approx. 50 ft below multiple perched water tables in an urban setting (very unique and challenging)

Technical Data

• Know-how Transfer of the Sequential Excavation Method (SEM) / New Austrian Tunnelling Method (NATM) for the construction of an underground station
• Main shaft with headhouse:
55 m (181 ft) deep x 15.8 m (52 ft) diameter
• Ancillary ventilation shaft with headhouse:
50.5 m (164 ft) deep x 9.3 m (30.5 ft) internal diameter
• Two platforms (north and south): 116.7 m (383 ft) long.
• Two councourse cross-adits: 19.5 m (64 ft) long
• Tunnels excavated by SEM/NATM, excavator in soft ground with shotcrete lining and excavation in multiple drift sequences (ground conditioning and pre-support where needed). Final lining by cast-in-situ steel fiber-reinforced concrete, with conventional bar reinforcement at junctions.

Further project related information

This deep “binocular” station is being mined through some of the most challenging soft ground conditions in the USA.
The excavated volume of the station is approximately 60,000 cy (46,000 m³) and the station comprises a variety of geometries and cross sections ranging from 235 SF (22 m²) up to 1,670 SF (155 m²).
This underground complex includes platform, concourse, cross-passage and emergency ventilation tunnels together with station egress and ventilation shafts. Various excavation sequences are in use for the different tunnels including the twin-sidewall drifts for the impressive 45 ft (13.7 m) wide by 42 ft (12.8 m) high concourse cross adits. This station has two train platforms approx. 165 ft (50 m) below the surface.