SEM Mining and TBM Tunneling progress at Sound Transit's Beacon Hill Station

Progress to date

Both shafts, the Main Shaft and the Ancillary Shaft have been constructed, and the Concourse Cross Adits North and South have been excavated. At 45ft width, these are the largest tunnels excavated in soft ground in the United States to date using the Sequential Excavation Method (SEM). To minimize the risk of ground loss in the extremely complex geology, the dual side wall drift excavation method was used. In addition, the ground was improvement by dewatering and jet grouting and special pre-support measures such as pipe umbrellas, spiling and grouting are implemented.

At present, tunneling of the South Bound Platform Tunnels is under way, and the works at the North Bound Platform Tunnel have recently been started.

The Tunnel Boring Machine (TBM) has completed its first drive from the West Portal to the South Bound Platform Tunnel. It is currently undergoing maintenance and will be pulled through the South Bound Platform Tunnel once the preparatory works are complete.

Project Overview

The Beacon Hill Station consists of a 181 ft deep, 46 ft inner diameter main shaft that will be connected to the head house and will house four high speed elevators, emergency staircases, ventilation shafts, mechanical and electrical equipment. A 26 ft inner diameter ancillary shaft will accommodate another set of emergency staircases and ventilation shafts.

From the main shaft, the 41 ft wide concourse cross adit will provide passenger and emergency access to the platform tunnels. The platform tunnels are 380 ft long by 32 ft wide and were designed to accommodate the platforms, artwork and architectural finishes, and the light rail tracks. Two cross adits will connect the platform tunnels, and ventilation tunnels will provide air flow in normal operation and for emergencies.

Geology

The Beacon Hill Station is being excavated within in glacial, overconsolidated, partly fractured or slickensided clays and tills. Intermittent sand and silt layers are present with multiple perched groundwater horizons. The high variability of the geology in the area of the future station poses the main design challenge for design and construction.

DSC Services

The Dr. G. Sauer Corporation (DSC) was retained as a specialty SEM subconsultant to the HMM/Jacobs JV for the design of the large station tunnels, adits, and platform Tunnels.

DSC’s design phase services included:

• Review and interpretation of the geologic information,
• design of excavation and support sequences,
• 3D Finite Element Analyses and structural and seismic design for the initial and final lining,
• development of the monitoring program for all mined structures,
• designed the membrane waterproofing system for all underground structures, including NATM tunnels,
  slurry wall shafts and headhouses. 

During construction, DSC is providing a senior NATM engineer, NATM shift engineers and inspectors, and an engineering geologist to perform resident engineering services and to support the Construction Management Team with the NATM expertise required for this complex project.  

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