Opportunity
Sixteen miles (25 kilometers) long, a light rail line called Link connects Seattle’s downtown area with SeaTac International Airport. One of the most challenging elements of this project was the deep mined Beacon Hill station and the one mile (1.6 kilometers) of twin “binocular” tunnels connecting it with Link.
The construction of the station required twin shafts and a complex configuration of vehicle, pedestrian, and ventilation tunnels ranging from 16 to 46 feet (5 to 14 meters) in diameter — all 160 feet (48 meters) underground.
Adding to the difficulty, the station and tunnels were constructed in ground that primarily consists of firm to hard clays but also includes zones of water-bearing sand and silt. According to the American Society of Civil Engineers, “the depth and dimension of the tunnels far exceeds anything done previously in soft ground in North America.”
Solution
As the lead joint venture partner, Mott MacDonald was responsible for overall project management and controls, and for the detailed design of all rail tunnels, portals, shafts, and mined station tunnels, including final linings and waterproofing.
The scope of work included scheduling, cost estimating, and contract drawings and specifications. Mott MacDonald provided design support during construction, including the engineering oversight of tunnels created using the Sequential Excavation Method (SEM).
The Beacon Hill station was mined from two vertical shafts, excavated to a depth of 185 feet (56 meters). The two shafts, one 46 feet (14 meters) in diameter and the other 26 feet (8 meters), act as entrances and exits and as ventilation structures.
SEM was used to construct the 380-foot (180-meter) platform tunnels, as well as concourse tunnels, cross-passage tunnels, ventilation adits, and concourse adits, which were 45 feet (13 meters) in diameter. Shotcrete was used for the initial linings.
The project included multistage excavation sequences, including twin sidewall drifts and single sidewall drifts. Stage-grouted barrel vault pipes and grouted pipe spiles formed the presupport for the SEM tunnels.
Twin 4,200-foot (1.3-kilometer) tunnels, one northbound and one southbound, were excavated using an earth pressure balanced tunnel boring machine. The tunnels were lined with one-pass precast segmental linings 18 feet 10 inches (5.8 meters) in diameter.
Outcome
According to Sound Transit, “The Beacon Hill Tunnel is very important to the Link light rail system. By the year 2020, approximately 3,000 people per day will board the train at the Beacon Hill Station, making it one of the most heavily used stations south of downtown Seattle.”
Optical survey and geotechnical instruments were used to monitor buildings and utilities during construction and detect any settling of the ground. A 24-hour work schedule allowed the ground to be excavated and supported in a way that maximized safety and the stability of the soil. A noise wall was constructed to reduce noise from the site.
The twin tunnels were designed with three cross-passages to meet the egress requirements of the National Fire Protection Association’s NFPA 130 standard.
The Beacon Hill station was opened for revenue service in July 2009. It includes four high-speed elevators, emergency staircases, ventilation shafts, and artwork on the platform, concourse, head house, and plaza. Riders can descend to the train in 20 seconds.
Awards
Reporting on the American Council of Engineering Companies’ Platinum Award for 2010, which was awarded to this project, the Society for Mining, Metallurgy & Exploration (SME) cited our use of a “risk-based design approach” in which “the design team actually planned the complete excavation, including the initial and final support systems for the very large diameter shafts and very deep tunnels.”
SME continued, “This approach to deep complex tunneling in very poor soils, at a depth and a diameter close to twice that previously done, resulted in the largest soft ground SEM tunnel in North America.”