Two Central Subway Projects in San Francisco win international awards

December 16, 2020

Each year since 2015 the International Tunnelling and Underground Space Association (ITA-AITES) has recognized the best tunneling and underground construction projects from around the world with its ITA Tunnelling and Underground Space Awards.

This year, two projects associated with the Central Subway Program in San Francisco, CA the Chinatown Station of the Central Subway Program and the Union Square Market Street Station, were honored by the ITA as Project of the Year (between €50M and €500M) and as the Innovative and Contributing Underground Spaces winner, respectively.

Chinatown Station is one of three stations for the Central Subway Project, a north-south LRT extension. Chinatown is a vibrant commercial and residential district with the city’s highest population density.

Chinatown station project was one of the most challenging engineering and underground construction projects in the United States due to its setting in narrow streets, historic buildings, numerous utilities and poor ground conditions. The station’s main elements are the Station Platform and Crossover caverns combined. The overall main cavern dimensions are 192 m (630 ft) long by 16.7 m (54.7 ft) wide and 13.1 m (43 ft) high with an excavated cross section of 202 m2 (2174 sq ft), making it one of the largest conventional tunneling excavation in poor ground and soil-like materials in the U.S. The main station entrance and service facility are located off street and connected to the platform cavern via a cross-cut cavern. This enables the construction of the station cavern in two directions simultaneously, running as many as four operations. Due to ground variability and the risk of conventional tunneling, a pre-support system comprised of grouted steel pipe arch canopies were provided and face bolts were used in the center drift. To protect adjoining buildings, including the Mandarin Tower and the historic Presbyterian Church, an instrumentation and monitoring plan was developed using multi-point extensometers, inclinometers, and total stationing. Compensation grouting was implemented to maintain the settlement in impacted structures to less than 12 mm (0.47 in.).

Also part of the Central Subway Program is the Union Square Market Street Station where battered drilled shafts were used a permanent ground support, earning this section of the project the award from ITA for Innovative and Contributing Underground Spaces.

The Central Subway Project in San Francisco, California. The entire project is a 2.7-km (1.7-mile) long subway extension with three underground stations and one surface station. The project will improve public transportation by extending the Muni Metro T Third Line through South of Market District, Union Square and Chinatown. The south entrance will be integrated with the existing BART/MUNI Powell Street Station, providing a direct connection between the two transit lines. To the north, the main entrance will be at the south-east corner of Union Square on Geary Street. A portion of the famous Union Square Garage and Plaza will be repurposed to house the main points of egress as well as two emergency ventilation shafts. The station is approximately 289 m (950 ft) long. Its overall configuration is divided into three areas: the North Concourse, the Platform and the South Concourse. The North Concourse is about 61 m (200 ft) long and 16 m (55 ft) below the roadway. The Platform is approximately 122 m (400 ft) long and ranges in depth from 90 feet to 100 feet. The South Concourse is about 250 feet long along Stockton Street and another 31 m (100 ft) as it wraps along Ellis Street. The depth ranges from about 6 to 9 m (20 to 30 ft) as it ascends north.
 

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