Aecom: Chinatown Station of the Central Subway Program – San Francisco

Chinatown Station is one of three stations of the Central Subway Program in San Francisco, U.S., as part of the Third Line LRT extension to downtown. The station is in the heart of Chinatown, a vibrant commercial and residential district with the city’s highest population density. The Chinatown Station is one of the most challenging engineering and underground construction projects in the U.S., due to its setting in a narrow street with heavy traffic, adjacent to historic buildings, numerous utilities and poor ground conditions.

Chinatown Station is one of three stations of the Central Subway Program in San Francisco, U.S., as part of the Third Line LRT extension to downtown. The total contract program value is $1.16B; Chinatown Station value is $350M. AECOM is the Owner’s Representative/PM/CM. Project stakeholders included FTA, agencies of the city of San Francisco and BART, as well as numerous local community associations. The station is in the heart of Chinatown, a vibrant commercial and residential district with the city’s highest population density. The Chinatown Station is one of the most challenging engineering and underground construction projects in the U.S., due to its setting in a narrow street with heavy traffic, adjacent to historic buildings, numerous utilities and poor ground conditions. The station was constructed using the New Austrian Tunneling Method (NATM). The station cavern excavation encountered mixed face conditions, with soft soils of the Colluvium and Colma Formations (dense, stiff and sandy clays) at the crown to weak and highly fractured rock of the Franciscan Formation (sandstone, shale, mélange) at lower levels. Due to ground variability and the associated risks, a robust design was implemented which included detailed Excavation and Support Class (ESC) and “toolbox” supports, as needed. Chinatown Station was mined using Conventional Tunneling (known as SEM or NATM), following the construction of two 6.2m diameter TBM tunnels. The station’s main elements are the Station Platform and Crossover caverns combined. The overall main cavern dimensions are 192m long by 16.7m wide and 13.1m high with an excavated cross section of 202m2, 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. The soil over the caverns varied from 16m at the north end to 25m at the south end. The excavation encountered mixed face conditions, with soft soils (dense, stiff and sandy clays of the Colluvium and Colma Formation) at the crown to weak and highly fractured rock of the Franciscan Formation (sandstone, shale, mélange) at lower levels. The structural support system comprises of fiber reinforced shotcrete and lattice girders as the initial lining and cast-in-place final lining over fully encapsulating waterproofing membrane. Due to ground variability and the risk of Conventional Tunneling, a robust design was implemented. A pre-support system comprised of grouted steel pipe arch canopies were provided and face bolts were used in the center drift. Probing ahead of the excavation was done because of ground variability and potential localized wet zones. To improve standup time, the Colma Formation was dewatered by deep wells supplemented from within the excavation by vacuum dewatering to reduce perched water pressures and to maintain the excavation stability. 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 of impacted structures to less than 12mm. To minimize risks and enhance collaboration, daily Excavation and Support meetings were held during construction. They provided an essential forum and quasi-concurrent agreement on the tunneling process and addressing risks.

This award seeks to recognise excellence in delivery of a tunnelling project or tunnelling-related element of a project up to the value of $500M that was completed between July 2019 and July 2020 Judges will be looking to reward the project that best meets or exceeds the design brief and/or client expectation but that also have sought to advance industry best practice and have worked in a truly collaborative way.