Central Subway Project, San Francisco – Railway Technology

This article explores the engineering and logistical complexities of the Central Subway project in San Francisco, a significant undertaking aimed at enhancing the city’s public transportation network. The project’s primary objective was to create a vital north-south light rail connection, alleviating congestion and improving transit times within a densely populated urban area. This involved the construction of a new subway tunnel, incorporating four new stations, and integrating the existing T Third Line. The project’s scope extended beyond simple tunnel construction; it encompassed extensive environmental impact assessments, complex utility relocation schemes, and the intricate coordination of numerous contractors and funding sources. Examining the project’s various phases, from initial planning and environmental review to the deployment of advanced tunnel boring machines (TBMs) and station construction, illuminates the challenges and triumphs inherent in large-scale urban rail development. This detailed analysis will highlight the project’s innovative engineering solutions, its impact on the San Francisco transportation system, and the lessons learned that can be applied to future transit infrastructure projects.

Tunnel Construction and Methodology

The core of the Central Subway project involved constructing a new subway tunnel using two TBMs, aptly named “Big Alma” and “Mom Chung.” These 6.3m diameter machines, manufactured by Robbins, excavated both soft and hard rock formations at a depth of approximately 140m. The southbound and northbound tunnels were bored respectively by the two TBMs, equipped with mixed ground cutterheads utilizing knife edge bits and carbide tools. Shaft-type screw conveyors facilitated efficient material removal. The deep tunnelling method was chosen to minimize surface disruption, a crucial consideration in a bustling urban environment. However, station construction employed different methods: the sequential excavation method (SEM) at Chinatown Station, and cut-and-cover techniques at Union Square and Moscone Stations, along with the tunnel portals. The SEM method, also known as the New Austrian Tunneling Method, involved constructing temporary support walls, slurry walls, and trenches, supplemented by secant piles for ground stabilization. The cut-and-cover approach allowed for a more conventional top-down or bottom-up construction process for these stations. A robust waterproofing system was installed between the primary and secondary tunnel linings after excavation, ensuring the longevity and structural integrity of the tunnel. Track installation utilized direct fixation for the tunnel sections and embedded track within concrete slabs for the surface street sections.

Station Construction and Design

Four new stations were integrated into the Central Subway system: 4th and Brannan Station, Yerba Buena/Moscone Station, Union Square/Market Street Station, and Chinatown Station. The design of these stations was a collaborative effort by a joint venture (JV) encompassing Parsons Brinckerhoff, Michael Willis Architects, and Kwan Henmi Architects. The stations were strategically located to serve key areas of San Francisco, providing convenient access to major transportation hubs and commercial districts. The construction of the stations, a significant component of the project, was undertaken by Tutor Perini under a contract valued at $840 million. This phase required meticulous planning and execution to minimize disruption to existing infrastructure and surrounding businesses.

Project Management and Funding

The Central Subway project showcased a complex web of collaborations and funding partnerships. A joint venture of Aecom and EPC Consultants managed the program and construction, guided by a $147 million contract. The project’s financial backing was diverse, demonstrating the collaborative nature of such large-scale infrastructure projects. The Federal Transit Administration (FTA) provided approximately half of the project’s budget through its New Starts fund, with the remaining funds secured through state and local agencies including the State of California, the Metropolitan Transportation Commission, the San Francisco County Transportation Authority, and the City and County of San Francisco. This diverse funding structure underscored the project’s importance to various governmental levels and its impact on the wider regional transportation network. The management of this complex funding stream was overseen by Hill International under a separate contract that focused on cost and schedule control, grant management, and risk mitigation.

Integration and System Deployment

The successful integration of the Central Subway required seamless coordination among various contractors specializing in different facets of the project. Parsons Brinckerhoff and Telamon JV handled utility relocation and tunnel design, while Synergy Project Management focused on utility relocation for Union Square/Market Street Station. HNTB and B&C Transit JV were responsible for system integration, ensuring that the new line functioned optimally with existing San Francisco transportation infrastructure. Auriga provided the crucial design for the Supervisory Control and Data Acquisition (SCADA) and communication systems, forming the nervous system of the new transit line. The meticulous management of these diverse tasks ensured that the final integration would run smoothly and efficiently, integrating with the broader Transbay Transit Center which aims to consolidate many city transit systems.

Conclusions

The Central Subway project stands as a testament to the engineering prowess required to build significant transit infrastructure in a dense urban environment. The successful completion of this project demonstrates the feasibility of integrating complex tunneling technologies and diverse construction methodologies to achieve ambitious transportation goals. The collaborative efforts of numerous contractors, governmental agencies, and funding bodies highlight the necessity of cross-sector partnerships in large-scale infrastructure development. The project successfully delivered an enhanced north-south transit corridor connecting key areas of San Francisco, reducing travel times and improving overall connectivity. The lessons learned from the Central Subway project, encompassing aspects of planning, execution, and funding, provide valuable insights for future urban rail projects worldwide. The project’s long-term impact will be felt for decades to come, as it contributes to improved mobility, economic growth, and enhanced quality of life for San Francisco residents and visitors. The innovative use of TBMs, the strategic deployment of various construction methods tailored to site-specific challenges, and the successful management of a complex financial structure, along with the smooth system integration all contributed to a successful completion.

Company Information:

• Robbins: Manufacturer of Tunnel Boring Machines (TBMs).
• Aecom and EPC Consultants: Joint venture providing program and construction management services.
• Tutor Perini: Contractor for station construction.
• Barnard Impregilo Healy: Joint venture undertaking tunnelling activities.
• Parsons Brinckerhoff, Michael Willis Architects, Kwan Henmi Architects: Joint venture responsible for station design.
• Parsons Brinckerhoff and Telamon: Joint venture performing utility relocation and tunnel design.
• Synergy Project Management: Utility relocation for Union Square/Market Street Station.
• HNTB and B&C Transit: Joint venture providing system integration works.
• Hill International: Provided capital program and controls system services.
• Auriga: Provided SCADA and communication system design.