The San Francisco Downtown Rail Extension (DTX) project, now known as The Portal, will include a 1.3-mile-long (2.1 km) rail extension being developed by the Transbay Joint Powers Authority (TJPA), in collaboration with the Program’s major stakeholders: the Metropolitan Transportation Commission (MTC), San Francisco County Transportation Authority (SFCTA), Peninsula Corridor Joint Powers Board–Caltrain, California High-Speed Rail Authority (CHSRA), the City and County of San Francisco, and the California Department of Transportation. The Portal will connect Caltrain’s regional rail system and the California High-Speed Rail Authority’s future statewide system to the Salesforce Transit Center in downtown San Francisco. The rail alignment will be constructed principally below grade to provide a critical link for Peninsula commuters and travelers on the state’s future high-speed rail system.
The Portal alignment begins in the below-grade Salesforce Transit Center rail station at 1st and Mission Streets. At the west end of the station, the station’s six tracks transition to two tracks through a cut-and-cover “throat” structure and continue in a mined tunnel southward under 2nd Street and westward under Townsend Street to a new underground station at 4th and Townsend Streets. West of the station, near 7th and Townsend Streets, the tracks ascend to grade via a U-wall, and the alignment continues southward at grade to 16th Street, just south of the existing Caltrain terminal station and railyard. A tunnel stub box extends side-by-side with the U-wall to allow for a connection to the future Pennsylvania Avenue Extension—a tunnel being developed by the SFCTA. Including revenue and nonrevenue at-grade trackwork and stations, the total construction length of The Portal is 2.2 miles (3.5 km).
The mined tunnel is 3,352 feet long (1,022 m) with a cross section varying from 50 to 60 feet wide (15–18 m) and 43 feet high (13 m) to accommodate two- and three-rail tracks, and will be excavated using sequential excavation method (SEM) in mixed-face soil and Franciscan Formation with ground cover ranging from 40 to 85 feet (12–26 m). As a subconsultant under the prime consultant Parsons, Delve Underground is responsible for preliminary design of the mined tunnel including two connecting ventilation adits and shafts.
To facilitate design of the mined tunnel, a detailed ground characterization was developed that identified rock mass types (RMTs), soil types (STs), and ground classes (GCs) based on available geotechnical data and interpretations from the project-specific geotechnical investigation. RMTs and STs were then grouped into four ground classes based on the similarity of anticipated ground behavior upon tunnel excavation. Each ground class was composed of RMTs expected to respond similarly to tunnel excavations and to require similar support systems.
The soil deposits present along The Portal mined tunnel alignment include artificial fill, Colma Formation and residual soil. The underlying bedrock is composed of Franciscan Formation (also referred to as Franciscan Complex in some published studies). The distribution of these units along the alignment within the tunnel horizon is indicated in the geological profile. The majority of the mined tunnel will encounter the Franciscan Formation. The north and south ends of the tunnel and a paleo valley in the central portion of the tunnel alignment will encounter soil deposits over Franciscan Formation bedrock (mixed face conditions).
Three support types with associated excavation sequences have been designed to address the initial support requirements for the anticipated ground conditions associated with four ground classes. ST I is applicable to GCs 1 and 2, and ST II is applicable to GCs 3 and 4 (at the buried valley). Finally, ST III is only applicable to GC 4 at the south and north portals.
Project development is currently being finalized, including packaging for different delivery methods, finalizing cost estimates, and contractor outreach. The tunnel, tunnel stub, and station excavation components of the project will be delivered through a progressive design-build contract. The cost estimate and project schedule are currently being updated and will be used to solicit funding for the project. The 30% design level cost estimate was presented to the TJPA’s Executive Steering Committee meeting on January 20, 2023, and an updated schedule for the project will be made public in early 2023.