Alameda Street Wet Weather Conveyance Tunnel in San Francisco

Introduction

The Alameda Street Wet Weather Conveyance Tunnel (AWWCT), approximately 3,800 feet long with a 12-foot-wide inside diameter, will accept and convey stormwater to existing downstream wet weather infrastructure. Lower-lying areas in the vicinity of 17th and Folsom Streets experience up to several feet of flooding during certain rain events. The new tunnel’s purpose is to improve reliability of the stormwater system in the flood-prone areas described, and it is designed to have a 100-year service life.

The AWWCT and associated infrastructure will manage stormwater to meet the San Francisco Public Utility Commission’s (SFPUC) Sewer System Improvement Program’s level of service within the project area to manage flows from a 5-year, 3-hour storm delivering 1.3 inches of rain.

The major design components of the AWWCT project include:

• Approximately 3,800 feet of 12-foot inside diameter tunnel
• Three shafts: Florida Street Receiving Shaft, Caltrans Launch Shaft, and De Haro Rotation Shaft
• Underground tie-in to the existing Channel Consolidated Transport/Storage (CCTS) Box
• Connection to a new upstream sewer box with passive flow control in the Florida Street Receiving Shaft
• Retrofit of four bents of US Highway 101 (US 101) viaducts to remove piles within the tunnel path
• Rebuild of the existing four-cell Division Street Sewer top-down and removal of piles
• Portions of segmental lining fully surrounded by soft marine clay

This article discusses the highway bent retrofit, the Division Street Sewer reconstruction, and the underground tie-in to the existing CCTS Box.

Project Considerations Launch Location and TBM Drives

As is typical in San Francisco, the bedrock (Franciscan Complex) undulates across the project alignment with approximately 1,300 feet entirely in rock and approximately 2,500 feet in sediments. Groundwater elevation is between 10 and 20 feet below the ground surface.

Because of San Francisco’s dense urban environment, the available property for launching is via amenable landowners, with a reasonable temporary easement agreement. This means launching the tunnel boring machine (TBM) from a central location, which is between two elevated highways: the northbound and southbound Interstate 80 (I-80) freeway. This creates two tunnel drives. The area surrounding the launch shaft contains parking lots, some of which include highway bents to work around.

The first TBM drive heads westward just over 1,800 feet along Alameda Street to Florida Street, where the TBM is received into a shaft that serves as a connection point for upstream sewer works designed as a separate project. The westward drive starts in soft soil and sands but is primarily within the bedrock. The primary obstacle with the westward drive is the Caltrans foundation piles, further detailed below.

The eastward drive is about 1,900 feet long heading down Alameda Street and onto Berry Street. The eastward drive is primarily within soft soils—Colma Sand and then into Young Bay Mud. The primary obstacles with the eastward drive are Young Bay Mud below the tunnel invert, a tight turn onto Berry Street, a 40-foot-wide four-cell box sewer on piles, a four-track commuter rail crossing, and an underground reception.

US 101 Foundation Modifications

The AWWCT alignment passes beneath four pile-supported bents of the Central Freeway Viaduct, connecting US 101 and I-80. The piled foundations were modified as part of a seismic retrofit program following the 1989 Loma Prieta earthquake. This retrofit placed several piles within the Alameda Street boundaries. Four of these piles, one at each of the four bents, conflict with the tunnel alignment.

Upon discovery of the conflict, the SFPUC worked with the California Department of Transportation (Caltrans) to determine the best path forward. With Caltrans’ concurrence, it was decided to keep the AWWCT alignment as originally proposed and to perform a highway foundation retrofit to remove the conflicting piles and replace them with ten new 24-inch cast-in-drilled-hole piles, compensating for any capacity loss. Construction would be staged to ensure stability of the highways and will require low overhead work to accomplish both pile installation and pile removal.

Division Street Box Sewer

The four-cell Division Street box sewer is a major component of SFPUC’s wastewater infrastructure, delivering a peak 21 million gallons per day (MGD) of dry weather flows and 1,560 MGD of wet weather flows. The tunnel alignment crosses this box sewer at an approximately 45-degree angle, making the crossing approximately 80 feet long. At this location, the bottom of the box sewer is about 5 feet above the crown of the sewer, and over 100 timber and steel pipe piles filled with concrete interfere with the tunnel alignment. To remove piles within the path of the tunnel, a top-down reconstruction of the box will be implemented with a new foundation system being established outside the tunnel limits.

CCTS Break-in

The AWWCT alignment terminates at an existing belowground structure—the CCTS Box. This terminus occurs within the City right-of-way, which is overlapped by a Caltrain franchise agreement area containing four commuter rail tracks (Caltrain provides local commuter rail services). The CCTS Box has an existing dry chamber that was constructed to accommodate future connections to the box. The chamber is only a few feet wider than the TBM but is long enough to allow for complete entry of the machine. The chamber is currently strutted crosswise at an elevation interfering with TBM reception.

To allow for TBM retrieval, the chamber will be backfilled, its struts removed, and it will be configured for TBM reception.

Summary

The new AWWCT is a key component of the SFPUC’s ongoing effort to improve flood resiliency for this low-lying, flood prone neighborhood. While the tunnel length is short, the project faces numerous complexities within this short stretch. This includes more than 100 piles within its path supporting critical existing infrastructure, navigation of rock and soil ground conditions, soft clay tunneling, a rail track crossing, and an underground reception. The project is expected to advertise a Request for Qualifications in Spring 2024.

Further information on this project and bidding information can be found at: https://webapps.sfpuc.org/bids/bidList.aspx

Renée Fippin, Delve Underground / Stantec, Joint Venture
Paul Louie, San Francisco Public Utilities Commission