Commercial Shoring at BART's Transbay Transit Center

Joe Lazzaretti
Sep 14
8 min read

Updated: Sep 16

The Salesforce Transit Center in San Francisco, previously known as the Transbay Terminal, is a major transportation hub connecting systems like BART, Caltrain, and Muni. Its construction required advanced shoring techniques to stabilize the ground during excavation in a dense urban area. This article outlines how engineers addressed challenges like seismic risks, space constraints, and soil conditions to complete the project.

Key Takeaways:

Shoring Techniques Used: Cement Deep Soil Mixing (CDSM), soldier piles and lagging, and high-strength steel tiebacks.
Urban Challenges: Limited space, traffic disruptions, and active transit operations required creative planning.
Seismic Safety: Shoring systems were designed to handle earthquakes and ensure long-term stability.
Construction Coordination: Temporary supports and phased construction minimized disruptions to city life.

This project highlights the importance of engineering precision and collaboration in urban construction. Read on for an in-depth look at the methods and lessons learned from one of San Francisco's largest infrastructure projects.

Engineering Challenges in Urban Shoring for the Transbay Transit Center

The Transbay Transit Center project encountered a series of tough engineering hurdles. Working within an urban environment meant dealing with tricky soil conditions, limited space, and the constant demands of transit operations. Let’s dive into how these challenges shaped the shoring design.

Soil and Underground Conditions

The geological makeup of the site presented one of the biggest challenges in Bay Area construction. The soil, a mix of clays and sands, was prone to significant movement during excavation, which was no small feat considering the below-grade excavation spanned an enormous 1,500 feet by 180 feet and reached depths of 60 feet[1][3].

To tackle this, engineers turned to the Cement Deep Soil Mixing (CDSM) method. They embedded the CDSM wall nearly 100 feet deep and reinforced it with steel H-piles, wales, and cross-lot struts[1][3]. This approach strengthened the soil to depths of up to 150 feet (45.7 meters)[1]. A prototype test was conducted to ensure the method would work, focusing on factors like strength, permeability, and continuity.

Given San Francisco's seismic risks, traditional methods that used "free field" ground motions weren’t sufficient for such a large, flexible underground structure. Instead, engineers conducted detailed two-dimensional soil-structure interaction (SSI) analyses. These included fully-coupled direct solutions and substructuring techniques to model "scattered" ground motions, soil springs, and dashpots at critical points along the structure[2].

Space and Traffic Limitations

Urban space constraints added another layer of complexity. The tight quarters meant engineers had to get creative with shoring design and installation. For example, the temporary shoring process required shutting down Fremont Street, a key artery in the city, which significantly disrupted traffic flow[4].

Even after the street reopened, limited space meant support beams had to extend to street level, reducing the number of traffic lanes available[4]. Transit operations were also affected. During construction, bus services were rerouted to a temporary terminal to keep public transportation running[4]. Shoring supports were carefully placed on pads on the asphalt, extending up to the bus deck level. This allowed for quick installation with minimal disruption at street level but required close coordination with city traffic management systems[4].

Construction Phase Coordination

Managing shoring activities alongside the phased construction schedule was a logistical puzzle. The project had to balance multiple phases while ensuring the safety of nearby buildings and infrastructure.

In September 2018, cracks in beams forced the Transbay Joint Powers Authority (TJPA) to implement emergency shoring and close streets for a week[4]. This incident underscored the need for flexible shoring systems that could adapt to unforeseen issues.

The phased construction approach required shoring systems that could be modified and relocated as needed. Engineers designed temporary supports that could be safely removed and reinstalled in new configurations without compromising the structure’s stability. Additionally, constant coordination with city officials, traffic management teams, and transit authorities was crucial. This included managing street closures, relocating utilities, and ensuring construction didn’t interfere with other urban projects.

To stay ahead of potential issues, engineers relied on advanced modeling tools like Plaxis FEM and consolidation analyses. These tools helped predict system performance and allowed for real-time adjustments as the project progressed[3].

Shoring Methods and Systems Used

The Transbay project required shoring systems that could handle the challenges of urban construction and seismic activity. Engineers relied on proven techniques while introducing tailored solutions to manage construction loads and withstand earthquake forces.

Main Shoring Methods

One of the key techniques used was cement deep soil mixing (CDSM). This method involved blending cement with the existing soil to create a reinforced wall. Steel H-piles, wales, and cross-lot struts were then added to enhance resistance to lateral forces. Rigorous prototype testing confirmed the system’s strength, permeability, and ability to maintain continuity.

In areas where CDSM wasn’t feasible, engineers turned to soldier piles and lagging. This approach used steel H-beams combined with horizontal lagging, offering flexible support while simplifying utility relocation efforts.

Additionally, high-strength steel tiebacks were tensioned at specific angles to reduce the pressure on shoring walls. These combined techniques addressed the site’s challenges, including unstable soil, limited space in a dense urban environment, and the ever-present risk of seismic activity. These solutions ensured smooth transitions between construction phases while maintaining structural stability.

Temporary Support Structures

Given the phased construction and the need to keep streets operational, temporary support systems were crucial. Cross-lot struts were used to brace shoring walls against each other, distributing loads evenly across the excavation. When unexpected repairs were needed, prearranged pads allowed for the quick deployment of support beams, minimizing disruptions to traffic.

To maintain safe bus operations during construction, engineers installed adjustable steel props and beam supports for a temporary bus deck. Real-time monitoring systems, including inclinometers, load cells, and settlement gauges, were implemented to track wall deflections and load distribution, ensuring safety and optimal performance throughout the process.

Connection with Foundation Systems

Temporary shoring measures were seamlessly integrated into the permanent foundation systems to ensure long-term stability. By incorporating temporary CDSM walls into the final foundation design, engineers reduced material waste and streamlined construction. Reinforcement connections were meticulously planned to create a continuous load path from the temporary systems to the permanent structural elements.

Waterproofing solutions, such as membranes and drainage systems, were added to combat groundwater challenges. To prepare for seismic activity, engineers designed specialized connections that allowed the system to endure expected ground movements while maintaining structural integrity for years to come.

Case Studies: Solutions and Lessons Learned

The Transbay Transit Center project faced a series of engineering hurdles that pushed the boundaries of shoring techniques. These solutions not only addressed immediate challenges but also left a lasting impact on urban construction practices.

Earthquake-Resistant Design

To tackle seismic challenges, engineers developed shoring systems designed to withstand earthquake forces. These systems featured flexible connections to permanent foundations, allowing them to absorb seismic energy while maintaining stability. To enhance safety, real-time monitoring systems were implemented, providing instant feedback during tremors and ensuring quick responses to any structural shifts.

Solutions for Urban Limitations

Working in a dense urban setting required inventive approaches to logistics and installation. With limited space for staging materials, engineers adopted just-in-time delivery systems, ensuring shoring materials arrived exactly when needed. A modular design approach further streamlined the process, allowing components to be installed or removed efficiently without causing major disruptions to traffic or utilities. Noise-reducing installation techniques enabled extended work hours without violating local noise ordinances, keeping the project on track.

Key Lessons for Future Projects

The Transbay Transit Center project highlighted several critical takeaways for future endeavors:

Early Geotechnical Investigation: Thorough analysis of subsurface conditions early in the process proved essential for accurate design adjustments and avoiding costly surprises.
Collaborative Planning: Regular coordination among contractors, supported by integrated digital modeling, reduced delays and minimized rework. This collaborative approach became a cornerstone of the project’s success.
Integrated Systems: Incorporating temporary shoring elements into the permanent foundation system not only improved efficiency but also enhanced long-term safety and performance. Redundant monitoring systems further ensured reliability throughout the project lifecycle.

These lessons have set a new standard for managing complex urban construction projects, offering valuable insights for future infrastructure developments. The innovative methods and expertise applied here pave the way for the advanced techniques discussed in the next section.

Tools, Materials, and Contractor Expertise

The success of the shoring work at the Transbay Transit Center was built on a combination of specialized equipment, carefully chosen materials, and the expertise of skilled contractors.

Specialized Equipment and Materials

The temporary support system relied on heavy-duty hydraulic jacks, steel towers, and steel-pipe towers, creating a multi-level structure that extended from street level to the bus deck. In late September 2018, specialized pads were placed directly on Fremont Street's asphalt to anchor the supports effectively [4][5][6][7][8]. These components worked in unison to redistribute loads from the damaged beams, with the choice of materials playing a key role in ensuring the system could handle the structural forces involved.

Role of Contractor Expertise

Beyond the equipment, contractor expertise was essential in adapting the shoring installations to the challenges of an urban environment. Skilled teams worked efficiently to place the supports, ensuring the structure's stability while navigating the complexities of a busy city setting.

Monitoring and Quality Control

To address potential risks, additional towers were installed on the First Street side [5][8]. This proactive step reflected a commitment to industry best practices, combining equipment, expertise, and continuous monitoring to ensure the Transit Center's stability and reliability during the repair process.

Conclusion: Effects on Future Transit Infrastructure Projects

The Transbay Transit Center shoring project highlights how temporary shoring and flexible engineering solutions can navigate the complex demands of urban transit systems.

Key Lessons from the Transbay Project

This project underscored several important principles for managing urban transit challenges. Swift problem-solving, adaptable engineering methods, and a strong focus on safety helped minimize disruptions while ensuring the structure remained secure in a bustling city environment.

A Blueprint for Future Transit Work

The insights gained from the Transbay project provide a solid framework for tackling future urban transit initiatives. Combining temporary shoring techniques with continuous monitoring and expert collaboration can keep essential systems operational during critical repairs. This strategy is especially valuable for maintaining aging transit infrastructure without resorting to complete shutdowns.

FAQs

What are the benefits of using Cement Deep Soil Mixing (CDSM) for shoring in urban construction projects like the Transbay Transit Center?

Cement Deep Soil Mixing (CDSM) plays a crucial role in urban construction, especially in bustling areas like the Transbay Transit Center. By enhancing the soil's load-bearing capacity and reducing settlement, it ensures the stability and durability of structures over time. This makes it a reliable option for supporting large-scale infrastructure projects.

What sets CDSM apart is its ability to limit vibrations and disruption during construction - an essential feature for densely populated city environments. It also improves ground conditions without the need for extensive digging and offers a practical way to address seismic challenges. These qualities make CDSM an efficient and adaptable solution for tackling the complexities of urban construction.

How do engineers design shoring systems to stay safe and stable in earthquake-prone areas like San Francisco?

In earthquake-prone regions like San Francisco, designing shoring systems with seismic safety in mind is absolutely essential. Engineers adhere to stringent building codes and seismic design standards that take the area's frequent seismic activity into account. These systems often incorporate features like shear walls, bracing, and reinforced concrete to effectively withstand lateral forces.

Before any construction begins, engineers develop detailed shoring plans that undergo thorough review and approval by licensed civil engineers. These plans ensure that all safety regulations are met, including those specified in California's trenching and shoring standards. By combining advanced engineering methods with strict safety measures, these systems are designed to remain stable, even during major earthquakes.

How were traffic and transit disruptions minimized during the construction of the Transbay Transit Center?

During the construction of the Transbay Transit Center, traffic management strategies were thoughtfully designed to keep traffic moving and reduce disruptions for residents, businesses, and commuters.

To limit the impact on daily life, major construction activities - like cable replacements and other intrusive tasks - were scheduled at night. This approach avoided peak traffic hours, ensuring daytime transit services could function with minimal disturbance. These measures played a key role in maintaining urban mobility and easing the project's effect on the surrounding area.