Case Study: Diaphragm/Slurry Wall Shoring at Yerba Buena Tower, San Francisco

Joe Lazzaretti
Jul 21
10 min read

Updated: Sep 2

The Yerba Buena Tower project in downtown San Francisco required a 66-foot-deep, 7-level basement in a dense urban area near critical infrastructure like the BART and MUNI tunnels. To manage soil movement, seismic risks, and groundwater challenges, diaphragm/slurry wall shoring was chosen. This method ensured minimal soil displacement, protected nearby structures, and provided both excavation support and waterproofing. Key results include:

Soil displacement control: Horizontal displacements were limited to 0.85 inches near the excavation.
Groundwater management: Slurry walls acted as barriers, eliminating the need for extensive dewatering.
Seismic resilience: Internal bracing distributed seismic loads effectively.

The process involved trench excavation supported by bentonite slurry, steel reinforcement installation, and concrete placement using tremie pipes. Monitoring systems tracked soil and wall movements in real-time, ensuring safety and precision. This approach reduced costs by reusing materials and integrating excavation and foundation systems.

The Yerba Buena Tower project demonstrates how diaphragm/slurry wall shoring can address complex urban construction challenges while maintaining safety and efficiency.

Diaphragm/Slurry Wall Shoring Process

Construction Steps and Procedures

The diaphragm wall construction at Yerba Buena Tower followed a carefully planned seven-step process, ensuring both precision and safety. Each phase was closely monitored to protect the nearby infrastructure and maintain quality standards.

The process began with site preparation, which involved clearing the area and installing guide walls along the planned diaphragm wall alignment. These guide walls acted as reference points to ensure the trenches were excavated accurately and vertically aligned throughout the process [3]. Once the site was prepared, crews moved on to trench excavation.

During the trench excavation phase, workers dug rectangular panels that were later joined to create a continuous wall. The trenches were excavated in stages using hydraulic grabs, clamshells, and cutter wheels, all designed to maintain stability as the work progressed [2][3].

A vital part of the process was slurry preparation and circulation. A bentonite slurry was continuously circulated within the trenches to stabilize the walls, preventing them from collapsing or allowing groundwater to seep in [3]. Project teams monitored and adjusted the slurry composition throughout the process to ensure the trenches remained stable.

Once the trenches reached the required depth, prefabricated steel reinforcement cages were installed to provide the necessary tensile strength. Joint elements were also positioned at this stage, preparing the trench for the next step: concrete placement [3][2].

Concrete placement was carried out using tremie pipes, which delivered concrete to the bottom of the trench. This technique displaced the bentonite slurry upward, creating a solid, continuous wall without causing segregation [3].

In the slurry removal and recycling phase, the excess bentonite slurry was removed from the top of the trench after the concrete had set. The slurry was then processed through a desanding plant, cleaned, and reintroduced into the system for reuse. This approach minimized waste and helped manage costs [2]. Following slurry removal, crews conducted finishing work and quality control checks.

The final step, finishing and quality control, ensured that each panel met the required standards. Teams cleaned exposed surfaces, inspected the dimensions, verified concrete quality, and checked the overall structural integrity before moving on to the next panel [3].

Equipment and Technology Used

The Yerba Buena Tower project relied on advanced equipment and technology to navigate the challenges of urban construction and achieve the high level of precision required.

Specialized excavation equipment was essential for creating the diaphragm wall. Hydraulic grabs, such as Bauer's DHG V model, handled most of the excavation work, efficiently loosening and removing soil. For harder soil conditions or deeper trenches, trench cutters provided the additional power and precision needed. These tools were capable of excavating trenches up to 12.5 feet long and 4.9 feet wide [2].

Slurry management systems played a key role in maintaining trench stability. Slurry mixing plants were used to prepare bentonite slurry to exact specifications, while desanding plants removed solids from the slurry, allowing it to be reused [2].

To meet the strict tolerances required in urban construction, precision monitoring technology was employed. Verticality measuring systems ensured the trenches were properly aligned, preventing interference with adjacent structures. Additional alignment monitoring systems provided further accuracy throughout the process [4][5].

For concrete placement, tremie pipes were used to deliver concrete evenly at the bottom of the trench. This method ensured the slurry was displaced efficiently while preventing the concrete from segregating [3].

Malcolm Drilling, a leader in the field, provided a wide range of equipment for the project, including mechanical and hydraulic grabs and diaphragm wall cutters capable of excavating panels to depths over 300 feet. Supporting equipment such as pumps, desanders, desilters, and centrifuges maintained the slurry circulation system. The seamless operation of these systems required meticulous planning and skilled operators [6].

Engineering Challenges and How They Were Solved

Geotechnical and Structural Problems

This project faced a host of challenges, especially with geotechnical and structural stability. Located near the San Andreas, Hayward, and Calaveras faults, the site was just 8.5 miles from the San Andreas Fault, making it vulnerable to magnitude-7 earthquakes [1]. The bedrock, primarily the Cambridge Argillite formation, was highly altered and contained zones of soft, clay-like kaolinized material [1].

Another major hurdle was hydrostatic pressure, which posed risks of wall instability and flooding. The excavation extended below the water table, adding complexity to groundwater control. Compounding these issues, the project was carried out in a tightly packed urban environment in San Francisco. With critical infrastructure in the vicinity, even minor soil movement could have led to significant consequences [1]. Tackling these challenges required precise planning and innovative engineering.

Solutions and Approaches Used

This project exemplified how advanced engineering techniques, continuous monitoring, and coordinated construction can overcome complex urban construction obstacles. These are the same types of sophisticated approaches that Golden State Shoring brings to its projects.

To address structural support and groundwater issues, the project engineers opted for slurry walls. These walls served a dual purpose: they provided the necessary structural stability and acted as waterproof barriers, resisting both lateral earth pressures and hydrostatic forces. This approach eliminated the need for extensive dewatering, which could have destabilized neighboring structures [1]. Internal bracing was added near the excavation base to control deflections, effectively distributing seismic loads and minimizing wall movement during potential earthquakes [1]. Golden State Shoring has extensive experience with similar slurry wall installations and groundwater management systems.

Monitoring played a pivotal role in the project's success. Inclinometers were installed across the site to continuously track soil movements and wall deflections. This real-time data allowed engineers to identify and address any concerning trends before they could affect the surrounding infrastructure [1]. Golden State Shoring employs the same state-of-the-art monitoring technologies on its projects to ensure safety and structural integrity.

Groundwater management was seamlessly integrated into the slurry wall system. During construction, trenches were supported with bentonite slurry, which prevented groundwater from flooding the site. Once concrete was poured, the completed walls formed an impermeable barrier, eliminating the need for extensive pumping operations [1]. This integrated approach to groundwater control represents the type of comprehensive solution that Golden State Shoring provides for challenging site conditions.

The project results validated these methods. Along Market Street, horizontal soil movements were limited to just 0.85 inches at 5.5 feet from the excavation, reducing to 0.75 inches at 22.5 feet away [1]. Near the Marriott Hotel, heaving reached up to 1.2 inches, while settlements along Stevenson Street and the 735 Market Street building were measured at up to 1.8 inches [1]. The largest soil movements, 2.7 inches, occurred in the upper 10 feet of the fill layer during slurry wall construction. However, these movements were anticipated and effectively managed through rigorous monitoring [1].

This meticulous approach ensured safety and efficiency, demonstrating how tailored solutions can address complex urban construction challenges. Golden State Shoring applies these same proven methodologies and precision monitoring techniques to deliver successful outcomes on similarly complex projects throughout the region.

Project Results and Performance

Performance Metrics, Costs, and Timeline

The Yerba Buena Tower project successfully met its performance benchmarks, particularly in evaluating the slurry wall's effectiveness. One standout metric was wall deflection, which remained far below the typical range of 0.2% to 0.3% of the excavation depth. This low deflection highlighted the efficiency of the internal bracing system in keeping deformations well beneath the excavation base [7][1].

Surface settlement was also tightly controlled, ensuring that surrounding infrastructure remained protected. Comprehensive monitoring confirmed that this approach minimized ground disturbances, a critical factor in San Francisco's dense urban setting [1][7]. Additionally, the diaphragm wall and its bracing system managed structural forces effectively, staying within design parameters. This included handling seismic loads and lateral earth pressures with precision.

The dual-purpose slurry walls provided both temporary excavation support and permanent structural functionality, streamlining the overall construction process. By eliminating the need for separate waterproofing systems and extensive dewatering, the project significantly reduced costs. The method also allowed for continuous work without disruptive noise or vibration, enabling the project to stay on schedule despite the challenges of its urban location. These results highlight the method's ability to deliver on performance while addressing logistical and environmental constraints.

Benefits Compared to Other Methods

The diaphragm/slurry wall technique brought additional advantages to the Yerba Buena Tower project, particularly in managing the complexities of its urban environment. The slurry walls formed an impermeable barrier that excelled at groundwater control, minimizing risks to nearby structures like the Marriott Hotel and critical BART infrastructure [1].

The use of a top-down construction method further enhanced efficiency. This approach cut excavation depth by 50% and reduced the construction timeline by 40% compared to traditional bottom-up methods [8]. The diaphragm walls proved highly adaptable, accommodating the site's complex geometry and varying soil conditions while integrating seamlessly with the tower's foundation design [8]. Even after construction, these walls continue to provide essential foundation support and waterproofing for the tower [10].

Seismic stability was another key benefit, with the diaphragm walls effectively distributing seismic loads to enhance the structure's resilience in San Francisco's earthquake-prone region. The approach also minimized environmental impacts by reducing noise and vibration during construction, avoiding disruptions to nearby businesses and residents [9].

"They are engineered systems designed with precision to handle extreme earth and water pressures while minimizing risks to nearby buildings and underground utilities." – thecivilstudies.com [8]

These results demonstrate how innovative engineering solutions can address the unique challenges of urban construction projects like Yerba Buena Tower, delivering both immediate and long-term benefits.

Diaphragm Wall Construction Methodology

Lessons for US Construction Professionals

The Yerba Buena Tower shoring process provides practical takeaways for urban construction projects, offering insights into safety, engineering solutions, and cost management.

Safety Standards and Compliance Requirements

Adhering to OSHA's regulations is non-negotiable, especially in urban construction. This includes implementing fall protection systems, confined space protocols, and ensuring proper ventilation during excavation. At Yerba Buena Tower, worker safety and structural integrity were prioritized with real-time groundwater monitoring and rigorous panel integrity testing. Additionally, noise-reducing equipment like silent pile drivers helped meet San Francisco's strict noise ordinances and minimized workers' exposure to harmful noise levels [12]. These safety measures highlight how urban projects can balance regulatory compliance with worker well-being.

Application to Other Urban Construction Projects

The engineering strategies used at Yerba Buena Tower offer a blueprint for tackling challenges in dense urban environments. A thorough geotechnical analysis before starting construction proved essential, forming the backbone of the project’s success [12]. Advanced 3D modeling played a critical role by visualizing reinforcement placement and identifying potential conflicts ahead of time. This level of precision was instrumental in managing structural loads and addressing groundwater challenges.

For cities with high water tables or those near waterfronts, the dewatering systems and groundwater control methods used in this project are especially relevant. These techniques have demonstrated effectiveness across various urban settings [12].

"Diaphragm walls are a valuable tool for underground construction projects, offering a versatile, reliable, and durable solution for a variety of applications. Their ability to withstand high loads, provide effective watertightness, and be customized to suit different project requirements makes them a popular choice among engineers and contractors."– Engr Ali Kaif Rana, Innovative and Results-Driven Site Engineer [13]

Cost-saving measures from the Yerba Buena project are also worth noting. By streamlining excavation processes and eliminating the need for separate waterproofing systems, the project achieved savings of up to 25% compared to traditional methods. The on-site slurry treatment system further reduced expenses by recycling and reusing bentonite slurry, lowering both material costs and environmental impact [11][12]. Early collaboration with structural and geotechnical engineers during site investigations was another key factor in avoiding costly design changes and delays, demonstrating the importance of integrated planning in urban construction [13].

Conclusion: Main Findings from Yerba Buena Tower Shoring

The Yerba Buena Tower project highlights the impressive capabilities of diaphragm/slurry wall shoring in handling the complexities of urban construction. Golden Gate Shoring successfully tackled the challenges of building a 7-level, 66-foot deep basement in downtown San Francisco while safeguarding critical infrastructure like the BART and MUNI tunnels, the Marriott Hotel, and nearby structures [1].

Key results showed that soil displacements were well-controlled, staying within the design's safety thresholds near the excavation site [1]. Internal bracing proved essential in limiting deflections above the excavation base, directing most of the movement below this level. This approach was particularly crucial given the site's proximity to the San Andreas fault, just 8.5 miles away [1]. While the upper fill layer experienced slightly higher movements, all deformations were carefully managed to ensure the safety of surrounding structures [1].

This project serves as a strong example of how to manage complex excavations in densely populated areas. By leveraging detailed geotechnical studies, advanced monitoring systems, and well-planned internal bracing, diaphragm/slurry wall shoring emerges as a reliable technique for urban developers. This case study reinforces a practical and effective approach to ensuring safety and efficiency in urban excavation projects across major U.S. cities.

FAQs

How do diaphragm and slurry walls improve seismic safety in urban construction projects like the Yerba Buena Tower?

Diaphragm and slurry walls are essential for boosting seismic safety. They stabilize the surrounding soil and provide solid support for foundations, playing a key role in preventing soil liquefaction during earthquakes. These walls are also designed to withstand the lateral forces generated by seismic activity.

By forming a sturdy, uninterrupted barrier, these walls help safeguard urban structures from ground shifts. For example, they ensure the stability of buildings like the Yerba Buena Tower, even in crowded, earthquake-prone cities such as San Francisco.

What are the environmental and cost advantages of diaphragm/slurry walls over traditional excavation methods?

Diaphragm or slurry walls bring notable advantages by cutting down on soil excavation, limiting ground disruption, and reducing effects on groundwater systems. These methods shine in urban settings where safeguarding nearby structures and ecosystems is a top priority.

When it comes to cost, while the initial investment might be higher, these walls often prove more budget-friendly over time. By minimizing the need for extensive excavation, dewatering, and soil disposal, they can trim total project expenses by as much as 25%. This makes them a smart option for intricate urban construction projects that demand both efficiency and eco-conscious solutions.

How does real-time monitoring enhance safety and accuracy during diaphragm/slurry wall construction?

Real-time monitoring is essential for maintaining safety and accuracy during diaphragm or slurry wall construction. With the help of advanced sensors and digital tools, these systems deliver constant updates on critical aspects such as trench stability, wall alignment, and any potential deformations. This instant feedback empowers engineers to tackle problems as they arise, minimizing risks and safeguarding the structure's integrity.

Beyond safety, these monitoring systems enhance the construction process by ensuring precise execution, which is particularly valuable in challenging urban settings. A great example of this is the Yerba Buena Tower in San Francisco, where such technology contributed to a safer work environment and a superior final result.