Bay Area Shoring Engineering: Custom Support System Design
- Joe Lazzaretti
- Jan 2
- 13 min read
Updated: Jan 3
The Bay Area demands shoring systems tailored to its unique challenges. With 728 earthquakes (M ≥1.5) in the past year, seismic risks, variable soil conditions, and dense urban environments make standard solutions insufficient. Custom-engineered systems address these complexities by ensuring safety, efficiency, and structural integrity during construction.
Key Takeaways:
Seismic Risk: Shoring systems must withstand intense forces from earthquakes, especially in areas with liquefiable soils.
Soil Challenges: Sites often transition between shallow bedrock and deep compressible deposits, requiring precise soil-structure interaction analysis.
Urban Constraints: Limited space and proximity to historic buildings or infrastructure like BART stations add complexity.
Regulations: California law mandates rigorous engineering standards for shoring systems taller than 14 feet or spanning more than 16 feet horizontally.
Techniques and Solutions:
Deep Foundations: Helical piles, micropiles, and secant pile walls handle high loads and seismic forces.
Temporary Supports: Systems like soldier piles with lagging and tieback anchors stabilize excavations effectively.
Advanced Methods: Deep Soil Mixing (DSM) and uplift-resisting micropiles mitigate liquefaction risks.
Custom shoring systems integrate advanced engineering, soil testing, and compliance with strict regulations to meet the Bay Area's demanding conditions.
Tutorial 4: Basics of Soil-Structure Interaction (SSI) in Retaining Wall Design
Soil-Structure Interaction in the Bay Area
Understanding soil-structure interaction is key to designing dependable shoring systems in the Bay Area. The region’s geology is anything but straightforward, with steep bedrock formations and compressible deposits that can reach depths of 60 feet [3]. This variability demands thorough pre-excavation analysis, which guides precise soil testing and customized support designs.
Soil Testing and Analysis Methods
Accurate soil testing is the backbone of safe shoring design. Engineers evaluate current site conditions while factoring in the effects of historical land reclamation on subsurface stability. Field testing plays a crucial role, validating design assumptions and assessing the performance of support systems [3]. In the Bay Area, these evaluations often encounter unique challenges like earthquake rubble, filled marshlands, and remnants of previous developments.
Vertical supports must rest on compact, level bases that match the soil’s load-bearing capacity [2]. However, factors like weather conditions and concrete pouring can lower the soil’s capacity below the design minimums, making continuous monitoring a necessity. To ensure safety, shoring systems are typically engineered to handle a total load of at least 100 pounds per square foot, covering both live and dead loads [2].
Seismic Retrofitting Requirements
Given the uncertainties of soil conditions, seismic retrofitting strategies must align with geotechnical findings. Diagonal bracing or similar systems are commonly used to counteract lateral forces [2]. In areas with liquefiable fill - such as those near historic shorelines or tidal mudflats - engineers must also address the risk of lateral spreading during earthquakes [3].
A great example of this approach can be seen in the 270 Brannan Office Building project in San Francisco. In 2017, engineers Wayne Magnusen and Leo Panian tackled a site with 60 feet of liquefiable fill near a historic shoreline. They implemented Deep Soil Mixing (DSM) panels to reduce liquefaction risks and used uplift-resisting micropiles to anchor shear walls against seismic overturning forces. This combination allowed for the use of conventional spread footings, even in challenging soil conditions [3].
Lateral Retention Systems
Excavation in the Bay Area requires lateral retention systems tailored to its unique geological conditions. California regulations specify that shoring systems taller than 14 feet or spanning more than 16 feet horizontally must be designed and approved by a registered civil engineer. Additionally, when motorized carts operate on the shoring system, design loads must account for an extra 25 pounds per square foot [2].
On-site documentation of shoring layouts and falsework plans is critical for ensuring compliance with these regulations and making timely adjustments as needed [2].
Custom Shoring Techniques and Systems
The Bay Area's unique geology demands advanced shoring methods. Engineers often combine various techniques to tackle challenges like liquefaction risks, seismic activity, and difficult soil conditions in San Francisco and nearby cities. These methods encompass deep foundations, surface applications, and temporary support systems, each tailored to meet the region's complex needs.
Helical Piles and Deep Foundations
Helical piles, equipped with spiral-shaped plates, anchor into stable soil layers to provide both vertical and lateral support. In seismic zones D, E, and F - which include much of the Bay Area - these piles act as damping systems during earthquakes[9].
Micropiles, small-diameter drilled piles, excel in high-load and lateral capacity scenarios. They are particularly effective when paired with Deep Soil Mixing panels in seismic zones. These piles can handle loads up to 500 kips and lateral capacities of up to 40 kips per pile[10].
Secant pile walls are indispensable in tight urban spaces. By overlapping concrete piles, they create a continuous barrier that prevents soil movement. Depending on project needs, these walls can be built with "hard" piles made of high-strength concrete or "soft" piles using self-hardening mixes. Verticality tolerances, typically kept at 1:100, ensure the wall's stability[8]. In many cases, secant pile walls serve dual purposes: temporary excavation support and permanent basement walls. This dual functionality helps reduce construction time and costs, making them a practical solution for the Bay Area's challenging soil and seismic conditions.
Shotcrete Applications
Surface applications like shotcrete play a vital role in stabilizing structures. Shotcrete, a type of concrete sprayed at high velocity, is used to create strong, reinforced walls for both temporary and permanent purposes. For example, engineers apply shotcrete to the exposed surfaces of secant pile walls after cleaning and chipping them to ensure proper bonding[8]. This process prevents gaps and movement that could weaken the wall. Shotcrete's high-velocity application also allows it to adapt to irregular surfaces and reach areas where traditional concrete pouring would be difficult.
Temporary Support Structures
Temporary shoring systems in California must meet rigorous standards while accommodating tight site constraints. Any system taller than 14 feet or spanning more than 16 feet horizontally requires approval from a California-registered civil engineer[2]. These systems must withstand lateral loads of at least 100 pounds per lineal foot of floor edge or 2% of the total dead load, whichever is greater[2].
Soldier piles with lagging are a common choice for excavation projects. Steel piles are driven or drilled into the ground, and timber lagging - typically at least 3 inches thick with a moisture content below 19% - is placed between them to hold back soil[8]. Timing is critical; the lagging must be installed in sync with excavation to prevent soil loss, especially in fine sand or silt.
For instance, at the 30 Otis project on Van Ness Street in San Francisco (January 2020), engineers used a soil-cement mix cut-off system combined with soldier beams and tiebacks for a 30-foot excavation. Slant-drilled underpinning was employed to protect nearby structures, including a three-story brick building and a two-story wood-framed structure[11][7]. Similarly, the Potrero Power Station redevelopment required shoring to stabilize 60-foot tall unreinforced brick walls during waterfront construction[6].
Tieback anchors are another essential component, transferring loads from retaining walls into deeper, more stable soil layers. Unlike internal bracing, tiebacks leave the excavation area clear for construction equipment. To ensure stability, the toe of these temporary systems must extend at least 5 feet below the excavation bottom or reach bedrock[8].
Case Studies: Bay Area Shoring Projects
These case studies highlight how custom shoring systems have been tailored to tackle the unique challenges of the Bay Area, such as stabilizing historic masonry in dense urban areas. They demonstrate how advanced techniques are applied to real-world projects in this region.
Potrero Power Station Shoring
The redevelopment of the Potrero Power Station in San Francisco presented a major challenge: stabilizing 60-foot unreinforced brick walls during interior demolition and waterfront construction work[12][6]. Tuan and Robinson, Structural Engineers, faced tight access restrictions, with three sides of the site being inaccessible for standard shoring installation[12]. Their innovative solution involved consolidating all shoring towers on the east side - the only area with access. To stabilize the walls on the inaccessible sides, they employed large pipe braces and walers, anchoring the walls to the eastern shoring towers[12][6]. This method effectively addressed the lateral stability issues and seismic risks associated with unreinforced masonry structures in the Bay Area.
"Shoring towers could only be located on the east side of the site. Large pipe braces and walers were used to tie the remaining three existing walls back to the shoring towers." - Tuan and Robinson, Structural Engineers, Inc.[12]
San Rafael Structural Shoring
In contrast to the urban demolition challenges of the Potrero project, the San Rafael project showcases how permanent shoring systems can be integrated into critical infrastructure. The Marin General Soil Nail Wall project in San Rafael/Greenbrae involved creating stable retaining walls for hospital-related facilities using soil nails. These walls were designed to meet strict seismic requirements and deflection limits, especially given their proximity to sensitive structures[7]. This project highlights how permanent shoring solutions not only provide immediate excavation support but also serve as essential components of the final structural system, accommodating the region’s variable soil conditions and seismic demands.
Designing Custom Support Systems: Step-by-Step
Engineering Assessment and Planning
The first step in designing custom support systems involves thorough geotechnical investigations. These are essential for defining shoring design parameters, especially in regions like the Bay Area, where seismic activity and soil variability are major factors. Engineers typically conduct test drilling and soil evaluations to gather critical data about slope stability and other site-specific conditions[10]. For projects near existing structures - such as historic facades or unreinforced masonry - engineers must carefully assess these buildings to determine the need for underpinning or lateral support[6]. A great example is the San Francisco Conservatory of Music project, where engineers evaluated a historic facade over 100 feet tall. This assessment helped plan the underpinning required for deeper basement excavation and the addition of a new mat foundation[6]. This upfront planning prevents unexpected issues during construction and ensures the shoring system is designed to handle challenges like seismic risks and varying soil conditions. Additionally, this phase lays the groundwork for meeting permitting and compliance requirements.
Permitting and Code Compliance
In California, shoring systems that exceed 14 feet in height or 16 feet in width must be reviewed and approved by a registered civil engineer[2][13]. Before any concrete is poured, a civil engineer must inspect the system to confirm it aligns with the approved plans. Additionally, projects are required to keep a falsework plan or shoring layout on-site at all times, as mandated by California Code of Regulations Title 8, Section 1717[2][13]. For permanent shoring systems intended to last more than two years, the designs must include seismic adjustments in earth pressure calculations and follow stricter guidelines for corrosion protection[14]. Once permits are secured and compliance is ensured, the focus shifts to carefully planning the construction phases.
Construction Phasing and Sequencing
Construction sequencing plays a vital role in ensuring safety throughout the project. For instance, protective guardrails must be installed around the perimeter immediately after supporting members are placed during formwork erection[2][13]. A noteworthy example is the 30 Otis project in San Francisco, where engineers managed a 30-foot-deep excavation using a combination of soldier beams, tiebacks, and a soil-cement mix cut-off system. This was paired with slant-drilled underpinning to stabilize nearby structures[11]. It's also critical that all vertical supports are placed on compacted, stable bases. Factors like weather and concrete pouring can alter the soil’s load-bearing capacity, so these conditions must be monitored closely[2][13]. Shores must remain in place until the concrete has gained enough strength to support both its own weight and additional loads. The removal process must be carefully sequenced to maintain structural integrity and protect adjacent structures throughout the project[2][13].
Tools and Technology in Shoring Engineering
Proprietary Systems and Job-Built Designs
Shoring projects in the Bay Area typically rely on two core approaches: proprietary modular systems and custom job-built designs. Proprietary systems, such as Doka SL-1, PERI, and EFCO, use prefabricated steel or aluminum components that are designed to snap together easily. These systems are reusable and come with pre-engineered lifting points, which improve safety during on-site operations[16]. Thanks to the use of high-grade steel, these systems can handle 20% to 30% more load for the same weight compared to standard structural steel[16].
On the other hand, job-built designs are tailored to fit specific site requirements. Using materials like timber or structural steel, these custom solutions are ideal for projects with irregular geometries. They offer the flexibility to accommodate unique shapes and ensure that surrounding structures remain intact during construction[6].
The choice between these two approaches depends on factors like project timeline, site complexity, and overall duration. Proprietary systems are favored for projects where speed is critical, as they eliminate the need for on-site cutting or welding. Custom job-built designs, however, are better suited for challenging layouts with irregular sections[16][17]. Many modern proprietary systems also feature advanced technologies like hydraulic rams for precision adjustments and real-time telemetry to monitor load forces - a critical feature in areas with seismic activity[16]. Regardless of the system, precise on-site adjustments are a must, and hydraulic jack systems make this possible.
Hydraulic Jack Systems and Load Testing
Once the shoring approach is selected, managing loads accurately becomes the next priority. Hydraulic jack systems play a vital role in maintaining construction stability, with the capacity to support at least 2.5 times the intended load for safety[15]. For operations like lift-slab construction, it’s crucial to keep all support points within half an inch of their required positions to avoid structural stress[15]. Modern hydraulic systems now include auto-leveling features that automatically pause operations if tolerances are exceeded, ensuring safety and precision[15].
To verify that design specifications align with on-site conditions, engineers often turn to Osterberg Load Cells. For example, during the foundation work for Salesforce Tower in San Francisco, two full-scale Osterberg Load Cell tests were conducted on 42 Load-Bearing Elements (LBEs) extending 310 feet below ground. These tests revealed that bentonite caking caused a decline in skin friction, leading to strict time limits for installation[4]. Such testing is especially critical in the Bay Area, where challenging soil conditions - like Bay Mud and Colma formation sand - combined with seismic risks demand precise data to prevent issues like excessive settlement or lateral spreading[4]. These technologies work hand-in-hand with custom shoring systems to address the region’s dynamic and complex construction challenges.
Conclusion
Custom Solutions for Bay Area Conditions
The Bay Area’s unique combination of seismic activity and challenging subsurface conditions demands shoring systems tailored to these complexities. With 728 seismic events (M ≥1.5) recorded in the past year [5], shoring solutions must prioritize damage control and ensure safety under California's rigorous standards [1]. Adding to the challenge, the region’s geology often transitions abruptly from shallow bedrock to deep, liquefiable fill [3]. To address these issues, integrated approaches - like combining Deep Soil Mixing panels with uplift-resisting micropiles - have shown success in local projects [3]. Urban constraints, such as those around critical infrastructure like BART stations, further necessitate designs that maximize below-grade space while maintaining structural integrity [7]. These tailored strategies, coupled with modern shoring technologies, are key to achieving both safety and efficiency.
Improving Safety and Efficiency
Advances in shoring technology have significantly improved project safety while streamlining construction timelines. For instance, innovative designs have saved clients over $100,000 in demolition costs and cut project schedules by more than two months [18]. Rigorous field testing ensures these designs perform as intended. During the Salesforce Tower project, engineers utilized 42 load-bearing elements socketed up to 70 feet into Franciscan bedrock to mitigate risks like liquefaction and lateral spreading [4]. Such methods not only reduce potential hazards but also meet California's strict standards, such as the minimum 100 pounds per square foot design load for formwork and shoring [2]. These advancements highlight the critical role of expert oversight at every stage of a project.
Working with Experienced Professionals
California law mandates that shoring systems exceeding 14 feet in height or 16 feet in horizontal span must have design calculations and working drawings approved by a registered California civil engineer [2]. This requirement is crucial for addressing the Bay Area’s complex site conditions. A prime example is the Salesforce Tower project, where engineers conducted a groundbreaking structure–soil–structure interaction analysis to ensure the foundation wouldn’t interfere with the adjacent Transit Center [4]. As Wayne Magnusen and his team emphasized in their Deep Foundations Institute case study, successful execution relies on:
"constructive collaborations between the project geotechnical engineer, structural engineer, general contractor, and specialty foundation and ground improvement contractor(s)" [3]
Before concrete is poured, a registered engineer must inspect and certify that the shoring system aligns with the working drawings and meets all material and workmanship standards [2]. This meticulous process ensures that each project adheres to the highest safety and performance benchmarks.
FAQs
How do custom shoring systems help manage seismic risks in Bay Area construction projects?
Custom shoring systems in the Bay Area are built to tackle the region's unique seismic challenges, such as intense ground shaking and unstable soil conditions. Engineers adhere to California’s strict safety standards and seismic guidelines, ensuring these systems can handle both vertical and horizontal forces during an earthquake.
For site-specific risks like soft soils or liquefaction, engineers often use ground-stabilizing techniques such as deep-soil mixing or micropiles. These methods strengthen the ground and improve the shoring system’s ability to withstand seismic stress. By combining cutting-edge engineering, tailored designs, and rigorous safety protocols, these systems play a critical role in protecting workers, structures, and nearby utilities during excavation and construction projects.
What are the main soil challenges in the Bay Area that impact shoring system design?
The Bay Area's soil conditions are a patchwork of complexities, making shoring system design a demanding task. Much of the land here is made up of reclaimed fill, loose sands, and soft deposits that can shift or even liquefy during earthquakes. This instability increases the risk of ground movement, creating challenges for maintaining structural stability. In some spots, shallow bedrock adds an extra layer of difficulty, complicating excavation and the design of support systems.
The region's historic shoreline and varied geology also bring layers of silts, clays, and older fill materials into play. These materials often lack cohesion and have inconsistent stiffness, which means engineers must account for higher design loads and use stronger bracing. Techniques like deep-soil mixing or micropiles are often necessary to improve the ground and ensure safety. Tackling these challenges is essential for building reliable shoring systems in cities like San Francisco, Oakland, and their neighboring communities.
Why is it important for shoring systems in California to meet state engineering standards?
Ensuring that shoring systems meet California's engineering standards is crucial for maintaining safety, compliance, and long-term reliability. These standards take into account critical aspects like seismic activity, soil conditions, and construction safety - factors that are particularly relevant given California's unique geological challenges and strict regulatory environment.
Following these regulations not only safeguards workers, nearby structures, and the public but also ensures legal compliance. It helps avoid expensive delays, fines, or even structural failures, making adherence a top priority for any construction project in the state.
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