When Do You Need Structural Shoring in the Bay Area?
- Joe Lazzaretti
- Jan 2
- 14 min read
Updated: Jan 2
Structural shoring is critical in the Bay Area for construction projects involving seismic retrofits, foundation repairs, or deep excavations. The region's earthquake risk, soft soils, and strict building codes make shoring essential to stabilize structures and ensure safety. Key points include:
Earthquake Risk: With 728 earthquakes (magnitude 1.5+) recorded in the past year, shoring helps buildings withstand seismic activity.
Older Buildings: Many Bay Area buildings, especially soft-story structures, require shoring during retrofits to meet modern safety standards.
Foundation Repairs: Deep excavations in areas like SoMa or the Marina often need robust systems to prevent soil shifts and building movement.
Excavation Rules: California law mandates shoring for excavations near existing structures, with strict limits on allowable movement (0.5 inches for buildings).
Shoring Methods: Techniques like soldier piles, soil nails, and trench-soil-mixing are tailored to the Bay Area's unique soil conditions.
Shoring isn’t optional - it’s a legal and practical necessity to protect workers, buildings, and nearby properties during construction in this earthquake-prone region.
How to Seismic Retrofit a House - Shear Wall Overturning Forces
Seismic Retrofitting Projects That Require Shoring
In the Bay Area, seismic retrofitting often relies on shoring systems to stabilize buildings while permanent safety measures are installed. These systems play a crucial role in maintaining structural integrity as engineers add components like fluid viscous dampers and braced frames [6][5]. Without this temporary support, buildings could shift - or even collapse - during the retrofitting process.
This kind of support is especially important for older buildings, which often have unique vulnerabilities. San Francisco, for instance, has flagged 4,921 soft-story buildings - structures with large first-floor openings like garages or storefront windows - that require retrofitting. By early 2024, 98% of these property owners had applied for permits, and 65% of the projects were completed [7]. These buildings are particularly at risk during earthquakes, as their design can lead to catastrophic instability. To ensure safety during construction, shoring systems must handle lateral loads of at least 100 pounds per linear foot of floor edge or 2% of the building's total dead load, whichever is greater [2].
Managing Soil Liquefaction and Lateral Spreading
In areas like the Marina and Mission districts, soil liquefaction presents another serious challenge. When loose, water-saturated soils are shaken during an earthquake, they can lose strength and behave like a liquid [8]. This often leads to lateral spreading, where the ground shifts horizontally toward open spaces, such as the Bay, potentially displacing building foundations during retrofitting.
Take the project at 270 Brannan in San Francisco as an example. Structural engineer Leo Panian faced a site with 60 feet of liquefiable soil sitting atop a historic shoreline. The team used a combination of Deep Soil Mixing (DSM) panels and micropiles to address the issue. The DSM panels helped control lateral spreading, allowing for the use of conventional spread footings, while the micropiles added resistance against seismic overturning [8]. Projects like this highlight the need for integrated shoring and ground improvement techniques to tackle these complex soil conditions.
Bay Area Seismic Retrofitting Case Studies
One standout example of seismic retrofitting in San Francisco is the Fox Plaza Tower on Market Street. In March 2019, the 30-story building underwent a voluntary retrofit led by Matt Skokan, Ph.D., S.E., and Plant Construction Company. This project addressed vulnerabilities in the tower's unique haunched beam-to-column connections. Engineers installed 14 fluid viscous dampers across the first seven floors and reinforced beam flanges on floors 17 through 21. These upgrades successfully reduced beam plastic rotations to below 0.02 radians, meeting "Collapse Prevention" standards for a 475-year seismic hazard event [6]. Extensive shoring was essential to support the structure while these permanent safety systems were integrated into its existing framework.
"The challenge is that the older buildings are not up to those codes. When it comes to building safety, it comes down to when it was built." - Richard Allen, Seismologist, University of California, Berkeley [7]
Since the 1989 Loma Prieta earthquake, the Bay Area has invested an estimated $1.2 billion in seismic retrofitting. These efforts have not only improved safety but also saved property owners between $10,000 and $200,000 in potential repair costs [7][1]. Proper shoring remains a critical component of these upgrades, protecting both workers and the buildings themselves during the retrofit process.
Foundation Repairs and Underpinning That Need Shoring
In the Bay Area, foundation repairs often demand shoring to stabilize excavations and protect workers from potential cave-ins [12]. When engineers dig deep to repair or strengthen foundations, the surrounding soil loses its natural support, becoming unstable. Without reliable shoring systems in place, nearby buildings can shift, settle, or even collapse.
In SoMa, the subsurface conditions are especially challenging, with abrupt transitions from tidal mudflats to steep bedrock that plunges over 60 feet [8]. Underpinning systems in this area must bridge the gap between compressible soil and stable rock, all while ensuring stability during excavation.
The California Building Code enforces strict rules to prevent structural damage. Excavation must stop if a building shifts by 0.5 inches or if soldier piles move by 1 inch [3]. If movement exceeds 0.75 inches (or 1.5 inches for soldier piles), additional shoring becomes mandatory [3]. These regulations are in place because the Bay Area's soft soils and seismic activity significantly increase risks during deep excavations, requiring robust and carefully designed shoring systems.
Shoring Systems for Deep Excavations
To meet movement limits, specialized shoring systems are designed to handle substantial lateral loads. Commonly used methods include soldier piles, lagging, and tie-back anchors, which provide critical support to adjacent structures [9][11]. These systems must resist a minimum lateral load of 100 pounds per linear foot of floor edge or 2% of the total dead load, whichever is greater [2]. Engineers also factor in surcharge pressure from nearby footings, traffic, and heavy materials.
In areas with soft soils, such as Van Ness Street in San Francisco, soil-cement mix cut-off systems are often used to stabilize the ground and control water during excavation [9]. For sandy conditions like Dune Sand, techniques such as permeation grouting or jet-grout cut-offs are employed to strengthen the soil before underpinning begins [11]. Tie-back anchors, which play a crucial role in these systems, are tested at 150% of their design loads and are built to handle a post-grout pressure of 300 psi [3].
A popular technique in San Francisco and Oakland is slant-drilled underpinning, which supports adjacent brick or wood-framed buildings during deep excavations [9][11]. By installing angled supports beneath existing foundations, engineers can avoid utility conflicts and stabilize structures without disturbing neighboring properties.
Bay Area Foundation Repair Projects
Several notable projects showcase the use of advanced shoring and underpinning techniques in the Bay Area:
30 Otis, San Francisco: In 2020, a 30-foot excavation utilized a combination of soil-cement cut-offs, soldier beams, tiebacks, and slant-drilled underpinning to stabilize adjacent brick and wood-framed structures [9][11].
Molecular Foundry at Ernest Orlando Lawrence Berkeley National Laboratory: This project involved a 68-foot deep excavation on a steep hillside between existing buildings. Engineers used tiebacks, soldier beams, and shotcrete for support [11].
Symphony Towers, San Francisco: A 35-foot deep excavation in Dune Sand required soldier piles, tiebacks, and slant-drilled underpinning to support five nearby buildings. Permeation grouting was also applied to stabilize the sandy soil and prevent settlement [11].
Lucas Center Expansion at Stanford University: This project included underpinning and temporary shoring for a 40-foot deep excavation next to an active 2-to-4 story medical research facility [11].
To ensure safety, weekly monitoring begins as soon as excavation starts. Measurements, accurate to within 0.01 feet, help detect any unexpected movement early, safeguarding workers and nearby structures throughout the foundation repair process [3].
Building Renovations and Excavations That Require Shoring
In the Bay Area, where seismic activity and foundation issues are common, shoring plays a critical role in ensuring safety during building renovations and nearby excavations. Renovation projects often involve altering load-bearing elements, which can compromise a structure's stability if not properly supported. Temporary vertical shoring, also known as falsework, is typically used to bear these loads while permanent changes are made [2]. This approach ensures that structures remain stable during the renovation process, especially in a region prone to seismic challenges.
Excavations near existing buildings present similar risks. If digging occurs within a distance equal to twice the depth of the excavation from a structure, that building is classified as "adjacent" and requires protective measures [13]. The Bay Area's soft soils further amplify these risks, potentially destabilizing foundations and causing settlement or lateral movement. Such shifts can impact not only the structure under renovation but also neighboring properties. These challenges echo those faced in other seismic and foundation-related projects in the region.
Interior Renovations That Need Temporary Support
Interior renovations, much like structural ones, often require carefully engineered shoring systems to maintain stability. California law mandates that shoring plans be approved by a registered civil engineer when the height of the shoring exceeds 14 feet or when horizontal spans are greater than 16 feet. These systems must be capable of supporting at least 100 pounds per square foot of combined live and dead loads [2].
As stated in the California Code of Regulations, Title 8, Section 1717: "Formwork and falsework or shoring for the support of concrete or other materials shall be designed, erected, supported, braced and maintained so as to assure its ability to safely withstand all intended loads during erection, construction, usage and removal." [2]
Before any concrete is poured or heavy loads are applied, a registered civil engineer must inspect the shoring system and provide written certification that it meets the approved plans. Vertical supports should always rest on compacted, level bases, with plates or pads used to distribute loads evenly based on soil conditions [2].
Bay Area Excavation Regulations and Shoring Requirements
The Bay Area has strict regulations governing excavation and shoring to address the region's unique challenges. Work must stop immediately if an existing building shows movement of more than 0.5 inches. If movement reaches 0.75 inches, additional shoring must be designed and installed before construction can continue [3]. These thresholds are critical in protecting structures in a region where seismic activity increases the risks associated with excavation.
When digging near existing footings, engineers must account for surcharge pressures caused by nearby structures or heavy loads [3]. Regular monitoring, typically on a weekly basis, is essential to detect and address any movement during the excavation process [3]. By adhering to these guidelines, construction teams can minimize risks and maintain the integrity of surrounding structures.
Bay Area Conditions That Trigger Shoring Requirements
The Bay Area's unique mix of seismic activity, soil characteristics, and strict construction codes creates specific conditions where shoring becomes essential. Recognizing these factors is key for contractors and engineers to plan effectively and avoid costly setbacks. These elements directly influence how shoring is approached in the region.
Geological Conditions: Colma Sand and Bedrock Depth
The Bay Area's geological profile plays a major role in determining when shoring is required. Loose soils, especially prone to liquefaction, increase the need for shoring during earthquakes [14]. Soil composition greatly impacts how ground shaking affects structures, making it a critical consideration in shoring decisions.
"Ground shaking is the primary cause of earthquake damage to man-made structures. Many factors influence the strength of earthquake shaking at a site including the earthquake's magnitude, the site's proximity to the fault, the local geology, and the soil type." - U.S. Geological Survey [14]
The depth of bedrock varies widely across the Bay Area, requiring site-specific studies to determine proper foundation support and shoring techniques [14]. Additionally, assessing the water table is crucial for identifying earthquake-related risks and classifying groundwater as static, perched, or artesian [14][16]. Geotechnical reports often uncover the presence of boulders or cobbles, which demand specialized drilling methods for installing soldier piles and tie-back anchors [3][15].
Building Codes and Site Investigation Requirements
Bay Area construction codes set clear guidelines for when shoring is mandatory. According to §1717, shoring systems taller than 14 feet or extending more than 16 feet horizontally - and supporting at least 100 pounds per square foot - must be designed and approved by a registered civil engineer [2].
Geotechnical reports provide critical details, including anchor tension capacity, allowable bond stress, seismic earth pressure increments, and acceptable levels of lateral deformation [3][15]. These reports are vital for tailoring shoring systems to each site. For permanent shoring systems in place for over two years, engineers must account for increased lateral pressure. Construction must stop if movement exceeds 0.5 inches or if soldier piles shift more than 1 inch [3][15]. These strict monitoring requirements reflect the region's seismic risks: as of January 2, 2026, the area experienced 728 earthquakes (magnitude 1.5 or greater) in the previous year [4]. Such detailed standards highlight the challenges of building in a seismically active region.
Shoring Methods Used in Bay Area Construction
In the earthquake-prone Bay Area, where soil conditions and seismic activity play a significant role, contractors carefully select shoring methods based on factors like soil type, excavation depth, and seismic demands. One of the most commonly used techniques is soldier piles and lagging, which is effective for both temporary and permanent earth retention. This method often includes tie-back anchors driven into soil or rock for added stability [3][10]. A prime example of its use is the Symphony Towers project at 724 Van Ness Avenue in San Francisco, where soldier piles with tie-backs supported a 35-foot deep excavation through the city's characteristic "Dune Sand" [11]. These examples highlight how contractors tailor shoring methods to specific site challenges.
Soil nailing is another effective method, particularly suited for steep slopes and harder soils. This approach involves installing anchors directly into the soil to create stability. For instance, the Lawrence Berkeley National Laboratory has successfully used soil nail walls. These walls, which can also serve as permanent solutions, have been constructed as tall as 50 feet to support reservoir tanks in the region [11].
Drilled pier shafts (also called contiguous piles) are a reliable choice for cohesive soils, offering excellent stability near footings and strong performance during seismic events [3][10]. Meanwhile, trench-soil-mixing has gained popularity for creating uniform soil-cement walls up to 52 feet deep. This method is not only efficient but also fast - one Bay Area project achieved production rates of 1 foot per minute using the Kemsolid KSI 16000 attachment, creating 33-inch wide walls up to 26 feet deep at speeds two to three times faster than traditional techniques like jet grouting or sheet piling [17].
Types of Shoring Systems
Different shoring systems are designed to address specific challenges in Bay Area construction:
Soldier Pile Walls: These use steel posts with wood or concrete lagging to retain soil during excavation. Tie-back anchors provide lateral support, extending into stable soil or bedrock. These anchors undergo rigorous testing, with performance tests at 200% of design loads and production tests at 150% [3].
Soil Nail Walls: Steel bars, or "nails", are installed at downward angles into the excavation face and secured with shotcrete. This creates a gravity-retaining structure that works well for permanent stabilization.
Contiguous Pile Walls: These consist of drilled concrete piers placed side by side, forming an almost continuous barrier. They are particularly useful in cohesive soils.
Hydraulic Shoring: This method employs adjustable pistons to provide temporary support in trenches, offering quick installation to ensure worker safety.
Trench-Soil-Mixing: Cement is blended directly into the soil, forming structural walls without the need for extensive excavation. This method is efficient and ensures consistent wall quality.
Shoring Method Comparison for Bay Area Conditions
Method | Depth Range | Ideal Soil Type | Seismic Performance |
Soldier Pile Wall | 10–35 feet | Sandy/Dune Soils | Strong lateral stability; common in SF urban cores [11][18] |
Soil Nail Wall | 15–68 feet | Rocky/Hard Soils | Great for steep hillsides; permanent stabilization [11][19] |
Trench-Soil-Mixing | Up to 52 feet | Challenging/Mixed | 2–3× faster than traditional methods; consistent mix quality [17] |
Contiguous Piles | 20–50 feet | Cohesive/Clay | High stability; frequently used for stitch piers [19] |
Hydraulic Shoring | Shallow/Temp | Various | Quick installation for worker safety in trenches [18] |
Bay Area soil conditions can vary widely, ranging from solid bedrock to soft clay and loose dune sand [18]. Contractors rely on detailed geotechnical reports to determine the most effective shoring method for each site. Locations near the bay, in particular, require special attention due to the risk of liquefaction - where saturated soils can behave like quicksand during an earthquake [18]. By understanding the strengths and applications of these shoring systems, contractors can make informed decisions to ensure stability and safety in Bay Area construction projects.
When to Use Structural Shoring in the Bay Area
Structural shoring is a must in the Bay Area for large-scale excavations, seismic retrofits, or renovations that temporarily compromise a building’s stability. Given the region's unique soil types and seismic activity, shoring isn't just for new construction - it’s a critical part of retrofitting older buildings that could be vulnerable to earthquake damage.
The challenges of earth retention in the Bay Area require careful consideration. Shoring becomes especially important when excavations are affected by external pressures, such as nearby structures or heavy traffic loads [3]. The area’s geological makeup - think Colma Sand, varying bedrock levels, and soils prone to liquefaction - makes geotechnical investigations a crucial first step in determining the right shoring method. California law also requires that any shoring taller than 14 feet or spanning more than 16 feet horizontally must be designed by a licensed civil engineer [2].
Vertical support (falsework) is another key component, particularly during concrete formwork installation. This support must handle both dead and live loads, with a minimum design load of 100 pounds per square foot [2]. For residential projects, shoring is especially vital for soft-story buildings (like those with living spaces above garages), hillside homes perched on tall posts, and post-and-pier foundations. Retrofitting these structures typically costs between $3,000 and $7,000 but can prevent repair costs ranging from $10,000 to $200,000 [1].
Once shoring is in place, monitoring protocols are non-negotiable. Construction must stop immediately if movement exceeds 0.5 inch - or 1 inch for soldier piles [3]. These strict limits reflect the seismic risks posed by the San Andreas, Hayward, and Calaveras faults. Choosing the right shoring method depends on factors like soil conditions, excavation size, and seismic concerns. Following these monitoring guidelines is essential to maintaining structural stability and ensuring worker safety throughout the project.
FAQs
What are the common structural shoring methods used in the Bay Area?
In the Bay Area, various shoring methods are widely used to maintain safety and stability during construction or renovation work. Here’s a quick overview of the most common techniques:
Raking shoring: Angled braces that redirect loads to the ground or a stable structure for support.
Flying shoring: Horizontal supports that span gaps or openings, holding loads without touching the ground.
Dead (vertical) shoring: Straight, vertical supports that stabilize walls, floors, or ceilings.
Trench shoring: Systems specifically designed to prevent soil from collapsing during excavation work.
For deeper excavations, earth-retaining shoring is a go-to solution. This often involves using soldier piles and lagging, sometimes strengthened with tie-back anchors, to hold back soil or rock effectively. These methods are particularly critical in the Bay Area, where seismic activity and densely packed urban settings demand shoring systems that comply with California’s stringent safety and geotechnical regulations.
Why is shoring necessary in the Bay Area when dealing with soil liquefaction?
Soil liquefaction happens when water-saturated, loose soils lose their strength and start acting like a liquid during intense seismic activity. This can lead to the ground sinking, shifting sideways, or even failing to support structures properly. In the Bay Area, with its history of reclaimed land and soft soil deposits, this issue poses a serious risk, especially in regions prone to earthquakes.
In such cases, shoring plays a crucial role in temporarily stabilizing the ground during construction. It helps protect workers, nearby structures, and excavation sites while more permanent solutions - like ground improvement techniques or deep foundation systems - are put in place. Beyond ensuring compliance with local seismic codes, proper shoring is key to maintaining the long-term safety and stability of your project.
Why is structural shoring important for seismic retrofitting in older Bay Area buildings?
Structural shoring plays a key role in seismic retrofitting for older buildings, especially in areas like the Bay Area. It provides temporary support while essential structural upgrades are carried out. Many older buildings in the region were constructed before modern seismic codes, leaving them vulnerable due to weak foundations, soft-story designs, or outdated materials. During retrofitting, critical load-bearing components may need to be removed, altered, or strengthened, which could compromise the building's stability without proper support in place.
Shoring helps by temporarily bearing the load of floors, walls, and beams, keeping the structure stable and safe for workers throughout the construction process. This is particularly important in the Bay Area, where seismic activity is a constant concern, and retrofitting projects must comply with strict safety standards. By implementing shoring, engineers and contractors can ensure that buildings are upgraded safely and effectively, without the risk of structural failure during the work.
Comments