Structural Shoring for Bay Area Basement Underpinning
- Joe Lazzaretti
- Jan 2
- 13 min read
Updated: Jan 2
Structural shoring is a key component in basement underpinning projects across the Bay Area, ensuring building stability during excavation. Given the region's seismic activity, variable soil conditions, and urban density, shoring systems must address unique challenges like soft soils (e.g., Bay Mud), proximity to transit systems, and earthquake risks. Techniques such as soldier piles, tiebacks, soil nail walls, and specialized methods like jet-grouting and slant-drill underpinning are tailored to meet these demands. Proper soil testing, seismic retrofitting, and water management further enhance project safety and efficiency. The article explores these methods, regional challenges, and case studies showcasing successful applications.
What is Underpinning? Types And Methods Explained | Underpinning Series - 1
Standard Shoring Methods for Basement Underpinning
In the Bay Area, basement underpinning often relies on three key shoring methods, each suited to specific soil conditions, excavation depths, and urban challenges. These methods are designed to address the unique demands of the region, offering practical solutions for complex construction scenarios.
Soldier Pile and Lagging Systems
This method involves the installation of vertical steel H-beams, known as soldier piles, which are either driven or drilled into the ground at regular intervals. As excavation progresses, horizontal panels made of timber or concrete - called lagging - are placed between the piles to retain the soil. Soldier piles must be installed with a vertical tolerance of no more than 0.5%, while timber lagging should have a moisture content below 19% and a minimum thickness of 3 inches [6].
This approach is particularly effective in Bay Area soils like Dune Sand, which is prevalent in San Francisco. For instance, at Symphony Towers in San Francisco, engineers used soldier piles and tiebacks for a 35-foot deep excavation in Dune Sand, along with slant-drill underpinning to protect five adjacent buildings [1]. A crucial step in this process is backfilling the gaps between the excavation face and the lagging with sand to ensure tight contact and prevent soil loss [6].
Tiebacks and Internal Bracing
Tiebacks are steel rods or tendons drilled into stable soil behind the shoring wall. They help transfer lateral earth pressure away from the excavation, creating an open workspace ideal for constructing new foundations. However, tiebacks often require drilling under neighboring properties, which can complicate the process due to property easement issues.
When tiebacks aren't an option, internal bracing provides an on-site alternative. This method uses steel struts, rakers, and wales installed within the excavation itself. Unlike tiebacks, internal bracing avoids impacting neighboring properties but does occupy valuable space within the excavation footprint. At 1100 Broadway in Oakland, engineers faced a tight challenge with the 12th Street BART station located just 4 feet from the property line. For this 20-foot deep excavation, they used a combination of soldier beams, wood lagging, tiebacks, and internal bracing to protect the nearby transit infrastructure [1][2].
Soil Nail Walls
Soil nail walls take a different approach by reinforcing the soil itself rather than merely holding it back. This process involves drilling holes into the excavation face, inserting steel tendons (nails), grouting them in place, and applying a shotcrete facing. The result is a reinforced gravity block that resists sliding and rotation. This method works best in soils with enough "stand-up time" to allow for vertical cuts of 3 to 6 feet before reinforcement is added [7][9].
"Soil nail walls are an excellent solution when vibration-free foundation installations are required such as construction next to existing structures, reconstruction projects and reinforcement of existing embankments." - Hardman Construction [7]
Since soil nailing produces no vibrations during installation, it’s especially useful in urban areas near sensitive structures. For example, the Molecular Foundry project in Berkeley utilized soil nailing on a steep hillside, achieving a 68-foot deep excavation with tiebacks, soldier beams, and shotcrete between existing buildings [1]. However, soil nails are passive elements that require slight soil movement to activate, so engineers must closely monitor lateral movement when working near existing foundations [8].
These methods form the backbone of basement underpinning in the Bay Area, addressing the region’s diverse geological and urban challenges with precision and care.
Specialized Underpinning Techniques for Bay Area Conditions
In the Bay Area, standard shoring methods often fall short due to unique challenges like unstable soils, high water tables, and seismic activity in densely populated areas. To address these issues, contractors rely on specialized underpinning techniques tailored to the region's demanding conditions.
Slant‑Drill Underpinning and Jet‑Grouting
Slant‑drill underpinning involves drilling at an angle beneath existing foundations to create support in tight spaces. A notable example is 555 Mission Street in San Francisco, where engineers combined slant‑drill underpinning with a jet‑grout cut‑off system to handle a 30‑foot-deep excavation in Bay Mud [2].
Jet‑grouting, on the other hand, uses high-pressure grout to form soil-cement columns that provide support and block groundwater. This method works particularly well in sandy soils or areas requiring strict groundwater management. For instance, at 30 Otis in San Francisco, the Van Ness Street revitalization project applied slant‑drilled underpinning alongside a soil‑cement cut‑off system and tiebacks to secure nearby structures during a 30‑foot excavation [2].
"Our design constraints included the presence of Bay Mud soil, high water table, and the proximity of the adjacent buildings." - Tuan and Robinson Structural Engineers [2]
Helical Anchors, Push Piers, and Steel Minipiles
Different methods are used to tackle specific site challenges and load requirements:
Helical Anchors: These are perfect for sites where bedrock is deep, as they gain support from dense soil at shallower depths. Hydraulic equipment drives the anchors into the ground, advancing about three inches per rotation, ensuring a vibration-free installation. To maximize load transfer, the upper helix blade should be installed at least 12 diameters below the surface [11][12].
Steel Push Piers: Designed for sites with reachable bedrock, these high-strength steel pipes can support heavy loads, with some piers handling over 57 tons. Each pier is individually load-tested, and a collar on the first pipe section minimizes soil friction during installation [12].
Steel Minipiles: These are ideal for mixed or challenging soil conditions, especially in commercial and industrial projects. For example, in May 2013, Caltrans engineers used micropiles for a seismic retrofit at the Route 92/280 Interchange. Poor-quality rock, high groundwater near the San Andreas Fault, and limited overhead clearance made micropiles the preferred choice over conventional Cast in Drill Hole (CIDH) piles [10].
Method | Load Capacity | Best Soil Conditions | Key Advantages |
Helical Anchors | High (100+ kips) [11] | Deep rock; dense soil at moderate depth [12] | Quick, cost-effective; torque-verified [12] |
Steel Push Piers | Very High (up to 57 tons) [12] | Reachable bedrock [12] | Maximum capacity; reliable in all weather [12] |
Steel Minipiles | Variable [10] | Mixed/difficult soils; poor rock quality [10] | Versatile; handles high groundwater [10] |
During installation, it’s crucial to monitor and record shaft torsion for helical piles to confirm they meet the required load capacity [11][12].
Reinforced Concrete Piers
Reinforced concrete piers offer permanent support by extending through unstable soils to reach stable, load-bearing strata [14]. In the Bay Area, where unstable deposits can run as deep as 60 feet, these piers provide long-term stability [3].
For sites with restricted access or existing spread footings, hand-dug underpinning piers are often used to establish deep foundation support [1][2]. In some cases, engineers design dual-purpose systems where underpinning serves as both temporary excavation support and permanent reinforced concrete basement walls [5]. Advanced modeling, like finite element analysis, helps predict foundation behavior and create reliable rehabilitation plans [5].
In challenging soils like Bay Mud, jet‑grout cut‑offs or soil‑cement mix systems are essential to control water inflow and stabilize the surrounding ground during construction [1][2]. For hillside homes, drilled concrete piers are particularly effective in countering "soil creep", the gradual lateral movement that causes foundation settlement on slopes [13].
"For micropile projects in poor ground conditions and with high structural load demands... it is essential to develop design parameters estimates such as pile tips and bond stress achievable for the site conditions." - Mahmood Momenzadeh, Caltrans [10]
To ensure safety, pier designs incorporate a margin of redundancy, allowing them to handle loads far beyond the expected maximum. This is especially important in seismically active areas, where cyclic loading and soil liquefaction remain ongoing concerns [12].
These advanced techniques, combined with standard methods, create a robust foundation strategy tailored to the Bay Area’s unique challenges.
Bay Area Structural Shoring and Underpinning Case Studies
These examples highlight how specific underpinning techniques have been successfully applied to tackle complex challenges in the Bay Area.
300 Grant Avenue, San Francisco
The project at 300 Grant Avenue involved basement excavation in a tightly packed downtown area with limited access. The work required careful coordination of shoring, demolition, and construction activities. Temporary supports were used to stabilize existing structures during the excavation, showcasing how phased construction and detailed planning can effectively address space and access limitations.
Symphony Towers, San Francisco
At Symphony Towers, the team managed a 35-ft deep excavation in Dune Sand, right next to five neighboring buildings. The solution included soldier piles, tiebacks, slant-drill underpinning, and permeation grouting to provide both vertical and lateral stability [1].
1100 Broadway, Oakland
The excavation at 1100 Broadway reached a depth of 20 feet, located just 4 feet from the 12th Street BART station. To ensure safety and stability, the project utilized soldier beams with wood lagging, tiebacks, and internal bracing to isolate the pile foundations from the nearby tunnel. Additionally, the historic Key System Building's facade was stabilized, and 100 existing piles were reused to improve efficiency and reduce waste [2][15].
The table below summarizes the main challenges and solutions for each project:
Project | Excavation Depth | Primary Challenge | Key Solutions |
300 Grant Avenue | Within existing basement | Limited access in dense urban area | Phased construction; innovative temporary supports |
Symphony Towers | 35 ft | Dune Sand; five adjacent buildings | Slant-drill underpinning; permeation grouting; soldier piles with tiebacks |
1100 Broadway | 20 ft | Proximity to BART station (4 ft away) | Soldier beams with wood lagging; isolated pile foundations; internal bracing |
These case studies highlight the importance of customized strategies to meet the Bay Area's challenging underpinning demands.
Best Practices for Bay Area Shoring Projects
The Bay Area's unique geological makeup and regulatory landscape make shoring projects especially challenging. The region's natural geology, combined with historical modifications, creates complex subsurface conditions that can shift dramatically over short distances. To tackle these challenges, three critical practices - soil testing, seismic retrofitting, and water management - are essential for ensuring project success.
Soil Testing and Analysis Requirements
Thorough soil testing is the foundation of any successful shoring project in the Bay Area. In areas like San Francisco's SOMA district, subsurface conditions can include up to 60 feet of compressible deposits and liquefiable fill near historic shorelines [3]. Testing is crucial to identify unstable soil layers and past human interventions.
Take the case of 270 Brannan in San Francisco. Engineers Wayne Magnusen and Leo Panian faced challenging conditions when designing a midrise office building that straddled a historic shoreline. Their soil analysis uncovered 60 feet of compressible deposits and liquefiable fill. To address this, they used Deep Soil Mixing (DSM) panels for liquefaction mitigation and uplift-resisting micropiles to anchor shear walls against seismic forces [3].
"Successful installation of the system relied on a program of field testing to validate the design and conformance with the design criteria." – Wayne Magnusen et al., Deep Foundations Institute [3]
The proximity of other structures can also complicate soil analysis. For instance, at 1900 Broadway in Oakland, excavations deeper than 30 feet required intricate shoring strategies due to the nearby 19th Street BART Station, located just behind the shoring wall [1]. In areas with high liquefaction risks, combining DSM with micropiles offers a dual solution: ground stabilization and foundation support in one system [3].
Integrating Seismic Retrofitting with Shoring Work
Modern shoring projects often combine seismic retrofitting with temporary support systems, addressing both construction needs and long-term structural stability.
At 30 Otis in San Francisco, Tuan and Robinson Structural Engineers designed a 30-foot excavation in January 2020. The project used a soil-cement mix cut-off system alongside soldier beams and tiebacks. To protect adjacent structures - a three-story brick building and a two-story wood-framed building - the team employed slant-drilled underpinning, ensuring stability both during construction and in future seismic events [4].
For residential buildings, seismic retrofitting is a more affordable but equally critical process. In San Francisco, retrofitting costs typically range from $3,000 to $7,000, and the Earthquake Brace + Bolt (EBB) program offers grants of up to $3,000 for homeowners in high-risk ZIP codes [16]. This work involves bolting structures to foundations and bracing crawl space walls to prevent shifting during earthquakes. Considering that 81% of housing units in San Francisco County were built before 1980 - before modern seismic codes - this type of retrofitting is especially important [16].
"Owners of retrofitted houses can save between $10,000 and $200,000 in repair costs resulting from a major earthquake by bolting, or bracing and bolting, their homes with a seismic retrofit." – Pacific Earthquake Engineering Research (PEER) Center [16]
Drainage and Water Management Strategies
Managing water inflow is just as critical as addressing seismic concerns. In the Bay Area, groundwater control is a key factor in excavation projects, especially in regions with Bay Mud and high water tables. Soil-cement mix cut-off systems have become standard for deep excavations of 30 feet or more, effectively controlling water infiltration and preventing soil movement [1].
At 555 Mission Street, a soil-cement cut-off system paired with slant-drill underpinning successfully managed high groundwater levels in Bay Mud conditions [1][2]. In sandy soils, techniques like permeation grouting can provide localized stabilization. For example, at Symphony Towers, a 35-foot excavation used permeation grouting alongside soldier piles and tiebacks to stabilize Dune Sand [1].
California regulations also play a significant role in water management during shoring operations. These rules require that weather conditions must not reduce the soil's load-carrying capacity below the design minimum [17]. Shoring systems must resist lateral loads of at least 100 pounds per lineal foot of floor edge or two percent of the total dead load, whichever is greater [17]. For projects with shoring heights exceeding 14 feet or horizontal spans over 16 feet, detailed calculations must be approved by a California-registered civil engineer [17].
Effective water management is an ongoing process. Matching grouting techniques to the specific soil type is essential - permeation grouting works well for sandy soils, while jet grouting is better suited for heavier Bay Mud or clay-rich conditions. Daily monitoring of soil stability and weather fluctuations ensures that vertical support systems remain reliable throughout the project [17].
Conclusion: Successful Basement Underpinning in the Bay Area
The Bay Area's unique and often unpredictable geology demands custom solutions for basement underpinning projects. With rapid subsurface variations and a high seismic risk, designing shoring systems that can adapt to these challenges is critical [1][3]. The stakes are high, and precision engineering plays a central role in ensuring success.
Take, for example, advanced projects like 270 Brannan and 1100 Broadway. These developments showcase how combining techniques like Deep Soil Mixing panels, uplift-resisting micropiles, and carefully planned shoring systems can effectively counteract liquefaction and seismic uplift forces [3]. At 1100 Broadway in Oakland, engineers faced the added challenge of designing shoring systems that avoided transferring loads to the nearby 12th Street BART station, which was just 4 feet from the property line [1][2]. These examples highlight the complex, site-specific demands of working in the Bay Area.
"The combination of natural geology and historical interventions can produce complex subsurface conditions that vary dramatically over short distances." – Wayne Magnusen, Priyanshu Singh, Ryan Nagle, Gina Carlson, and Leo Panian [3]
Collaboration among engineering and construction teams is another cornerstone of success [3]. Field testing ensures that designs perform as expected before full-scale implementation, and ongoing monitoring helps maintain the integrity of shoring systems throughout construction. Foundation repair expert Steve Egloff underscores the importance of tailored solutions:
"A foundation replacement is rarely required... an experienced foundation repair contractor can fix most foundation problems" [13]
These strategies not only provide immediate stability but also protect long-term investments. With 81% of homes in San Francisco County built before 1980 - predating modern seismic codes - integrating retrofitting with underpinning work is vital for long-term structural safety [16]. Seismic retrofitting can save property owners between $10,000 and $200,000 in earthquake repair costs and increase property values by roughly 10% [16][18].
FAQs
What are the best shoring methods for stabilizing Bay Mud during basement underpinning in the Bay Area?
Stabilizing Bay Mud - a soft, silty-clay material that’s common along the San Francisco Bay shoreline - requires precise techniques to ensure safety and structural stability during excavation. One highly effective method pairs deep-soil-mixing (DSM) panels with uplift-resisting micropiles. DSM forms a sturdy soil-cement mass that helps reduce settlement and lowers the risk of liquefaction. Meanwhile, micropiles provide vertical load support and prevent wall overturning. This combination offers a reliable and cost-conscious solution, especially in seismically active areas with Bay Mud.
For projects that face space or budget limitations, diaphragm walls or soil-cement cut-off walls are often the go-to options. These methods create strong, water-tight barriers that are well-suited to the soft, water-saturated nature of Bay Mud. They also make dewatering safer and easier to manage. In cases where excavations are deeper or space is limited - like in dense urban areas - additional support systems, such as micropile-supported underpinning or slant-drilled underpinning, can be used. These techniques transfer loads to deeper, more stable soil layers, ensuring the surrounding structures remain secure.
How are seismic retrofitting and basement underpinning combined in Bay Area projects?
Seismic retrofitting often goes hand-in-hand with basement underpinning in Bay Area projects, creating a stronger foundation and boosting earthquake resistance. Techniques like helical piers, micropiles, and drilled concrete piers play a key role. These methods anchor the foundation into deeper, more stable soil layers, ensuring it can handle both the building's weight and the lateral forces generated during an earthquake - all while meeting California Building Code requirements.
Temporary shoring is a critical step during construction, keeping the site stable and safeguarding nearby structures while underpinning elements are installed. Once the new supports are securely in place and tested, the existing foundation can be lifted, leveled, and integrated with the upgraded seismic system. This streamlined process addresses settlement issues and enhances earthquake resilience in one go.
What are the main benefits of using soil nail walls in urban construction projects?
Soil nail walls bring a host of benefits to urban construction projects. One of their standout features is their ability to fit into tight, confined spaces, making them a perfect solution for areas where space is at a premium. Plus, their installation process causes minimal disruption to nearby traffic or buildings - a big win in busy urban settings.
Another advantage is the speed and cost-effectiveness of their construction. Using smaller equipment, these walls can be installed efficiently, saving both time and money.
What sets soil nail walls apart is their ability to handle differential movements and seismic loads, offering a combination of flexibility and stability that’s especially valuable in earthquake-prone regions. For walls over 15 feet tall, they often outperform traditional retaining systems in terms of both cost and efficiency, making them a go-to choice for many urban development projects.
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