Seismic Engineering Challenges for San Diego International Airport's New Terminal 1

• How seismic hazards like liquefaction, lateral spreading, and fault rupture influenced the design of Terminal 1 at San Diego International Airport.

• What site-specific seismic response analyses are and why they’re critical for airport construction in high-risk zones.

• Which deep foundation and ground improvement techniques were used to mitigate seismic risk, including ACP piles, CIDH piles, DSM, and jet grouting.

• Why regional airports in seismically active or coastal areas must go beyond standard code requirements to ensure long-term structural resilience.

San Diego International Airport's Terminal One expansion is a transformative infrastructure project for the San Diego region that includes a new terminal structure, an expansive parking garage, and a comprehensive network of new access roads. The new Terminal 1 (T1) structure includes 30 gates for aircraft and will significantly expand San Diego International Airport’s capacity and improve access for travelers across the San Diego region. The network of access roads for the new terminal includes nine new bridges, multiple retaining walls, and on-grade and elevated roadways. The design-build project team was led by a joint venture of Turner and Flatiron, with Gensler as the lead architect and Kleinfelder serving as both the landside civil engineer and the airside and landside geotechnical engineer. Kleinfelder worked with Turner Engineering Group for both design optimization and risk mitigation of the site’s challenges and strategies described herein. The joint venture project team has completed the design phase and the project is currently in construction, with plans to complete the first phase of the terminal in 2025.

The project’s complexity was heightened by the terminal’s location near the Rose Canyon Fault Zone (RCFZ). While it’s not as active as the better-known San Andreas Fault, past earthquakes and recent studies suggest that the RCFZ is capable of generating significant earthquakes, potentially reaching magnitudes of 7.0 or higher. Additionally, the airport is located on hydraulically filled reclaimed land next to San Diego Bay, resulting in shallow groundwater and loose and compressible soil conditions which have the potential to liquefy, lose strength, and seismically settle during a significant seismic event. 

Given the site's geological conditions and seismic setting, innovative earthquake engineering solutions were necessary to ensure seismic life safety and long-term serviceability for the project’s structures.

Understanding the Site’s Seismic Challenge

Due to the site’s location in the seismically active Southern California region, a detailed geotechnical investigation was necessary to assess the seismic risks associated with the terminal, parking garage, and roadway access network. Four primary seismic considerations influenced the design of Terminal One and its associated infrastructure, which consisted of the following:

·       Liquefaction 

·       Lateral Spreading 

·       Seismic Ground Motions

·       Fault Rupture 

Liquefaction occurs when loose, saturated granular soil loses its strength due to earthquake shaking, temporarily behaving more like a liquid than a solid. Structures founded on or above potentially liquefiable soils may experience significant damage or even failure due to the temporary loss of foundation support, total and differential seismic settlements, and/or lateral spreading displacements. Given the loose, hydraulic fill composition of the near-surface soils at the site, the shallow groundwater from the nearby bay, and the potential for significant seismic shaking during an earthquake, the risk of liquefaction was high for the project site. 

Furthermore, when combined with sloping ground or “free face” conditions, such as at a river or bay edge, the loss of soil strength that is associated with liquefaction can result in significant lateral displacements through a phenomenon known as lateral spreading (i.e., a form of seismic slope instability), even for very small ground surface inclinations on the order of a few percent. These lateral spread displacements can be on the order of several feet, imposing substantial lateral loads on foundations and retaining walls and potentially causing irreparable damage or failure. Accordingly, bridges and other structures at the project site that are on inclined ground surfaces or near retaining walls founded on potentially liquefiable material were at risk of significant lateral soil movement due to lateral spreading during seismic events.

Due to the potentially liquefiable soils at the site, development of seismic ground motions to be used for structural design of the T1 building and new bridge structures required complex modeling techniques known as site response analyses. Unlike standard seismic design approaches, which are based on firm ground conditions and mapped seismic design values, site response analyses modeled the anticipated vertical propagation of earthquake waves through the underlying soft and loose saturated soils at the site during multiple earthquake scenarios. This site response analysis provided the seismic design parameters for the structures for the T1 project.

Lastly, one of the more significant and unique challenges that drove design decisions for the project was the presence of the active RCFZ running through the airport site. This required careful design considerations, as regulations mandate specific setbacks from faults with the potential for surface rupture to prevent catastrophic structural damage in the event of an earthquake. 

Mitigation Strategies for Seismic Stability

To address the project site’s four seismic challenges, the following mitigation strategies were implemented:

·       Liquefaction – Deep foundations, extending into the firm non-liquefiable materials at depth below the site, were implemented for the T1 building, parking garage, and bridge structures at the site.

·       Lateral Spreading – Foundations for bridges were designed to resist lateral loads imparted on the structure due to lateral spreading. Ground improvement was used at select bridge approach areas where seismic slope instability risk was high.

·       Seismic Ground Motions – As required by the California Building Code and Caltrans standards for liquefiable sites, site response analyses were performed to model the propagation of seismic ground motions through the soft and liquefiable ground conditions implementing site-specific seismic design parameters.

·       Faults –Strategic placement of bridge structures and placement of the T1 building footprint were implemented such that a proper clearance was provided to the surface trace of the RCFZ to comply with fault setback zone requirements

Deep foundations consisting of Auger Cast-in-Place (ACP) piles were implemented for the T1 building and parking garage, while Cast-in-Drilled-Hole (CIDH) piles were implemented for the bridge structures for seismic support. ACP piles are deep column elements that are cast in place using a hollow stem auger with continuous flights pumping grout from the bottom of the drilled hole as the auger is removed upon completion of drilling, then reinforced with a steel reinforcement cage. The CIDH piles are large-diameter, high-capacity column elements in which grout is pumped into an open drilled hole that is supported by polymer slurry or by temporary or permanent casings. These foundations were installed to depths well below the loose and soft liquefiable soil layers to resist both structural loads and seismic loads caused by vertical and lateral soil movement from liquefaction and lateral spreading. The piles were designed to withstand downward forces due to seismic settlement of the surrounding ground.

Ground improvement strategies were implemented at select bridge approaches and retaining walls to mitigate the potential for seismic slope instability. The ground improvement techniques implemented for the project consisted of Deep Soil Mixing (DSM), a technique where the existing soil was mixed with grout to increase strength and stability, and jet grouting, a method that involved injecting a column of grout placed using high-pressure nozzles at depth in the ground, creating a pile-like grout column without requiring extensive reinforcement. These strategies provided a cost-effective alternative to deep piles while ensuring the necessary ground stability for seismic resilience.

Lastly, to mitigate surface fault rupture hazard, the eastern edge of the T1 building was designed with a footprint to maintain a minimum distance of 25 feet from the active strands of the RCFZ, resulting in an angled eastern end of the structure.

Broader Implications for Seismic Engineering in Airport Development

While the seismic challenges at San Diego International Airport are particularly complex, they are not unique to this project. Earthquake risks extend along the entire West Coast, including California, Oregon, Washington, and Alaska, as well as in regions further east, such as the New Madrid Seismic Zone in the Missouri/Tennessee/Arkansas area and in South Carolina. Additionally, many airports, especially those located in coastal areas, are built on reclaimed land, making them vulnerable to similar liquefaction and lateral spreading risks.

The solutions implemented during the design and construction of the San Diego T1 expansion provide valuable insights for future airport developments nationwide. Key takeaways include:

 Importance of Site-Specific Seismic Analysis: Traditional regional seismic design factors may not be sufficient for complex sites, especially where liquefaction hazards exist. More complex sites may require detailed, location-specific studies that account for the unique soil and seismic conditions of the project.

·       Adaptability in Foundation Design: Incorporating deep foundations and/or ground improvement techniques can mitigate the risks of liquefaction and lateral spreading.

·       Innovative Engineering Solutions: Using techniques like DSM and jet grouting can provide effective alternatives to deep pile foundations, balancing cost and safety considerations.

·       Importance of Local Geologic and Seismic Expertise: Engaging a team of engineers and geologists with an in-depth understanding of the local geologic and seismic hazards allowed for the design-build team to implement mitigation measures early in design development, avoiding potential schedule or cost overruns.

The expansion of San Diego International Airport’s T1 represents an impressive engineering achievement that successfully navigated significant seismic challenges to better meet the growing demand for air travel while improving overall airport infrastructure in the San Diego region. By leveraging site-specific seismic analyses, deep foundation solutions, and innovative ground improvement techniques, the design-build team’s geotechnical engineers delivered cost-effective solutions for the new terminal, bridges, and roadways capable of withstanding the region’s earthquake risks.