SRWs and Seismic

Segmental Retaining Walls (SRWs) and geogrid reinforcement were used to stabilize a hillside in a seismic region, while highlighting key design practices and seismic considerations.

Slope Stabilization in a Seismic Region

Since the emergence of the Segmental Retaining Wall (SRW) industry in the 1980s, modular block systems have steadily grown in popularity. Compared to traditional retaining wall construction, SRWs offer reduced costs and faster installation timelines. As adoption has increased, so has industry knowledge and innovation.

A major advancement came in 2002, when Columbia University and Allan Block partnered with Professor Hoe Ling and Professor Dov Leshchinsky to conduct full-scale seismic testing on shake tables in Japan. The results significantly improved the understanding of dynamic earth pressures and ultimately reshaped how SRWs are designed in seismic environments.

Project Overview: Diamond Bar, California

Slope stabilization in a seismic region became a key concern for the Dua family in Diamond Bar, California. Their planned residence was to be built into a hillside overlooking the surrounding area. Due to the significant grade change, the project required a series of terraced retaining walls, along with careful consideration of global slope stability.

ABI Engineering Consultants was engaged to evaluate the site and develop a feasible solution. Early in the process, ABI identified that global stability—not wall design—was the primary challenge. Before any wall construction could begin, the existing slope needed to be excavated and reinforced.

To address this, geogrid reinforcement was installed within the slope below the proposed walls. This stabilization method mitigated potential slope failure and created a reliable foundation for the retaining wall system.

Design Approach and Construction

Given the project’s location in a highly seismic region, ABI Engineering selected an Allan Block SRW system due to its inherent flexibility and ability to accommodate movement during seismic events.

For the seismic design, ABI used a horizontal peak ground acceleration (Ao) of 0.4g, which falls within the typical industry range of 0 to 0.4g. This value reflects a conservative and site-appropriate design approach.

Jimmy Wang of ABI Engineering served as the primary designer. By utilizing AB Walls design software alongside ReSSA Global Stability modeling, he was able to confidently design a complex terraced wall system for a seismically active site.

Once the design was finalized, the Dua family partnered with Orco Block and secured a Certified Contractor to complete construction. With ABI overseeing the installation, the Allan Block wall system was successfully constructed. The stabilized slope and completed walls allowed the project to move forward with residential construction on a secure foundation.

Selecting an Appropriate Seismic Acceleration Value

A common question in SRW design is: What seismic acceleration value should be used?

Typically, this value is determined by a local geotechnical engineer familiar with site-specific conditions. However, when additional guidance is needed, industry standards provide a clear direction.

Allan Block, along with organizations such as the National Concrete Masonry Association (NCMA) and the Federal Highway Administration (FHWA), references AASHTO guidelines for seismic design.

According to AASHTO, seismic acceleration—often referred to as A, Ao, or As—is defined as the Peak Ground Acceleration (PGA) adjusted for site effects.

Key Considerations:

• The horizontal acceleration coefficient (kh) is typically more significant than the vertical coefficient (kv), and kv is often neglected.
• The US Geological Survey (USGS) provides tools to calculate site-specific PGA values.
• AASHTO allows a 50% reduction in seismic coefficient when 1–2 inches (25–50 mm) of deformation is acceptable.
• Since SRWs can tolerate up to 3 inches (75 mm) of lateral movement, this reduction is commonly applied in SRW design.
• For rigid wall systems, AASHTO recommends using the full PGA value.

The NCMA Design Manual (3rd Edition) further limits seismic acceleration (As) to 0.45g, supporting the commonly used industry range of 0 to 0.4g, which has demonstrated excellent performance in both laboratory and field conditions.

Code Guidance and Industry Standards

In some regions, such as California, building codes (e.g., the 2013 California Building Code) may not provide sufficient guidance for SRW design. In these cases, engineers commonly rely on AASHTO standards, which serve as the primary reference across the United States for seismic retaining wall design.

This consistent reliance on AASHTO ensures uniformity and reliability in SRW engineering practices nationwide.

Best Practices for Seismic SRW Design

To further support engineers, Allan Block developed Best Practices for SRW Design in 2014. This work contributed to the NCMA’s Segmental Retaining Wall Best Practices Guide, created in collaboration with industry leaders.

Key recommendations include:

• Maximum geogrid spacing of 16 in. (406 mm)
• Use of structural fill throughout the reinforced zone
• Minimum geogrid lengths:

  • 60% of wall height for upper and middle layers
  • 90% of wall height or +3 ft (0.9 m) for top layers
• Apply the same seismic acceleration to both wall and slope stability analyses
• Recognize limitations of the Mononobe-Okabe (M-O) method for steep slopes
• Modify grading or increase wall height if slope angles exceed M-O limits
• Consider advanced analysis methods (trial wedge, finite element) when needed

These guidelines are based on over 25 years of research, testing, and field performance, with the ultimate goal of achieving zero wall failures in SRW applications.