Best Practices for Segmental Retaining Wall Design

The intent of this material is to communicate the best practices for design of Segmental Retaining Walls (SRW) as determined by Allan Block Corporation based on 25 plus years of research, design and field experience. This is not meant to be a final authority as each project has its own set of unique situations.

The local engineer of record must use their best engineering judgment to account for those situations that present themselves and provide a safe and efficient design for the customer. At no time does the contractor or local building official have the authority to override the approved plans and specifications provided from the local engineer of record.

It is the recommendation of Allan Block Corporation that the local engineer of record work for and be paid by the project owner. It has been determined that the local engineer of record should be the Project Site Civil Engineer as they are best suited to take responsibility for the design, and how it affects the site, whether they do the design in-house or use an outside consultant to do the design for the project.

The Project Site Civil Engineer has control of several of the overall aspects of the project and therefore is most able to properly handle the integration and communication required to ensure the performance of the wall complies with the needs of the site. For wall design applications that are outside of the experience level of the Project Site Civil Engineer, a wall designer with the appropriate knowledge and experience should be contracted with by the Project Site Civil Engineer. It is recommended that the wall contractor not be responsible for securing the engineering.

last updated: 1/14/2022

Chapter 11: Seismic Considerations

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updated: 5/29/2018

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11.1 Recommendations Associated with Seismic Loading

11.1 For walls with dynamic loading:

Designer must understand the local seismic code requirements before starting design.
Closer spacing of geogrid is recommended, maximum 16 in (40 cm).
Extension of the top layers of geogrid. Typically the top layers will be extended to roughly 90% or more of the wall height to satisfy design requirements.
Using select/structural backfill will reduce the effects of the dynamic loading.
For more information on seismic design and the effects on segmental retaining walls, see the Allan Block Executive Summary of the independent full-scale seismic testing conducted by Columbia University and the National Research Institute of Japan.

11.2 Slope Above Seismically Loaded Walls

11.2 When dealing with a slope above a wall with seismic loading applied to the wall, the same acceleration coefficient applied to the wall must also be applied to the stability calculations of the slope.

11.3 Monotube-Okabe Slope Above Limitations

11.3 The Mononobe-Okabe (M-O) seismic methodology places limits on the steepness of any slope above the wall. If during the design phase it is determined that the desired slope is not allowed, the site grading should be altered or the wall height should be increased to reduce the steepness of the slope above. See section 12.4 for description of slope steepness.

11.4 Alternate Design Approach – Trial Wedge Method

11.4 While the M-O method is the standard used in the seismic design of segmental retaining wall, a trial wedge method may be utilized to investigate walls when the M-O method becomes limited due to slopes above. Using methods similar to a global stability model the trial wedge method determines the weight of the soil wedge above the failure plan and determines the active earth pressure of the soil wedge acting at the back of the soil mass. Using the trial wedge method is not without limitations as well. The solution it provides is purely mathematical and understanding soil mechanics and the slope steepness limits is also important. The limitation to the steepness of a slope discussed in section 12.4 still apply, and if the designer chooses to utilize the trial wedge method for walls with slopes above that exceeded the soil mechanic limits, they must review the slope stability in a global stability program and possibly reinforce the slope above the wall. For a greater discussion on the Trial wedge method see Chapter 5, Seismic Design, in the Allan Block Engineering Manual.