How Can I Determine My Retaining Wall Project's Embedment Depth for More Accurate Final Project Costs?

One element sometimes overlooked when preparing project estimates is the necessary amount of embedment the wall design will require. Read on to learn more about determining the embedment depth for your wall project.

What is Embedment in a Retaining Wall?

Retaining wall embedment is the portion of a retaining wall buried below ground level that provides support and stability for the wall. The embedment required for any retaining structure depends on factors such as wall height, grading at the top and bottom of the wall, groundwater elevations, loading conditions, and many others. The minimum embedment depth for a Redi-Rock wall is 6 in (152 mm). The maximum could require multiple courses.

When the preliminary project design doesn't consider proper embedment, retaining wall block counts can be shorted on the estimate. This results in an estimate that doesn’t accurately reflect the material quantities necessary to complete the job and creates a noticeable discrepancy between the initial estimate and the final cost.

Why is Embedment Important?

The standard design guidelines require embedment depth for all retaining walls regardless of the product. The proper embedment depth is an important element in design that helps ensure the wall's foundation is stable. It ensures the wall does not move laterally, the bottom of the wall is not undermined, and the base is at a depth that ensures the proper bearing capacity of the foundation soils while limiting settlement.

How Can I Determine My Wall Project's Embedment Depth to Provide a More Accurate Project Estimate?

The embedment depth for your retaining wall depends on several factors, including soil conditions, wall height, loading conditions, groundwater elevations, and frost depth. A common rule of thumb for different wall types is:

• Gravity and Segmental Retaining Walls. Embedment depth is typically 10% of the wall height, with a minimum of 6 in (152 mm).
• Cantilever Cast-in-Place Retaining Walls. Embedment depth is usually 33-50% of the wall height for stability.

What Site-Specific Characteristics May Also Influence Embedment Depth?

To ensure accurate embedment estimates, also consider the following:

• Soil Bearing Capacity. If the soil is weak or expansive, deeper embedment may be required to reach the depths where the foundation soil has enough bearing capacity.
• Slope Conditions. If the wall is on a slope, embedment depth may need to be increased.
• Seismic Considerations. Earthquake-prone areas may require extra depth and reinforcement.

The National Concrete Masonry Association’s Design Manual for Segmental Retaining Walls (3rd Edition) suggests that embedment depth should be increased when any of the following special conditions are present:

• Significant settlement potential or weak bearing capacity of underlying soils
• Steep slopes near or below the toe of the wall
• Potential scour at the toe of the wall in waterfront and submerged wall applications
• The maximum depth of seasonal soil volume change extends below the bearing pad
• Seismically-active sites

The standard design guidance also indicates that wall embedment depth for modular block walls need not extend below seasonal frost depth if the soils under the wall are not susceptible to significant movement due to freeze/thaw cycles.

Any retaining wall expected to exceed 6 ft (1.8 m) should be reviewed by a qualified geotechnical engineer familiar with site soils.

Learn more about embedment and other important retaining wall design considerations.

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Proper accounting of embedment depth in your retaining wall estimate ensures accuracy in material calculations and project costs. By considering soil conditions, wall height, loading, and site-specific characteristics, you can better determine the necessary embedment depth for stability and long-term performance… and not be surprised by a higher final project cost.