Proper Embedment Guidelines for Retaining Wall Construction

This article covers proper embedment for segmental retaining walls, factors that affect depth, and best practices for preparing a stable foundation to ensure long-term wall performance.

When you know the geometry of your wall, do you know how much block needs to be placed below ground? Understanding proper embedment for a segmental retaining wall (SRW) is essential for achieving a stable design and preparing accurate material estimates.

Determining the correct embedment depth requires evaluating overall site conditions. Important factors include:

• What slopes exist on the site?
• When will the wall be constructed within the project schedule?
• What are the soil conditions at the site?
• Will water be present at the base of the wall?

Each of these factors can influence how deeply the wall must be embedded to provide a stable foundation.

General Embedment Requirements

Segmental retaining wall block is buried to improve stability and reduce the potential for undermining at the base of the wall.

A general guideline is:

• Minimum embedment: 6 in. (150 mm) of buried block
• For walls over 6 ft (1.8 m): add 1 in. (25 mm) of embedment for each additional 1 ft (0.3 m) of wall height

This guideline assumes flat grade conditions in front of the wall and no complicating site factors. When site conditions change, additional embedment may be required.

Toe Slopes

When a slope exists below the wall, the stability of that slope must be considered. Even relatively small slopes can affect the stability of the retaining wall if the base is not embedded deeply enough.

Reduced embedment decreases the shear resistance along the potential failure surface and reduces the stabilizing weight of soil supporting the structure.

A common recommendation when a toe slope exists is to bury enough block so that a 5 ft (1.5 m) horizontal benchcan be created from the toe of the wall to daylight.

Example:

• With a 2:1 slope below the wall
• 2.5 ft (0.8 m) of embedment is required
• This creates the recommended 5 ft (1.5 m) bench at the base of the wall

Depending on soil type and slope conditions, a global stability analysis using specialized engineering software may be required.

Future Excavation Below the Wall

Retaining walls are often installed early in a construction project. Later phases may include pavement or other improvements placed near the toe of the wall.

If excavation is required for these improvements, typical embedment depths may not be sufficient and the excavation could undermine the wall.

To avoid this problem:

• Embed the wall at least one full block course below the lowest planned future excavation.

Potential for Erosion or Scour

Sites exposed to flowing water or wave action may experience scour or erosion at the base of the wall. In these conditions, additional embedment is often necessary.

A qualified engineer should evaluate the potential scour depth and recommend the appropriate embedment. The bottom of the wall should generally extend at least 2 ft (0.6 m) below the anticipated scour depth.

Additional scour protection may also be required, such as:

• Riprap
• ScourStop™ transition mats
• Articulated concrete mats

Weak Bearing Soils or Settlement Potential

If the soils beneath the wall have low bearing capacity, deeper embedment can improve wall stability.

Additional burial:

• Increases lateral support
• Helps reduce eccentricity
• Reduces the potential for forward rotation if settlement occurs

However, there are limits to the benefit of deeper embedment. If weak soils are present, a local geotechnical engineer should be consulted to evaluate foundation conditions.

Shrink/Swell and Frost-Susceptible Soils

Segmental retaining walls are flexible structures, which allows them to accommodate minor soil movement without damaging the block system.

In most cases:

• Wall embedment does not need to extend below frost depth.
• The granular leveling pad can accommodate movement caused by freeze–thaw cycles.

If necessary, the thickness of the leveling pad can be increased to extend below seasonal frost depth without increasing the design height of the wall.

If local building codes require it, the base of the wall may also be placed below the frost penetration depth.

Soils susceptible to seasonal shrinking and swelling can be addressed in a similar manner.

Constructing a Proper Foundation for a Retaining Wall

Proper base preparation is critical for the long-term performance of a retaining wall. A well-prepared base ensures the wall has a stable and uniform foundation.

Use a clean granular material, such as wall rock, to create a firm base.

Site Preparation

1. Excavate the area by removing all vegetation and organic materials. These materials should never be used as backfill.
2. If reinforcement is required, excavate behind the wall to accommodate the full design length of the geogrid. Refer to the approved plans for exact lengths.
3. Starting at the lowest point of the wall, excavate a base trench 24 in. (600 mm) wide along the entire wall length.

Base Material Requirements

The base material depth should be:

• 4 in. (100 mm) minimum for walls under 4 ft (1.2 m)
• 6 in. (150 mm) minimum for walls over 4 ft (1.2 m)

The base trench depth must include:

• the base material thickness
• plus 1 in. (25 mm) for every 1 ft (300 mm) of wall height to accommodate the required buried block.

Compaction

Compact the base trench with at least two passes of a walk-behind plate compactor to create a firm foundation.

The foundation soils at the bottom of the trench must be solid and stable. If the soils consist of heavy clay, wet soils, or previously disturbed material:

• remove the unsuitable soils
• replace them with granular material
• compact in lifts of 8 in. (200 mm) or less