Technical Reference: Segmental Retaining Wall (SRW) Design and Construction

Disclaimer and Scope of Use

Important Notice:
This document is provided for informational purposes only and reflects general industry best practices established by the Concrete Masonry and Hardscapes Association (CMHA) and the National Concrete Masonry Association (NCMA). It does not constitute a site-specific engineered design.

Retaining walls are structural elements. Walls exceeding approximately 3 to 4 feet in height, or walls supporting surcharge loads such as slopes, driveways, parking areas, fences, or structures, typically require a design prepared and sealed by a Licensed Professional Engineer (P.E.) and approval from the local Authority Having Jurisdiction (AHJ).

The author assumes no liability for the use or misuse of this information.

1. System Overview, How Segmental Retaining Walls Work

Unlike rigid reinforced concrete walls, a Segmental Retaining Wall (SRW) is a flexible, gravity-based system. Stability is achieved through the self-weight of dry-stacked concrete units and, where required, the mass of a reinforced soil zone behind the wall.

Key Structural Components

The Units
Dry-cast concrete blocks manufactured in accordance with ASTM C1372. Depending on the system, units interlock using rear lips, pins, shear lugs, or molded geometry.

The Batter
An intentional setback established by the block geometry, typically on the order of approximately 2° to 6°, depending on the system. The batter shifts the center of gravity toward the retained soil, reducing active earth pressure acting on the wall face.

The MSE Zone (Mechanically Stabilized Earth)
For taller or more heavily loaded walls, geogrid reinforcement creates a reinforced soil mass that acts together with the wall units as a single structural block.

2. Foundation and Base Engineering

The most common cause of SRW distress is differential settlement resulting from inadequate base preparation. This mechanism is closely related to bearing capacity and embedment behavior in loose or disturbed soils, similar to those encountered in drilled shafts and soldier pile systems.

Excavation and Embedment

To prevent toe sliding and frost-related movement, a portion of the wall must be buried.

• Standard practice: Embedment of approximately 10 percent of the exposed wall height

• Minimum embedment: 150 mm (6 inches) for any wall

Granular Leveling Pad

The leveling pad provides uniform bearing support for the wall.

• Material: 19 mm (3/4 inch) crushed, angular stone
  Sand or pea gravel must not be used, as these materials lack shear resistance and can deform under concentrated loads.

• Dimensions: Minimum 150 mm (6 inches) thick, extending 150 mm (6 inches) in front of and behind the block

• Compaction: Typically 95 percent Standard Proctor Density, or as required by manufacturer specifications

3. Soil Reinforcement, Geogrid Integration

Geogrid reinforcement is required when the gravity mass of the wall units alone is insufficient to resist lateral earth pressures.

When Geogrid Is Typically Required

• Wall height exceeds approximately 3 to 4 feet

• Surcharge loads such as parking areas, pools, fences, or structures are present within a distance behind the wall approximately equal to the wall height

• Sloping ground exists above the wall, such as 2H:1V or 3H:1V slopes

Actual reinforcement requirements depend on soil conditions and manufacturer design tables.

Installation

Geogrid must be installed perpendicular to the wall face and pulled taut. It is held in place by the block units and activated by compacted backfill through friction, not by adhesive.

4. Drainage and Hydrostatic Control

Water is the primary contributor to retaining wall distress. Saturated soils impose significantly higher lateral pressures than dry soils.

Drainage Components

Drainage Column
A minimum of 300 mm (12 inches) of clean, angular, free-draining stone must be placed directly behind the wall units for the full height of the wall.

Filter Fabric
A non-woven geotextile must separate the drainage stone from native soil to prevent migration of fines and long-term clogging.

Drain Outlet
A perforated drain pipe should be installed at the base of the drainage column and sloped at approximately 1 percent to daylight or an approved discharge point.

Note: Segmental retaining walls rely on internal drainage and do not typically incorporate weep holes through the wall face.

5. Proper Backfilling and Compaction

The long-term performance of an SRW depends heavily on proper backfilling practices.

• Lift thickness: Backfill should be placed in 150 to 200 mm (6 to 8 inch) lifts

• Compaction equipment: Use heavy plate compactors for bulk backfill. Within approximately 1 m (3 feet) of the wall face, use lighter or hand-operated equipment to avoid pushing the wall out of alignment

6. Construction Tolerances

Although SRWs are flexible systems, they must still meet accepted industry tolerances.

• Horizontal level: Variation not exceeding 1 percent along the wall length

• Vertical alignment: Total deviation not exceeding 1 percent of wall height

• Bulging: Limited to approximately 6 to 12 mm (1/4 to 1/2 inch) over a 3-meter length

Deviations beyond these limits are typically associated with base settlement or inadequate backfill support. Such conditions are commonly evaluated through a structural engineering inspection to determine causation and appropriate corrective measures.

7. Summary Checklist for Quality Assurance

Closing Note

Segmental retaining walls are highly durable when installed in accordance with manufacturer guidelines and sound engineering practice. The majority of performance issues are not material related, but instead result from inadequate base preparation, poor drainage, or improper compaction.