Wall Penetrations for Retaining Walls

Guide to designing and constructing wall openings in Allan Block retaining walls, including culverts, pipes, and structural penetrations.

The widespread use of segmental retaining walls (SRWs) around the world has led to a variety of design solutions. With diverse project applications comes the need for different types of wall openings—ranging from simple drainpipe openings to large, corrugated steel bridge culverts. Most openings allow water to flow from behind the wall, while others provide ventilation for buildings, traffic passage for bridges, or access for utilities.

Culverts and Wall Openings

Any wall opening requires two key considerations: proper sizing for its intended use and structural support for the surrounding wall.

• Small openings: Pipes of modest size often require no external support—cutting the wall facing to fit the opening may be sufficient.
• Larger openings: Stability can be ensured with reinforced solutions such as bond beams built directly into the wall or fully reinforced concrete headwalls. These measures add support to both the wall and the structure passing through it.

Designers must first determine the pipe or culvert capacity. Once the opening size is known, the appropriate wall design can be selected and engineered.

For large openings, differential settlement is a critical concern. Large structures are typically built on frost footings, while walls are not, so additional foundation stabilization may be necessary. Minimizing differential settlement reduces the risk of stress cracks in the wall face. Vertical control joints may also be required to manage potential cracking.

Design & Construction Tips for Culverts and Openings in Allan Block Retaining Walls

The design of a wall penetration depends on the opening’s size and purpose.

• Bridge structures: The retaining wall’s weight contributes to the overall stability of the structure. Design focus is usually on sliding or shear along the culvert surface, which can be addressed with a concrete lip or “beauty ring” formed between the culvert edge and the Allan Block facing. Reinforcement bars and anchoring devices secure the lip to the culvert. Anchoring methods vary depending on culvert material, such as concrete, corrugated steel, or HDPE.
• Large pipes or box culverts: Fully reinforced concrete headwalls with structural footings may be required. Vertical control joints, cut into every other block course, help manage potential surface cracking from differential settlement.

For water-carrying pipes, additional washed drainage rock should be placed around the pipe in the reinforced zone behind the wall. This ensures that any leaks are safely drained, preventing internal wall damage. Designers should also evaluate outflow and use rip-rap or similar erosion control measures in front of the wall.

• Rigid structures: Concrete box culverts are relatively straightforward because they do not flex.
• Flexible structures: Corrugated steel culverts may deform if construction is not methodical. To maintain symmetry, contractors should build one course at a time on both sides of the culvert simultaneously.

When constructing a concrete headwall, it is common to pour in stages. The final pour is made after the Allan Block wall is built to the adjacent height, allowing subsequent courses to sit directly on top. Any beauty ring casting is completed near the end of construction.

Planning and coordination are essential, especially for large openings where multiple contractors may be involved. Preconstruction meetings should address sequencing to ensure a smooth and successful project.

Don’t Forget About Lighting

Surface-mounted lighting and low-voltage equipment may also require wall penetrations for fixtures, wires, and conduits. These should be carefully planned before wall construction to ensure proper installation.

AB Walls Elevation Views and Estimating

AB Walls is Allan Block’s SRW design software, offering advanced features beyond simple cross-section design. Key capabilities include:

• Detailed elevation and plan views
• Multiple cross-section cuts along the wall length
• Export to DXF format for any CAD program
• Material quantity calculations for the entire project

AB Walls is available through your local Allan Block producer or the Allan Block Engineering Department.

DID YOU KNOW?
Anchoring signage, handrails, or equipment to the Allan Block facing is also considered a wall penetration. Standard masonry concrete anchoring bolts can be used, sized according to project requirements. Larger anchors may require additional engineering consideration.