Best Practices for SRW

Guidelines for designing safe, durable segmental retaining walls in residential and commercial projects.

Best Practices for SRW Design

Allan Block Corporation has been a leader in the segmental retaining wall (SRW) industry for nearly 30 years. Throughout this time, we have advanced the fields of design, construction, and research to elevate the industry as a whole. Working closely with the Concrete Masonry & Hardscapes Association (CMHA) and leading academic researchers, Allan Block Corporation has developed the Best Practices for SRW Design manual. This newsletter highlights a selection of key topics from that comprehensive document.

The manual is intended for all SRW applications, including both residential and commercial projects, and is organized into twelve chapters:

• Best Practices: Design Guidelines and Pre-Construction Considerations
• Typical Wall Construction
• Water Management – Typical
• Water Management – Alternate Drain
• Water Applications
• Soils and Compaction
• Geogrid Reinforcement
• Tall Wall Considerations
• Global Stability – General
• Global Stability – Terraced Walls
• Seismic Considerations
• Above-Wall Considerations

The purpose of the Best Practices document is to communicate effective SRW design techniques and to guide the industry toward the goal of zero wall failures due to product quality, design, or construction. While the manual provides extensive guidance, it is not a final authority. Each project has unique conditions that require the judgment of the local engineer of record to ensure a safe and efficient design. Allan Block Corporation recommends that the engineer of record be employed by the project owner.

In most cases, the project site civil engineer should serve as the engineer of record. They are best positioned to oversee the design and its impact on the site, whether completing the design in-house or contracting an experienced wall designer. The wall contractor should not be responsible for securing engineering, nor do contractors or local building officials have authority to override approved plans and specifications.

Design Guidelines

Design guidelines cover the full spectrum of considerations for SRWs: understanding the wall’s purpose, geotechnical reports, soils analysis, site plans, drainage, loading conditions, unit specifications, construction drawings, site visits, and formal construction reviews. Special attention should be paid to site soils and soils used within the reinforced mass.

• Preferred soils: Cohesionless, free-draining materials with less than 10% fines, PI < 6, and LL < 30.
• Alternative soils: Low-plasticity fines (CL, SM, SC with PI < 20, LL < 40) may be acceptable if additional design measures are taken to reduce water infiltration.

Typical Wall Construction

Effective wall construction requires attention to all aspects of the wall system:

• Soils below the wall and in the reinforced mass
• Wall rock column size and material type
• Verification of soils used
• Wall embedment depth, base size, and base material
• Geogrid minimum lengths, initial placement, and spacing
• Compaction requirements
• Capping requirements

Wall rock should be placed in the cores of hollow SRW units and in the first 12 inches (30 cm) behind the wall. This well-graded, compactible aggregate (0.25–1.5 in / 6–38 mm, ≤10% fines) aids compaction, reduces drag forces on geogrid, and provides a pathway to remove incidental water.

Geogrid lengths are measured from the front face of the wall and should extend at least 60% of the wall height. Geogrid should start on top of the first course, with spacing no more than 16 inches (40 cm). This spacing improves wall performance, particularly in seismic conditions, compared to the traditional 24-inch (60 cm) minimum.

Tall Wall Considerations

Walls 10–15 ft (3–4.5 m) or taller require additional attention, depending on surcharge conditions. A 10-ft wall with a slope or structure above it is considered tall, while the same wall without surcharge may not require additional design considerations.

Water Management

Proper water management is critical, as typical design methodologies often overlook hydrostatic forces. Effective designs identify water sources and use a combination of:

• Blanket and chimney drains
• Drainage pipes
• Above-grade water management, including swales

These strategies help minimize the impact of water on the reinforced mass.

Global Stability

Global stability is frequently overlooked. All walls with slopes above or below, as well as terraced walls, should undergo a global stability analysis. Local codes often require a 5–7 ft (1.5–2.1 m) level bench in front of the wall to aid stability.