Segmental Retaining Walls vs Cast In Place

Discover how a project originally designed with cast-in-place concrete was redesigned using an Allan Block segmental retaining wall system to lower costs and expand design options.

Upgrading to Allan Block – Cast-in-Place No More

When a retaining wall is required, many engineers still lean toward specifying a cast-in-place concrete wall. This project was no exception. ROV Consulting, Inc. originally designed this high-end residential project with a traditional concrete retaining wall.

However, after speaking with the local representative from Allan Block about the benefits of segmental retaining wall (SRW) design, the engineering team began to recognize the advantages the Allan Block system could provide.

The flexibility of the system allowed the retaining walls and stairs to be easily integrated into the landscape, which opened up new design possibilities. Additional patios and terraces were explored, giving the homeowner more opportunities to enjoy the backyard space. Using their newly acquired knowledge of Allan Block systems and the AB Walls Design Software, the engineers at ROV Consulting evaluated several structural design options.

The general contractor was understandably pleased when multiple bids from AB Certified Wall Installers came in significantly lower than the original cast-in-place concrete wall estimate. The Allan Block system eliminated the need for extensive formwork and continuous concrete footings, greatly reducing labor and installation costs.

To help satisfy the homeowner’s aesthetic preferences, the contractor arranged a visit to a local dealer’s yard to review available color and pattern options in the AB Europa Collection. The homeowner quickly selected the AB Abbey Pattern, and construction soon began.

By choosing Allan Block products, all three key decision-makers—the engineer, the contractor, and most importantly the homeowner—were extremely satisfied with the outcome. The final result was a beautiful and functional retaining wall that met all project requirements while remaining cost-effective to design, plan, and build.

SRWs vs. Cast-in-Place Concrete Walls

For decades, commercial developments commonly relied on reinforced cast-in-place concrete walls to retain soil. Since the mid-1980s, however, segmental retaining walls (SRWs) have emerged as a more efficient and flexible alternative.

Some key advantages SRWs offer over traditional cast-in-place walls include:

• Lower installation costs
• Reduced excavation requirements since frost-depth footings are typically unnecessary
• Faster installation
• Simplified design processes
• Improved aesthetics
• Multiple reinforcement options (geogrid, no-fines concrete, wall anchors, etc.)
• Flexible structural behavior rather than rigid performance

Beyond these benefits, SRWs often outperform rigid walls under extreme conditions, particularly during seismic events. Their flexible design allows them to absorb and dissipate seismic forces more effectively than rigid concrete structures.

Proven Seismic Performance

Full-scale seismic testing conducted by Allan Block demonstrated that SRWs can withstand significant seismic loading. Test walls were subjected to forces up to 0.8g and experienced only minimal deflection and settlement.

The testing showed that the block facing, reinforced soil mass, and geosynthetic reinforcement move together as a single, coherent system during seismic loading. According to Dr. Hoe Ling of Columbia University, structures that are both flexible and coherent are particularly well suited for seismic conditions (AB Seismic Research Summary, Reference Document #R0505, August 2003).

Real-world performance has supported these findings. SRWs installed in seismically active regions have repeatedly demonstrated superior resilience compared to rigid wall systems. This was further documented in a report by the Federal Highway Administration summarizing the effects of the 2010 Maule Earthquake in Chile, which evaluated impacts to transportation infrastructure.

The Evolution of SRW Design

Segmental retaining wall design and construction practices have evolved significantly over time. Improvements in block manufacturing—such as higher compressive strengths and lower absorption rates—have increased durability and long-term performance.

Design methodologies have also advanced. Early reinforced wall designs often used geogrid spaced every four courses with reinforcement lengths equal to about 50% of the wall height. These approaches no longer meet modern design standards.

Although external stability calculations often produced acceptable factors of safety, field performance revealed that construction practices—such as inadequate compaction or large vertical spacing between reinforcement layers—sometimes resulted in suboptimal wall behavior.

Full-scale testing conducted by Allan Block demonstrated the advantages of placing geogrid reinforcement at two-course (16 in / 400 mm) spacing. Closer spacing improves overall wall performance by:

• Creating a more cohesive reinforced soil mass
• Allowing lighter-weight geogrid to be used
• Interrupting potential slip planes between reinforcement layers

Testing also refined the understanding of seismic load distribution, showing that a rectangular load distribution along the wall face better represents actual seismic forces. As a result, design guidelines evolved to reduce primary reinforcement lengths from 70% of wall height to about 60%, while extending the top reinforcement layer to approximately 90% of wall height to bridge the retained and reinforced soil zones.

Advancements in Stability Analysis

Historically, retaining wall designs focused on three primary analyses:

• Internal stability
• External stability
• Bearing capacity

Today, many engineers also perform Internal Compound Stability (ICS) analysis. ICS evaluates potential failure surfaces that pass through the retained soil, reinforced soil mass, and wall facing within the design envelope.

These calculations consider:

• Strength of infill and retained soils
• Individual geogrid layer strengths and spacing
• Shear and connection strength provided by the SRW facing

By analyzing both internal and external stability simultaneously, ICS provides a higher level of verification for reinforced soil structures. Allan Block has played a leading role in promoting ICS analysis and was the first to incorporate it into its design software.

A Reliable and Cost-Effective Solution

Segmental retaining walls have become a dependable and cost-effective option for nearly all retaining wall applications. As design methods, materials, and construction practices continue to improve, SRWs will remain at the forefront of retaining wall technology.

Much of this advancement has been driven by the ongoing research and development efforts of Allan Block, helping engineers, contractors, and owners deliver better performing retaining wall solutions for projects around the world.