Installing an SRW properly will avoid a range of problems and costly corrections. Here’s how to do it.

by Gabriela Mariscal, Geotechnical Engineer, National Concrete Masonry Association

Modular block walls, or segmental retaining walls (SRWs) are a type of mechanically stabilized earth (MSE) structure built with mortarless concrete block facing commonly reinforced with geosynthetics.

Because they use no mortar and are conveniently sized, modular block walls can be assembled quickly and without heavy construction equipment. SRW blocks are “dry stacked”, relying on their weight and frictional, mechanical or shear interlock, along with the reinforcement and soil, to engineer durable MSE walls that offer facings available in a range of patterns, textures, colors, and finishes.

The variety of colors and textures available for SRW blocks allows designers to envision walls that integrate aesthetically with their surroundings, such as this colorful retaining wall in El Paso, Texas.

Due to their modular nature MSEs can tolerate much larger settlements than reinforced concrete walls¹, without showing signs of distress. Additionally, they can be reused or recycled. They do not contain the chemicals used to preserve wood or need the extensive form work and specialized labor required for cast in place retaining walls. The constant research of the block manufacturers is allowing them to produce higher quality products able to perform in different conditions.

Reduced labor time and cost, durability, and low maintenance requirements have made modular block walls a competitive design option for landscaping, commercial property development, and, of course, highway construction projects.

SRWs are a cost effective solution for stabilizing slopes near roads. Multiple SRWs are used to protect this road in Wisconsin from hillside erosion.

According to the Federal Highway Administration (FHWA)², the first MSE walls used in highway projects appeared in the early 1970s and the first modular block wall was used in the 1980s.

Today, there are more than 60,000 MSE walls in excess of 35 feet high along U.S. highways. Another 9 million-square-feet of MSE walls are being constructed annually using different materials. The tallest MSE wall, built along a runway at the Seattle-Tacoma International Airport, towers 140 feet high.

For the aforementioned reasons, MSE walls are used increasingly as support structures for ramps, bridge abutments, road elevations, bridges, grade changes, waterfront protection and it is important to know the mechanics of their analysis.

Analyzing MSE Wall Stability

A core feature of MSE wall design software is stability analysis, which is key to preventing MSE wall failure.

Attractive, low-maintenance bridges using SRWs are effective spans over challenging terrains. This two-lane bridge spans a creek in Beaverton, Ore.

A recent article in the Journal of Geotechnical and Geoenvironmental Engineering documents such a MSE wall failure in Korea.³ An investigation revealed that “the inappropriate design and the low-quality backfill were mainly responsible for the wall failure, although the primary triggering factor was the rainfall infiltration.”

To minimize the possibility of MSE wall failure, designers should have a basic understanding of four important stability analyses: external stability, internal stability, compound and global stability.

External stability examines the reinforced soil zone and the facing column to determine the appropriate geosynthetic length to overcome base sliding, overturning about the toe, and foundation-bearing capacity.