Landscape Architect Articles

2006 November LSMP PMBR - Segmented Retaining Wall Options12-04-06 | News

Segmented Retaining Wall Options

by National Concrete Masonry Association

An example of a terraced wall system, showing how the necessity to break up a vertical wall with a terrace can be turned into an advantage. This terrace (center) has been turned into a seating and viewing area. Image courtesy of Keystone Retaining Wall Systems, Inc.

The development of interlocking concrete retaining wall units has led to a vastly expanded number of options. That?EUR??,,????'?????<

The following introduction to retaining wall products and anchoring systems is based on text supplied by the National Concrete Masonry Association (NCMA).

With the photos that accompany it here, the article attempts to supply common information that applies to all retaining wall designs. More specific information and specifications, of course, can be found at manufacturer web sites or at the NCMA web site: www.ncma.org.

This photo illustrates the difference between structural and decorative retaining walls. Terraces are required to add strength for big walls that hold back hillsides. While it also adds some strength, this terrace structure appears to have been built to create an attractive planter. Photo courtesy of Risi Stone Systems.

Segmental Anchoring Systems

Segmental retaining walls (SRWs) are gravity retaining walls that rely primarily on their mass (weight) for strength and stability. The system consists of concrete masonry units which are placed without the use of mortar (dry stacked), and which rely on a combination of mechanical interlock, unit-to-unit friction or shear capacity and mass to prevent overturning and sliding.

The units may also be used in combination with horizontal layers of soil reinforcement which extend into the backfill to increase the effective width and weight of the gravity mass.

Segmental retaining walls are flexible structures, so the footing does not need to be placed below the frost line, provided there is sufficient foundation-bearing capacity. SRW units are manufactured in conformance with standards and specifications to assure that units delivered to a project are uniform in weight, dimensional tolerances, strength, and durability?EUR??,,????'?????<

This big-block product is heavy enough to hold back slopes up to eight feet high without special excavation and reinforcement. These blocks weigh 240 pounds each and are installed with a custom clamp attached to a backhoe. This Versa-Lok product is called Brute. Photo courtesy of Versa-Lok Retaining Walls.

Fitting It Together

The SRW system is composed of units whose size and weight makes it possible to construct walls in the most difficult of locations. Curves and other unique layouts can be easily accommodated. Segmental retaining walls have the ability to function equally well in large-scale applications (highway walls, bridge abutments, erosion control, parking area supports, etc.) as well as smaller residential landscape projects.

Since SRW units are available in a variety of sizes, shapes, textures and colors, segmental retaining walls provide designers and owners with both an attractive and a structurally sound wall system.

Segmental retaining walls can be designed as either conventional or as reinforced soil. The structural capacity of the SRW system will vary with the SRW unit size, shape, batter, etc. Manufacturers recommendations should be followed regarding the capacity of their particular system for the soil loads under consideration.

This view shows a good mix of the practical and aesthetic. Terraces spread out the forces that can cause a vertical wall to fail, while providing opportunities for plantings. A variety of erosion-control and water feature hardware can be paired with most wall block products. Photo courtesy of Keystone Retaining Wall Systems, Inc.

Conventional Walls

Conventional SRWs are constructed with either single or multiple depths of units. For stability, the conventional SRW structure must have sufficient mass to prevent both sliding at the base and overturning about the toe of the structure. Since the system consists of individual units dry stacked one atop another, shear capacity is an important component to assure that the units act together as a coherent mass. Shear capacity provides a means of transferring lateral forces from each course to the succeeding course. This is provided by the frictional resistance between SRW units. Grip and strength are provided in the form of ?EUR??,,????'?????<

Structural stability can be increased by increasing the wall batter. Batter is achieved through the setback between SRW units from one course to the next.

In most cases, the batter is controlled by the location of shear pins or leading/trailing lips. Taller walls can also be achieved by using multiple depths of units. The multiple depths of units increase the weight of the wall system and provide a more stable base and greater resistance to soil pressures created by gravity.

Most standard wall blocks come with capstones that create an attractive finish. Segmental retaining blocks are almost never grouted into place (to let water pass through to insure stability) but capstones are usually cemented into place for safety and to deter vandalism. Photo courtesy of Versa-Lok Retaining Walls.

Reinforced Soil

Reinforced soil walls should be specified when the maximum height for conventional gravity walls is exceeded or when lower structures are surcharged by sloping backfills, live loads, and/or have poor foundations.

A reinforced soil SRW is designed and constructed with multiple layers of soil reinforcement placed between the SRW courses and extending back into the soil behind the wall at designated heights and lengths. The geosynthetic (fiber mat) reinforcement and the soil in the reinforced zone acts as a composite material, effectively increasing the size and weight of the gravity wall system.

System Components

The basic elements of each segmental retaining wall system are the foundation soil, leveling pad, segmental retaining wall units, retained soil and drainage fill. For reinforced soil SRWs, include the mass of soil reinforcement as well.