The Right Block For the Job: Capabilities of Retaining Wall Products

By Becky Felling, Keystone Retaining Wall Systems, Inc.

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 by designers and builders. This terrace (center) has been turned into a seating and viewing area. Photo courtesy of Keystone Retaining Wall Systems, Inc.

There are dozens of segmental wall block products on the market. It makes sense to learn about the fundamentals of retaining wall design before you choose one for your project. There are two basic types of segmental retaining wall structures: gravity walls and reinforced walls.

Gravity Walls

Gravity walls are structures that rely solely on mass and geometry to resist the earth pressure forces acting on it. Landscaping wall products and structural wall units not utilizing soil reinforcement are considered gravity walls by definition.

Limiting wall heights to safe working heights for the size of block unit selected and avoiding additional surcharge conditions can avoid simple overturning failure. A simple ?EUR??,,????'??rule of thumb?EUR??,,????'?? is to restrict wall heights to no greater than three times the unit depth unless referring to design charts for site-specific design recommendations. This ?EUR??,,????'??rule of thumb?EUR??,,????'?? leads to the following rough guidelines:

Wall block manufacturers produce products in a variety of textures and colors. Versa-Lok employs a top-pinning system. The slots where the securing pins fit can be seen on the top of the blocks shown here. Photo courtesy of Versa-Lok Retaining Walls

Reinforced Walls

Reinforced soil walls are composite structures that use structural units and geosynthetic soil reinforcement (typically geogrids) to create a stable mass that can be designed and constructed to much greater heights than simple gravity walls. These structures are commonly referred to as MSE (mechanically stabilized earth) structures or reinforced soil SRWs (segmental retaining walls). Note that only pinned units are specifically designed to properly accommodate earth reinforcement and provide facial stability and connection strength for these larger and more critical structures.

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

Reinforced walls rely on the mass of the composite structure to provide external stability (sliding, overturning, etc.) and the strength of the soil reinforcement, connections, and block units to be internally stable. The principal modes of failure observed in reinforced wall structures are:

Reinforced walls are considerably more complex than simple gravity walls and must be designed accordingly. The soil strength and stability component of the design takes on much greater importance as the structures become taller requiring more attention to site specific soils information, proper design considerations, and contractor quality assurance provisions. Reinforced walls should be constructed using a wall design provided by a qualified engineer based on actual site conditions.

Most segmental retaining wall blocks are light enough to be placed by hand, making installation easy and preventing damage to existing turf and plantings. A rubber mallet assists seating in a sand bed in this view. Many wall block products have grooves that assist placement, as seen here. Photo courtesy of Versa-Lok Retaining Walls

What are the Most Important Considerations for the Wall Builder to Understand?

The most important considerations in successfully building segmental retaining walls are:

?EUR??,,????'???? Addressing drainage issues
?EUR??,,????'???? Compaction of the backfill behind the wall
?EUR??,,????'???? When and how to use soil reinforcement (such as geogrid) for walls that exceed the height or soil conditions where a gravity wall may be used

Connection System

Some retaining wall systems use pins, clips, or other means to align and connect the blocks. Others simply use a lip or knob on the block as an alignment aid. Several manufacturers offer pinned units for the construction of structural and gravity walls. A pin allows a means of connection to geogrids in reinforced walls and assures easy alignment.

Solid or Cored

Some block systems are essentially solid while others have cores. Where a core is used, the block should be core-filled with free-draining rock, which enhances unit stability, drainage, and shear connection.

Common Elements

All segmental retaining wall products are dry-stacked, and are essentially free-draining due to the presence of joints between wall units.

This big-block product is heavy enough to hold back slopes up to eight feet high without special excavation and reinforcement. Don?EUR??,,????'???t strain your back, though?EUR??,,????'??+these blocks weigh 240 lbs. 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