gravity 101: the force behind the wall



the maximum height without  reinforcement is approximately twice the depth of the retaining wall unit 

Before building a retaining wall, it's important to recognize the role gravity plays in successful wall construction. That's because gravity, the unseen force that attracts mass to the surface of the earth, can make or break your wall.

According to Mike Simac, principal author of the National Concrete Masonry Association's Segmental Retaining Wall Installation Guide, every wall must harness the forces of gravity to create stability.

"A common misconception," notes Simac, "is that a soil-reinforced wall is fundamentally different than a 'conventional gravity wall' constructed with block alone. All segmental retaining walls are gravity walls, relying on a combination of mechanical interlock and mass or weight for stability. Reinforced walls use layers of synthetic soil reinforcements such as geotextiles or geogrids to create a gravity mass extending back behind the wall, giving strength and stability to the whole structure."

In the case of smaller segmental retaining walls, block alone may be heavy and wide enough to do the job. Says Simac, "The ideal conditions for a conventional gravity wall are a level slope below the wall, plus level backfill a the top of the wall. If these conditions are met, in most lower height cases a conventional gravity segmental retaining wall will do nicely for applications such as a typical tree ring or low garden border."

In general, the maximum that can be safely reached without soil reinforcement is approximately twice the depth of the segmental retaining wall unit. Given this "formula," if your wall unit is 24 inches deep, the approximate maximum height of your conventional gravity structure is two to four feet. In addition, any heavy surcharge in the area should be at least twice the height of the wall away from the back of the wall. A reinforced wall should be specified when these basic criteria cannot be met.

Keep in mind that any units laid above the final layer of reinforcement in a reinforced wall behave essentially like a conventional gravity wall, since these units are supported by their mass alone. For this reason, this portion of the structure should be constructed in adherence to the height criteria for a conventional gravity wall.

A registered design professional can help you assess factors such as the strength of the soil behind and at the foundation of the wall, the surcharge at the top of the wall and drainage behind the wall to design a suitable structure. The laws of physics, conditions of the soil and fundamentals of gravity will ultimately determine whether your wall can stand on its own or with the help of soil reinforcement. When you build your wall correctly, the forces of gravity will work for you.

Mike Simac is a principal engineer with Earth Improvement Technologies, Cramerton, NC. Reprinted with permission of Anchor Wall Systems

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