Change-of-Grade Know-How

Sept. 29, 2010

About the author: Charlie Hall is chief executive officer and president of Charles Hall Co. Hall can be reached at c.hall@charleshall or 615.896.7132.

Retaining walls are systems used for implementing erosion control on unrestrained slopes. This column is intended to provide an overview of some of the options available for situations that involve the change-of-grade structures.

One option is to build gravity walls. A gravity wall is any wall with adequate mass incorporated into its construction to resist forces exerted on a slope, ultimately preventing movement and sliding. A gravity wall can be as simple as a small boulder wall, or it can consist of larger “big-block” units in which the sheer mass and size of the wall system contains the soil mass. These walls commonly are used where it is not feasible to utilize reinforcing components such as geogrids, geotextiles, metal straps or mesh, soil nails or other products. In these cases, property lines, right of ways, utility easements or other circumstances will restrict the use of a reinforced retaining wall.

One of the most commonly used reinforced retaining wall systems is a segmental retaining wall (SRW), developed in the early 1980s.According to the National Concrete Masonry Assn., 300 million sq ft of dry-cast SRWs were produced for the commercial and do-it-yourself markets in the U.S. in 2009. These products, sometimes referred to as modular systems, vary in appearance and adapt well to many different situations. Specific product brands often are specified in a plan, but most brands are very comparable and utilize similar design parameters and engineering. In the U.S., some of these walls have been constructed to a height of more than 70 ft, and their acceptance is almost universal within the construction industry.

There are situations in which an SRW is not the most effective application. These circumstances typically arise when the soils to be retained or reinforced are not suitable for this type of solution, or when proper reinforcement cannot be installed due to space restrictions.

Mechanically stabilized earth (MSE) walls are accepted within the construction industry as well. This type of wall is particularly desirable when drainage and water needs to migrate through the wall face. Some MSE walls utilize a metal reinforcement alternative in dealing with erosion control.

Here are some tips for working successfully with retaining walls: Always insist that an engineer licensed in the state where the wall is to be constructed examines the site and designs the walls, and make sure that the installer and the salesperson(s) representing the products are both knowledgeable and experienced.

I personally visit many of the projects where my products are used, often working on site with the installers. Oversight is especially important when there is a new or less-experienced installer on the job site, or when the wall is different from the installer’s past installation experiences.

Remember that there are a variety of retaining walls from which to choose, and the assistance of a knowledgeable product representative, a licensed engineer and a competent and experienced installer are imperative for a successful solution when selecting the best structure for erosion control.

Download: Here

About the Author

Charlie Hall