Have you ever looked around your property to see if you have retaining walls? If you have, what’s their purpose? Have you checked your neighbor’s property, looking for retaining walls and what, if any, effect these walls may have on your property? As you drive to work in the morning, have you noticed any retaining walls? What’s their purpose?
When you study these walls, maybe for the first time, do you see flaws, distress or signs of movement (such as leaning or cracking)? Should you be concerned?
Let’s start with your home. Does your home have a basement? Those walls “retain” the earth surrounding it. Is your driveway higher or lower than the surrounding grade? Are there walls supporting the driveway? Do you have landscaped areas that are stepped or terraced? What separates the different levels?
A retaining wall is a man-made structure, designed and constructed to interrupt a naturally sloping earth surface and to restrain the pressures created by the changes in elevation. Retaining walls are used to create a level surface such as a roadway designed to pass through a steep side slope. Retaining walls are also constructed to hold back earth when a building wall is constructed to install a basement. Small retaining walls are used for terracing earth grading to create landscape areas around residential and commercial buildings and properties.
Perhaps the most important aspect in the construction of any style retaining wall is the pressure built up behind the wall due to water accumulation. Water pressure against the rear of a wall can be significantly greater than the pressure of the earth. Therefore, drainage behind and through a wall is extremely important. Walls are usually coated with waterproofing; more recently, a filter fabric has been used. Perforated PVC piping is installed behind the wall, and weep holes are installed to allow trapped water to escape. In backfilling the wall, crushed stone is installed, at least two feet thick, to provide a positive draining material directly against the wall. Sometimes, a filter fabric is installed vertically between the crushed stone fill and the earth backfill. A clayey material should never be used as backfill material.
FAILURE
Retaining walls can and do fail. Usually a failure is caused by deterioration due to age, the imposition of increased loads not predicted during design or improper construction techniques or materials.
What should you watch for? Landscape walls move over time. They are usually constructed by landscapers or homeowners without attention to proper construction details. Bulging of these walls is common, as are tipping and sliding. The higher the landscaping wall, the more concern you should have about movement since a complete failure could have other consequences.
Watch your foundation for movement, especially leaning, that would suggest a failure in the strength of the retaining wall. Also, vertical or horizontal (not random or “spider web”) cracks are often evidence of movement.
Retaining walls along your driveway will most often offer evidence of movement by beginning to lean.
Many walls will move a little and then stabilize, so taking measurements of the amount of leaning (using a plumb line) will tell if any movement is ongoing. When in doubt, consult a licensed Professional Engineer with structure skills.
All failures cannot be prevented, but “due diligence” is the catch phrase. Pay attention to the surroundings, and observe changes as they occur.
DESIGN
Retaining walls are complex structural elements that are usually part of a more elaborate design. It is important that they be designed by a structural engineer. Many states require retaining walls over four feet high to be designed or approved by a qualified, licensed Professional Engineer. It is always important to adhere to local building codes and to secure the proper building permits, as well as engage a qualified, experienced contractor.
Once the need for a retaining wall is recognized, it is wise to hire professionals to ensure proper compliance with any codes. Starting with a licensed structural Professional Engineer is strongly recommended and, in most cases, required.