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What is a retaining wall? To retain, according to Webster’s Dictionary, means “to hold secure or intact.” A retaining wall, then, holds something “secure or intact.” Typically, as it matters to homeowners, the “something” is soil on a slope or at a higher elevation that, if left on its own, will not remain “secure and intact.”
Retaining walls come in many shapes, sizes and materials. This issue of YOUR HOME will examine a few of them.
Why Retaining Walls?
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?
Purpose: 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.
The use of retaining walls is as old as man. Examples of retaining walls are found in all early civilizations. An early example is the construction of the Temple Mount in Jerusalem in 20 BC. One of these walls, still left standing, is called the Wailing Wall. Of course, the Romans used retaining walls to build both their roadways and aqueducts throughout Europe and the Middle East. During the construction of many of the great cathedrals of Europe, buttressed walls were engineered to support the high domed interiors. One of the earliest examples and perhaps the largest retaining wall, 5,500 miles long, is the Great Wall of China.
Man’s ingenuity through the centuries created needs for retaining structures: railroads, the Saint Lawrence Seaway, the Suez Canal, the Panama Canal, subways, the Holland and Lincoln Tunnels…on and on…
Every building material possible has been used in building retaining walls. Earth itself has been used effectively. By categorizing soils (sands, silts, clays) and compacting them in shallow layers, effective walls have been built. Of course, erosion may cause a maintenance problem. Most recently, earth structures have been built by mixing or injecting cement into the mix.
Early walls were built of stone, either dry laid up (no mortar) or with mortar. Concrete and cement have been mainstays in construction since the ancient Romans. Reinforced concrete, pre-stressed concrete and pre-cast concrete are all variations. Both wood and steel pilings have been used not only for temporary construction but also for permanent walls.
The most common retaining wall is the gravity wall. This wall retains the horizontal pressures of the earth (and/or water) by the wall’s own dead weight. This weight must be sufficient to resist both overturning and sliding. The shape of these walls is basically trapezoidal, with the base wide and both faces sloping inward and upward to a narrower top, usually approximately two feet wide. The depth is usually several feet below the front grade (to protect from frost), and the material in front of the wall also adds resistance to prevent the wall from sliding. Gravity walls have been constructed of earth, stone and plain concrete (no reinforcing steel bars).
The cantilever wall consists of a concrete footing and a concrete wall keyed (connected) into the footing. The footing usually extends beyond the front of the wall to aid in resisting the overturning forces. Essentially, a cantilever wall retains soil because the heel (or rear) of the footing is held down by the same soil trying to push the wall over. Cantilever walls use less material than gravity walls but require a strong connection between the footing and the stem (a moment connection), which usually relies on steel reinforcing within the wall. Many times a key is extended below the footing (one foot) to aid in resisting sliding. Most of the cantilever walls you see are freestanding. However, some basement walls are considered braced cantilever walls because the first floor framing is connected to the top of the wall. This bracing needs to be taken into account during the design of the wall.
The counterfort wall looks exactly like the cantilever retaining wall from the front (exposed) face. The difference is that a counterfort or tee section is constructed perpendicular to the main wall and the tee section acts as a beam, supporting the main wall. With the tee section extending over the rear footing, it strengthens the main wall, and that wall is designed differently from a cantilever wall; less reinforcing is required.
A buttress wall is designed very much like a counterfort wall. The main difference is that the tee section is on the front of the wall, and the footing shape is reversed so that the larger part is in front of the wall.
Finally, most homeowners are familiar with landscape, driveway or stairway retaining walls. Many of these are constructed of wooden landscape ties, stone or interlocking pre-cast concrete blocks. The important issue with regard to the landscape ties is that they usually need tiebacks (ties constructed perpendicular to the main wall) to resist overturning and sliding.
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.
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.
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.
A special thank you to Anthony P. Sousa, P.E., of Criterium - Brown Engineers, for his technical and writing contributions to this issue of Your Home. His willingness to share his structural experience and knowledge is an example of the expertise within the Criterium organization.
Copyright 2013 Criterium Engineers