In many residential properties, retaining wall structures are used to alter topography for landscaping considerations, or for controlling storm water. Retaining walls can range widely in size and be quite elaborate. Various materials such as timbers, native stone, colored concrete bricks and concrete blocks or placed concrete can be used in their construction. Retaining walls are commonly used to level, retain, or terrace a sloping area or to maintain an existing grade around a tree or some other landscape feature.
There are some general basic engineering principals that apply to retaining wall construction. Failure to comply with any of these basic-engineering principals will frequently cause long term problems. When planning and constructing a retaining wall, some factors that should be considered are design and building code issues, construction materials, type and purpose of the wall, on site placement, drainage, and foundation or footing.
Experience indicates that the following factors are at issue in most retaining wall failures:
Design and Building Code Issues:
Retaining walls should always be viewed as load bearing members first and aesthetic landscaping features second. Design of any retaining wall requires knowledge of lateral earth pressures. The effect of the earth pressure, including the contribution of water, is a direct function of the height of the retaining wall. In some states, retaining wall designs taller than a prescribed height must be designed or approved by a qualified licensed professional engineer. Additionally, it is important to ensure that all applicable building code requirements are followed even for shorter walls.
Footing and Foundation Design:
A properly designed footing and foundation system is required to ensure that the retaining wall resists the imposed lateral forces. When properly installed these features also prevent the retaining wall from sinking or settling excessively. In a well designed and built wall the footing and foundation are tied to the wall by utilizing steel reinforcing bars or other methods.
Most retaining walls fail because of the build-up of pressure behind the wall caused by groundwater or soil-moisture accumulation. The hydrostatic pressure on the wall increases as the depth of the groundwater increases. Failure of the retaining wall occurs when the imposed forces of the groundwater exceed the resisting forces of the retaining wall. Common sources are storm water infiltration, an elevated groundwater table, or a leaking water supply line. A sloping gradient behind the wall will direct storm water towards the wall structure. All retaining walls should be constructed to freely remove all groundwater that collects behind the wall and eliminate the build-up of hydrostatic pressures. This can be accomplished by providing a gravel drainage blanket and pipe behind the wall. The pipe should be diverted to a low spot beyond the retaining wall structure to ensure proper drainage away from the wall. In addition, properly constructed weep holes at set intervals will provide drainage through the wall and help prevent the build-up of hydrostatic forces.
As you can see, a functional and long lasting retaining wall installation requires consideration of many factors. Among the most important and often neglected, is a proper and complete engineering design evaluation. In addition, qualified contractors are necessary to ensure that a secure retaining wall structure is constructed.
Chapski Engineering Services will be happy to answer any questions that you may have concerning this topic or any other engineering issue.