Landscaping can be expensive, especially when it comes to retaining wall construction. Very often we run across excessive movement and settlement in retaining walls due to improper drainage, improper construction practices or improper roof runoff control. In this post I hope to provide a few tips on retaining wall construction and some do’s and don’ts.
The most important item to consider when designing your landscaping is how you are going to control drainage. We often find downspout leaders depositing roof runoff onto or right behind retaining walls. This excessive moisture causes settlement and other movement. This can also lead to the buildup of hydrostatic pressure which can lead to total wall failure or failure of the wall over a period of years. Generally the roof drainage system should avoid the retaining wall area altogether or be piped through the wall to prevent these issues.
I guess now would be a good time to explain what hydrostatic pressure is. Hydrostatic pressure as defined by Webster’s Dictionary is: “Pressure exerted by or existing within a liquid at rest with respect to adjacent bodies.” Ok so what does that mean? When water is present and is being forces by gravity it will find the easiest path, the path of least resistance. Often times that will be at a structure (foundation wall) or at a retaining wall. If you ever look at a wall that is bowed or leaning it was most likely caused by hydrostatic pressure. Foundation walls first bow because they are secured at the top and bottom. Eventually they will crack and fail. Retaining walls generally lean until they simply fall over. To prevent this you will need to design proper drainage into the wall so the water has an easy path through the wall, thus preventing hydrostatic pressure.
Wall drainage is also an important consideration when building a retaining wall. Back-filling the wall with native soil is almost never a good idea. The wall must be able to drain moisture either through the wall, or around the wall to prevent hydrostatic pressure. Proper slope and drain tile with adequate drainage is necessary. Clear Stone or washed stone is good example of materials the drain effectively. another mistake that is very common is that the ends of the drain tile get buried in the soil. This causes the buildup of pressure in most cases due to the water not being able to flow to the surface. The drain tile should be exposed at the end of the wall.
There are many types of retaining wall materials. Poured concrete, Timber, Stone, Etc. All of these must have a provision for drainage to prevent movement, generally with gravel or washed stone and drain tile. The large stone retaining walls are generally just stacked with gaps between to allow moisture to escape, thus preventing the buildup of pressure. With dry stacked block walls it is also essential to gave a good gravel base to prevent settlement. Any movement with this type of wall can be catastrophic and lead to wall failure.
So to sum thus up, water control, water control, water control. If you control water flow through grading, and drainage, you should be in good shape. Water is one of the largest causes of settlement issues in residential properties, whether it be settlement cracking in the driveway slab, foundation wall or retaining wall, it usually points to poor drainage or moisture control.
As always, I hope you have taken some value from this post. If you have a suggestion for a maintenance topic please submit them to my email below.
-AZ
Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection, Residential Thermal Imaging, and Manufactured Home Foundation Certifications. He also manages several rental properties through Zuehlke Properties, LLC. He can be reached by email at Aaron@Zuehlkeinspections.com or by calling the office at 608-931-7485.