How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
How do I recognize structural problems in a retaining wall?
Thursday, July 12, 2018
When we show homebuyers structural defects—and sometimes even outright structural failure—of a retaining wall during a home inspection, they are often quite surprised. It’s not something they look at when examining a house to purchase. Retaining walls are out in the yard, sometimes all the way at the back or side of the property, and considered to be just part of the landscaping.
But they are much more than that. Retaining walls hold back tons of soil on a sloping hillside site to create a reasonably level area to build a home, pool, or driveway. When one fails, it can undermine the foundation of nearby structures. Plus, a retaining wall next to the property line that fails can also cause damage to your neighbor’s property for which you could be liable.
So be sure to take a careful look at any retaining walls on the property. Also, a neighbor’s retaining wall that abuts the property will be a concern if it is showing signs of structural problems. Some structural defects are easy to spot, while others require careful observation. Here’s our list of 7 tips on what to look for:
1) Sight down the wall - Look for any bulges or leaning. Bulges can indicate a band of excessive pressure, possibly from soil saturation after heavy rain, and are a structural defect. As a retaining wall leans out-of-plumb, the pressure in the top area increases and tends to accelerate the rotation of the wall. A spirit level can help you determine if a wall is leaning, and a digital “SmartLevel” can tell you by how much. In the photo below, the 86.5º reading indicates the wall is leaning out-of-plumb by 3.5º.
Some retaining walls, like the one below, do not require tools to determine that wall “A” is tilting away from wall “B” by way more than a few degrees.
2) Beware of a concealed retaining wall surface - When a retaining has had a decorative facing applied over it, there may be concealed damage underneath. The concrete block wall that was faced with diagonal wood siding in the photo below gives an upgraded look to the patio it surrounds. But the small areas of the backside of the wall that were visible indicate structural distress, shown in the second photo below.
3) All wood retaining walls eventually succumb to rot and termites - Landscape timbers are an inexpensive solution for low retaining walls, but don’t last long. In the photo below, the failing retaining wall is supporting soil around a pool deck. While the imminent failure of the wall is not going to cause a epic disaster, it was already causing fractures and undercutting of the pool deck behind it, which will progress to slippage of sections of fractured deck if not repaired. Also see our article How long do timber railroad tie retaining walls last?
4) Check for hidden retaining walls - They can be under a deck or behind a fence. The one under a deck in the photo below is in failure, and only viewable after climbing down the hill and looking back.
5) Look for stair-stepping cracks in masonry retaining walls - Narrow zig-zag crack lines indicate the beginning of a problem. When the cracks open wider and one side is forward of the other, it’s time to take action.
6) Look for a collapsed soil level or holes in the ground on high side of retaining wall - Stacked large rocks (called rip-rap) are sometimes used to make a retaining wall. While they create a handsome and natural-looking barrier, pockets can open up between rocks that allow soil to flow out during a heavy rain. Similarly, soil can migrate under the footing of a cantilever retaining wall—especially if the bottom is in water or mud. What looks like a rabbit-hole in the photo below is actually a washout gully that runs through an opening in a rip-rap wall on the other side of the wood fence.
7) Non-standard retaining walls are rarely successful - The building code allows retaining walls less that four-feet high to be constructed without an engineer’s design specs. But stacked concrete block, without mortar or a footing, is doomed to failure. Likewise, a retaining wall made from a scavenged mobile home I-beam and old car hoods, like in the photo below, will have a short life. The customer on that inspection suggested that maybe mixing Chevy and Ford body panels was the real problem.
As a retaining wall gets higher, the weight pushing against the back of the wall increases geometrically. So a wall that is twice as tall has way more than twice the pressure against it. The diagram below shows the wedge of soil pressing against a retaining wall that would collapse if the wall was removed. This is why a high retaining wall requires both professional design and installation to survive, while a low wall can be a do-it-yourself project.
Also see What is batter in a retaining wall?
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To learn more about exterior walls and structures, see these other blog posts:
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