It’s rarely one issue that causes a retaining wall to fail. A properly constructed drainage system will typically mask most other common issues. A lack of proper drainage will amplify any other miscues on the wall. Today’s example is no different with a basic retaining wall not designed to handle a pool. In the picture below, there are three issues to point out that could have been avoided.
Drainage
The first issue is one that is not apparent from looking at this picture. The wall does have drainage stone behind the wall which is good. However, there are no clear pipe outlets and no evidence of a drainage pipe behind the wall. A wall of this height must include a full drainage system with a drain pipe that has properly designed outlets.
Block Placement
The second issue is block placement. Each row of blocks should stagger so that the midpoint of one block lines up with the joint between the two blocks above and below. This forms the desired running bond pattern. The additional overlap would also provide more surface area for friction to keep the blocks from separating. Although the wall would still have shifted out, properly placed blocks would help mask the issue aesthetically. Installing blocks with as little overlap as the wall above is just poor construction.
Surcharge Not Accounted For
The third issue is this wall supports a surcharge, ie. the above ground pool pictured below. For a more in-depth definition of a surcharge, click on the link. The wall was not designed to support the pool. In a previous post, we talked about when it is necessary to contact an engineer. Anytime a wall supports a surcharge, it needs to be engineered. The wall also appears to be more than 4-ft tall which also requires engineering. The current issues could have been avoided had the homeowner or landscaper contacted an engineer.
What is an Engineered Wall?
When an engineer is contracted to design a retaining wall, they take all the site parameters into consideration. This includes the soil type, surface water flows, the height of the wall, slopes both in front of and behind the wall, and any surcharges the wall may support. All engineered walls will include a full drainage system.
For a segmental block wall like the one above, the capacity of the retaining wall is increased by adding geogrid. The geogrid is installed between rows of blocks and extends back into the backfill. Geogrid is a strong plastic material (see picture to the right). The depth of the geogrid will be 60% of the wall height or a minimum of 4-ft. This will increase if the wall supports a surcharge, is founded on clay or other soft material, or if your property is in a seismic region. A wall like the one above may require geogrid that is 70% or 80% of the wall height.
A concrete or cinder block wall will include rebar in the wall tied to a footing. The amount of rebar and the size of the footing will increase as the wall height increases, to support a surcharge, or for different soil types. Wood walls require specific design depending on the site conditions.
Although this wall didn’t collapse (at least not yet!), this is still considered a wall failure. The wall will slowly creep until it eventually collapses. If you want to ensure your retaining wall project stands the test of time, checkout services to talk to an experienced engineer.
