It's time for a post about retaining wall installation - best practice. Retaining walls are something that home inspectors see frequently. And just as frequently those walls have problems.
A recent post prompted many to think what is necessary to the proper installation of a retaining wall. This is a very good question, and one deserving a response.
Retaining walls can be constructed of many things - metal, wood, brick and block, man-made blocks, and huge boulders and stone. The idea is to retain the weight of the soil behind the wall, and divert water, and its pressure, away from a structure or pathway.
An engineer I have known and referred for many years is Ken Fraine, of Soil and Structure Consulting, Inc. When asking him his thoughts regarding this post, he said,
"Nearly every failed retaining wall I’ve ever inspected failed due to an improper drainage system or a complete lack of one. Note that [concrete] block walls should also have a waterproofing membrane placed on the backside to prevent deterioration of the mortar joints and the blocks themselves."
Drainage, therefore, is significant. The the single-most reason walls will eventually fail.
Properly constructed, retaining walls are very complex. Many things work together to insure their success, from retaining wall material, back fill, angle of the wall setback, stabilization into the rear structure and drainage, etc..
I have seen retaining walls made from the man-made blocks that are 30' high and more. Of course, with that height, more drainage is necessary than the single collection pipe shown here. Those blocks intersect, or have lips which rest on each other such that one cannot be moved as regards its neighboring blocks.
The collection pipe would be a 4" plastic tube with pre-drilled holes to collect and then divert water. Of course it would be wrapped with a silt-filtering mesh and then protected with gravel (called Unit Core Fill/Drainage above).
Even with different wall materials, the components shown above would still apply. And any wall constructed like the one in the diagram above would be
My recommendation: just from a visual inspection or walk around, it might be difficult to determine if a retaining wall has all the components and structural capability of the diagram above. But you can ask! Find out who did the work, try to obtain any permits and receipts and investigate the company that put the wall in. Doing all that will go a long way toward the peace of mind needed to know if a wall will last.