How much support does a retaining wall need?

Answered by Jeffrey Anderson ~ November 30, 2012 ~ No Comments

If I have an 18inch high retaining wall eighty feet long and I put the post 36inches apart and 30inches in the ground will it be properly supported?

Dale - Aurora, CO

Jeffrey Anderson

Dale, an 18 inch retaining wall isn't very tall so in my opinion, placing the posts every 36 inches might be more often than is needed. I suggest you have an engineer take a look at the soil and wall design -- a lot of the posts may be able to be eliminated which should save labor and expense. I believe your post depth is good as you want to be below the soil's frost line. I would think that 30 inches would do it in Colorado, but you might want to double-check that as well.

There are two issues with retaining walls that are just as important as the vertical support: drainage and lateral bracing. Even though your wall isn't very high, it can be very depressing to see a project you worked so hard on begin to fail after a year or so. It can also be costly if the project needs to be redone.

You should provide drainage ports in the wall every 6 feet or so along the length of the wall to allow water to pass through from the higher to the lower grade. If water is allowed to build up, the pressure can force the wall to lean. I would think this could be a big issue in the spring when all the snow Colorado usually receives begins to melt.

The spacing of the holes can depend on your soil type. If you have a variety such as clay that doesn't drain very well, adding a little gravel along the base of the wall on the high side to promote water movement may also be a good idea. Again, this is something a local professional engineer should be consulted about prior to beginning the wall construction.

Lastly, there needs to be some sort of lateral support installed to tie the wall into the higher grade. Retaining walls constructed with blocks use a specially designed fabric mesh that is installed every several courses to act as an anchor. If you're building your wall out of treated 6-by-6s or railroad ties, a short section of the material running perpendicular to the wall can be installed into the higher grade at intervals.

Attach a piece of the lumber about 12 inches long to the end of the perpendicular section to form an anchor prior to backfilling. I would install one of the anchors about every eight or ten feet for an 18-inch wall, but once again, an engineer should be consulted. You may not need them that frequently with your soil type.

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