A retaining wall can sometimes be a necessary landscape feature in a yard with a severe slope. This can help level a yard to create a nice, even, usable surface.
For a retaining wall to stand the test of time, however, proper drainage behind the structure is essential. Backfill that’s too porous and lack of a drainage outlet can cause curves, bulges, overturning and, ultimately, wall failure when the amount of water being collected is too much for the wall alone to handle. Ensuring proper drainage during construction can save thousands of dollars further down the road.
Homeowners can undertake retaining wall projects themselves, but any construction over three feet is best left to the professionals. Before digging anywhere in your yard, dial 811 to get the local utility companies to mark utility lines in the yard. Additionally, in most places around the country you need a permit for outdoor construction projects such as retaining walls so make sure you’re not in violation of any local ordinances.
Start by digging an even trench about 4 inches deep for the bottom course of the wall. The bottom course (layer of rock, railroad tie, etc.) of a retaining wall should be below ground level. Thoroughly tamp the trench and even it out with 1–2 inches of leveling sand or paver base to ensure a sturdy foundation.
- Lay a perforated drain pipe on 1–2 inches of drainage aggregate directly behind the above-ground or second course in your wall. Aggregate, or course roughly ¾-inch free-draining, angular gravel, is used behind all retaining walls to keep water pressure from building up behind the wall face.
- Place more drainage aggregate on top of the perforated pipe to a depth of 12 inches behind the wall and up to the same level as the second course. Place backfill behind the aggregate to the same level as the aggregate. Tamp both the aggregate and backfill to create an even surface from the second course all the way to the ground behind the retaining wall.*
- Additional courses are similarly installed and backfilled until you reach the top, where the wall is capped and topsoil is placed with sod or whatever landscaping is desired.
- The drainpipe itself can run along the course of the wall and be directed out to the sides wherever the wall ends.** Make sure any drainage above the wall is diverted into the drainpipe behind the wall or off to the sides of the wall to avoid the accumulation of more water than your drainage system can handle.
*Make sure you aren’t tamping more than 2–4 inches of backfill and aggregate at a time, as when these components are poorly compacted, it results in increased pressure on the retaining wall, especially when water begins to collect behind it.
**Before you let water from your wall drain flow just anywhere, research proper places to divert water.
Depending on the length of the wall, you may consider incorporating weep holes into your retaining wall drainage system using T/Y fittings.