A retaining wall holds back soil that wants to slide downhill. On a hillside lot around Rapid City, that’s often what makes a yard usable: a flat spot for a driveway, a patio or a lawn.
Most walls that fail look fine for the first year or two, then start leaning, bulging or cracking. The cause is almost always something you can’t see once the wall is built.
1. Water behind the wall
Water is the most common reason retaining walls fail. Soil soaked with water is heavier than dry soil and pushes harder on the wall, and water trapped behind a wall adds pressure of its own. In a Black Hills winter, that water can also freeze and push some more.
A wall that lasts lets water out:
- Clean, free-draining rock directly behind the wall, so water can drain down instead of soaking the soil against the wall.
- A perforated drain pipe at the bottom of that rock, with an outlet where the water can leave. A drain pipe with nowhere to go doesn’t help.
- Grading on top of the wall that sheds surface water away from it, so rain doesn’t soak straight in behind it.
- Downspouts and sprinklers aimed away from the area behind the wall.
2. A weak base
A wall is only as straight and stable as what it sits on. Walls built on loose soil, topsoil or a thin layer of sand settle unevenly, and the wall tips or cracks.
A good base means digging below the first course, building a compacted gravel leveling pad, and burying the bottom course partly below grade. The block manufacturer’s installation guide spells out how deep for its product.
3. Too tall for how it’s built
A wall that relies on its own weight, like a plain stack of block or boulders, can only hold back so much soil. Taller walls usually need geogrid, layers of reinforcement fabric laid back into the compacted soil behind the wall, which tie the soil and the wall together. The manufacturer’s charts or an engineer’s design say where geogrid goes and how far back it extends.
The load above the wall matters too. A driveway, parked vehicles, a building or another slope sitting on top of the wall (a surcharge) adds pressure that a wall holding up lawn doesn’t face.
4. Backfill that wasn’t compacted
Soil dumped behind a wall all at once settles for years. Backfill should go in thin layers, compacted as the wall goes up, with care right behind the block so the compactor doesn’t push the wall out.
Warning signs
- The wall leans or bulges out in the middle.
- Cracks or blocks separating, shifting or popping out.
- Water seeping out of the face or the base of the wall after rain.
- Soil washing out through joints or around the ends.
- Sinking ground or gaps opening behind the wall.
A wall that has started to move rarely fixes itself. Rebuilding it the same way usually gets the same result, so a rebuild should fix the drainage, base and reinforcement at the same time.
Block or boulder?
- Segmental block walls give a clean, even face and fit tight spaces. They’re the usual choice for taller walls that need geogrid and an engineered design.
- Boulder walls look natural on a Black Hills lot and suit hillside cuts and landscaping. They still need a good base and drainage behind them.
Do you need a permit?
It depends on the height and what the wall holds up. Section R105.2 of the International Residential Code, which many cities adopt, exempts retaining walls up to 4 feet tall, measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge. Taller walls, and walls holding up a driveway, building or slope, generally need a permit and often an engineered design.
Local rules vary and change, so check with your city or county building department before you build. In Rapid City, that’s the city’s Building Services Division. Outside city limits, it’s usually the county.
Need a wall built?
Black River Contracting builds block and boulder retaining walls around Rapid City, with the drainage and base work done first. See retaining walls in Rapid City or request a quote.