- Water pressure behind the wall is the number one cause of failure.
- Every wall needs drain stone behind it and a way for water to get out.
- Taller walls usually need geogrid reinforcement, engineered design, and a permit.
- The bottom course belongs below grade on a compacted base.
On sloped lots from Toccoa to Cornelia, retaining walls are how you turn a hillside into a usable yard. They're also one of the hardscape features we rebuild most often. A leaning or bulging wall almost always failed for the same few reasons, and nearly all of them come back to water and what's below grade.
Why clay is hard on walls
Clay holds water. After a heavy rain, saturated soil behind a wall weighs far more than dry soil and pushes harder. That pressure, called hydrostatic pressure, is what bows walls outward. Clay also expands when wet and shrinks when dry, adding seasonal movement. A wall that ignores water is fighting its own backfill.
Element 1: Drainage behind the wall
A properly built segmental block wall has a zone of clean, crushed drain stone directly behind the blocks, commonly about a foot thick, running the full height. At the bottom, a perforated drain pipe collects water and carries it to an outlet beyond the end of the wall or to daylight downhill. Filter fabric between the drain stone and the clay backfill keeps fine soil from clogging the stone.
The most common thing we find when we take apart a failed wall: clay backfilled directly against the blocks, no drain pipe, or a pipe with no outlet.
Element 2: A real base
The first course of block is the foundation of everything above it. It should sit on a compacted aggregate leveling pad on firm soil, and the bottom course should be buried below finished grade. Manufacturer guidelines typically call for burying a portion of the wall's height, with a minimum depth even on short walls. A first course sitting on top of the ground, or on soft fill, will settle and tilt.
Element 3: Geogrid for taller walls
As walls get taller, block weight alone can't hold back the soil. Geogrid is a strong mesh laid in layers between courses and extended back into the compacted backfill, tying the wall and the soil into one reinforced mass. Block manufacturers publish height limits above which grid is required, which vary by block and soil. Taller walls, and walls supporting driveways, structures, or steep slopes above them, typically need engineered design.
Element 4: Setback
Segmental walls are designed to lean slightly back into the hill, a built-in setback created by the block's lip or pins. That batter helps the wall resist the soil pushing on it. Walls built perfectly vertical, or blocks installed backward to make the face look straight, give up that advantage.
Permits and engineering
Under the building codes most Georgia jurisdictions follow, short retaining walls are often exempt from permits, while taller walls, generally over about four feet measured from the bottom of the footing, and walls supporting additional loads like a driveway or slope above, commonly require a permit and engineered design. Local rules vary, so check with your city or county building department. We'll tell you what applies and build to the engineer's plan when one is required.
Warning signs on an existing wall
- Bulging or bowing in the middle of the wall
- Leaning outward at the top
- Cracked or displaced blocks
- Soil or mud washing through joints
- Water seeping from the face with no outlet pipe in sight
- Settling or gaps at the top of the wall
Leaning walls rarely straighten themselves. In most cases, the wall needs to be taken down to the problem area and rebuilt with proper drainage. Catching it early limits how much has to be rebuilt.
Natural stone and boulder walls
Boulder walls follow the same principles: a firm base, the largest stones at the bottom, a lean into the slope, and drainage behind. Their weight and irregular shape make them a great fit for natural-looking foothill landscapes. See using boulders and natural stone on a sloped lot.