Boulder Retaining Walls in Sandy Springs, GA

A boulder wall uses the weight of large stones to hold a slope in place. We build new boulder walls and reset ones that have shifted, with the base and drainage done properly so the wall stays where it was set. Below is what to watch for and how we approach the work.

When a boulder wall is the right choice

A boulder wall holds soil back with the weight of large stones set in tiers. There is no mortar and usually no concrete footing. Each stone sits on the one below, tilted back into the slope, and the gaps are packed with smaller stone. Done well, it looks like part of the hillside rather than something built on it.

Boulders suit sloped yards where you want a natural look, a planted terrace, or a wall that follows a curve. They also handle tree roots and uneven ground better than a straight block wall, because the stones can be placed around obstacles instead of through them.

They are not the answer for every slope. A boulder wall takes up more depth than a block wall of the same height, because the base stones are large and the face leans back. On a tight lot next to a driveway or a property line, that lost space matters. Taller walls holding back a lot of soil, or walls carrying a load such as a parking pad, often need an engineered design. In those cases we will tell you so before anything is set.

Signs a boulder wall needs attention

Most boulder walls give warning before they fail. Walk the wall after a heavy rain and look for these signs:

• Stones that have rolled forward or rotated so their face tips toward you
• Soil washing out between stones, leaving dark gaps or small piles at the base
• A bulge in the middle of a run that used to be straight or evenly curved
• Settling at one end, often where a downspout or a low spot sends water
• Wet soil or standing water at the foot of the wall days after the rain stops
• Cracks opening in the lawn or patio along the top of the wall

What usually causes the trouble

Water is behind most failures. When rain soaks the soil behind a wall and has nowhere to go, it adds weight and pressure the stones were never set to hold. Boulder walls drain better than solid walls because water can pass through the joints, but that only works if there is clean gravel behind the stones and the joints are not packed with clay.

The second cause is the base. Boulders need a firm, level footing of compacted stone, with the first course set partly below grade. When the bottom row is placed on loose fill or topsoil, it settles unevenly and the stones above follow.

Other common causes we find:

• Stones too small for the height of the wall, so there is not enough weight to resist the soil
• Little or no backward lean, so the wall stands upright instead of leaning into the hill
• Downspouts or a sump line discharging at the top of the wall
• Tree roots growing behind the stones and pushing them out
• New soil, a patio or a shed added above the wall after it was built

Repair or rebuild

If a short section has moved and the base is sound, we can usually pull the affected stones, correct the drainage behind them, and reset them with the proper lean. The original boulders can almost always be reused, which keeps the cost down and the wall looking the same.

A rebuild makes more sense when the bottom course has failed along much of the wall, when the stones are too small for the height, or when the wall was built without any drainage stone at all. Resetting the top of a wall with a bad foundation only buys time. In that case we take the wall down to the base, build a proper footing, add drainage, and set the stones again, bringing in larger boulders where the old ones were undersized.

We will explain which situation you have and why, and you can decide from there.

Boulder walls on Sandy Springs lots

Much of Sandy Springs sits on rolling ground that drops toward the Chattahoochee and its creeks. Many homes are on wooded lots that slope front to back, and plenty of older ranch and split-level houses were built with a cut into the hill behind them. Those cuts are often where a boulder wall ends up.

The soil here is mostly red clay. Clay holds water and swells when wet, then shrinks in a dry spell. That cycle works stones loose over time, so the gravel layer behind the wall and a clear path for water to leave matter even more than they do in sandy ground. Summer thunderstorms can drop a lot of rain in a short time, and a wall that handles a gentle soaking may not handle fast runoff from a roof and driveway above it.

Mature trees are another local factor. Oaks and poplars on many lots send roots well past the drip line, and digging a footing can damage them. We plan the wall line with the trees in mind, and the flexibility of a boulder wall helps here. If your property is near a stream or the river, there may be buffer rules on what can be built close to the water, and taller walls may need a permit. We will point out when you should check with the city before work starts.

What a visit involves

We start by walking the slope with you. We look at how high the wall needs to be, where water comes from and where it should go, what sits above the wall, and how equipment can reach the area. For a repair, we check the base course and probe behind the stones to see what fill was used.

After the visit, you get a written description of the work: the wall line, the approximate height, the drainage plan, and whether we expect to reuse stone or bring new boulders in. Setting large boulders takes a machine, so we will also talk about access, lawn protection and where material will be staged.

On the day of the work, the sequence is usually the same. We excavate, build and compact the base, set the first course, place drainage stone and pipe where needed, then set each course and backfill as we go. The last step is grading the top so water runs away from the wall instead of toward it.