A retaining wall may be built from heavy block, masonry, stone, or timber, but the strength of the visible wall is only part of what determines how well it performs. What happens to water behind and around the wall can be just as important as the material holding the soil in place.
When rainwater moves downhill and enters the soil behind a retaining wall, it needs somewhere to go. If that water cannot drain effectively, moisture can accumulate within the retained soil. The additional pressure can place forces on the wall that it was never intended to handle.
That is why a properly planned retaining wall is not simply a row of attractive blocks installed against a slope. Drainage aggregate, grading, backfill, drain pipes, outlets, surface water management, and the conditions of the surrounding property all need to work together.
For Georgia homeowners dealing with sloped yards, erosion, heavy runoff, or plans to create more usable outdoor space, understanding drainage can help explain why two retaining walls that look similar from the front may perform very differently over time.
Hearn Landscape designs and installs custom retaining walls for properties throughout its Georgia service area, using options including modular blocks, masonry blocks, landscape timbers, and other materials. The company also has an in-house licensed engineer involved in retaining wall design, inspection, and permitting when appropriate.
A Retaining Wall Has to Manage More Than Soil
The basic purpose of a retaining wall is to hold soil at different elevations.
A homeowner may install one to create a flatter backyard, control an eroding slope, support landscaping, or make room for a patio, lawn, garden, or other usable area.
But soil does not remain in exactly the same condition throughout the year.
Water enters it.
Rain falls on the slope above the wall. Irrigation may add additional moisture. Roof downspouts, driveways, patios, and other surfaces may direct runoff toward the area. Water can also move through the soil beneath the surface.
As soil becomes wetter, the forces acting behind the wall can change.
The Concrete Masonry & Hardscapes Association explains that water should be directed away from segmental retaining walls whenever possible. When water does reach the wall system, drainage components are needed to reduce erosion, soil migration, seepage forces, and hydrostatic pressure. CMHA’s segmental retaining wall design guidance specifically identifies drainage as a site-specific part of retaining wall design.
What Is Hydrostatic Pressure Behind a Retaining Wall?
Hydrostatic pressure is the pressure created by water.
If water accumulates in soil behind a retaining wall and cannot escape, that water adds pressure against the structure.
Imagine the difference between holding back a pile of relatively well-drained soil and holding back that same soil while water is collecting within it. The wall now has another force to resist.
Retaining walls can be designed for specific water conditions, but ordinary landscape retaining walls generally should not depend on the wall face alone to hold back uncontrolled water buildup.
Federal Highway Administration design guidance emphasizes the importance of intercepting water and preventing hydrostatic pressure from building behind retaining structures. FHWA also notes the importance of free-draining backfill and underdrains when water could accumulate behind a wall. FHWA retaining wall drainage guidance reflects how seriously drainage is treated in engineered retaining structures.
For a homeowner, the practical lesson is straightforward: water behind the wall needs a planned path out.
Drainage Begins Before the Wall Is Built
A good drainage plan starts with understanding where the water is coming from.
Before construction, the property should be evaluated for conditions such as:
- The direction the yard naturally slopes
- Low areas where water collects
- Runoff coming from neighboring portions of the property
- Roof downspouts
- Irrigation
- Driveways and patios
- Existing drainage pipes
- Natural drainage channels
- Soil conditions
- The location of the wall relative to the house
- Where collected water can safely discharge
This broader site evaluation matters because a drain behind the retaining wall should not be expected to solve every drainage problem on the property.
CMHA specifically notes that drainage aggregate and a drain pipe behind a segmental retaining wall are intended to handle incidental water and should not necessarily serve as the property’s primary drainage system. Surface drainage and groundwater conditions may require additional solutions.
That is why retaining wall planning and yard grading and drainage often need to be considered together.
Why Free-Draining Material Is Installed Behind Many Walls
The soil directly behind the wall is extremely important.
If poorly draining material is placed immediately against the back of the wall, water may have difficulty moving downward toward the drainage system.
Free-draining aggregate creates spaces through which water can move more easily.
For segmental retaining walls, CMHA recommends a zone of free-draining gravel immediately behind the wall units. This material can help incidental water move downward, reduce migration of retained soil through the wall face when properly designed as part of the soil filter system, and reduce pressure from trapped water.
The exact drainage detail depends on the wall design.
The important point is that what homeowners cannot see behind the wall can be every bit as important as the blocks they see from the yard.
What Does the Drain Pipe Behind a Wall Do?
Many retaining wall systems include a perforated drain pipe near the base.
Water moving through the drainage aggregate can enter this pipe and be carried toward an appropriate outlet.
The pipe does not create drainage by itself. It has to be part of a complete system.
That means considering:
- Proper pipe placement
- Correct slope
- Suitable surrounding material
- An outlet that remains open
- Protection against clogging
- The amount of water expected
- The surrounding site grade
A perfectly installed pipe that has nowhere to discharge water will not accomplish much.
Likewise, a drain outlet that becomes buried under mulch, soil, leaves, or landscaping may stop working as intended.
The system needs both a collection method and a discharge strategy.
Surface Water Should Not Be Sent Directly Toward the Wall
One of the most important parts of retaining wall drainage happens above the wall.
If the yard is graded so that runoff continuously flows directly toward the retained soil behind the wall, the drainage system has to deal with far more water than necessary.
Good site planning tries to control surface water before it becomes a problem.
The University of Georgia Extension identifies retaining walls and terracing as tools that can help stabilize slopes and control erosion, but it also recommends managing stormwater across the entire slope. Vegetation, grading, strategically placed materials, and other techniques can slow runoff and reduce erosion. UGA Extension’s guidance for slopes and hillsides emphasizes treating water movement as a landscape-wide issue rather than focusing on one structure.
This is why the grade above a retaining wall matters so much.
Downspouts Can Overwhelm a Retaining Wall Area
Roof runoff deserves particular attention.
A downspout can concentrate water from a large roof area into one relatively small section of the yard.
If that outlet terminates directly behind a retaining wall, every substantial rain event may send a concentrated volume of water toward the wall’s backfill.
The solution is not necessarily to eliminate the downspout. The water simply needs to be directed appropriately.
Depending on the property, that may involve extensions, underground drainage, grading, catch basins, or another designed drainage route.
UGA Extension notes that stormwater runoff from roofs, driveways, patios, and other impervious surfaces can contribute substantial amounts of water to residential landscapes and can cause erosion and flooding when it is not managed properly.
The retaining wall should fit into that drainage plan rather than becoming the place where all of the property’s runoff collects.
Georgia Soil Makes Drainage Planning Especially Important
Georgia properties can contain a wide range of soil conditions, but clay-heavy soil is common in many areas.
Clay soil generally drains differently from sandy or gravelly material. Water may move more slowly through dense or compacted soil, while heavy rainfall can produce significant surface runoff on slopes.
UGA Extension notes that drainage can be challenging in heavy clay soils and that steep slopes can increase runoff and erosion concerns.
For retaining walls, that means the existing soil cannot simply be ignored.
An installer needs to understand how water is likely to behave around the wall and provide appropriate drainage rather than assuming rainfall will naturally disappear into the ground.
Poor Drainage Can Cause a Wall to Bulge or Lean
One of the more recognizable retaining wall problems is a wall that begins moving outward.
The face may develop a visible bulge. Portions may lean. Blocks may shift out of alignment. Cracks can develop in some types of walls.
Water pressure can be one contributor to these problems.
Poor drainage is not the only possible cause. Wall movement can also result from inadequate footing preparation, improper backfill, poor compaction, insufficient reinforcement, excessive loads above the wall, unsuitable materials, or design problems.
That is why diagnosing an existing retaining wall problem requires looking at the entire structure.
Simply pushing blocks back into place does not correct the underlying issue if the soil and water behind them are still applying excessive pressure.
Water Can Wash Soil Out From Behind the Wall
Too much water can create another problem: erosion behind or around the retaining wall.
Water follows available pathways.
If soil is not properly separated from drainage aggregate, or if runoff is concentrated in the wrong location, fine soil particles can migrate with moving water.
Over time, homeowners may notice:
- Depressions forming behind the wall
- Soil washing through openings
- Sink areas
- Exposed aggregate
- Gaps along the top of the wall
- Erosion near the ends of the wall
These symptoms suggest that water is moving soil rather than simply draining through the system.
A properly designed filter and drainage arrangement is intended to allow water movement while helping keep retained soil where it belongs. CMHA specifically identifies controlling migration of fines as one of the functions of appropriate retaining wall drainage materials.
The Ends of the Wall Need Drainage Planning Too
Drainage problems do not always appear directly behind the center of a retaining wall.
The ends matter.
If runoff is directed around the end of the wall at high velocity, it can erode the soil beside the structure.
A good retaining wall design therefore considers what happens when water leaves the system.
Where does the pipe discharge?
Where does surface runoff go?
Will discharged water create erosion downhill?
Will it flow toward a neighboring property?
Will it end up against the house?
Moving water away from one problem area only to create another problem is not a complete drainage solution.
Hearn Landscape’s grading and drainage services include customized runoff management, French drains, grading, and natural-stone drainage features intended to manage water throughout the landscape.
Retaining Wall Drainage Matters When Building a Patio Above or Below the Wall
Retaining walls are often part of larger outdoor living projects.
A wall may create the level area needed for a patio above it. In other cases, the wall may hold back a slope beside a patio below.
Either arrangement changes the drainage picture.
Pavers and concrete surfaces can influence where rainfall travels. Patio grading needs to move water in a planned direction instead of sending it toward the retaining wall or house.
If the wall, patio, and drainage system are designed independently, one improvement can unintentionally create a problem for another.
Hearn Landscape provides paver patios and outdoor living installations in addition to retaining walls, allowing these features to be considered as parts of a larger landscape design.
Irrigation Can Create Problems Behind a Retaining Wall
Rain is not the only water source in a landscape.
Irrigation can contribute moisture behind a retaining wall every week throughout the growing season.
Sprinkler heads aimed toward the wall, leaking irrigation lines, or drip irrigation installed too heavily along the upper edge of the wall can increase soil moisture.
Landscape planting above a retaining wall therefore needs to account for both the plants’ water needs and the wall’s drainage design.
The goal is not to keep the retained soil completely dry. That would be unrealistic outdoors.
The goal is to prevent unnecessary saturation and provide a path for water that does enter the area.
Drainage Is Especially Important With Taller Walls
As retaining walls become taller or support larger loads, design considerations become more significant.
A short decorative landscape wall is not the same engineering problem as a tall wall supporting a substantial slope, driveway, structure, or other surcharge above it.
Wall height, soil characteristics, drainage, reinforcement, loading, and local permitting requirements may all affect the design.
Hearn Landscape states that its retaining wall team includes an in-house licensed engineer to assist with designing, inspecting, and permitting the retaining walls it builds.
That engineering perspective is particularly valuable because drainage cannot always be treated as an accessory added after the structural design is complete.
It is part of the design.
Signs an Existing Retaining Wall May Have a Drainage Problem
Homeowners should pay attention to changes around an existing wall.
Potential warning signs include:
- Water continuously seeping through the wall
- Soil washing through joints
- Bulging sections
- Leaning
- Cracks
- Standing water above the wall
- Erosion near the ends
- Depressions developing behind the wall
- Drain outlets that remain clogged
- Water pooling at the base
- Sections shifting after periods of heavy rain
One symptom does not automatically identify the cause.
For example, some drainage details intentionally allow water to exit through designated locations. The concern is when the wall begins showing movement, soil loss, uncontrolled water, or other evidence that the drainage system may not be functioning as planned.
Do Not Treat Drainage as an Add-On
One of the biggest mistakes in retaining wall construction is thinking about the wall first and deciding what to do with the water afterward.
Drainage should influence the project from the beginning.
Before construction starts, the design should answer questions such as:
Where does water currently flow?
How much runoff reaches this slope?
What happens during heavy rain?
Where will water behind the wall be collected?
Where will it discharge?
Does the property need additional grading?
Are downspouts affecting the area?
Will irrigation be installed above the wall?
Will a patio or other hardscape change runoff patterns?
What soil is being retained?
What loads will exist above the wall?
Answering those questions before installation is much easier than trying to add drainage after a completed wall starts moving.
The Best Retaining Wall Works With the Entire Landscape
A retaining wall should do more than hold back dirt.
It should work with the slope, drainage, soil, landscaping, and future use of the property.
That is especially important on Georgia properties where rainfall, sloped terrain, clay-heavy soils, patios, roofs, irrigation, and other landscape features can direct substantial amounts of water toward one area.
The blocks or masonry may be the part everyone sees, but drainage stone, backfill preparation, drain pipes, grading, outlets, and water-management planning are what help the wall perform behind the scenes.
For homeowners planning a retaining wall, the right question is not simply, “What will the wall look like?”
It is also, “Where will the water go?”
Hearn Landscape designs and builds retaining walls along with complete grading, drainage, landscape design, and outdoor living solutions for homeowners throughout Brooks, Fayetteville, Newnan, Peachtree City, Senoia, and surrounding Georgia communities. Property owners dealing with a slope, erosion, poor drainage, or plans for a new outdoor space can request a consultation with Hearn Landscape to have the property evaluated before construction begins.