Retaining wall drainage explained: How to prevent costly damage

Retaining walls are structures that hold back soil and prevent erosion on sloping ground. They are popular in landscaping and construction as well as on roadways as a road barrier to prevent vehicles from driving over the edge of an embankment. But retaining walls are not problem free if not adequately drained on the back side. Pressure buildup caused by water behind the wall can result in structural damage, erosion, or flooding.

To prevent these problems, good drainage systems are necessary. These are made of things like perforated pipe, gravel and weep holes, and they help to divert water away from the wall so the water doesn't cause damage. With the installation of these drainage systems people can stabilize walls, save the surrounding landscape and avoid costly future repairs.

Understanding Retaining Wall Drainage

And here’s why the drainage is very significant for retaining wall pipe system, and some of the typical elements used to achieve proper drainage.

1. Prevents Water from Collecting Behind the Wall

When water pools behind a retaining wall, it exerts pressure on the wall, especially during periods of heavy rain or snowmelt. But without drainage, this water accumulates and can cause the wall to crack, settle or fall over. A proper drainage solution directs the water away from the wall preventing it from pooling and reducing the risk of damage.

2. Reduces Pressure that Can Lead to Wall Failure

Water pooling behind the wall can create hydrostatic pressure, or a force pushing against the wall. If the pressure gets too high, the wall can crack or even collapse. Good drainage helps to stop this from happening by permitting water to drain freely so it will not accumulate and become issues for the walls of the basement. 

3. Protects the Surrounding Landscape and Buildings

It not only saves the wall, but it saves the area surrounding it. Without good drainage, water can collect and flood the surrounding area, drowning plants or even flooding nearby structures. Water travels through this proper drainage so it does not become flooded and the soil beneath does not wash away.

Common Drainage Components

A drainage system is made up of several components that work together to aid in the proper collection and management of water.

• Perforated Drain Pipes: Perforated drain pipes are installed behind the retaining wall to drain water. These pipes have tiny perforations that allow the water to run into them and out of the wall region. The pipes are typically located at the base of the wall or at various locations behind it to facilitate easy flow of water and to prevent water from accumulating behind the wall.
• Gravel Backfill: Gravel that is put behind the wall allowing water to run through the back of the wall. This alleviates the pressure on the wall. The gravel complements the drain pipes by giving water a place to flow in such a way that it doesn’t pool directly against the wall. It also acts to retain the soil and prevent erosion behind the wall.
• Weep Holes: Sometimes the weep holes are found at the base of the wall through which water can drain out from behind the wall. These holes also alleviate pressure by allowing water to escape, so it doesn’t get trapped behind the wall. And they don’t allow moisture to be trapped in the wall, potentially creating mold and mildew.
• Filter Fabric: Filter fabric is placed over the gravel and drain pipes to prevent soil from infiltrating the system. Without it, soil can clog the pipes and gravel, inhibiting the draining process. The filter fabric allows water to pass through and prevents soil from clogging it.

Consequences of Poor Drainage

A retaining wall without drainage can cause serious issue, not only to the wall itself, but to everything around it.

Structural Issues

Water accumulates behind the wall with no place to go causes undue stress on the wall. That pressure can force the wall to bulge, or lean, and weaken it as a barrier to soil. With time, the weight in the wall would cause it to become unstable and fail if left alone.

The trapped water can also exert pressure on the wall and make it crack, and cause the blocks or material to move. These movements may disrupt the general alignment of the wall, close enough and unsafe for safety. Cracks frequently indicate that a wall is being overpressed and may fail shortly if drainage isn’t improved.

Landscape Damage

If water does not drain properly, it can cause erosion of the soil behind and beneath the retaining wall. This erosion is not just detrimental to the wall, but it also takes away plants and foliage that the area needs. Plants rely on sturdy soil, and erosion destroys the root systems, which leads to the death of plants.

Water seepage behind and around the wall can result in stagnant conditions that are conducive to mold and fungus. When these conditions spread out into the surrounding landscape, they can lead to plant diseases. When soil is regularly waterlogged, the plant is nutrient-deprived, and additional damage is caused to the area around the wall.

Financial Implications

Improper drainage can cause damage to retaining walls such as; cracking, leaning, or bowing. Once things are getting to the point of really damaging things, it's not cheap to fix. The wall itself might have to be replaced entirely, at which point costs can skyrocket.

Potential breakage caused by insufficient drainage, slanting walls or cracks or soil erosion can cut the value of the property. Expensive repairs could scare off potential buyers and the look of the landscaping will be ruined. It may also be difficult for property owners’ to sell or rent their properties for a good price.

How to Plan Effective Drainage Systems

The wall drainage design guide behind retaining wall is critical to its long term stability. It is a guide to the important stages of developing a stable drainage design.

1. Evaluating Soil Type and Slope

The key to designing good drainage is knowing the site. What kind of soil people have is what determines the flow and drainage of water. For example, sandy soils have more effective drainage than the more water-retentive clayey soils. The tilt of the land also dictates how the water will flow on its own. 

• Steep slope: Water tends to run off quickly.
• Flat area: Water may pool and accumulate.

Considering such factors will help the designer to design the drainage system to cope with the particular site.

2. Identifying Water Flow Patterns

It is important to monitor water around the retaining wall and surrounding area. Knowing natural drainage flow and low spots where moisture collects can be helpful in determining which direction to drain the water. Key considerations include:

• Depressions that hold water.
• Places where, instead of running from the wall, water could flow toward the wall.

Through this evaluation, potential problem areas can be identified and studies appropriate water-handling strategies.

3. Determining Pipe Size and Placement

Once the site has been evaluated, the next step is to design the installation of the perforated pipes. These pipes must be correctly sized to accommodate the planned water volume in the area. And the location of these pipes is important as well:

• Pipes should be located at the bottom or the back of the walls in critical areas.
• Catching water from the roof early, before it accumulates behind the wall, is crucial, and that requires proper depth.

4. Planning for Gravel Backfill and Filter Fabric Installation

Gravel backfill for drainage is critical for allowing the water to drain correctly. To ensure effective drainage:

• Choose permeable gravel that allows water to filter through, yet can support the wall.
• Cover the pipes or gravel with filter fabric to prevent soil from restricting flow.

When installed correctly, these parts help maintain long term system efficiency.

5. Ensuring Proper Grading

The correct drainage relies on having the right grading around the wall:

• Ground should slope away from the retaining wall.
• A slight grade of about 1-2% is typically enough to direct water away and prevent pooling.

This slope assists in keeping water from pooling behind the wall.

How to Install Retaining Wall Drainage

Good drainage behind the retaining wall will help guarantee it stands the test of time. The following is a guide to the most significant steps of the installation process:

1. Digging Trenches

Dig behind the wall to expose the clogged drainage system. The right digging makes the system work. Key considerations include:

• Trench depth: The depth of the trench should, at least, be the thickness of the perforated pipe and gravel bed.
• Trench width: The trench should be wide enough to accommodate water to flow by without blockage through the gravel and the pipes.

Proper digging will guarantee that the drainage parts will fit and function as designed.

2. Laying a Stable Gravel Base

A solid gravel base is a key to the construction, and sets the stage for how long they will last after they have been excavated. This provides:

• A footing for the retaining wall to rest on.
• A sturdy stage that holds the pipe while aiding a good flow of water.

The gravel base should be leveled and compacted to prevent settling over time.

3. Proper Placement of Perforated Pipes and Gravel

Once the gravel base is placed, the next step is to lay the drainage pipes. Correctly placed perforated pipes are essential for a wall-directed dewatering:

• Lay pipes along the bottom of the wall or behind it in strategic locations.
• Surround pipes with gravel to create water flow into the pipes unencumbered.

The pipes should be tapped gradually for drainage to follow the natural water flow to the discharge point.

4. Securing Filter Fabric to Prevent Soil Infiltration

To keep dirt from clogging the drains, installation is needed (a filter fabric should be used around the gravel and pipes):

• Place fabric over the gravel backfill for drainage behind block wall for additional material build up.
• Make sure that the fabric is overlapped, so there’s a barrier to keep the soil from growing into the gravel, and let water through.

It does so as a means to keep the system efficient over a period.

5. Installing Weep Holes at Strategic Intervals

Weep holes are essential for allowing water to escape from behind the retaining wall:

• Weep holes in the base of the wall to drain water.
• Provide for a maintenance-free drainage by installing weep holes every so often along the wall.

These holes drain hydrostatic pressure and retaining wall water buildup fix of the wall.

6. Backfilling and Compaction

Backfilling refers to the filling of the space behind the wall with soil or other material. Correct backfilling will help keep the wall and the drainage in place:

• Backfill the trench, layering in small amounts and tamping it down so there are no air pockets under the wall.
• The levels of gravel around the pipes and weep holes must be the same.

After backfilling, the soil should be compacted to prevent any future settlements and to ensure stability of the wall.

• Use a tamping machine to tamp down the soil.
• Pack the soil well to eliminate future settling, which can cause the wall or drainage to fail.

Maintenance and Monitoring

When it comes to preventing costly repairs and re-construction of a retaining wall, it is important to properly maintain and monitor the drainage system behind the wall.

Regular Inspections

Periodically check the space behind the wall for signs of the water collecting or a change in the wall’s position. Water collecting is indicative of bad drainage, and movement in the walls could mean more water pressure or damage to the structure. Look for blockage of weep holes and drainage pipes from time to time. Cleaning these areas of debris and mud will allow water to escape from behind the wall and not build up pressure.

Seasonal Considerations

Before the rainy season or freezing temperatures, make sure that the drainage is working. Clean and clear the pipes and weep holes to prevent them from becoming clogged with heavy rain or ice. Keep an eye out for plants growing adjacent to the retaining wall. Plants can block the drainage and reduce the movement of water. Trim back plants and weeds to reduce water management. 

Repairing Damage Promptly

Repair any cracks or any weakened areas inside the wall or drainage. Don't let these problems linger or you could end up with costly damage. Replace any damaged drainage parts (pipe and weep holes) to maintain water flow and system efficiency.

The Bottom Line

Good drainage is essential to the preservation of retaining walls. Using components such as perforated pipes, gravel backfill and weep holes can prevent water from accumulating, thereby lowering the pressure and preventing the wall from failing.

For individuals interested in landscaping, wall installations, or both, Growcycle has valuable information and products to help with garden maintenance, landscape design, and suggestions on how to optimize space utilization. Their commitment to sustainability can also help in the development of green drainage systems for walls and other landscape elements.

Disclaimer: This material is for informational purposes only and should not be relied on for legal, medical, financial, or any other form of professional advice.

About the Author

Victor Miller

Environmental Journalist Focused on Sustainable Agriculture

Victor Miller is an environmental journalist at Growcycle, specializing in sustainable agriculture. With a passion for storytelling, he highlights the latest trends, innovations, and practices that promote ecological balance and responsible farming.