Drainage for a retaining wall is absolutely paramount to include. It is a consideration that must always be made, and if neglected, it can mean your retaining wall does not last long, and could hurt yourself, or someone else.
In this blog, we will show you why drainage matters for retaining walls (and how to get it right).
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Is drainage the same for every kind of retaining wall?
No, the drainage system isn’t identical for every type of retaining wall. The specific drainage design will vary depending on factors like the wall’s height, the type of soil, local climate, and the overall complexity of the project. However, the core principle remains the same: to effectively divert water away from the wall to prevent hydrostatic pressure build-up.
Here are some examples of how drainage systems can vary:
Gravity Retaining Walls
These walls rely on their weight to resist soil pressure. A simple drainage layer of gravel behind the wall is often sufficient.
Cantilever Retaining Walls
These walls have a stem and a base slab. The drainage system typically involves a perforated pipe wrapped in geotextile fabric, placed behind the wall to collect and divert water.
Sheet Pile Walls
These walls are composed of interlocking steel sheets driven into the ground. Drainage can be achieved through weep holes in the sheet piles or a drainage layer behind the wall.
Mechanically Stabilised Earth (MSE) Walls
These walls use reinforced soil to create a stable structure. Drainage is often incorporated into the reinforced soil layer, with perforated pipes and geotextile fabric to collect and divert water.
In all cases, the drainage system should be designed to prevent water from pooling behind the wall and to ensure that it is properly directed away from the structure. A well-designed drainage system is essential for the long-term stability and durability of any retaining wall.
What do I do if my retaining wall is leaking?
Start by identifying the source of the leak. Inspect the wall for cracks, gaps, or damaged drainage pipes. Once you’ve pinpointed the problem, you can take the necessary steps to repair it.
If the leak is minor, you might be able to fix it yourself by sealing cracks with a waterproof sealant or replacing damaged drainage pipes. However, for significant leaks or complex issues, it’s advisable to consult a professional contractor. They have the expertise to assess the damage, determine the best course of action, and implement effective repair solutions.
Why drainage matters for retaining walls (and how to get it right)
A well-designed retaining wall is a testament to engineering prowess, but its longevity and structural integrity hinge on one critical factor: proper drainage. When water accumulates behind a retaining wall, it exerts immense hydrostatic pressure, gradually weakening the structure. This pressure can lead to soil erosion, cracking, and ultimately, the collapse of the wall.
How to Get Drainage Right
To ensure the longevity of your retaining wall, consider these crucial drainage techniques:
- Drainage Layer: A layer of coarse gravel or crushed stone should be placed behind the wall. This layer allows water to permeate through the soil and drain away from the wall.
- Weep Holes: These small openings, typically located at the base of the wall, allow water to escape. They should be spaced evenly along the wall’s length to facilitate efficient drainage.
- Drainage Pipes: In cases of heavy rainfall or poor soil conditions, a perforated drainage pipe can be installed behind the wall to collect and divert water away from the structure.
- Filter Fabric: This geotextile fabric prevents soil particles from clogging the drainage layer and pipe, ensuring optimal water flow.
- Slope the Backfill: The soil behind the wall should be sloped away from the structure to encourage water run-off.
By incorporating these drainage strategies into your retaining wall design, you can significantly extend its lifespan and minimize the risk of costly repairs or failures. Remember, a well-drained retaining wall is a resilient and reliable structure that can withstand the test of time.
Does every retaining wall need drainage?
Water pressure behind a retaining wall can cause significant stress on the structure, leading to erosion, cracking, and potential failure. A well-designed drainage system redirects water away from the wall, preventing hydrostatic pressure build-up. This is especially crucial in areas with heavy rainfall or high water tables.
While very small retaining walls in well-draining soil might not strictly require drainage, it’s generally considered best practice to include one. Even a simple drainage layer of gravel behind the wall can significantly improve its longevity.
Can my retaining wall’s drainage overflow?
While a well-designed drainage system is crucial for preventing hydrostatic pressure and maintaining the structural integrity of a retaining wall, it’s not infallible. A few things can lead to drainage overflow:
- Heavy Rainfall: Excessive rainfall can overwhelm the drainage system, causing it to overflow.
- Clogged Drainage Pipes: Debris and sediment can accumulate in drainage pipes, reducing their capacity and leading to overflow.
- Inadequate Drainage Design: A poorly designed drainage system may not be able to handle the volume of water, resulting in overflow.
- Soil Erosion: Erosion can damage the drainage system, reducing its effectiveness and causing overflow.
- Landslide or Soil Movement: These events can disrupt the drainage system and cause water to overflow.
To prevent drainage overflow and protect your retaining wall, it’s essential to:
- Regularly inspect the drainage system for any signs of damage or blockage.
- Clean the drainage pipes regularly to remove debris and sediment.
- Ensure the soil behind the wall is sloped away from the structure to facilitate water run-off.
- If you live in an area with heavy rainfall, consider installing a larger drainage system to handle the increased water volume.
- A professional engineer can assess your specific situation and recommend appropriate drainage solutions.
Where does the drainage go?
The drainage from a retaining wall typically drains into a designated area where it can safely dissipate. This area could be, a natural drainage area: This might be a nearby stream, river, or a naturally low-lying area on the property.
In urban or suburban areas, the drainage can be connected to a drainage system. Or, a dry well, which is a pit filled with gravel that collects water and allows it to slowly percolate into the ground.
The specific drainage solution will depend on local regulations, soil conditions, and the overall landscape design. It’s crucial to ensure that the drainage system is designed to handle the anticipated water volume and to prevent water from accumulating near the foundation of buildings or other structures.
Create a structurally sound retaining wall today
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In this blog, we hope to have answered the question ‘Why drainage matters for retaining walls? (and how to get it right)’. Whilst also providing additional information including construction solutions that we offer.
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