A retaining wall, by its very nature, is holding back soil that would naturally fall into the space where the retaining wall has been erected. That means it’s made to deal with the constant weight of the retained soil. 

Seems simple right? So what happens when water soaks into the soil behind the retaining wall? Well, after heavy rainfall, or consistent amounts of rainfall, the soil absorbs the rainfall, resulting in an even heavier load for the retaining wall to hold. If the water can’t escape, it builds up, creating immense force known as hydrostatic pressure.

This combined weight and pressure can easily exceed the wall’s design capacity, especially in the high-rainfall we can experience regardless of the season!

Worrying wet waterlogged walls

So how does some heavier soil destroy a wall designed to support heavy weight? Well, it doesn’t just tip over instantly, there are a few warning signs that manifest, that tell you your retaining wall needs better drainage. 

Here are the warning signs:

1. The wall is leaning or has bulges: This is the most immediate sign. The hydrostatic pressure physically pushes the wall outwards, causing it to lean or bulge. Once a wall starts to visibly shift, it’s only a matter of time for when the wall falls. 
2. Cracks in the wall: Water pressure forces cracks to form. These cracks allow more water in, accelerating the eventual failure of the wall.
3. Footing failure (settlement): Saturated soil can wash away or soften the soil beneath the wall’s footing, leading to differential settlement where one part sinks lower than the other.
4. Efflorescence Even if the wall doesn’t collapse, trapped moisture will be forced through the wall, bringing salts to the surface, otherwise known as efflorescence. This ruins any like paint or render and indicates structural moisture problems.

How drainage can save you thousands for your retaining wall

So now we know how having no, or poorly constructed drainage can damage your retaining wall. How exactly does drainage work so well that it can save you thousands that would be spent on repairing your retaining wall?

Drainage is a two-fold application. Here are the key elements of internal and external drainage

1. Internal drainage: The backfill system

This backfill system’s job is to collect water before it reaches the wall face and channel it downwards towards the external drainage

• Free-draining backfill: The first 300 mm to 600 mm layer directly behind the wall must be filled with a free-draining granular material (like crushed stone, coarse gravel, or clean single-size aggregate). This creates a permeable highway for water to quickly travel down.
• Filter fabric : A filter fabric should be placed between the native soil and the granular backfill. This critical fabric prevents the finer native soil particles from washing into and clogging the gravel, ensuring the drainage system remains functional for decades.

2. External drainage: pipes and outlets

The internal system collects the water; the external system takes it away.

• Perforated pipe: A slotted or perforated pipe must be laid at the base of the wall, inside the granular backfill, and resting on the footing. This pipe collects the water that filters down through the gravel.
• Weep holes: These are small, deliberate openings left in the wall face, usually near ground level, allowing minimal amounts of water that penetrate the wall to escape directly. They are a secondary measure and should not be relied upon as the sole drainage solution, especially for large walls.
• Outfall: The perforated pipe must connect to a non-perforated pipe that runs through or around the wall to a safe outfall— like a lower, non-critical area of the garden. 

The drainage checklist for your retaining wall

If you are planning to repair a wall, or if you are checking an existing one, use this checklist to understand the state of the wall’s current drainage:

• Is there granular backfill? The wall should not be sitting directly against clay or compacted soil.
• Is there a functional perforated pipe at the base? Can you see the outfall where the pipe drains to?
• Are the weep holes clear? Use a piece of wire to gently poke into any visible weep holes to ensure they haven’t been blocked by soil or debris.
• Does the ground slope away from the top of the wall? The surface immediately behind the wall should slope slightly away from the wall to direct surface water away, preventing it from saturating the soil right next to the structure.

Of course, there may be an instance where all of these checks have come back positive, but the wall still has signs of poor drainage or structural instability. For the best assessment, you should consult a structural engineer. 

Can I install drainage for my retaining wall myself, or should I get a professional?

Because drainage is a structural issue, professional advice/installation is often a small price to pay to avoid massive failure costs later. It can be done by yourself, but it is not recommended at all if you do not have the experience, expertise, or equipment to do this. 

For any wall over 1.2 metres tall, or a wall showing signs of distress (leaning, large cracks), always consult a qualified structural engineer. They will calculate the lateral earth pressure, design the required footing, and specify the exact drainage requirements necessary to meet UK building standards and prevent the failure that could cost you thousands.

In the UK, walls over a certain height (often 1.0m to 1.5m, depending on local authority and location) may require Building Regulations approval. This process inherently ensures that structural stability and, critically, drainage, meet required safety standards.

Don’t let inadequate drainage be the mistake that undermines your retaining wall and costs you thousands of pounds, if not more. 

Investing time and material in a robust drainage system is the difference between a durable, lifelong garden asset and a dangerous, costly liability. Contact PPC for a professional approach for structurally sound retaining walls, whether it’s a garden or a public space, PPC has the tools, services and knowhow, to help you get the best possible retaining wall. 

Retaining wall creation with durability and drainage in mind