Soakaways: How to Prevent & Repair Blockages

A soakaway is popularly used to manage rainwater where no convenient connection to a main stormwater system is available. Capable of temporarily holding a large quantity of water, the soakaway will then slowly release this into the surrounding soil at a rate that will prevent waterlogging or flooding.

A properly installed soakaway generally requires little maintenance, but if it becomes blocked this will reduce how efficiently it manages rainwater. If left unresolved, this can eventually lead to problems.

In this blog post, we look at the most common causes of blocked soakaways, how to prevent blockages and how to clear them if they occur.

What causes a soakaway blockage?

There are three main causes of blockages in a properly constructed soakaway: compacted soil, accumulation of silt and a collapsed drain.

Soakaway blockages caused by compacted soil

Before a soakaway is installed, a percolation test will (or should) have been carried out on the surrounding soil to determine its permeability and therefore its ability to absorb rainwater at a reasonable rate. The permeability will depend on the composition and compactness of the soil; dense, clay-heavy soil will be less efficient than light loam or sand.

However, soil compaction can occur if heavy vehicles are driven repeatedly across it, or during an unusually long period of hot, dry weather. This can also happen if the soil around the soakaway was backfilled while comparatively wet and has since drained off.

Soakaway blockages caused by silt accumulation

Rainwater is rarely comprised of pure water by the time it reaches a soakaway. Dust, moss, dead leaves and other debris from roofs and gutters are all washed into the system and will accumulate over time.

Silt from the surrounding soil can also penetrate a soakaway if the geotextile membrane surrounding the soakaway is the wrong type, has been fitted incorrectly, has shifted or has been damaged.

Soakaway blockages caused by a collapsed drain

Drains leading to soakaways can sometimes collapse or shift due to excessive pressure from vehicles, ground movement or soil erosion. Rainwater may no longer flow so freely through the drain, creating sluggish spots that allow silt to accumulate. If the drain has been broken through, silt will also be able to enter through the break.

How can a soakaway blockage be identified?

The most common indicators of a blocked soakaway are flooding and waterlogging, as the blockage will prevent the soakaway from performing efficiently. If the soakaway was installed to rectify waterlogging over a patch of ground, this will return as previously. If it is being used to manage rainfall from non-porous areas such as roofs and driveways, the water will back up along the drain during spells of rain, causing gutters and gullies to overspill and flat surfaces to flood.

An indicator of a blocked soakaway caused by soil compaction is dips appearing on the ground surface and flooding during rainy spells.

How can soakaway blockages be prevented?

As prevention is always better than cure, it makes sense to ensure that a soakaway has been correctly installed, in suitable soil, by a qualified and experienced contractor. 

The composition of the surrounding soil is critical if a soakaway is to function correctly, and a percolation test should be carried out, ideally by a professional.

The soakaway system and the geotextile membrane used to wrap it must be correctly specified, and the dimensions of the soakaway should be sufficient for the size of the property or ground it will serve. While a domestic soakaway system may be acceptable for a dwelling, a heavier-grade soakaway system may be required for high-flow commercial use. Soakaway crate systems are modular and relatively inexpensive, so it makes sense to over-specify the capacity required rather than take a risk to save a bit of extra money and digging.

Installing a silt trap before the soakaway will help capture debris passing through the drain before it empties into the soakaway. Regular emptying of the silt trap will also be necessary to ensure this works efficiently.

How can a soakaway be unblocked?

Unfortunately, unblocking a soakaway isn’t always as easy as unblocking a stormwater or sewer drainage system. By the time the blockage may have been exposed, the damage is likely to be extensive enough to require unearthing the soakaway to inspect and possibly replace it.

Start by inspecting the silt trap (if one has been installed). Does it need to be emptied? Is water backing up from the soakaway to the silt trap? If you add water to the pipe upstream from the silt trap, does it flow freely into the trap?

Now examine the area around the drainage pipe and soakaway for any signs of damage such as sunken earth or localised waterlogging that might indicate the location or cause of the blockage.

If the problem appears to be in the area of the drainage pipe, excavate around this and inspect it for signs of damage. Having eliminated a blocked or damaged pipe as a possible cause, you now have no option but to dig up the whole soakaway to expose the crates and membrane.

Minor damage may be repairable and silt can be removed, but if the damage is extensive you will have to replace the soakaway. 

If the soakaway is full and not draining into the soil but sediment doesn’t seem to be the issue, it’s possible that either the soakaway is too small for your needs or the soil has become too compacted.

How do I replace a soakaway?

Building Regulations stipulate that a replacement soakaway should be placed in a different location to the one it is replacing. This can be a problem, particularly on a small site, as it must also be at least 5 metres away from any buildings. 

The soil in the new location must also be percolation tested, as permeability can vary dramatically even over a short distance.

We therefore recommend you contact your local authority’s Building Control Department before replacing a soakaway.

The process of replacing a complete soakaway system is the same as that of installing a new one. For advice on how to do this, you can read our article Soakaway Systems - An Installation Guide.

We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call our friendly team of experts on 01420 555600 or email [email protected]. 

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!