Soakaway Systems: A Guide

This is most commonly achieved by installing a connection to a main stormwater drain, a nearby watercourse or foul water drain (with the permission of a local water authority) but if none of these options is feasible, a soakaway is used.

In this blog post, we will look at what a soakaway does, how it is constructed and everything you need to know before installing one on your property.

What is a soakaway?

Soakaways have been used for decades, often comprising a simple pit dug into the ground within the property’s boundary and backfilled with gravel or rubble. This coarse material creates many small spaces that store water during rainfall and then allow it to gradually discharge into the surrounding soil at a rate that prevents flooding and waterlogging. This process is called rainwater attenuation.

Modern, more efficient soakaway design replaces the aggregate backfill with sturdy interlocking plastic crates. These are also known as sustainable drainage systems (SuDS) and stormwater attenuation crates.

How does a soakaway crate function?

A soakaway crate connects to the stormwater drainage system on the property and is constructed from a series of durable interlocking plastic crates. 

The internal geometry of the crates maintains its strength and stability even when a vehicle drives across it, and wrapping the entire soakaway in a permeable geotextile allows the water to exfiltrate from the crate while preventing the soakaway chamber from silting up and becoming less effective.

Why are crates better than backfill for a soakaway?

While aggregate is often plentiful on a construction site, particularly if former buildings have been demolished to make way for the development, there are many reasons why stormwater attenuation crates are a better option.

Soakaway crates hold more water

Soakaway crates offer a much higher water storage capacity than aggregate backfill – up to 97% of their total volume. For this reason, a shallower pit is required to allow for greater water storage capacity, taking less time and therefore saving money. The crates are also lightweight and easier to handle, making installation faster – another cost-saving benefit.

Soakaway crates perform more efficiently

In terms of performance, the structure of a soakaway crate accelerates the flow of water through the crate, meaning that sudden, heavy rainfall is going to be less of a problem. 

Any particles carried into the soakaway with the rainwater will freely fall to the bottom of the soakaway rather than amassing around the inlet and reducing its efficiency.

Soakaway crates are also a cleaner solution than aggregate, which naturally contains sediment that will impair the soakaway’s ability to store and discharge rainwater efficiently.

Soakaway crates are more dimensionally stable

All aggregate tends to compress when subjected to heavy surface loads over time, reducing its water storage and drainage capabilities. 

As they are installed in a wide range of locations, soakaway crates are designed to withstand the weight of light pedestrian traffic, private cars and heavy commercial vehicles without compression.

What locations are not suited to installing a soakaway? 

Certain conditions apply to installing a soakaway and these may affect the suitability of a site for stormwater attenuation.

Soakaways need space

Regulations stipulate that a soakaway must be at least 5 metres from the wall of any building and at least 2.5 metres from the boundary of the property.

A soakaway should also not be placed near another soakaway (they both need space to release water into the surrounding soil) or anywhere the water may become contaminated.

Soakaways need to drain

For a soakaway to function correctly it must be surrounded by soil that is normally dry and porous enough to absorb the water discharged by the soakaway.

Locations with a naturally high water table may therefore be unsuited to installing a soakaway as it cannot discharge into soil that is already saturated. The water table must be low enough for it to not rise as high as the bottom of the soakaway pit at any time of the year.

Dense, less porous soil may also affect the performance of a soakaway. A percolation test (see our separate blog post) will establish whether the soil is suitable. The percolation test will also help calculate the size and volume of the soakaway that will be required based on the surface area it is drawing stormwater from.

What are the options if a soakaway can’t be installed?

If you are considering a soakaway you have presumably ruled out the possibility of connecting to a main stormwater drain, so other options include discharging into an existing nearby watercourse such as a ditch, stream, river, pond or lake. If none of these is available and you are developing a site for several properties, you may be able to get planning permission to incorporate a pond or swale into the development.

Failing all these options, some local water authorities allow stormwater drains to be connected to main sewer drains. This will largely depend on whether the sewers will have the capacity to take this additional volume (bear in mind that your development will also be increasing the demand for foul water drainage) without creating problems. An annual charge may also be levied for this connection.

How long will a soakaway last?

The lifespan of a soakaway depends on several factors. It’s important to ensure that the soakaway has been correctly installed, with filters to prevent large debris such as leaves and moss from entering and clogging it up. Regular checks and maintenance will also extend the lifespan – clearing silt traps, jet washing pipework leading to the soakaway and installing a submersible pump that can remove silt and debris that may accumulate in the soakaway are all worthwhile measures – but under normal circumstances, it’s reasonable to expect a soakaway to last for at least as long as the building it is serving.

We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call the friendly team of drainage experts at Drainfast 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!