What is a Soakaway?

Soakaways have been used for a long time as a method for the disposal of surface water runoff.

Historically soakaways were just simple pits excavated in the ground and filled with various types of rubble. Rainwater pipes were then connected to these underground pits allowing water to soak into the ground.

Most of the time they were just filled with left over building material and little thought was given to capacity requirements or the long term operation and future maintenance of the soakaway.

Why Soakaways and how do they work?

Soakaways are one of the earliest forms of sustainable drainage systems (SuDS). They provide surface water attenuation by storing water in the various voids within them, they provide surface water treatment by via filtration and they prevent flooding by discharging surface water to the ground as opposed to a watercourse or sewer.

Soakaways act as self contained drainage outfalls. In a typical residential scenario a soakaway would be connected to all rainwater pipes, driveway gullies, channel drains etc. When it rains, water is channelled from the property and into the soakaway where it then slowly infiltrates into the ground.

Building Regulations

Under 'Part H' of the Building Regulations, soakaways must be considered as the primary method of surface water disposal. Only when then use of soakaways have been dismissed on the grounds of technical reasons will a new connection be permitted to a watercourse or sewer. This criteria is also applied by most Water Authorities, Lead Local Flood Authorities and Environment Agencies.

In order to comply with building regulations requirement and the requirements of BRE Digest 365, soakaways must be located at least 5 metres from a building or structural foundation, must half empty within a 24 hour period, must contain the required storage volume to meet the design criteria and must be suitable to withstand the relevant surface loading above them. Soakaway Solutions can assist by designing your soakaway to ensure it meets these requirements.

What are the Different Types of Soakaways? 

Today there are various different ways to construct a soakaway, each with different advantages and disadvantages. Some of the main types of soakaway are listed below:

Rubble Filled Soakaway 

As mentioned above, these are the oldest type of soakaway. They are just simple holes filled with rubble. Modern day rubble soakaways tend to be constructed using material such as MOT Type 1 X or drainage aggregate. It is important that rubble soakaways are not constructed using aggregates containing fine particles such as MOT TYPE 1 or mill waste. These fine particles will compact and prevent water from infiltrating out of the soakaway and into the surrounding sub soil. Rubble filled soakaways are seldom used in modern day drainage designs, this is mainly down to inefficiency. A rubble soakaway will only typically provide a 30% void ratio, most cellular storage crates offer in excess of 90% making them far more efficient at holding water. As with all soakaways, rubble filled soakaways should be lined with a non-woven geotextile membrane, this prevents fine particles from the surrounding subsoil from entering the soakaway. We rarely design rubble filled soakaways for our clients unless they are specifically requested.

Manhole Soakaways

These are special manhole chambers, usually made out of concrete or plastic. They have perforated walls that allow water to escape into the surrounding subsoil. They are usually surrounded by large particle drainage aggregate on the outside to offer additional storage volume. Manhole soakaways are good for use in areas such as public highways (where permitted) as they are very strong and can be easily accessed through the manhole lid. The disadvantage is they are heavy, require heavy duty plant to manoeuvre the rings and offer limited storage volume. We would typically look to specify manhole soakaways in HGV trafficked areas or where they have been specifically requested.

Soakaway Crates

The most popular choice for constructing modern day soakaways. They contain a void ratio in excess of 90% that makes them extremely efficient for providing surface water storage volume. They are also lightweight, making them easy to manoeuvre. Some models come with pre-formed pipe inlets to assist when connecting them to the drainage pipework. Most soakaway crates are also modular allowing them to be arranged in different configurations to provide the specific volume required. It is essential that soakaway crates are wrapped in a non-woven geotextile membrane to prevent silt from entering and blocking the crates. Soakaway crates must be installed correctly, especially where they are to be installed under areas that will be subject to loads from vehicles and HGV's. We always recommend you consult the manufacturer to check that the crate is fit for purpose prior to installation. Soakaway crates are predomianlty the main type of system we use when designing soakaways.