Sustainable drainage systems (SuDS) are drainage solutions that are designed to manage surface water run-off to maximise its benefits and minimise its negative impacts, often by mimicking natural drainage systems.
SuDS systems operate by managing the run-off within a site, before disposal. The disposal options, in order of preference, are: infiltrating into the ground, discharge into a body of water (river or sea), to a surface water sewer, or to a combined sewer.
Soakaways are a buried drainage system that manages surface water run-off by infiltrating it into the ground. Traditionally the run-off is collected from roofs and hard surfaces by gutters and gullies and then conveyed through a network of pipes below ground to the soakaway.
Soakaways are typically the preferred method of disposal for run-off but require careful consideration of the location and surrounding soil conditions before building. If your soakaway is not working as it should be, we have listed some reasons below as to why that might be.
The soil that a soakaway is built in must be permeable for the water to infiltrate effectively. Permeable soils include sands, gravels, and some chalks. It is important that the water from the soakaway infiltrates into the ground quickly enough to accommodate concurrent incidents of rainfall.
If soil is impermeable it will prevent water from infiltrating into the ground. Examples of impermeable soils are clays, silts, loam and certain rock types. The presence of clay and silt within sands and gravels can severely impact the permeability of the soil – a complication often not considered if the majority of the soil is permeable. Soakaways in impermeable soils are unlikely to function because they just fill up with water, so an alternative method of disposal is required.
Why not take a look at our Horatio’s Garden drainage case study for more information.
A soakaway should be placed deep enough in the ground to reach well-draining soil. It is not uncommon in areas of chalk bedrock to encounter a layer of impermeable clay over the top. If the chalk is not reached then the soakaway will not function as intended. It is important to have a good understanding of the site geology when determining the depth of a soakaway.
The presence of groundwater can have a big impact on the function of a soakaway. Areas with a high water table can be liable to groundwater flooding, but even water below the surface can prevent water from infiltrating if it’s too high. Many areas with chalk bedrock, which are otherwise permeable and well suited to soakaways, can have seasonal problems with high groundwater. Over the winter a big recharge of water can go into the aquifers and the water table rises. The base of a soakaway should be 1.0m above the water table, to stop the soakaway flooding and to avoid contamination of groundwater.
There are various options for design when it comes to soakaways; they can range from a simple pit filled with rubble to a perforated concrete ring surrounded by stone, to a series of stacked, plastic crates. If the design is not suitable for the location (i.e. under a driveway) it can eventually fail and prevent it from working effectively.
It is important to prevent silt and debris from entering a soakaway as this could block it up and reduce the soakage capacity. Effective pre-treatment should form part of the conveyance system to capture and filter the debris, this can take the form of gullies with sumps and dedicated silt trap chambers. These pre-treatment devices protect a soakaway, so it is important to inspect them and empty them regularly as part of a good housekeeping regime.
A drainage soakaway is generally the preferred option for disposing of surface water run-off, so it’s important that the design is suitable for the local ground conditions and it is maintained, to maximise the service life.
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