What is a soakaway & what can they be used for?
Soakaways are a common way of managing surface water runoff as part of Sustainable Drainage Systems (SuDS). Whether a soakaway is suitable depends heavily on the ground conditions, groundwater levels and characteristics of the site.
In this guide, we look at what a soakaway is, how it works, where it can be used and what needs to be considered when designing one, including infiltration testing and current SuDS requirements.
What is a soakaway?
A soakaway is a below-ground drainage feature that collects surface water runoff from areas such as roofs, driveways and other hard surfaces, before allowing it to gradually infiltrate into the surrounding ground.
Rather than directing rainwater straight into a sewer or watercourse, a soakaway temporarily stores runoff and releases it into the soil. This can help manage rainfall close to where it falls and reduce the volume of surface water entering drainage networks.
Soakaways can take different forms depending on the site and drainage requirements, including systems constructed using geocellular crates or other suitable infiltration structures.
For a soakaway to work effectively, the surrounding ground must be able to accept water at a suitable rate. Its feasibility therefore depends on factors such as soil and geology, infiltration rates, groundwater levels, contamination and nearby buildings or infrastructure.
These conditions should be properly assessed before a soakaway is designed, typically through site investigation and infiltration testing.
How does a soakaway work?
When rain falls onto impermeable surfaces such as roofs, driveways and paving, it creates surface water runoff. This water is directed into the soakaway, where it is temporarily stored.
The stored water then gradually passes through the soakaway and infiltrates into the surrounding soil.
The rate at which this happens depends largely on the permeability of the ground. Free-draining soils may allow water to infiltrate relatively quickly, while less permeable soils, such as some clays, can make infiltration drainage unsuitable.
This is why the suitability of a soakaway should not be assumed based solely on the appearance of the ground or the presence of soakaways on neighbouring sites.
Are soakaways a Sustainable Drainage System?
Yes. Soakaways are one form of Sustainable Drainage System.
SuDS are designed to manage surface water in ways that more closely mimic natural drainage processes rather than simply conveying runoff into underground sewers.
The National Standards for Sustainable Drainage Systems in England establish a hierarchy for the destination of surface water runoff. Rainwater collected for non-potable reuse is the priority, followed by infiltration into the ground where practicable.
This means infiltration drainage such as soakaways can form an important part of a wider SuDS strategy where site conditions allow.
However, a soakaway will not necessarily be the most appropriate SuDS solution for every development. Other measures can include rainwater harvesting, permeable paving, rain gardens, swales, basins and other attenuation or infiltration features.
What are the advantages of a soakaway?
Where ground conditions are suitable, and the system is correctly designed, soakaways can offer several benefits:
- Manage rainfall close to where it falls, reducing the need to convey surface water elsewhere.
- Reduce pressure on drainage and sewer networks by preventing or reducing surface water discharge into them.
- Support natural infiltration and the movement of water back into the ground.
- Provide below-ground storage, which can make them useful where above-ground space is constrained.
- Form part of a wider SuDS strategy for a development.
- Manage runoff from roofs and other impermeable surfaces, subject to appropriate design and water-quality considerations.
Like any drainage feature, these benefits depend on appropriate site assessment, design, construction and ongoing maintenance.
What are the disadvantages of a soakaway?
Soakaways are not suitable for every site.
Potential limitations include:
- Ground conditions may have insufficient infiltration capacity.
- High groundwater levels can restrict their use.
- Poor siting or design can create risks to building foundations and nearby infrastructure.
- Infiltration may be inappropriate where it could mobilise existing contamination.
- Polluted runoff can present a risk to groundwater.
- Sediment can accumulate and reduce performance if appropriate pre-treatment and maintenance are not provided.
- Site investigation and infiltration testing may be required before the drainage design can be confirmed.
Where infiltration is not suitable, an alternative drainage solution may need to be considered.
An infiltration trench is usually filled with permeable granular material, designed to promote infiltration of surface water to the ground. An infiltration basin is a dry basin or depression designed to promote infiltration of surface water runoff into the ground. Soakaways are the most common type of infiltration device in the UK, where drainage is often connected to oversized square or rectangular, rubble-filled voids sited beneath lawns.
Where can a soakaway be used?
Soakaways are commonly associated with residential development, where gardens, amenity areas and other open spaces can provide potential locations.
However, they can also be considered for commercial and other developments where the site’s ground conditions and drainage requirements are appropriate.
Whether a soakaway can be used depends on more than simply having enough space.
Consideration needs to be given to:
- The permeability of the underlying soil and geology.
- Groundwater levels.
- Nearby buildings, roads and other infrastructure.
- Land stability.
- Existing contamination.
- The quality of runoff entering the system.
- Other infiltration features nearby.
- The volume of runoff that needs to be managed.
Site-specific assessment is therefore essential.
Do you need infiltration testing for a soakaway?
Infiltration testing establishes whether the ground can accept surface water at a suitable rate and provides information for designing an infiltration system.
The National SuDS Standards state that geotechnical investigations should be undertaken to establish whether ground conditions are suitable for infiltration.
Testing can provide an infiltration rate, which helps drainage engineers determine how quickly water can pass into the surrounding ground and informs the size and design of the proposed system.
Testing should be undertaken using an appropriate methodology and at locations and depths representative of the proposed infiltration features.
GeoSmart provides infiltration testing to help determine whether infiltration drainage is feasible and provide the data required to inform drainage and soakaway design.
How is a soakaway designed?
A soakaway needs sufficient storage capacity to accommodate runoff during a rainfall event while allowing water to infiltrate into the surrounding ground at an appropriate rate.
The required design will depend on factors including:
- The area draining into the soakaway
- Rainfall and design storm events
- Climate change allowances where applicable
- The measured infiltration rate
- The dimensions and storage capacity of the system
- Groundwater conditions
- The characteristics of the surrounding ground
- The consequences if the system exceeds its capacity
The National SuDS Standards also require infiltration features to be appropriately sized for the design event based on the site’s ground conditions and contributing drainage area.
This is why a soakaway should be designed using site-specific evidence rather than selecting a standard size.
How big does a soakaway need to be?
There is no single standard soakaway size that will be suitable for every development.
The dimensions required depend on the amount of runoff entering the system and how quickly the surrounding ground can accept that water.
For example, a development with a large area of impermeable surfacing or relatively slow infiltration rates is likely to require a different drainage solution from a small roof area over highly permeable ground.
Infiltration testing and drainage calculations are therefore important parts of determining the appropriate size.
GeoSmart can undertake the required investigations and calculations to inform the design of an appropriate infiltration drainage system.
How far should a soakaway be from a building?
The appropriate distance between an infiltration system and buildings, roads or other infrastructure depends on the nature of the proposed system and the site’s ground conditions.
The National SuDS Standards require the potential risk to buildings, roads, embankments and other infrastructure to be assessed where point infiltration systems are proposed within 5 metres.
Where unstable geology is present, a greater separation distance may be necessary.
Rather than relying on a universal distance in every situation, the location should therefore be assessed based on the proposed drainage system, geology, groundwater conditions and surrounding infrastructure.
How does groundwater affect a soakaway?
Groundwater levels are an important consideration when determining whether a soakaway is suitable.
Under the National SuDS Standards, the base of an infiltration drainage feature should not be within 1 metre of the maximum likely groundwater level.
Groundwater conditions can change seasonally, which means observations taken at a single point in time may not necessarily represent the highest groundwater level that could occur at the site.
Appropriate groundwater information may therefore be needed to demonstrate that sufficient unsaturated ground will remain beneath the infiltration system.
Can you use a soakaway on contaminated land?
Infiltration drainage requires particular care where contamination is present.
Allowing water to infiltrate through contaminated ground can potentially mobilise pollutants and create a pathway into groundwater or nearby surface waters.
The National SuDS Standards state that proposed infiltration through contaminated land should be assessed and should only take place where there is no unacceptable risk of mobilising pollutants.
The potential for contamination should therefore be considered as part of the site assessment before deciding whether a soakaway is appropriate.
When might a soakaway not be suitable?
A soakaway may not be appropriate where:
- Infiltration rates are too low
- Groundwater levels are too high
- There is insufficient unsaturated ground beneath the proposed system
- Infiltration could create an unacceptable risk to nearby buildings or infrastructure
- The ground is unstable
- Contamination could be mobilised
- Runoff entering the system presents an unacceptable pollution risk
- The proposed system cannot safely manage the required runoff
If infiltration is not feasible, other SuDS or drainage options can be considered as part of the site’s drainage strategy.
What problems can arise with a soakaway?
One of the main risks to the long-term performance of a soakaway is clogging.
Sediment and other material entering the drainage system can accumulate over time and reduce its ability to store and infiltrate water. If performance deteriorates sufficiently, water can back up within the drainage network or cause localised flooding.
Current National SuDS Standards require infiltration features to incorporate, or be preceded by, appropriate pre-treatment to help protect the infiltration surface from clogging.
Maintenance should therefore be considered from the design stage.
A suitable maintenance plan should identify requirements for inspection, sediment removal and any other activities necessary to keep the system functioning throughout its intended lifetime.
Soakaways and planning applications
Surface water drainage is an important consideration for new development.
The National Planning Policy Framework requires development proposals that could affect drainage on or around a site to incorporate sustainable drainage systems proportionate to the nature and scale of the proposal.
SuDS should be designed in accordance with the National Standards for Sustainable Drainage Systems and should have arrangements in place for their maintenance over the anticipated lifetime of the development.
Where infiltration drainage is proposed as part of a planning application, sufficient evidence may therefore be required to demonstrate that the solution is feasible and appropriately designed.
Depending on the development, this could include infiltration testing, groundwater information, drainage calculations and a wider drainage strategy.
Local requirements should also be checked with the relevant planning authority and Lead Local Flood Authority.
Do soakaways need maintenance?
Yes. Although soakaways are below ground, they should not be treated as maintenance-free.
Inspection and maintenance can help identify problems such as sediment build-up, blocked upstream drainage or deterioration in performance before they result in flooding.
The drainage design should provide suitable access for inspection and maintenance, while upstream treatment features such as catchpits may need periodic cleaning to prevent sediment reaching the infiltration system.
The maintenance requirements should be documented and retained for those responsible for the drainage system over its lifetime.
Need help with a soakaway or infiltration testing?
Determining whether a soakaway is suitable starts with understanding the ground.
GeoSmart can support developments with infiltration testing, Sustainable Drainage Systems (SuDS) strategies and detailed drainage design, helping establish whether infiltration is feasible and informing an appropriate surface water drainage solution.
Find out more about our infiltration testing services, or speak to our team about the drainage requirements for your site.
