Soakaway Design Ultimate Guide (2023)

What is a soakaway?

A soakaway is a subsurface structure for the temporary storage of water before it soaks in to the ground. A soakaway is essentially a point feature; it does not have a channel. A soakaway is part of the Sustainable Drainage Design

When is a soakaway possible?

A soakaway is possible when:

• The ground infiltration rate is greater than 1×10-6m/s. Ideally you prefer a soakaway rate of 1 x 10-5m/s in order to achieve the half emptying time
•  The ground is not contaminated
• The water table is 1m below the base of the soakaway
• The distance from the soakaway to a building is 5m or more

What is infiltration rate?

The infiltration rate velocity at which water disperses into the ground. It is usually measured in m/s.

The lowest percolation rate at which a soakaway is still efficient is 1 x10-6 m/s.

The infiltration rates depend on the type of soil encountered on site.   The table below shows a description typical infiltration values. 

When it is proposed to use permeable paving the CBRs should also be undertaken to determine if the soil will be stable once is saturated. The table below shows the relationship between CBRs infiltration rates and type of infiltration systems. 

What is the purpose of a soakaway?

A soakaway main purpose is to disperse the water produced by an impermeable area within the soil.

The second aim of a soakaway is to help to recharge the aquifer.

It also forms part of your drainage strategy system for the whole development

How long do Soakaways last?

The life of a soakaways depends on the amount of silt or sediment that enter it and the maintenance of the structures that protected it.

The soakaway problems are in relation to increase sediment within it. The drainage should conform with the below ground drainage building regulations.

In general, you can find soakaways from 1960 and still working well.

This is because the silt traps were working adequately and most of the water was being filtered by a series of smaller soakaways.

Can a soakaway get blocked?

One of the main risks of a rainwater soakaway is silting that causes clogging of the surrounding soil.

This is a common problem with newer soakaways in which the pre-treatment systems are not in place. For instance, silt traps are not installed or gutters are not maintained.

Nevertheless, a well design soakaway should have a minimum risk of getting blocked as the design should not allow for infiltration at the base of the soakaway.

How deep can a soakaway be?

The depth of the soakaway depends on the soil.

Usually, the council will only accept soakaways of up to 2.5m deep for dwelling. This is because deeper soakaway tends to carry risk to the stability of houses should they need to be removed in the future.

In industrial settings soakaways can reach deeper depths and can have a combination of crates and deep concrete soakaways.

Nevertheless, a well design soakaway should have a minimum risk of getting blocked as the design should not allow for infiltration at the base of the soakaway.

Soakaway Rules

The soakaway regulations UK are:

Soakaways should not be built:

• Within 5m of a building
• Within 10m to 15m away of a building if soakaway is within Chalk
• In areas where ground stability is a problem
• In contaminated soil or near landfills in which gas accumulation can be an issue
• In the proximity to drainage fields. Ideally a soakaway should be located 10m from a drainage field
• When the water table is less than 1m below the base of the soakaway

Permits

The following permits may be required if you have a soakaway within your development: 

• Direct discharge into the chalk requires a permit from the Environment Agency
• Discharge in a SSSI area
• Discharge within in a ground water Source Protection Zone 1

Standards

There are 4 main standards. 

Standard 1: Building Regulations

•  Method is used for areas less than 100m2
• Use a return period of 10 years
• For areas less than 25m2 a rainfall of 10mm in 5min should be used

The calculation method should use the soakaway standard specified within the approved document H2.

We will discuss this method in the next chapter.

Standard 2: NHBC

You can use this method only if your house is being approved by NHBC and tests are not required by the Lead Local Planning Authority. Otherwise use a BRE365.

The use of this method is only for small areas of less than 150m2.

It is recommended that this method is use as an starting point and then complete soakaways test to BRE365.

Standard 3: Adoption Design and Construction Guidance

The main points of this guidance are:

• The soakaway must be fed by an upstream channel that is legally a sewer or lateral drain that is also proposed for adoption
• If a soakaway is adopted the whole structures should be included such as the rubble and membranes
• Design is as per the CIRIA Report C753 ‘The SuDS Manual’.
  Access for maintenance and replacement should be included within the development

Standard 4: CIRIA Report C753

The key standards for the design of the soakaway under this process are:

• It can be used for any size soakaway
• Infiltration is only allowed at the sides
• Design is based on a Soakaway tests as per BRE365
• A factor of safety should be included as part of the design 

There are 3 tests that can be done:

Test Type 1 – To building regulations standards

• Excavate a hole to the base of the potential soakaway
• Dig a 300mm x 300mx30mm deep at the base of the excavation
• If the above to steps are difficult, use a 300mm earth auger
• Fill in the holes with 300mm of water and leave to drain overnight
• The following day, repeat the test by infilling the hole up to 300mm height
• Take the time it drains away
• Repeat the test two times more

Test Type 2 – To NHBC Standards

This test is done by using a borehole of 150mm diameter.

The process is as follow:

• Bore a hole 150mm dia to 1m below ground level
• Pour water to 300mm in depth, measure from the bottom
• Take the time that water takes to soakaway
• Repeat test three times
• Once the tests are completed then bore the hole 1 m lower (2m bgl)
• Repeat the process above
• The average figure is used for the design of the soakaway

Test Type 3 – To BRE 365

This is more used test in the UK. It is approved by most of the Lead Local Flood Authorities. The process is as follow:

• Excavate a hole 1m x 1m x Depth of potential soakaway
• Pour in min 1m3 of water (Instantaneously). You will need a water bowser
• Record Time & Depth until the hole has emptied
• Repeat test three times. Depending on the flows the test can take 1 to 3 days

The soil infiltration rate from the time taken for the water level to fall from 75% to 25% effective storage depth in the soakage trial pit, using the lowest f value of the three test results for design. 

Who can do a percolation test?

The tests to building regulations and the BRE365 can be completed by builder, a farmer or  a Soil Investigation Company.

The tests to NHBC standard should be completed by a Soil Investigation company.

How much does a percolation test cost uk?

The costs vary within the regions and depends on the soil conditions.

For instance, areas with brush soil and low permeability soils, the percolation tests tend to cost more.

This is because each test takes longer to complete.

In general, these are the prices within the south east with a medium permeability soil.

• Test to building regulation standards: £1,500.00. This test depends on the depth of the excavation
• Test to NHBC standard: £900.00
• Test to BRE365 standard: £1,500.00

Soakaway Crates

Soakaway Plastic Crates are currently the most used type of soakaways this is because the if the easy installation. However, these products have the highest whole life carbon value in comparison with other materials.

There are around 5 manufactures in the UK alone of these products and a number of foreign manufacturers, particularly German, Dutch and Polish.

There are clear differences in quality and performance in units and their specification should be in the context of a detailed design, where highway and hydraulic performance is fully considered.

Performance specification is not normally possible without corresponding design and calculations.

It should be considered as exceptional that a contractor or manufacturers have the wide design knowledge to take responsibility for the main design performance.

A number of major manufactures do offer “free design” services, with although possible to use, should be considered with caution and a structural and hydraulic check independent of this should always been undertaken.

If in doubt, ensure that a product is required to have BBA certification.

Manhole Rings

This a manhole with holes on the walls. There are different manufactures such as Stanton Bonna.

The manholes will be surrounded with a 300mm of shingle which will help to distribute the flow uniformly within the soil.

Gravel Trenches

Gravel trenches use a shingle with a perforated drainage pipe at the base. Infiltration takes place on the sides and base of the trench.

Usually, these soakaway drainage pipe and trenches are used to intercept flows.

When use as an infiltration system, they trend to be very long or draining small section of impermeable areas.

Basins or Rainwater Gardens

Basins and rainwater garden are becoming more prevalent due to its dual purpose of infiltration and providing an amenity space.

They also tend to be very efficient at distributing water in the proximity to building and be fully integrated in draught reliance landscape design.

Basins tend to accept greater impermeable areas (more than 100m2) while Rainwater garden are usually used for small roof (50m2) and about 3m away (depending on soil conditions) from buildings.

This a detail of a rainwater garden, there are more rainwater garden details on this page from RHS. 

There are three principal methods to designing a soakaway.

• Method 1: BRE 365
• Method 2: NHBC method
• Method 3: Building Regulations

BRE365 – Soakaway Calculations

The most reliable method is the BRE365. This method is recommended for any size developments and it is approved by all the Lead Local flood authorities and Building regulations and insurers.

The following recommendations are applicable:

• Infiltration on the sides should only be allowed
• The half time of emptying within 24 hours in readiness for subsequent storm inflow
• It should not flood for rainfall events up to 1 in 30-year return period in accordance with Document H of the Building Regulations
• Soakaways can be designed to 1 in 100-year return period plus an allowance for climate change as per the NPPF. In this case, the half empty time must be within 24 hours
• Flooding can occur for the 1 in 100year storm event +CC; however, overflows should be retained within the site and avoid risk to residents and emergency services
• The allowance required to take climate change into account is dependent on the design life of the development. For dwelling it is 40% for industrial building with less that 50 years design life is 30% however each LLFA has their own requirements.

It is recommended that an adequate software system is used to calculate the soakaway including the climate change requirements.

NHBC method

The NHBC only take into account storms of up to 10 years without climate change.

The NHBC method is based on the soakaway rate obtained as per section 2.

The table below determines the size of the concrete ring by knowing the impermeable are to drain in sqm and the time it took for the borehole to drain (as per section 2).

The example in red below shows that an area of 150m2 with a drain time of 900minute will require a concrete soakaway of 2.8m diameter.

Building Regulations

The building regulations soakaway calculations are given in section 3.27 of their regulations. Key points:

• Only used for storms up to 1 in 10 years
• Used for small areas up to 25m2

How can you manage flooding?

Soakaway construction varies depending on:

• The type of soakaway
• Soil conditions
• Depth and size of soakaway 

The following principles are related to geocellular storage system or plastic crates as these products are the most used within the industry. 

Geocellular systems or Plastic Crates

It is critical that the ground preparation and system installation are carried out to appropriate quality control conditions.

This includes installation of 100mm base gravel and levelling of the soakaway to avoid movement.

Detailed guidance on installation is provided by the manufacturer and designer of the soakaway.

After installing the crates. In untrafficked situations, excavations could be backfilled with well compacted selected, as-dug material that does not contain large particles or sharp materials.

In trafficked areas, the use of well compacted backfill and cover such as 6F2 or Type 1 depending on loads and depths. The material should be selected in accordance with standard highway works specifications
Poor quality backfill can cause lateral earth pressures and collapse of the structure.

The following should be avoided:

• Running heavy plant over constructed tanks
• Stockpiling material over them during construction. These loads may have not been included within design calculations
• Runoff should be prevented from entering the modular blocks during construction
• Damage of both the modular structure and the geotextile. Follow-on trades can also cause damage and put the integrity of the structure at risk

As in all construction, installing a soakaway will require a health and safety risk assessment to be undertaken. The risk assessment should identify the hazards and put into place the measures to manage risk.

Maintenance

All soakaways should be provided with some form of inspection access, so that the point of discharge of the drain to the soakaway can be seen.

Lined soakaways (concrete rings) have the advantage of access for inspection and cleaning, and this should be a feature of soakaways.

The location should also be clearly identified on any development plans, therefore allowing a point of reference for future property owners or those involved in maintenance.

Monitoring of soakaway performance can be informative about changes in the soil infiltration rate and in warning of soakaway blockage in the long term.

The inspection access should provide a clear view to the base of the soakaway, even for filled-type soakaways. For small, filled soakaways, a 225 mm perforated pipe provides a suitable inspection well.

Trench-type soakaways should have at least two inspection access points, one at each end of a straight trench, with a horizontal perforated or porous distributor pipe linking the ends along the top of the granular fill.

It may be convenient with a trench-type soakaway to have several drain discharge points along the length of the trench, each connected to the soakaway via an inspection access chamber.

Hope you found this piece informative!