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Should you include soakaway testing in your ground investigation?

As of April 2015, any development is required to consider the potential use of Sustainable Urban Drainage Systems (SUDS). As a result, agb Environmental has started to see a significant increase in the number of ground investigations specifying soakaways testing; an important step in assessing the viability and design of SUDS. We discuss the background to why soakaway testing is important, and how the data is used.

Background to Soakaways
The surface water drainage arrangements for any development site should be such that the volumes and peak flow rates of surface water leaving a developed site are limited and in some cases no greater than the rates prior to the proposed development.

For new developments, it may be necessary to provide surface water storage and infiltration systems to limit and reduce both the peak rate of discharge from the site and the total volume discharged from the site.

Discharging surface water into a drain of a development will only be permitted if other SUDS techniques have been considered and prove to be unsuitable.

In considering the disposal of surface water from a site, Part H of the Building Regulations requires that the first choice of surface water disposal should be to discharge to infiltration systems where possible.

There are many types of infiltration systems available, but soakaways are the most commonly used type in the UK. They store rapid runoff from a single house or from a development and allow its efficient infiltration into the surrounding soil.

Soakaways can reduce the volume of water that needs to be disposed of by downstream drainage and facilitates groundwater recharge.

The time taken for storm water to exfiltrate through the base and / or sides of the device will depend upon the soakaway shape and size, and the infiltration characteristics of the surrounding soil.

The Soakaway Test

A soakaway test is a method for calculating the infiltration characteristics (specifically the permeability) of the ground in preparation for a possible soakaway within a surface water SUDS. Understanding the permeability of the ground is a critical step in ensuing that the correct drainage system is chosen, and is now forming a key step in obtaining planning permission.

BRE Digest 365 details a test method to allow the calculation of the soil infiltration rate and has generally been adopted as the standard for soakaway testing. This test requires that a trial pit is excavated and accurately measured and is then filled with water. The rate of the falling water level can then be used to determine the soil infiltration rate. To meet BRE 365, ideally this test should be repeated three times to establish a realistic value. There are numerous technical requirements to this approach, including size of the pit, what to do if the pit may collapse, the depth to groundwater etc.

There are alternative tests which provide lower quality data, and in some cases are technically incorrect, therefore specifying a soakaway to BRE 365 is the best option to obtain the data needed.

When Soakaway Testing Isn’t Required

The two key exceptions to this are:

  • A site investigation (excluding a soakaway test) can clearly demonstrate that the ground conditions are unlikely to support soakaways. For example, if the ground conditions are known to be practically impermeable or groundwater levels are particularly high. This should be proved through appropriate ground investigation and testing.
  • If, during the soakaway test, the water level is not dropping, or is dropping at a very slow rate, the completion of the soakaway would be impractical from a time perspective.

The resultant soakaway data is then used within the drainage strategy to establish various design requirements. With the prevalence and heightened attention on surface water flooding, a surface water drainage strategy will be scrutinised by the regulators.

agb Environmental Ltd has significant experience of undertaking soakaway testing to BRE 365, providing advice based on ground investigations and a preliminary assessment within Phase 1 Desk Studies. The agb Environmental team can advise on the requirement for soakaway testing when scoping a ground investigation.

If you would like to understand your requirements for soakaway testing or any other aspect of ground investigations, please contact Alex Brearley via email on or by calling 01638 663226.