Soakaway Testing & Design

Soakaway testing
Soakaway Testing

Soakaway Testing

What is a Soakaway?

A soakaway is an effective method to store excess water run off and control the infiltration into the surrounding soils rather than the local sewage system or water courses.  A typical soakaway should store the immediate volume of water for an impermeable area expected of typical storm for that location.  This water should then infiltrate into the surrounding soil completely over a 24-hour period in preparation for the next storm.

There are two types of soakaways: Conventional or Borehole Soakaway

  • Conventional Soakaway; comprises a partially perforated cylinder, or rectangular chamber.
  • Borehole Soakaway: deep bored hole is lined with plastic or steel pipe and is used to transfer water from a storage area to depth. These are typically used when impermeable strata overlie permeable strata.

Infiltration Testing and Design

There are three methods used to determine the properties of the soil surrounding the soakaway in the form of an infiltration test.

The most widely known is to carry to your test to BRE Digest 365, in this method a trial pit is excavated and filled with water, the rate at which the water dissipates is used to calculate the infiltration rate.

An alternative, but very similar method is the one used primarily for designing the soakaways from septic tanks, BS6297:2007 (+A1:2008).

For conventional soakaways using BRE365, trail pits are excavated to reach suitable representative strata for the infiltration test to be conducted in. The trail pit is filled with clean water from a bowser or tanker and the time taken for the water level to fall is recorded. An infiltration test is considered successful if the water volume decreases to 25% of the initial volume. The rate of decrease between 75% and 25% is then used to determine the infiltration rate.

Depending on the rate of infiltration of the strata, the test should be repeated two to three times, to ensure an accurate infiltration rate can be calculated. Upon completion of the infiltration test any remaining water is pumped out and the trail pit backfilled with arisings and made safe.

For borehole soakaways, a test borehole is drill and a variable head test is conducted upon completion or at intervals within each stratum.  There are two types of infiltrations tests that can be carried out in a borehole, the most typical one is a falling head test, which is conducted in a borehole above the water table, by filling the borehole with clean water and recording the depth of water at intervals within an hour.

The other test is a rising head test, which is conducted in a borehole with a high standing ground water, in which the water is pumped out and the rising water is recorded over time until it has reached its original level.

For a more accurate result, these tests should be repeated two to three times, to ensure an accurate infiltration rate can be calculated.

Many local authorities often require developers to have considered and demonstrate a sustainable urban drainage system (SUDs) before approving planning permission. This can be achieved by undertake infiltration testing to assess the suitability of soakaway installation.


Soakaways are designed to ensure that they can hold sufficient water volume that the site can generate. This water volume is calculated based on the area of impermeable ground for the site and the typical amount of rainwater generated from the typical storm of the site location and other water sources. Thus, by calculate inflow versus outflow based of the drained characteristic the size of the soakaway can be determined for the site.

GES can provide the necessary information to allow for the design for a soakaway at your site.



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