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Soakaways and the percolation test


What is a soakaway?tank_with_infill

A soakaway is where sewage effluent is discharged and then soaks into the ground via a pipe when access to a watercourse (a stream, wet ditch or a drainage pipe that leads to a water source) is not possible.

The Soakaway

1.27 Part H2 of the Building Regulations 2000: Drainage and waste disposal  (go to page 32) states that


A drainage field or mound serving a wastewater treatment plant or septic tank should be located:

  • At least 10 m from a watercourse or permeable drain;
  • At least 50 m from the point of abstraction of any groundwater supply and not in any Zone 1 groundwater protection zone;
  • At least 15 m from any building;
  • Sufficiently far from any other drainage fields, drainage mounds or soakaways so that the overall soakage capacity of the ground is not exceeded.

Before a soakaway can even begin to be constructed, 3 tests must be carried out to determine the suitability of the potential site. These tests are designed to conform with Section H2 of The Building Regulations and BS 6297 : 2007 & Amendment 1 2008.

  • Firstly you must find out if the potential soakaway site is an area designated by The Environment Agency as Groundwater Source Protection Zone. If it is then you will be unable to pollute the water underneath with sewage effluent as it is used for drinking water.
  • Secondly, you must dig a Trial Site Assessment Hole. This must be a minimum of 2.1 m deep, and if you hit groundwater or any rock with 2.2 m of ground level then a soakaway will not be permitted.
  • Finally you must perform a percolation test to judge how quickly the soil absorbs 1mm of water. If the effluent soaks away too quickly or slowly then a soakaway will not be permitted.

Further information on and explaining how to perform a percolation test is listed below in more detail. 

The percolation test


Why is it needed?


A percolation test is required to be carried out on the land where the soakaway is intended to be built.  It is a test that determines the rate in which soil absorbs a known volume of water.  This is to ensure that the land (soil) is suitable for a soakaway, and to ensure that the soakaway is designed properly.




How do you carry out a percolation test?


The percolation test method is set out at 1.33 to 1.38 Part H2 of the Building Regulations 2000: Drainage and waste disposal (Go to page 33).  A simplified method is set out below, but for the full method, please click on the link above. 

A percolation test should not be carried out during abnormal weather conditions.


The percolation test method


  1. Excavate hole 300 mm square and 300 mm below the proposed outlet from the sewage treatment plant.
  2. Bore the test hole vertically to the appropriate depth. Remove all loose debris.
  3. Fill the test hole with water to a depth of at least 300 mm. Allow to seep away overnight.
  4. Next day, refill the test section with water to a depth at least 300 mm. Observe the time, in seconds, for the water to seep away from 75% full to 25% full.
  5. Divide this time by 150 mm.
  6. The answer gives average time in seconds (Vp) required for the water to drop 1mm.
  7. Carry out the test at least 3 times, with at least 2 trial holes.
  8. The average figure from the tests should be taken.
  9. The average value of Vp should be between 12 and 100 and the preliminary site assessment report and hole tests favourable to use drainage field disposal.
  10. The minimum value ensures untreated effluent cannot percolate too rapidly into groundwater. Where Vp is outside these limits, effective treatment is unlikely to take place. However, where there is an alternative form of secondary treatment to treat the effluent, it may still be discharged into a soakaway.


PLEASE NOTE that some Building Regulation Authorities will not accept your own readings from the test.  Therefore, you may be required to contact a contractor to carry out the percolation test.  Please check with your Local Authority.


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