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 300mm square and 300mm 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 300mm. Allow to seep away overnight.

4)           Next day, refill the test section with water to a depth at least 300mm. Observe the time, in seconds, for the water to seep away from 75% full to 25% full.

5)           Divide this time by 150mm.

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.