How to Carry Out a Percolation Test

aimee huxley

Why do a Percolation Test?


A percolation test is required to be carried out on the land where the septic tank system or sewage treatment plant soakaway is to be installed. It is a test that determines the rate in which soil absorbs a known volume of water. This is to make sure that the soil is suitable for a soakaway, and to make sure that the soakaway is designed correctly.


A percolation test is ALWAYS required for septic tanks.


The Percolation Test Method

  1. Excavate a hole 300mm square and 300mm below the proposed outlet from the septic tank or sewage treatment plant.

  2. Dig the test hole vertically to the proper 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 is taken.

The average value of Vp must be between 15 and 100 and the preliminary site assessment report and hole tests favourable to use drainage field disposal.


The minimum value ensures untreated effluent cannot percolate too rapidly into groundwater before tertiary soil treatment has taken place. Where Vp is outside these limits, effective treatment is unlikely to occur. However, where there is an alternative form of tertiary treatment to treat the effluent, it may still be discharged into a soakaway.


NOTE: Some Councils insist that Percolation Tests are carried out by 'qualified persons'. You may be required to contact a contractor to carry out the percolation test. Please check with your Local Authority.


A Percolation Test fails if the results are under 15 or over 100 seconds/mm drop.


Please contact us for further information.


Tel: 01948 840030

A percolation Test fails if the results are under 15 or over 100 seconds/mm. drop.


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