Percolation Test Method

A preliminary assessment should be carried out including consultation with the Environment Agency and local authority to determine the suitability of the site, the natural vegetation on the site should also give an indication of its suitability for a drainage field.

A trial hole should be dug to determine the position of the standing ground water table and the trial hole should be a minimum of 1m2 in area and 2m deep, or a minimum of 1.5m below the invert of the proposed drainage field pipework.

The ground water table should not rise to within 1mtr of the invert level of the proposed effluent distribution pipes. If the test is carried out in summer the likely winter groundwater levels should be considered.

A percolation test should then be carried out to assess the further suitability of proposed area.

A hole 300 mm square should be excavated to a depth 300 mm below the proposed invert level of the effluent distribution pipe., where deep drains are necessary the hole should conform to this shape at the bottom, but may be enlarged above the 300 mm level to enable safe excavation to be carried out.

Where deep excavations are necessary a modified test procedure may be adopted using a 300 mm earth auger, bore the test hole vertically to the appropriate depth taking care to remove all loose debris. Fill the 300 mm square section of the hole to a depth of at least 300 mm with water and allow it to seep away overnight.

Next day refill the test section with water to a depth of at least 300 mm and observe the time in seconds for the water to seep away from 75% full to 25% full level (i.e. a depth of 150mm). Divide this time by 150mm and the answer gives the average time in seconds (Vp) required for the water to drop 1 mm.

The test should be carried out at least three times with at least two trial holes, the average figure from the tests should then be taken.

The test should not be carried out during abnormal weather conditions such as heavy rain, severe frost or drought.

Drainage field disposal should only be used when percolation tests indicate average values of Vp of between 12 and 100 and the preliminary site assessment report and trial hole tests have been favourable. This minimum value ensures that untreated effluent cannot percolate too rapidly into ground water.