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The Percolation test, what it is it, why it is important and how to do I it 

Do I need a Percolation test? 

A Percolation test is needed where the Septic tank or Sewage treatment plant cannot discharge into a ditch. In these circumstances the discharge must be made directly into the land. A Percolation test is intended to give an idea on the ability of your soil structure to absorb water. This reading will help match the rate and expected volume of discharged water from your Sewage treatment plant to the lenght and dept of any soakaways 

What is a Percolation test. 

1) A hole 300mm square should be excavated to a depth 300mm 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 300mm level to enable safe excavation to be carried out. Where deep excavations are necessary a modified test procedure may be adopted using a 300mm earth auger. Bore the test hole vertically to the appropriate depth taking care to remove all loose debris. 
 
2) Fill the 300mm square section of the hole to a depth of at least 300mm with water and allow it to seep away overnight. 
 
3) Next day, refill the test section with water to a depth of at least 300mm and observe the time, in seconds, for the water to seep away from the 75% full to 25% full level (that is a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp) required for the water to drop 1 mm. 
 
4) The test should be carried out at least three times per trial hole with at least two trial holes. The average figure from the tests should be taken. 
 
5) 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.