The Percolation test method explained

The percolation test is the LAST of the 3 mandatory tests to determine if a new or replacement soakaway can be installed. You must do the Groundwater Source Protection Zone search and the Trial Site Assesment Hole first.

There are two very different types of soakaway – foul water soakaways and surface water soakaways. Foul water soakaways deal with small volumes over a long period, whereas the rainwater soakaway may have to handle large volumes in a short time and the percolation test explained here is ONLY for foul water soakaways.( A typical test for a surface water soakaway would be a pit 1000 x 1000 x 2000mm filled to the brim and the level recorded at various times until the pit is empty.) 

Be warned, soakaway crates and tunnels are NOT allowed for septic tank and sewage treatment plant soakaways. They are only for rainwater and surface water soakaways. Beware of the 'Cowboy Traders!'


Percolation Test Stage 1

Percolation Test Stage 2

Percolation Test Stage 3

Percolation Test Stage 3

Percolation Test stage 4 
Percolation Test Stage 5 
(i.e., a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp) required for the water to drop 1mm.

The percolation test calculations for a  soakaway are as follows

Area (A) = V X P X 0.20  for sewage treatment units and

Area (A) = V X P X 0.25 for septic tanks

V = the time is seconds for the water in the test hole to drop by 1mm.

P = the max. number of persons that the unit is designed to serve 

The calculation gives the AREA in SQUARE METRES required for the soakaway trenches.  Further calculations are required to give the length of pipes required depending on the width of the trench, e.g. for a 600mm wide trench, the AREA would be divided by 0.6

Please call us if you are unsure of your calculations as we design the soakaway FREE OF CHARGE

See wastewater legislation 

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