The following procedure should be used to determine the area of trench floor required for disposal of effluent via a sub-surface irrigation system.

Excavate a hole 300mm square to a depth of 250mm below the proposed invert level of the land drain. Where deep drains are necessary the hole should conform to this shape at the bottom but may be enlarged above the 250mm level to enable the excavation to be carried out safely. Alternatively, a 300mm auger may be used to bore a hole to the appropriate depth, taking care to remove all loose debris.

Fill the hole to a depth of at least 250mm with water and allow it to soak away overnight.

Next day, refill the test section with water to a depth of at least 250mm and record the time, in seconds, for the water to seep away completely. Make water level observations referring to a fixed datum using a dipstick or other suitable water level indicator.

Divide this time by the depth, in millimetres, of water placed in the hole. This figure gives the average time for the water level to drop 1mm and is called the ‘percolation value’ (Vp).

If the percolation value exceeds 100, ground conditions may be unsuitable for discharge from a septic tank system and alternative means of disposal should be considered to avoid surcharging the tank or ponding of septic effluent on the surface.

For residential properties the area of trench floor required can be calculated using the following formula:

A = P x Vp x 0.25

where:
A = area of trench floor (in square metres)
P = number of persons served by the tank
Vp = percolation value

Repeat the test at least three times, preferably in several locations, and use the average percolation value in the calculation. Avoid carrying out the test under abnormal weather conditions, such as heavy rain, severe frost or drought.