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
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
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
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