Percolation Test


Below is a guide of how to perform a percolation test to check the suitability of your ground for a soakaway and the drainage area required.

You can download a calculation form HERE.

  1. Excavate at least two holes, 300mm square to a depth of at least 300mm below the proposed invert level (bottom of the infiltration pipe) and space them evenly along the proposed line of the subsurface irrigation system.

  2. Fill each hole with water to a depth of at least 300mm and allow this to seep away overnight.

  3. If the water drains rapidly, within 10 minutes, the hole should be refilled up to a maximum of 10 times. If the water continues to drain away rapidly, the ground is unsuitable.

  4. If the water has not soaked away within 6 hours, the area is not suitable.

  5. Next day, refill each hole with water to a depth of at least 300mm and observe the time in seconds for the water to seep away from 75% full to 25% full (i.e. depth of 150mm).

  6. Divide this time in seconds by 150. This gives the average time in seconds (Vp) required for the water to drop 1mm.

  7. Repeat the test at least three times in each hole.

  8. Take the average figure from the tests to produce the percolation value Vp (in seconds).

  9. Obtain the average figure for the percolation value (Vp) by summing all the values and dividing by the number of values used.

  10. Record these results in the form provided. These may be required by the regulator and the property owner.

  11. Where the Vp results vary widely (50% above or below the average figure), make further tests on a minimum of three different locations in the area of the proposed drainage field.

  12. Drainage field disposal can only be used when percolation tests indicate average values of Vp between 15 and 100 and the preliminary assessment of the trial hole tests has been favourable.

  13. The minimum value of 15 ensures that untreated effluent cannot percolate too rapidly into groundwater.

  14. Where Vp is above the limit of 100, effective treatment is unlikely to take place in the drainage field as there will be inefficient soakage leading to wastewater ponding on the surface.

The Vp is used to determine the total floor area of the drainage trenches and therefore the total length of irrigation drain.
For domestic premises, the floor area of the drainage field required may be calculated as follows:

A = p x Vp x 0.25 for septic tanks

A = p x Vp x 0.20 for package sewage treatment plants (i.e. 20% less, because the effluent has received additional treatment)

A = required drainage field floor area in square metres (m2).

P = number of people served by the tank (for domestic applications this should be the maximum number of people that could live in the dwelling).

Vp = percolation value.

The calculated area A should be converted to an amount of linear trench based on the width of the trench which is usually between 0.3m to 0.9m. The layout of the trench network will depend upon the soil porosity and the availability of land but the legs of the trenches should be connected so as to form complete loops.