A Percolation test (also called a porosity test) should be carried out prior to the installation of a septic tank and / or drainage field.
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​The purpose of the test is to establish the suitability of the ground for the installation of the drainage field.
Sometimes the ground can be unsuitable if there is a high water table or the soil is mainly clay.
If the soil is suitable, the test will prove the rate at which the water will disperse.
Depending on what the calculation determines we will be able to design a drainage field accordingly.
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All tests should be carried out during normal weather conditions and if undertaken in summer an allowance should be made for winter weather.
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The test is carried out over two days, the first day is to saturate the ground and the second to record the seepage.
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​DAY 1

• Excavate three holes approx 1m x 1m and 1m deep. This allows for the smaller excavation of 300mm square and 300mm below the proposed outlet from the septic tank in which to measure the perosity.
• Dig the test hole to the correct depth. Remove all loose debris.
• Fill the test hole with water and allow to seep away overnight.

​DAY 2

• Refill the test section with water. Observe the time, in seconds, for the water to seep away from 75% full to 25% full.
• Divide this time by 150mm.
• The answer gives average time in seconds (Vp) required for the water to drop 1mm.
• Carry out the test at least 3 times to take the average on each excavation.

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If the average value of Vp falls below 15 or over 100 the ground will not be suitable for a drainage field.

The installation of a packaged treatment plant may be a better option in this situation.

The drainage field must be a minimum of 10m from a watercourse, 15m from a building and 50m from a borehole or spring.
Also they should not be constructed under driveways, parking areas or buildings.
Tress in the vicinity will potentially cause a problem.

For full building regulations please click below.

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