A percolation test is used to work out whether the ground is suitable for a drainage field for septic tanks and sewage treatment plant systems or a soakaway for surface water and is done as follows:
Step 1: Excavate a test hole 300mm square x 300mm deep below the proposed invert level
Step 2: Fill the test hole with water and allow to drain away overnight
Step 3: Refill to a depth of 300mm and note the time taken in seconds to drain away from 75% full to 25% full (i.e. 150mm drop in level from 225mm to 75mm)
Step 4: Carry out the test at least three times, using at least two test holes, and take the average. Do not test during abnormal weather such as heavy rain, severe frost or drought
Step 5: Calculate the Vp (average time in seconds for the water to drop 1mm) for example: If the average time from the tests was 2,700 seconds
- divide 2700 / 150mm (depth of water) = 18 Vp* (* a drainage field should only be used when the average Vp is between 12 and 100. Outside this range, effective treatment in a drainage field is unlikely and another form of secondary treatment will be needed)
- Area of drainage field (At) for a septic tank = number of persons to use property (p) multiplied by Vp multiplied by 0.25 i.e. 5 persons x 18 x 0.25 = 22.5 square metres
- To calculate the total length of the trench or trenches divide the Area of drainage by trench width depending on the size of the digger bucket i.e. 22.5 square metres / 0.6m = 37.5m
- Trenches should be 300mm to 900mm wide, with 2 metres of undisturbed ground between parallel trenches, and laid at a uniform gradient no steeper than 1 in 200 (paragraphs 1.40 to 1.42)
See Building Regulations Part H2, paragraphs 1.26 to 1.44 (pages 31 to 33) for more information or contact us.
For a surface water soakaway.
The percolation test result (Vp) is also used to work out the infiltration rate of the soil for a surface water soakaway (Approved Document H3, paragraph 3.28).
The calculator below gives a rough first estimate only: Volume of soakaway = Area to be drained x (Rainfall rate / 3000). Approved Document H says soakaways should be designed for a storm return period of once in ten years. For small soakaways serving 25 square metres or less, a design rainfall of 10mm in 5 minutes may be assumed. Soakaways serving larger areas should be designed in accordance with BS EN 752 or BRE Digest 365 (paragraphs 3.27 to 3.30).
See Building Regulations Part H3 (page 39 – 47) for more information regarding rainwater drainage