A percolation test is used calculate if 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 over night

**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**: Repeat the procedure (if necessary in two more test holes) and calculate the average

**Step 5**: Calculate the Vp (average time in seconds for the water to drop 1mm) for example: If the average time for the 3 test holes was 2700 seconds

- divide 2700 / 150mm (depth of water) = 18 Vp* (* Vp should range between 12 and 100, if not then the system should be designed by a drainage specialist)
- Area of drainage (At) = 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 meters squared
- 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 meters squared / 0.6m = 37.5m
- Minimum permitted area is 30m long by 0.6m wide

See Building Regulations Part H2, sections 1.26 to 1.44 (page 29 – 36) for more information or contact us.

**For a surface water soakaway.**

If the Vp calculation exceeds 140 seconds then BS6297 would consider the ground unsuitable.

To determine the size (volume) of a soakaway the calculation is Volume of soakway = Area to be drained x (Rainfall Rate / 3000)

See Building Regulations Part H3 (page 39 – 47) for more information regarding rainwater drainage