Percolation test for proposed surface water soak away or French drain (for areas under 25sq
Guidance Notes for this Test Method For Rain Water Disposal under 25 square meters.
1. A hole 300mm square should be excavated to a depth 300mm below the
Proposed invert level of the incoming pipe or the level at which the French drain sits. Where deep drains are necessary, the hole should conform to this shape at the bottom, but may be enlarged above the 300mm level to enable safe excavation to be carried out. Where very deep excavations are necessary, you should seek specialist advice.
2. Fill the 300mm square section of the hole to a depth of at least 300mm
with water and allow it to seep away overnight.
3. Next day, refill the 300mm square section of the 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 level (i.e. a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp)required for the water to drop 1mm. For example 60mins divided by
150mm = 24 seconds (Vp = 24 seconds).
4. The test should preferably be carried out at least three times with at
least two trial holes. The average figure from the tests should be taken.
The test should not be carried out during abnormal weather conditions
such as heavy rain, severe frost or drought.
Where the test is carried out as described above, the soil infiltration rate (f) is related to the value (Vp) derived from the test by the equation:
Formula to calculate the Soil Filtration rate
The storage volume should then be calculated so that, over the duration the storage volume is sufficient to contain the difference between the inflow volume and the outflow volume.
The inflow volume Percolation test is calculated from the rainfall depth and the area drained. The outflow volume (O) is calculated from the equation:
O = as50 x f x D
Where as50 is the area of the side of the storage volume when filled to 50% of its effective depth (for example (1m2/2) x 4 = 2m2) and (D) is the duration of the
storm in minutes (for example 5 minutes).
Drained area = 25m2
Incoming water is 25m2 area x 10mm of rainfall = 0.25m3 of water to be disposed of.
Outflow volume is: O = as50 x f x D therefore O = 2x 0.0002 x 5 and thus O = 0.002m3 (Meters Cubed. The 3 stands for Cubed)
Therefore required capacity is 0.25m3 - 0.002m3 = 0.248m3. Which means that a traditional rubble filled soak away, measuring 1m3 below the inlet pipe and with Say, 20% void, i.e. 0.2m3 storage capacity, will not be adequate and the soak away volume should increase to 1.24m3 capacity.
This information is mainly based on the advice contained in section H2 of
Building Regulation. Approved Document H 'Drainage and Waste Disposal Percolation test 2002 edition. This leaflet does not substitute for the Approved Document, or BRE Digest 365, which should be referred to for full information about the sizing and design of soak ways.
For those of you who do not have the patience for the Percolation test there is an easier way.
Dig down to where your French drain pipe is going to sit and dig one extra foot deeper, square it off so it’s a foot square.
Now fill this hole to the top with water and leave overnight, do the same the next day if the waters gone by night fall that’s a great sign the drain will be fine.
If not give it till the morning if it’s still not gone, don’t worry you may have to carry the drain further i.e. an extra 3-5 meters.
finished reading the Percolation test