Percolation Test To BS6297 : 1983
To determine the area of land required, a percolation test is essential; the following method should be adopted:
Evacuate a hole 300mm square to a depth of 250mm below the proposed invert level of the land drain. Where deep drains are necessary, the hole should conform to this shape at the bottom but may be enlarged about the 250mm level to enable safe excavation to be carried out. Fill the 300mm square section of the hole to a depth of at least 250mm with water and allow it to seep away completely.
Divide this time by the depth in millimetres of water placed in the hole. The answer gives the average time required for the water to drop 1mm. Take care, when making the test to avoid abnormal weather conditions such as heavy rain, sever frost or drought. Carry out the test at least 3 times and take the average figure.
Where deep excavations are necessary a modified test may be adopted using a 300mm earth auger. Bore the test hole vertically to the appropriate depth taking care to remove all loose debris. Now make water level observations referring to a fixed datum using a dip stick or some suitable alternative water level indicator.
The value found in this way is called the percolation value of the soil and can be used to determine the area of drainage trench floors required to disperse effluents. If the peculation value exceeds 140, the soil is not suitable for land. From 140 to 100, land drains are desirable.
The floor area of subsurface drainage trench required to disperse effluents from the treatment plant calculated from:
At = p x Vp x 0.2
P is the number of persons served by the tank and
VP is the percolation value obtained as describe.
The area determined should be used to calculate either a length of land drain, or the floor area of one or more shallow soakaways. Drainage trenches should be from 300mm to 900mm wide and areas of undisturbed ground should be maintained between parallel trenches.