To determine the area of land required a percolation test is essential; the following method should be adopted.
Excavate a hole 300 mm square to a depth 250 mm 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 above the 250 mm level to enable safe excavation to be carried out. Fill the 300 mm square section of the hole to a depth of at least 250 mm with water and allow it to seep away overnight. Next day, refill the test section with water to a depth of at least 250 mm and observe the time, in seconds, for the water 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 1 mm. Take care, when making the test, to avoid abnormal weather conditions such as heavy rain, severe frost or drought. " The percolation test consists of carrying out the above measurement three times, and taking an average of the three results. In the event of any of the measurements in a test being 50% or more above or below the average, make a further three measurements and calculate a further average. Unless an average value of Vp of 24 s/mm or less is obtained (in which case no further tests are needed) make further tests on a minimum of three different locations on the route of a land drain, or at least three tests on separate days on the site proposed for a soakaway" Where deep excavations are necessary a modified test procedure may be adopted using a 300 mm earth auger. Bore the test hole vertically to the appropriate depth taking care to remove all loose debris. Make water level observations referring to a fixed datum using a dipstick or some suitable alternative water level indicator.
The value found in this way is called the percolation value (Vp in s) of the soil and can be used to determine the area of drainage trench floors required to disperse effluents. If the percolation value exceeds 140 s the soil is not suitable for drainfields. From 140 s to 100 s (about 10 h to 7 h to fall 250 mm), underdrains are desirable. Underdrains. Where underdrains are necessary, drainage trenches should be constructed not less than 600 mm deeper than the above trenches, and the lower part filled with pea gravel. A second system of drainage pipes should be laid on the bottom of the trenches to convey surplus drainage to an outfall in a surface ditch or watercourse. Underdrains are costly, and a secondary treatment system able to produce an effluent suitable for surfaces discharge may be preferable. Disposal on land. Disposal of effluent on land involves the distribution of the effluent overground so that loss occurs by seepage into the ground and by evapo-transpiration. The area of land required, which should be divided into two portions to permit each to be rested in turn, will vary according to its nature. Up to 100m2 per person may be required where the ground is of heavy clay. The floor area of subsurface drainage trench (At in m2) required to disperse effluents from septic tanks may be calculated from:
- At = P x Vp x 0.25 where P is the number of persons served by the tank; and Vp is the percolation value obtained as described in the second paragraph
- For effluents, which have received secondary treatment followed by settlement, this area should be reduced by 20%, i.e At = P x Vp x 0.2
"The area determined should be used to calculate either the total floor area of the drainage trench and therefore the length of land drain, or alternatively the floor area of one or more shallow soakaways"
