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DETERMINATION OF SOIL ABSORPTION CAPACITY
Percolation test is done to determine the absorption capacity of soil. This test is important for the design of septic system. This test is also carried out while purchasing land, to understand the behavior of soil under moisture conditions. This test is also called as perc test. Every country may have different regulation for the exact calculation of length of line, depth of pit, etc, but the procedure for the percolation test is same. In India, this test is carried out as per IS2470- Part-II.
As per IS2470- Part-II, the percolation rate is time required in minutes for water to fall 25 mm in the test-hole. A test in trial pits at more than one place in the area should be undertaken to permit deriving an average figure for percolation rate.
PROCEDURE FOR PERCOLATION TEST:
Percolation test is done to determine the permeability of soil at the depth at which the effluent needs to be disposed.
A 100 to 300mm diameter or side of a circular or a square hole is bored to the required depth of proposed absorption test. The bottom of the hole is carefully scratched to remove any smeared soil surface and to provide a natural soil interface into which water may percolated.
All the lose material is removed from the hole and coarse sand or fine sand of 50mm thickness is added to protect the bottom of the bore from scouring.
Water is then poured upto a minimum depth of 300mm over the gravel.This test shall be carried out during the wettest season of the year to make sure that the soil has ample opportunity to swell. The percolation of the soil is determined after 24 hours of the adding water in the bore. If the water remains in the test hole after the overnight swelling period, the depth is adjusted to 150 mm over the gravel. Then from fixed reference point the drop in water level is noted over a 30 minute period. This drop shall be used to calculate the percolation rate.
If no water remains in the hole, water is added to bring the depth of the water in the hole till it is 150 mm over the gravel. From a tied reference point, the drop in water level is measured at 30 minutes intervals for 4 hours, refilling 150 mm over the gravel as necessary. The drop that occurs during ' the final 30 minutes period is used to calculate the percolation rate. The drops during prior periods provide information for possible modification of the procedure to suit local circumstances.
In sandy soils or other porous soils in which the first 150 mm of water seeps away in less than 30 minutes after the overnight swelling period, the time interval between measurement is taken as 10 minutes and the test run for one hour. The drop that occurs during the final 10 minutes is used to calculate the percolation rate.
Percolation Rate -Based on the final drop, the percolation rate, that is, the time in minutes required for water to fall 25 mm, is calculated.