DETERMINATION OF SOIL ABSORPTION CAPACITY
PERCOLATION TEST
The percolation test, also known as a perc test, is an important method used to determine the absorption capacity of soil. It is commonly performed for designing septic systems and assessing the behavior of soil under moisture conditions when purchasing land. The procedure for conducting the percolation test is generally the same across countries, although specific regulations regarding parameters such as the length of the test line and depth of the pit may vary. In India, the percolation test is carried out according to the guidelines provided in IS2470-Part II. In this standard, the percolation rate is defined as the time required in minutes for water to fall 25 mm in the test-hole. It is recommended to conduct the test at multiple trial pits within the area of interest to obtain an average figure for the percolation rate.
PROCEDURE FOR PERCOLATION TEST:
The percolation test is conducted to determine the permeability of soil at the proposed depth for effluent disposal. It involves boring a circular or square hole with a diameter or side length of 100 to 300mm to the required depth. The bottom of the hole is carefully cleared of any smeared soil surface to create a natural soil interface for water to percolate.
After removing loose material from the hole, a layer of coarse or fine sand, about 50mm thick, is added to protect the bottom of the bore from scouring. Water is then poured into the hole to a minimum depth of 300mm above the gravel. This test is ideally performed during the wettest season of the year to ensure that the soil has sufficient opportunity to swell.
The percolation of the soil is determined after a 24-hour period from adding water to the bore. If water remains in the test hole after the overnight swelling period, the water depth is adjusted to 150mm above the gravel. The drop in water level is then measured from a fixed reference point over a 30-minute period, and this drop is used to calculate the percolation rate.
If no water remains in the hole, water is added to bring the water depth to 150mm above the gravel, and the drop in water level is measured at 30-minute intervals for 4 hours from a tied reference point. The drop that occurs during the final 30-minute period is used to calculate the percolation rate. The drops during earlier periods provide information for possible modifications of the procedure to suit local conditions.
In sandy or porous soils where the first 150mm of water seeps away in less than 30 minutes after the overnight swelling period, the time interval between measurements is reduced to 10 minutes, and the test is run for one hour. The drop that occurs during the final 10-minute period is used to calculate the percolation rate, also known as the time in minutes required for water to fall 25mm.