Percolation Test

A percolation test is conducted with an objective to gauge the rate at which water gets absorbed in the soak away area. Knowledge of the rate of water absorption is used for designing a sewage treatment facility or private septic tank. This test helps us understand whether a particular soil is suitable for draining out water efficiently.

One can devise a proper design for different septic system types on the basis of results of percolation test (also known as 'perc' test). Suitability of the site for effluent disposal can, therefore, be determined with this test.

The dimensions of area used and holes drilled for this test might differ. However, the basic idea (measuring the percolation rate) behind conducting it, remains the same. Here is a step-by-step explanation for running a percolation test.

• Number of holes to be drilled may vary for different tests. However, drilling a minimum of 3 holes is mandatory in any given test. The commonly used leach line testing makes use of 3 holes.

• Diameter of holes range from 6-8 inches. Accuracy of test results is directly proportional to the number of holes being drilled.
• Different patterns are used to drill holes for the perc test. Holes are mostly drilled in a straight line. In some cases, 5 holes are drilled for testing; 1 hole is drilled in each corner of the testing area and fifth at the center.

• The depth of holes can vary, but it cannot be greater than 24 inches.
• Holes are filled with water up to the point of saturating the soil. Water is then allowed to drain away completely. Refilling is done after this procedure.
• Rate at which water percolates down the holes is calculated next in this activity. Drop in the level of water is measured at regular intervals in a total time period of 30 to 60 minutes.

• The value of slowest uniform rate of percolation is important from the point of designing a proper septic or sewage system. The unit for calculating this value is minutes per inch. This value indicates the number of minutes required for water level to drop by one inch.
• The value of water percolation rate is used in the calculation of area required for septic tank construction.

These pits are characterized by their large size. The size of holes drilled for these pits is different from that in other methods. Holes can be 3-4 feet in diameter; their depth can range from 20-30 feet. Fire hose is used to fill these vertical seepage pits. Rate at which water level drops down is measured after filling these pits.

The number of holes drilled in a horizontal pit range from 5 to 8. These holes are dug 3-10 feet below the ground surface. It is important to note that method used for calculation of water absorption remains the same, irrespective of the pit size and holes being dug.

The terms, soakaway area and area for subsurface drainage mean the same. Activity of determining the soakaway area gives you an idea about the space to be reserved for septic tank. The formula, At = P x Vp x 0.25 is used to find the area for septic tank.

The symbol, 'Vp' is used in reference with value of percolation rate in sec/mm. The symbol of 'P' stands for total number of people served with the proposed septic tank. 'At' stands for the area in (sq. meters) required for construction for subsurface drainage trench. The story on septic tank costs should help in determining the budget of such projects.

The percolation test is important to determine whether a particular area can be successfully converted into a soakaway. Apart from the rate of water absorption, many other factors are taken into account. The level of water table and seasonal variations are only a few of these.

Although different methods and formulas are used to determine area for septic tank construction, the information presented in this article should give you a general idea of the procedure being followed.