A percolation test is a test to determine the absorption rate of soil for a septic drain field or “leach field”. The results of a percolation test are required to properly design a septic system. In its broadest terms, percolation testing is a simply observing how quickly a known volume of water dissipates into the subsoil of a drilled hole of known surface area. While every local authority will have its own laws regarding the exact calculations for the length of line, depth of pit, etc the testing procedures are generally the same. In general, sandy soil will absorb more water than soil with a high concentration of clay or where the water table is close to the surface.
A percolation test consists of digging holes in the soil of the proposed drainage area to a specified depth, pre-soaking the holes by maintaining a high water level in the holes, then running the test by filling the holes to a specific level and timing the drop of the water level as the water percolates into the surrounding soil. There are various empirical formulae for determining the required size of a leach field based on the size of facility, the percolation test results, and other parameters. The drainage or percolation area is designed based on these test results. For leach line testing, test holes are drilled, most commonly twelve inches in diameter, Ideally, these should be drilled to depths matching your trench invert level. For better, more conclusive results, holes are used in a pattern of one hole at each corner of the proposed leach field and one test hole in the centre. Testing of these holes will result in a value with units of minutes per inch. This value is then correlated to a predetermined county health code to establish the exact size of the leach field. After the test is completed the results are used to design a suitable polishing filter or percolation area for the site, though the end result is a different type of septic system, established through a different calculation if needs be.
Finally, for leach line systems and horizontal seepage pits, a “deep hole” (TRIAL-HOLE) is drilled to find the water table or the approximately twelve feet (dry). Exact depths will again depend on local health codes. In the case of a vertical seepage pit, local groundwater data may be used, or if the drill hole reaches groundwater, the pit will be backfilled again according to county health codes. In Ireland there is a requirement for an area to be mapped around the site, photographic recording of the test, BS5930 soil analysis, detailed test reporting as well as level related detailed sewer system construction drawings to be included in the full site assessment report.