Perc Test | Septic System Design
Perc Tests & Soils Testing are conducted in order to determine if sewage from a septic system will soak (percolate) into the ground in an onsite septic system. While percolation testing and soil borings are not technically considered land surveying, land surveyors are one of three professionals that are qualified to perform these tests.
Professional Soil Classifiers and Professional Engineers (with soils experience) are also qualified to determine the suitability of soils for a septic system. In Alabama they can evaluate the soils without performing a perc test.
If a perc test (sometimes spelled perk) is carried out by a surveyor, the first step is to determine at what depth to do the test. The soil boring helps the surveyor determine this depth. The soil boring might indicate water or rock, a limitation to the placement of field lines. Once that is determined the perc test hole is dug out to that level.
We then put a little gravel in the bottom of the hole and fill the hole up and keep it saturated for at least 4 hours. After the saturation time, the water in the hole is taken down to about 6-inches depth. We then measure how much the water falls during a 30 minute period. This measurement should be done for 4 hours, or until the absorption rate stabilizes.
The percolation test results will determine the length of field lines needed.
Alabama Department of Public Health – Appendix A Table 3
This table provides the required length of field lines based on the perc rate obtained.
Using the above Table 3, a 3 bedroom home with a percolation rate of 50 minutes per inch would require 300 linear feet of effluent field lines. This is for conventional gravel field lines in a 36-inch wide trence.
If you choose to use another approved field line type (EZ Flow, Infiltrator, Multi-pipe) the length may be able to be reduced. Speak to your surveyor and/or the Health Department about this. Make sure that your septic installer is familiar with the type of field lines chose.
If you are in a situation where a conventional septic system won’t work, an Engineer designed septic system may be required to overcome your site limitations. Some of these limitations are slow percolating soil, high ground-water, rocky soils, steep slopes, or limited area to place the septic system.