trial&error
Senior Member
According to Polk county health department you do not do a perc test. Where you dig a hole and pour water and measure how long til it drains. You have a soil scientist do a soil morphology where they examine the different types of clay present and tell you where and how deep you must put your drain field. Mine had to be installed 6 ft deep due to varying clay topographies. https://polkcountyhealthcenter.org/septic-permits-and-inspections/The Soil scientist performs a perc test.
Too much clay can ruin a site and cause it to not perc.