Test Holes and Perc Tests


   Test Holes are holes dug with a backhoe in the area where new or additional leaching might be added. They are for visual inspection and are backfilled as soon as the information is recorded.

   With test holes we are looking for:
   1. The depth, or distance from the top of the ground, to ledge rock. Code requires the bottom of the leaching to be four feet above ledge rock.
   2. The depth to water or mottling. Code requires the bottom of the leaching to be eighteen inches above "maximum groundwater level".
   3. The soil types and where they change.

     Test hole results might look like:
   12 - 30" red -brown loam
   30 - 96" medium to coarse sand with small stones
   no ledge, no water, no mottling, roots to 58"

   The example shows ideal conditions for "subsurface sewage disposal". The extent of root penetration is a good indicator of two things: a "restictive layer" (see "Groundwater...") or too little water for the roots to go down any further. A restrictive layer, while stopping water from soaking down further, also prevents roots from seeking the soils below. In the example above however, there is so little moisture in the soil below 58" that the roots will not penetrate further.

Perc tests, or percolation tests, are small diameter holes often dug with a post-hole-digger. They are used to measure the rate water soaks down ("percolates") at the depth where the leaching will occur.

The hole is "presoaked" by filling it with water from a hose or container. It is later re-filled and a measurement is taken of the distance from a known point (often a stick inserted in, or laid across, the top) to the water level. Ten minutes later another measurement is taken and recorded. This is repeated every ten minutes for over an hour as the water level drops. The final reading determines the perc rate.

   Perc rates are stated as ten minute intervals, the best rate being 1" in 10 min..* That means that the water in the perc hole dropped at least one inch during the final ten minute segment. The next best rate is 1" in 20 min., and so on till 1" in 60 min.. Soils that percolate slower than 1" in 60 minutes are not suitable for leaching systems.

   Perc rates are the way we size the leaching area of a septic system. The slower the soakdown capability of the soil, the greater the area needed for soakdown. (see page titled "Sq. Ft. of Effective Leaching Needed")
   Perc test data is shown on plans as two vertical columns - the time and the measurement - such as:
   12:48 - 10 1/4"
   12:58 - 12 3/4" etc. for 6 or 7 readings

*A perc rate of 1" in 5 minutes, though no longer allowed for leaching calculation, may be used for M.L.S.S. purposes. (see M.L.S.S. page)


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©2019 by G. Scully