Perc Test Hole Specifications for Soil Percolation Tests for Septic Field Design

  • PERC HOLE SPECIFICATIONS - CONTENTS: What are the specifications for soil perc test holes: how deep, what size, etc. What are "perc tests" or soil percolation tests? How do we perform a soil perc test when designing or testing a septic system?
  • POST a QUESTION or READ FAQs about the specifications for septic system test hole tests, deep hole tests, and percolation tests for drainfield qualification and soil testing

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This septic system design chapter provides specifications for soil percolation holes used for testing for septic system absorption system or drainfield design. We explain how to dig a hole for soil percolation tests, deep hole tests, and perc testing where there is rocky soil. We provide perc test safety recommendations.

Soil perc tests may also be performed in order to evaluate soils when a septic system is believed to have failed, and when repair or septic field replacement are being considered. Readers should also see our example of state-regulated soil percolation tests at the New York State Septic System Design Regulations 75-A.4 - Soil and site evaluation for septic system design page.

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Specifications for Digging Holes for Soil Percolation Tests for Septic System Design and Approval

How to Dig a Soil Test Pit

  • Soil Perc Test Hole Depth: A typical soil perc test hole or pit is dug five feet (5') deep. On occasions a 6' hole is used if the engineer so requires.
  • Perc Test Shape: The bottom of the pit should be level and about 2 ft. x 4 ft. in size. At least one side should be sloped so that if someone falls into the pit they can easily exit. (See more safety notes below).
  • Perc Test and Water or Rock: If the excavator encounters water or bedrock the excavation will normally stop at that depth regardless.
  • Soil Perc Test SAFETY:
    • In dry soils, for safety and ease of access (to be able to climb in and out of the hole, one side of the hole must be sloped to permit someone to easily enter and leave the pit. The other side(s) of the hole are roughly vertical.
    • In wet soils, for safety and ease of access in and out of the hole, one side of the pit must be dug stair-stepped with each step no greater than 2 ft. in height.
    • Soil Percolation Test SAFETY: Earth which has been removed to create the hole must be piled at least 2' away from the hole edges, again for safety.
    • [DF opinion]: SAFETY: engineers or workers should not enter a soil test pit (nor any other excavation) while working alone.
    • Re-fill the test holes after testing has been completed.

[Oregon's State DEQ has some nice sketches of a soil test pit. These notes paraphrase the Oregon DEQ's text except for [DF opinion].]

Also see this excellent Percolation Testing Manual from Saipan in the Mariana Islands.

Where to Dig the Soil Test Pit for Septic Systems

Usually two percolation-rate test holes are dug, 50' to 100' apart in order to evaluate the proposed septic leachfield area. Evidence of the seasonal high water table is noted (possibly based on changes in soil color at various depths).

For safety, septic soil drainfield perc test holes must be re-filled after the test is complete. If the hole must be left open and unattended during the test it should be barricaded to prevent anyone from falling in. Here are some hints from Callum County, Washington:

  • Dig the perc test hole downhill from the house site if possible.
  • Stay away from swales and drainage ways, and areas that are seasonally wet.
  • Keep 100 feet away from all wells and surface water, including irrigation ditches.
  • Septic systems cannot be located on slopes in excess of 45% (24 degrees).
  • It is helpful if property boundaries and the proposed house site are marked ahead of time.

What is the Soil Test Pit Showing and How Does the Percolation Test Impact Septic System Design?

"Soil percolation or perc tests are used to determine the ability of a soil to transmit wastewater effluent through the soil profile.

The soil percolation rate is the amount of time water takes to move through soil, measured in minutes per inch. Finer textured soils have slower percolation rates; it takes longer for water to drain from a test hole. These soil types need larger drainfields than soils with faster percolation rates, such as sandy soils, to handle a given amount of wastewater.

Soils with very slow percolation rates may not be suitable for drainfields. In Nebraska [and other jurisdictions], if soils perc at a rate slower than 60 minutes per inch, consider installing a lagoon system if the lot is at least 3 acres.

Otherwise, an engineer must design a specialized [alternative design] septic system. Soils with very fast percolation rates, less than 5 minutes per inch, must be modified by adding a loamy sand liner to the drainfield, so that proper treatment can occur." [Univ. of Nebraska, Lincoln Extension, Institute of Agricultural & Natural Resources, "Residential Onsite Wastewater Treatment: Site Evaluation]

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at REFERENCES. This article is part of our series: Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

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