Sloped side soil perc test pit schematic adapted from Oregon DEQ cited in this article (C) InspectApedia.comPerc Test Hole Specifications for Soil Percolation Tests for Septic Field Design

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Soil absorption test or percolation rate test hole specifications:

What are "perc tests" or soil percolation tests? How do we perform a soil perc test when designing or testing a septic system?

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.

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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.

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

Stepped side soil percolation test pit for wet soils areas adapted from Oregon DEQ at InspectApedia.comHow to Dig a Soil Test Pit

Where to Dig the Soil Test Pit for Septic Systems

Backhoe in operation  doing excavation (C) Daniel FriedmanUsually 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:

Number of Soil Test Pits Required

The number of soil test pits required to obtain an approved septic system installation may be as few as one, and depends on

Often where I have visited building sites and have observed multiple test pit excavations that's because the excavator moved the test pit location until she found one that gave acceptable results. That's where the absorption bed should be located.

Here is a typical specification, from New York State's Appendix 75-A:

(2) If a subsurface treatment unit such as an absorption field is planned, at least four feet of useable soil shall be available over impermeable deposits (i.e., clay or bedrock). Highest groundwater level shall be at least two feet below the proposed trench bottom.

Where systems are to be installed above drinking water aquifers, a greater separation distance to bedrock may be required by the local health department having jurisdiction. At least one test hole at least six feet deep shall be dug within or immediately adjacent to the proposed leaching area to insure that uniform soil and site conditions prevail.

If observations reveal differing soil profiles, additional holes shall be dug and tested. These additional holes shall be spaced to indicate whether there is a sufficient area of useable soil to install the system. Treatment systems shall be designed to reflect the most severe conditions encountered.

If the percolation tests results are inconsistent with field determined soil conditions, additional percolation tests must be conducted and the more restrictive tests must be the factor used for the system design. 

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

How much water is needed to perform a soil perc test?

Enough clean water, typically several 5 gallon pails, is poured into the soil percolatoin test pit hole to provide 1-2 feet of depth of water in the actual test pit (a 2' x 4' flat bottomed hole).

1 cubic foot = 7.5 U.S. gallons

Since the standard test pit hole is 2' deep x 2' wide x 2' across, that's 8 cubic feet.

8 cu.ft. x 7.5 U.S. gallons = 60 gallons of water to fill the hole.

I have never seen a soil test performed in which the test engineer poured 12 5-gallon joint compound buckets of water into the hole. But certainly you need enough water in the hole to permit a reliable measurement of the percolation rate.

In many jurisdictions and where there is less supervision, a smaller test pit is what's usually dug, perhaps half the bottom area as the standard given above. And typically a single 5-gallon bucket of water is poured into the hole.

How is the Soil Percolation Rate Measured?

Water is poured into the standard test pit.

The depth of water in the pit is measured using a tape measure or a calibrated rod or stick and that depth is recorded.

Depending on the rate at which water is absorbed into the soil, subsequent measurements of water depth are made at intervals ranging from a few minutes (very sandy, fast-absorbing soil) to several hours (slow perc rate soils) to overnight (terrible perc-rate soils).

At each subsequent perc test water level measurement the tester notes the elapsed time and the number of inches of water remaining in the pit.

Using a standard percolation test pit hole size of 2' x 2' bottom area, we have (24 x 24) 576 square inches of area to absorb water - allowing a somewhat standardized report of the soil's percolation rate.

A minimum usable perc test hole area accepted by some authorities (PERC TEST STANDARDS) is about 230 square inches (a bit less than half of the area given above).

Soil perc test procedures: pouring water, making observations, reporting results, are described in detail at SEPTIC SOIL & PERC TESTS - topic home.

Percolation rate examples:

Percolation rates are given in minutes per inch of water in the test pit.

Watch out: The stated or observed "soil percolation rate" only makes sense if the test pit bottom is flat, the sides of the test pit hole are relatively vertical, and if both the area of the test pit bottom in inches and the volume of water poured into the hole are known.

If 1 gallon of water is poured into this smaller tet pit hole (1" x 1" x 231") and if the water is absorbed in 2 minutes, the soil percolation rate, given in minutes per inch of water, is 2 minutes.

If 5 gallons of water (one full joint compound bucket) is poured into this smaller test pit and if it takes a full day (24 hours) for the water to absorb into the ground, that's a soil percolation rate of 288 minutes.

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. [adapted from Univ. of Nebraska, Lincoln Extension, Institute of Agricultural & Natural Resources, "Residential Onsite Wastewater Treatment: Site Evaluation]

Soil Percolation Test Pit Design Specification Guidelines

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: SEPTIC SYSTEM INSPECTION & MAINTENANCE COURSE an online book on septic systems.


Continue reading at PERC TEST STANDARDS - how to calculate and find acceptable percolatin rates for septic system soils, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SEPTIC DRAINFIELD SIZE for wastewater application rate tables & septic trench design tables


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.

Or see these

Septic Soil Percolation Test or Wastewater Disposal Field Testing Articles

Suggested citation for this web page

PERC HOLE SPECIFICATIONS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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