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This chapter discusses the specifications for soil percolation holes used for testing for septic system absorption system or drainfield design. 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.

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 document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

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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, 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]

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SEPTIC SYSTEM DESIGN BASICS
  Septic Soil PERC TESTS
  WHAT ARE PERC TESTS
  PERC HOLE SPECIFICATIONS
  PERC STANDARDS
  SEPTIC TANK SIZE
  SEPTIC TANK DEPTH
  SEPTIC TANK TEES
  SEPTIC DRAINFIELD SIZE
  SEPTIC DRAINFIELD SHAPE
  SEPTIC CLEARANCES
  SEPTIC TANK PUMPING FREQUENCY
  SEPTIC TANK PUMPING PROCEDURE
  SEPTIC PUMPING REPAIR
  SEPTIC TREATMENTS
  STEEP SLOPE SEPTIC DESIGNS
SEPTIC DESIGN ALTERNATIVES

• Percolation Testing Manual, CNMI Division of Environmental Quality, Gualo Rai, Saipan provides an excellent English Language manual guide for soil percolation testing. Original source: www.deq.gov.mp/artdoc/Sec6art108ID255.pdf
• Soil Test Pit Preparation, fact sheet, Oregon DEQ Department of Environmental Quality, original source www.deq.state.or.us/wq/pubs/factsheets/onsite/testpitprep.pdf The Oregon DEQ onsite water quality program can be contacted at 811 South Ave, Portland OR 97204, 800-452-4011 or see http://www.oregon.gov/DEQ/
• Thanks to reader Michael Roth for technical link editing 6/29/09.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
• Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)
• ...

More Information on Building Diagnostic Inspections and Repairs

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
• Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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