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Perc Tests: Soil Percolation & Soil Depth Requirements for Septic Absorption Systems / Septic Drainfields

  • SEPTIC SOIL & PERC TESTS - CONTENTS: What are "perc tests" or soil percolation tests? Perc-hole / soil test pit standards & specifications. Cost of a soil perc test.
  • POST a QUESTION or READ FAQs about the procedures used for septic system test hole tests, deep hole tests, and percolation tests for drainfield qualification and soil testing
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Septic soil percolation tests or soil absorption test procedures:

The ability of a drain field, also called leach field, or drain field, to absorb septic effluent determines the size, location, and type of effluent absorption system which can be installed at a property. Septic drainfield percolation test procedures: this article describes the need for and process of "soil testing" or the preparation and use of soil test pits for septic system absorption system or drainfield design or repair.



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WHAT ARE PERC TESTS: What is a septic system soil percolation test? What is a "deep hole test"?

Backhoe in operation  doing excavation (C) Daniel FriedmanPerc 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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In specifying the size and type of absorption field (leach field, seepage pits, galleys, other) a septic engineer or health department official will require that a soil percolation test or "soil perc test" be performed. You may hear it described as a "deep hole test."

In brief, one or more holes are dug in the soil of the property where (or near where) a septic leach field is to be installed. Water is placed in the hole, and the engineer observes the amount of time it takes for the soil to absorb the water, or for the water to "percolate" through the soil. The engineer will also examine the exposed soil layers to obtain additional site design information. (Details follow).

What is a Soil Test Pit or Perc Test Pit or Deep Hole Test Pit?

Steps in a Perc Test / Soil Percolation Test

  1. A hole, 5-7 feet deep is dug in an area to be tested for future use as a drain field, or near the drain-field area in representative soils.
  2. Water is poured into the hole
  3. The soils or septic engineer or contractor observes the rate at which soil absorbs the water by noting the time that it takes for the level of water in the hole to drop one inch (for example). This observation is simple.
    • To be reasonably accurate a board or stick is placed roughly-level on the ground, crossing over the top of the soil perc test hole
    • The engineer measures the height from the stick under-side (or top - it doesn't matter as long as she's consistent) to the top of the water when water is first poured into the perc test hole
    • The engineer measures the height from the stick to the bottom of the water in the hole at the same point in time - at the start of the test.
    • If the soil is extremely porous so that water begins to disappear almost immediately the engineer watches the water level drop and times that event.
    • If the water level drops more slowly, then the engineer waits a specified time, typically 30 minutes or an hour and at the end of that interval the engineer measures the distance from the stick to the top of the water again.
  4. The soil percolation rate is the number of inches that the water level dropped in the soil perc test hole over the specified interval, typically 30 minutes, or 60 minutes for a slow-draining soil.
  5. The septic engineer reports the number: EXAMPLE: the soil in test hole #1 showed a percolation rate of 3 inches in 30 minutes.

More precise "perc tests" may involve using a specific quantity or volume of water or a perc test hole of specific dimensions to make these observations. Some perc tests may specify the inclusion of a couple of inches of gravel in the hole bottom - depending on the nature of the soil itself.

The first time I participated in a soil perc test procedure I found myself smiling with surprise at how low-tech the procedure actually was (in New York State.) After identifying the most-likely location on the lot for placement of a septic drainfield, the excavator used a backhoe to dig a very rough hole about 5 ft. deep. Happily no groundwater immediately filled in the hole (which would have been bad news). Perhaps this is why builders try to have this test done in July which is the period of most-dry weather and lowest groundwater table levels.

After digging this rough hole, the septic engineer poured a 5-gallon (joint compound) bucket of water into the hole. In some cases a few buckets might be dumped therein. After that sophisticated move, the observers simply watched the rate at which the water disappeared. a one-inch drop in water level in this hole in three minutes was considered very good.

If the water was found still in the hole at no drop in level the next morning, this was considered seriously bad and probably requiring some soil exchange or other special design measures.

When is a Soil Perc Test Required vs Performed?

There are two different questions here:

  1. A soil perc test or percolation test is going to be required in most jurisdictions when a builder or property owner is going to install a new or replacement septic system that requires local health or building department approval. This might be a perc test for a new building site or a perc test to permit approval of a septic design for a replacement soakbed or drainfield at an existing property whose existing septic fields must be replaced.
  2. A soil perc test is usually performed during wet weather or during the wet season - a time that varies depending on where you live.

    In North America that's typically April-June

    In Australia the wet season is during Worrwopmi, the humid time - a period that depends on where in Australia you're located. In the Australian tropical region the wet or monsoon season extends over six months between November and March.

    In northern and central New Zealand the wet season extends through winter while on the South Island winter is the season of least rainfall.

    In the U.K. in my view most of the year is "wet season", though in Scotland May is probably the wettest month of all.

    In western Europe in the Alps region the wet season is principally between March and December.
    In western Europe along the Mediterranean Sea the wet season extends from October through March.
    Heaviest rain coming from the Atlantic occurs during the European Monsoon during the European winter and again in June.

Why Soil Perc Testing during Wet Weather is Important

Why not do our perc tests during the dry season when a site is most-likely to "pass" local soil perc test requirements? I've absolutely seen builders use this trick to pass a marginal site for locating a septic soakbed.

Unfortunately designing a septic effluent disposal system based on "dry season" perc testing results means that the septic system design is likely to be inadequate: that is, during the wet season when soil water tables are higher and perc rates are slower, the septic system is going to discharge un-treated effluent into the environment: basically your are peeing and pooping into the water supply.

Septic Soil Perc Test Costs (soil percolation rate test cost)

Question: what's the cost to get a perc test performed?

2016/08/29 Anonymous said:
what s the cost to get a perc test

Reply:

Perc test costs vary depending on where you live and also on just what local regulation require. For example, a deep hole perc test in some jurisdictions will require the use of a backhoe - you'll be paying the hourly rate for the backhoe operator that might be $100. or more, plus the cost for the consulting engineer or septic designer to introduce water and watch and measure the percolation rate. The number of perc test holes, hole depth, and ease of site access all affect the total cost for a soil perc test for septic system design and approval.

Actual costs range from a low of about $100. (presuming you're in the U.S.) for a shallow hand-dug perc test hole to $1000. for several perc test holes dug deeper (typically using a backhoe) plus the septic designer's measurements and report of soil percolation rates on the proposed septic field site. In some municipalities, the backhoe rate alone can be higher: $300. to $500./hour.

In VERY general terms, I'd figure $500. for the backhoe for 2-3 deep hole excavations and another $500. to pay the septic engineer.

We discuss perc tests in detail at inspectapedia.com beginning at SEPTIC SOIL & PERC TESTS but you'll also want to see these two articles as the specifics of exactly what sort of soil perc test your local health or building department will require for septic system design approval will determine the amount of trouble, work, time and cost of your particular soil perc test:

Our sister site the Building Advisor (Steve Bliss) also discusses SOIL AND PERC TESTING at http://buildingadvisor.com/buying-land/septic-systems/soil-and-perc-testing/

This InspectApedia.com article series on soil percolation rate testing continues at PERC HOLE SPECIFICATIONS

Septic Soil Percolation Test or Wastewater Disposal Field Testing Articles

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Continue reading at PERC HOLE SPECIFICATIONS 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

Or see SEWAGE FLOW & DESIGN FLOW ESTIMATES

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