Having completed a number of perc test in soil for a septic leach field design, and doing some research on the subject, I would like to share with you what I have learned.
What is a perc test also known as a soil percolation test. A perc test is a method to determine how much time it will take a soil type to disperse or absorb a certain amount of water. The test is used to design a septic system leach field. The septic system designer determines the depth and location of the perc holes. A specific amount of water is poured into each hole. The time is recorded for the water to drop 1 inch, the water is replenished to the original depth. This test is performed until the 1 inch water drop times stabilize, usually 4-5 times.
In the rest of this article I will describe how to do a soil percolation test also known as a perc test.
* a percolation test and perc test are the same*
*a leach field and drain field is the same*
Where to do dig a percolation test hole and how many do you dig
A perc test is done in the area on your building lot where you would like to place the leach field. If your building lot is large enough you might want to do a test on each side of the house to determine the best spot to place the septic leach field. If the soil conditions on the right side of the house contain a lot of clay and the soil on the left side of the house contains sand the design for a leach field on the left side of the house would be smaller and cost less. Sandy soil percolation times are much shorter than clay.
The official perc test locations will be provided by the designer of the system who must follow local building rules and regulations. The number of holes can range from 2 on up. The size and use of the building will dictate the size of the proposed leach field and the size of the leach field dictates the number of test holes required.
What tools do you need to do a perc test
You need these tools to do a perc test
- Shovel
- Bucket
- Stop watch
- Tape measure
- Pencil and paper
- Perc measuring board* you can make
The perc measuring board is made out of a 1 inch by 2 inch or so material about 24 – 30 inches long. As it shows in the picture you take and sharpen the bottom to a point and attach a short cross board (it needs to fit in the test hole) about 3 inches up from the bottom. From the cross board you place a screw or nail 5 inches up and another one 1 inch above the first one. Measuring board is now complete.
How big and deep to dig the perc test holes
You will dig the holes with the hand shovel about 12 inches round and 12 to 16 inches for a shallow system and 24 to 30 inches deep for a standard system. The holes must be dug by hand as to not compact the sides and bottom of the test hole that a mechanical post hole digger might do. The compaction of the sides and bottom would not give a accurate percolation test time.
How to start the perc test
Once you have your hole dug and a small amount of washed stone in the bottom of the hole then you should pour some water in the hole to fill it up 6 inches or more above the bottom. This is the pre-soak and it will take a minimum of 4 hours, overnight is better.
After the pre-soak insert the measuring board into the test hole so the cross board stops on the top of the stone on the bottom.
Add water to the test hole until it reaches the top hole. Start the timer and record the time it takes to reach the bottom nail that is one inch down. Add more water until it reaches the top nail and record the time again. Repeat this procedure until the times are running close to the same. This means the times have stabilized.
How many times should you do the perc test
You do the test until the times stabilize. This could take 4 to 7 times. If you pour water in the test hole at least 4 hours and up to 12 hours before you start the test this will help stabilize the time rate a little faster.
Will the results vary from hole to hole
Each hole may have a different time. The soil type and if it has been compacted in the past will have an effect on the test.
What soil type should give what perc results
The percolation of water thru soil depends on how far apart each grain of soil is. Clay being the worst soil for percolation of water because of the fine particulates it is made out of. Coarse sand and gravel will have a extremely fast percolation rate.
Gallons of water absorption
Soil Type Per sq.ft. in 24 hours
Course sand or gravel 5 gallons
Fine sand 4 gallons
Sandy loam 2.5 gallons
Sandy clay 1.7 gallons
Clay with considerable sand or gravel 1.1 gallons
Clay with small amounts of sand or gravel .8 gallons
What this means is if you have coarse sand your test times may be 10 minutes where if you have clay it may take hours. All soils have a mix that is unique to the area.
What do the results mean for the size of the leach field design
Perc rate minute per inch square foot needed per each gallon a day
0 – 30 minutes .4 sq. ft.
31 – 45 minutes .7 sq. ft.
46 – 60 minutes 1 sq. ft.
61 – 120 minutes 2 sq. ft.
This is what this means for the design of a leach field. The NYS Building Code has been used for a reference for this example:
3 bedroom house 450 gallons per day of wastewater that will enter the leach field
Perc test in front of the house stabilized at 57 minutes leach field required 450 ft trench length.
Perc test in the rear of the house stabilized at 27 minutes leach field required 375 trench length
To visit the webpage click below
NYS Department of Health Appendix 75-A
As the example shows it may be well worth your time to do a perc test on your building site to locate the most efficient area to locate the leach field
How much does it cost to have a perc test done
$100 – $1000 +
The price depends on the local building code requirements.
Add in the difficulty and size of the project and prices in your area.
This is why I can not provide a hard price but only an estimate.
I hope this article has helped provide you with a answer
Gary 10/12/2018
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