A percolation test is one way of determining a design infiltration rate. If a site disturbance must be minimized, this test can be performed in a small augered well. Adding a permeameter may speed up analysis.
Percolation test results cannot be translated into field saturated hydraulic conductivity (Kfs) by simple manuipulation of the units. However, an acceptable approximation can be made by accounting for the soil texture and by making measurements under non-saturated conditions. The required calculations have been written into an easy to use spreadsheet, formatted for printing:
For reference, the relationship between measured percolation test (PT) and field saturated hydraulic conductivity (Kfs) used in the spreadsheet is:
- H = Mean water depth during a test
- a = the radius of the test pit
- α* = the representative soil sorptive number, and
- C = is a shape function.
Additional details may be found in the original paper.
- Shovel or other excavation apparatus
- Study paper bags (or similar craft paper)
- Clean, fine gravel e.g. pea gravel or HPB, not 'screenings' or 'crusher run'.
- Water supply (on site?)
- Record sheets for field measurements
- Perc test holes should be made at points and elevations selected as typical in the area of the proposed infiltration facility.
- Typically, test holes are dug at each end of the area of the infiltration facility and near the centerline. Testing of the receiving area may also be necessary. Further holes could be needed, depending upon the nature of the soil, the results of the first tests and the size of the infiltration facility footprint.
- Initial excavations should be made to the proposed depth of the infiltration facility (or as instructed by the designer). It is easiest to dig a larger hole part way down, then dig a 18 - 20 cm deep accurately sized test hole in the base of the larger hole.
- To make the percolation test more accurate, any smeared soil should be removed from the walls of the test holes. This is best achieved by digging the hole approximately 5 cm undersized and then enlarging the hole to the accurate size as follows: using a rigid knife, insert the blade into the top side of the hole opposite you approximately 2.5 cm deep, holding the blade with its cutting edge vertical. Pull the blade away to break out a chunk of soil, repeat every ~ 2.5 cm around the hole, then repeat for another “ring” below until reaching the base. The result will be a hole with a ragged inner surface which looks like a freshly broken clod of soil.
- The base of the hole should be cleaned of debris and be approximately flat, use a metal scoop or similar. It should also be picked to present a natural surface. Note that a picking action (use a pointed tool) is needed, not a scratching action (which just produces smears that are indented).
- Place 5 cm of clean fine gravel in the bottom of the hole. If the sidewalls are likely to collapse, use a paper basket to support the sidewalls.
- Place a piece of white plastic or similar provided with clear marks at 12.5 and 15 cm (5" and 6") from the bottom of the test hole prior to adding the gravel. For greater accuracy a float and pointer arrangement can be set up.
- If the soil contains considerable amounts of silt or clay, and certainly for any soil with “clay” as part of the texture description, the test holes should be pre-soaked before proceeding with the test. Pre-soaking is accomplished by keeping the hole filled with water for 4 hours or more. The water should be added carefully and slowly to avoid disturbing the soil (including the sidewall soils). The test should be carried out immediately after pre-soaking;
- To undertake the test, fill the test hole (the accurately sized test hole) with water. The water should be added carefully and slowly to avoid disturbing the soil (including the sidewall soils). When the water level falls anywhere below the 12.5 cm mark, refill the hole to the top. No recording of time needs be done for these 2 fillings.
- After the second filling, when the water level falls below 12.5 cm, add enough water to bring the depth of water to 15 cm or slightly more. Note that these measurements are from the base of the soil bottom (using the installed marker), not the gravel layer.
- Observe the water level until it drops to the 15 cm depth, at precisely 15 cm, commence timing, when the water level reaches the 12.5 cm depth, stop timing, record the time in minutes.
- Repeat the procedure until the last 2 rates of fall do not vary more than 2 minutes or by more than 10% (whichever is less).
- Backfill the holes with the excavated soil and flag and label their locations so you can pick them up for the plan.
- If a test hole is discarded due to flow in a root channel or similar, record the information and make a replacement test. If there is a large variation (greater than or equal to 50%) between tests in the same soil layer, increase the number of tests.
Paper basket to protect hole
If sidewalls of the hole are likely to collapse, one option is to make a paper basket to protect and support the sidewalls as follows:
- Cut the bottom out of a large paper bag (grocery bag) and cut the bag open along a side.
- Lay bag on a soft surface. Punch holes in the bag about 5 to 7.5 cm apart using a pencil or similar.
- Roll into a tube, with the short dimension being the axis of the tube, and place in the test hole.
- Open the tube until the paper is in contact with the sidewalls of the test hole, then roll the top of the tube over to stiffen it.
- After placing the tube in the hole, place the plastic marker and add the base gravel layer.
- Reynolds, W.D., Galloway, K., and Radcliffe, D.E. (2015). "The relationship between perc time and field-saturated hydraulic conductivity for cylindrical test holes.", National Onsite Wastewater Recycling Association (NOWRA) 2015 Onsite Wastewater Mega-Conference, Virginia Beach, VA, USA, November 3-6, 2015
- Registered Onsite Wastewater Professional Program BC, PERCOLATION TEST PROCEDURE & FORM (2016) https://owrp.asttbc.org/wp-content/uploads/2016/11/Percolation-Test-Procedure-and-Form.pdf