How to carry out a percolation test

Posted by Callum Vallance-Poole, on November 2, 2020.

If you are thinking about installing a soakaway or sewage treatment on your land you will have to first conduct a percolation test. Below we explain what this involves and how to carry out a percolation test to produce accurate results.

What is a percolation test?

A percolation test is used to determine the water absorption rate of soil. The test is carried out before a new or replacement soakaway is prepared and installed, to ensure the ground is suitable for its construction. 

It is the last of three tests that have to be conducted, following on from Groundwater Source Protection Zone search and a Trial Site Assessment Hole to analyse the local water table.

The results of a percolation test will vary depending on the condition of the soil. For example, sandy soil will drain water at a quicker rate compared to soil with thick clay. 

How do you carry out a percolation test?

Carrying out percolation test is a straightforward process that involves digging a pit, filling it with water and then timing how long it takes to drain away. Before carrying out the test make sure the weather conditions are suitable – avoid conducting the test in heavy rain or on frozen ground. 

You can use the following steps to carry out a percolation test:

  • Excavate a hole: You will need to dig a hole 300mm square with a depth of at least 300mm below the invert, taking care to space them along the line where the soakaway will be installed. While you are digging the hole ensure to notice any changes to the makeup of the soil as it will affect the drainage time. It is advised to dig more than one test whole to produce the best results. 
  • Fill the hole with water: Empty at least 300mm of water into the hole – be sure to start the timer right away as the soil will begin to absorb the water immediately.
  • Refill the hole: If the water drains within a 10 minute period, refill the hole up to 10 more times. The soil should be deemed unsuitable if the water continues to drain away at this pace.  
  • Calculate the percolation rate: The hole should be refilled again with 300mm of water and time how long it takes to drain from three-quarters full (225mm) to one-quarter full (75mm).
  • Determine the average time: Divide the above time by 150 (mm) which will produce the average time in seconds for the water to drop 1mm. This is referred to as the ‘V’.
  • Repeat the test again: This final stage will need to be repeated at least three more times at different points during the day. To get the average test figure, add the results together and divide by the number of test holes to get the ‘VP’.
  • Final results: If the VP results are significantly different from each other, carry out additional tests on at least three more new holes to produce new average times you can compare to. 

Tests that produce an average VP between 15 and 100 will mean the ground is suitable for a soakaway. The minimum of 15 ensures that untreated effluent is not able to percolate too quickly into the soil which could result in the pollution of groundwater. 

Where the VP is above 100, this means the ground absorbs water too slowly which could lead to wastewater pounding on the surface, and potential pollution and flooding issues. 

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