Soil percolation tests, known casually as perc tests, determine how fast soil absorbs water. It’s used to work out which method of drainage the area should use as different soils absorb water at different rates. If the soil doesn’t absorb water very well then you’re unable to install a soakaway system, for example.

A percolation test should not be carried out in adverse weather and if it’s done in summer then the times produced should be increased just to make sure that there’s scope for winter rainfall.

DIYTable of contents:

  1. What do I need?
  2. How do I carry out a soil percolation test?
  3. How do I interpret the results of a soil percolation test?

What do I need to carry out a soil percolation test?

A percolation test doesn’t require any specialist tools. You need:

  • Shovel
  • Tape measure
  • Container for transporting water or a hose that will reach the percolation test site
  • Stopwatch
  • Notepad and pen

How do I carry out a soil percolation test?

  1. Dig a hole that’s 300mm x 300mm and 300mm deep at your drainage site
  2. Remove any loose stones or litter from the hole
  3. Fill the hole up to 300mm deep with water and allow it to drain overnight
  4. Fill the hole again to 300mm and note in seconds how long it takes to go from 3/4 full to 1/4 full
  5. Divide the seconds it takes by 150. This number is known as the Vp and it’s the average time it takes for the water to drain by 1mm
  6. Perform this test in a similar area with different patches of the ground at least another 2 times
  7. Take the average figure from the tests at your final Vp


How do I interpret the results of a soil percolation test?professional

Before some types of drainage products (sewage treatment plants, for example) are given approval by the local authority, they require a soil percolation test. Some councils will accept the results of your own test, but some will want a qualified professional to carry this out. If you’re unsure of your own test results it’s advisable that you contact a specialist.

To begin to consider using a soakaway system as a drainage method you must achieve a minimum Vp of 15. The design of a soakaway system is dependent on the Vp too. The longer it takes to drain, the longer the pipework design field should be.

For help or advice with percolation tests and drainage fields, contact our drainage experts on 01752 692 221 and they will be happy to help you. 

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