Why it’s important to perform a percolation test for drainage

A percolation test is an important initial inspection before installing new drainage, especially sustainable drainage systems (SuDS) that make use of a soakaway.

The results of a percolation/permeability test tell you how efficiently the surrounding soil can absorb water from the soakaway or from the surface above.

Light sandy soils can take in larger amounts of water quickly, whereas dense clay soils can only absorb small amounts in any one space of time.

This influences SuDS design because, particularly in systems that use a soakaway or similar, you need to be confident that any wastewater will disperse into the soil before the soakaway overflows.

How does a percolation test work?

A percolation test involves pouring water into a hole of standard size and measuring how quickly it is absorbed into the soil.

The hole is typically a 300mm cube and should drain completely within a few hours. For a precise measurement of permeability, the tester records the time taken in seconds for the water level to drop a specific distance.

By dividing that distance by the time taken, a measure of permeability is obtained in mm/s, which can then be used to calculate the size of soakaway needed to avoid any risk of flooding.

Why is a permeability test important?

Hopefully, the benefits of a permeability test are already clear as it’s a relatively easy way to calculate how fast water will soak into the surrounding soil.

This allows you to make an informed decision when installing drainage systems and especially when installing SuDS with soakaways, which must have sufficient capacity to avoid overflowing before the water can naturally percolate away.

Your local authority may also demand a percolation test before they will grant permission for any new SuDS to be installed – so in many cases it is a mandatory part of the process.

Best practice for percolation tests

There are some simple things to remember when carrying out a percolation test:

  • Conduct testing under normal weather conditions, not when the soil is especially dry or already waterlogged.
  • Use a Trial Site Assessment Hole to find the level of the natural water table, as your soakaway will need to be above this.
  • Repeat the test several times and calculate the average percolation rate for a more accurate result.

If you are considering installing a new septic tank, sewage treatment or any kind of SuDS with a soakaway, contact us today on 0151 236 0707 and make sure you get a professional percolation test carried out before you begin any work.

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