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Calendar 10 October, 2023 (Updated 10 January, 2024)

Soil Percolation Tests: A Guide

If you are installing a soakaway, septic tank or sewage treatment plant that will discharge water into the soil, you will need to carry out a soil percolation test.

 In this blog post, we discuss what a percolation test is, when it is required and how to perform one.

What is a soil percolation test?

Percolation is defined as “the movement and filtration of liquids through porous materials”. By carrying out a percolation test on the soil that surrounds your proposed installation, you will be able to determine its water absorption rate and therefore its permeability.

This is important for two reasons: it establishes whether or not the ground has sufficient capacity to absorb the expected volume of water that will be discharged; it also indicates how capable the soil will be of removing contaminants from wastewater before it reaches the water table.

When do I need a soil percolation test?

A percolation test should always be carried out before installing a septic tank or sewage treatment plant which discharges wastewater to a drainage field. You should also run a percolation test if you are installing or even replacing a soakaway as this is likely to be in a slightly different location to the previous one and the soil conditions can vary greatly even over a short distance.

What soil types are suitable for a percolation test?

Not all soil types are suitable for discharge drainage and therefore a percolation test will be a waste of time and money. If the soil is too dense, drainage will be slow and result in flooding or waterlogged soil. On the other hand, if it is too porous the wastewater will drain too quickly for the soil to properly filter out contaminants.

Chalky, loamy and sandy soils are generally the best types for discharge, whereas soils that are silty or have a high clay content are less suitable.

Is a soil percolation test all that’s needed?

A soil percolation test is the third stage of assessing site suitability for installing a discharging stormwater or sewage management system and should only be carried out once you have conducted a Groundwater Source Protection Zone[JJ1]  search to establish protected groundwater supplies and a Trial Site Assessment Hole[JJ2]  to find the depth of the localised groundwater table.

Can an amateur carry out a soil percolation test?

While some local councils accept the findings of DIY soil percolation tests, others will insist you use a qualified professional using specialised equipment. This will typically cost in the region of £350 to £500.

We would also recommend using a professional if you are unsure of the results of your testing.

When is the best time for a soil percolation test?

A soil perforation test is ideally carried out during a wetter time of the year as this will more accurately allow for natural soil saturation and water table levels, although subzero temperatures, snow and rain should be avoided.

If you can only perform the test in warmer, drier weather you will need to increase the testing time to allow for this difference.

How do I do a soil percolation test?

While qualified professionals use specialised equipment that gives more accurate results, a soil percolation test can be carried out with basic tools – shovel, tape measure, bucket or hose, stopwatch, notepad and pen are sufficient.

  1. Digging your test holes

Dig at least two test holes that are 300mm square and excavated to at least 300mm deep (or 300mm below the invert level of the proposed outlet pipe if you’re installing a septic tank or sewage treatment plant). 

If the outlet pipe is quite deep, using a backhoe or auger rather than a hand shovel might save time and backache. You may also need to excavate a wider area to give safe access (as per Building Regulations Part H2 1.34).

Remove all loose stones and debris from the holes and ensure the sides and base are smooth.


  1. Saturating the soil

Use a tape measure to mark a depth of exactly 300mm from the base of the holes and fill them with water to this level.

This water should saturate the soil overnight so if it drains away too quickly, refill the holes several times and monitor how long it takes for the water to disperse entirely. If it is still draining away too quickly after 10-20 minutes, the soil is too porous. On the other hand, if the water hasn’t drained away after 6-12 hours the soil isn’t porous enough.

If either of these things happens and you are planning on installing a wastewater drainage field, there is no point in continuing with the rest of the percolation test.

If you are planning on installing a stormwater soakaway, only test holes that retain water at this stage present a problem.

  1. Verifying the percolation rate

The next day, fill the holes with water up to the 300mm mark. When the water level drops to 225mm, start timing it and see how long it takes to fall to 75mm. This step could take quite a while and if you are monitoring several holes simultaneously you may prefer to mark lines on sticks at 75mm, 225mm and 300mm from the end and leave the sticks in the holes.

Divide the elapsed time (in seconds) by 150 to determine the fall rate per millimetre; this is known as the Vp rate.

  1. Repeating the test

To gain an accurate result, step 3 should be repeated at least three times per hole, ideally at different times of the day. Calculate the average Vp number of all readings from all holes to establish your soil’s percolation rate.

What do the soil percolation test results mean?

If you are installing a storm water soakaway, you will need a minimum Vp of 15 seconds. 

For a drainage field linked to a septic tank or sewage treatment plant, you will need a Vp of between 100 and 140 seconds. The percolation rate will also influence how long the drainage field pipework will need to be.

We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call the friendly team of drainage experts at Drainfast on 01420 555600 or email [email protected]

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!

Mark Chambers Drainfast Team Portrait

Written by
Mark Chambers

Marketing Manager

As Marketing Manager, Mark plays an active role in running strategic projects to increase our brand profile.

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