What is a Soil Percolation Test?
Soil Percolation Tests determine how fast soil absorbs water. The results of the tests determine which method of drainage the site should use as different soils absorb water at different rates. For example, light sandy soils or chalk can take in larger amounts of water quickly, whereas dense clay soils can only absorb small amounts in any one period of time.
Soil Percolation Tests determine how fast soil absorbs water. The results of the tests determine which method of drainage the site should use as different soils absorb water at different rates. For example, light sandy soils or chalk can take in larger amounts of water quickly, whereas dense clay soils can only absorb small amounts in any one period of time.
How does a percolation test work?
A percolation test involves pouring a fixed quantity of water into a hole of standard dimensions and measuring how quickly it is absorbed into the soil. The hole is typically 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 for a new roof (ie; a house) needed to avoid any risk of flooding.
What will the results of a soil percolation test determine?
The results of a soil percolation test will determine two things; how large your soakaway needs to be as well as if you can even use this method in your surface drainage strategy. The results of this test can also be used to determine the required size of your drainage field for an Anaerobic Digester unit. The results can also be used to demonstrate to the Lead Local Flood Authority (LLFA) that you have carried out your test correctly and that the drainage solutions (foul and surface water) are sensible and not likely to cause issues later on.
How does a percolation test work?
Here are the steps for undertaking a percolation test. Before doing this yourself note that some LLFAs wont accept a DIY percolation test but in the very least it allow decisions to be taken about your soil:
1) Dig a cubic hole which is 30cm square and at least 30cm below the proposed invert, spacing them along the proposed line of where the soakaway or drainage would be. While digging the hole, look out for changes in soil characteristics at different depths. This could be useful later.
2) Fill each hole with 30cm of water.
3) If the water drains within 10 minutes, the hole should be refilled with water up to 10 times. If the water continues to drain away then this soil is unsuitable for a drainage field for a foul water but it will be suitable for regular surface water (ie; rainfall), if it hasn't drained away within 6 hours it is also not suitable.
4) Refill the test holes with the water to a depth of at least 300mm and then using a stopwatch check the time for the water to seep away from 75% to 25% full level.
5) Divide the seconds by 150, the answer gives the time in seconds (V) required for the water to drop 1mm.
6) Repeat the test 3 times for each hole at different times of the day. The average figure from the tests should be taken by adding all the results together and then divided by the number of holes giving the speed that the water percolates through the soil (Vp).
If the test results shown are average Vp value between 15 and 100 then the ground is suitable for a drainage field (ie; Foul water) soakaway.
Best practice for percolation test:
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.
- Try to dig holes roughly where the soakaways or drainage field are likely to go, not a garage, house, access or swimming pool etc.
- Repeat the test several times and calculate the average percolation rate for a more accurate result.
My site failed a Percolation test, what are my options?
If a percolation test fails, meaning that a drainage field is not appropriate for the property, a package treatment plant would not be suitable for the property. Septic tanks are already outlawed in the UK since 2020.
The options available are:
- Mains sewer connection
- A treatment plant discharging to a nearby ditch or watercourse (if one is available)
- A treatment plant discharging to a bespoke soakaway arrangement. You would need to obtain a Permit for Consent to Discharge from the Environment Agency if the property is in England, NRW in Wales, SEPA in Scotland, NIEA in Northern Ireland.
How does a soakaway system work?
A soakaway system (or drainage field) discharges the effluent from septic tanks and sewage treatment plants to ground. It is a series of perforated pipes placed in trenches and arranged so that the effluent disperses through the ground for further treatment. This way there is no risk for contamination. The solids from the sewage sink to the bottom of your septic tank , or package treatment plant, and get removed when the tanks are emptied every 3-5 years. Licensed waste disposal companies do provide this maintenance service if you need it.
For more information and advice on Soil Percolation Tests, please Contact us