How To Carry Out A Soil Percolation Test
Very occasionally your local authority may ask you to carry out or arrange a soil percolation test in order to determine whether or not your soil conditions are suitable for the installation of a soakaway. Some soils such as heavy clay soils do not support soakaway filtration. Also if you have a particularly high water table this also may render a normal soakaway unsuitable, although in such cases you could also consider our shallow dig soakaway options.
This test is sometimes referred to as a porosity test. By following the instructions below you will be able to do the test yourself, although if you are unsure about it we would always recommend seeking professional assistance.
You should carry out the test at a time when weather conditions have been normal – no drought, prolonged rain or heavy frost.
Start by digging a hole to depth that roughly the same as your proposed soakaway. For our Ellipse soakaways this would be as follows;
- Standard 1 Cubic Metre Soakaway sited in a garden – 1310mm
- Standard 1 Cubic Metre Soakaway sited under a driveway – 1510mm
- Shallow Dig 1 Cubic Metre Soakaway sited under a garden – 890mm
- Shallow Dig 1 Cubic Metre Soakaway sited under a driveway – 1090mm
The test hole should be 300mm square. Take care when digging and shore up the hole if required. We suggest that the easiest way to construct this is to dig the hole so that it is large enough to stand in for part of the way, then narrow it so that your test hole is 300mm square by 300mm deep.
Now, insert two six-inch nails into the wall of the 300mm deep test hole. One should be 75mm from the base and the other should be 75mm from the top.
Pour water into the hole to a depth of 300mm. Leave this to drain into the soil for 12 hours.
Refill the test hole right to the top (300mm) and time how long it takes for the water to drain between the two nails. Start the timer when the water drops to the higher nail and stop it when it reaches the lower nail.
Divide the time in seconds by 150 (the difference in height between the two nails).
The resulting figure is the average time taken in seconds for the groundwater to drop 1mm.
If this number is less than 100 your soil conditions are suitable for installing a soakaway.
We recommend that you repeat this test two more times to ensure that your results are consistent.