When planning a new drainage field for a septic tank or sewage treatment plant, a percolation test is a critical step. Results may be requested by building control inspectors, drainage engineers or even planning officials before work can be completed. But why are percolation tests important – and what should you do if yours ends up as a fail?
What is a percolation test?
A percolation test determines the rate at which water drains away from soil, as we’ve covered in detail previously. It involves digging a hole to a specified depth, filling it with water and measuring how long it takes to soak into the earth.
This enables assessment of the soil’s ability to absorb and filter wastewater, ensuring any installed system will be able to function properly. An accurate rate of drainage, or ‘percolation value’ (Vp) is essential to enable the design of an efficient and compliant drainage field. And in order to gain an accurate Vp, you should do multiple tests, to give an average output.
Why do I need a percolation test?
Your Vp value will help determine the size and layout of the drainage field for your new system. This number needs to fall between 15 and 100 seconds per millimetre to indicate suitable soil conditions. Outside of this range, you could face the following problems:
- Faster than 15 seconds per mm – This indicates that the soil is too permeable, meaning effluent could rapidly infiltrate and potentially pollute groundwater sources.
- Slower than 100 seconds per mm – The opposite effect – i.e. soil not being permeable enough – can result in pooling of wastewater, unwanted odours and above ground pollution.
What do I do if my percolation test fails?
If your percolation test results are outside the ranges specified above, there are several steps you can take.
- Consider your test conditions
If you tested during a prolonged dry spell, or close to recent heavy rainfall, this could have affected soil conditions and skewed the results. Try re-testing during more typical weather, in order to get more useful numbers.
- Explore alternative drainage options
If your test data appears accurate, you may require a less standard option, tailored to poorer quality soil.
Be sure to seek professional advice or Environment Agency guidance before commencing any works.
- Consult a professional
If you performed your percolation test yourself, it might be worth engaging a reputable professional to conduct a new one.
Ultimately, even with all due time and care taken, there’s no substitute for experience.
Time for a percolation test?
If you’re not completely confident in performing a percolation test (or what to do if it fails), you should speak to a professional. A certified drainage engineer will ensure correct technique and definitive results that are more likely to be accepted by third parties. If you’d like to discuss support with percolation testing or any aspect of drainage field design, please contact us.