When designing Sustainable Urban drainage systems, engineers must first consider the discharge of surface water from impermeable areas into the ground in line with National Planning Practice Framework NPPF. Although it is possible to carry out desktop studies that will highlight if infiltration is feasible, on-site testing should be carried out to confirm this prior to construction, this is known as percolation testing or infiltration testing.

The Percolation Test Procedure

Surface water infiltration testing should be carried out in accordance with BRE365 soakaway testing:

The trial pit should be excavated at the same depth of the proposed SuDS report feature
The trial pit should be 1m to 3m long, 0.3m to 1m wide and should have vertical sides and if required filled with granular material for stability.
A lot of water will be used to determine the soil infiltration rate so a water bowser may be needed. The inflow should be rapid so that the pit can be filled to its maximum effective depth in a short time. Take care that the inflow does not cause the walls of the pit to collapse and empty. Each time, record the water level and time from filling, at intervals sufficiently close to clearly define water level versus time.
The soakaway test should be repeated at least three times, the three fillings should be on the same or consecutive days.

Documenting the Test

It’s crucial to maintain detailed records of the percolation test, as this information will be required when applying for planning permission and demonstrating compliance with BRE365. Ensure your documentation includes:

The location of the test site
Date and time of the test
Depth and dimensions of the trail pit
Measurements of water level and percolation rate
Any other relevant observations
Don’t forget to take pictures

Infiltration Rates

Once the raw data has been recorded the times taken for the water level to fall from 75% to 25% can then be used to calculate the soil infiltration rate. Infiltration rates are required to size soakaway storage volumes based on the impermeable area.

Percolation tests are a critical aspect of designing sustainable drainage systems in line with BRE365 guidelines. You can assess the suitability of the soil for soakaway designs and help contribute to effective surface water management, reducing the risk of flooding and protecting the environment.

If you would like to discuss infiltration testing or any drainage related matters on your site such as soakaway calculations, or if you’re in need of a drainage strategy, please feel free to contact us.