What is Infiltration Testing?

What is Infiltration Testing and Why is it Necessary?

In line with the drainage hierarchy, Sustainable Urban Drainage Systems (SuDS) should first consider the discharge of surface water runoff from impermeable areas to ground. This is also known as infiltration testing for SuDS or percolation to ground. GeoSmart’s SuDSmart Plus report is a desktop based assessment of the Site conditions, which can provide a preliminary opinion on whether infiltration to ground is likely to be feasible at the Site.

The Role of Infiltration in SuDS and Planning Applications

For many developers and architects, a soakaway is the primary choice for managing surface water. However, relying on assumptions rather than site-specific data can be a costly mistake. If our SuDSmart Plus report identifies that ground conditions are likely to be suitable, it is essential to carry out formal infiltration testing before your site designs are finalised.The results of a BRE 365 test directly dictate the required storage volume and depth of your drainage features. By testing early in the process, you avoid the risk of expensive redesigns or failed layouts later on. Furthermore, while most Local Planning Authorities (LPAs) require proof of infiltration rates to discharge planning conditions, they may also demand prohibitive testing to prove a site is not suitable for infiltration if the desktop data is inconclusive.

Understanding Soil Permeability and Infiltration Rates

The calculated soil infiltration rate is the critical variable used to determine the feasibility of soakaway drainage. It directly influences the design storage volume, the required attenuation capacity, and the total number of SuDS features needed for a site.

Soil types across the UK vary significantly; for instance, free-draining sandy soils offer high permeability, whereas heavy clays may be virtually impermeable. To get an accurate result, infiltration testing must be carried out in the specific area of the site where the proposed soakaways or infiltration basins are located.

Seasonal Variables and Design Safety

It is vital to account for seasonal fluctuations when testing. Factors such as high groundwater levels and increased soil moisture content during winter months can significantly reduce a soil’s ability to accept runoff. For a robust drainage strategy, infiltration rates should be calculated based on a “worst-case scenario” to ensure the system remains effective during prolonged periods of wet weather.

How to Conduct an Infiltration Test (BRE Digest 365 Standard)

Most planning conditions recommend that infiltration testing should be undertaken in accordance with BRE Digest 365. There are several key stages when undertaking an infiltration test:

Step 1: Excavating the Soakage Trial Pit

• A soakage trial pit to the same depth as the anticipated SuDS feature.
• The soakage trial pit should be 1 m to 3 m long and 0.3 m to 1 m wide. It should have vertical sides trimmed square and, if necessary, for stability, should be filled with granular material. When granular fill is used, a full-height, perforated, vertical observation tube should be positioned in the soakage trial pit so that water levels can be monitored with a dip tape.
• It should be possible to construct a suitably-dimensioned pit with a backhoe loader or mini-excavator. For safety reasons do not enter the soakage trial pit.
• 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 soakage trial pit can be filled to its maximum effective storage depth in a short time. Take care that the inflow does not cause the walls of the soakage trial pit to collapse.

 Step 2: Filling and Recording Water Levels

• Fill the soakage trial pit and allow it to drain three times to near empty. Each time record the water level and time from filling, at intervals sufficiently close to clearly define water level versus time. The filling of the soakage trial pit should be on the same or consecutive days.

Step 3: Calculating the Soil Infiltration Rate

• Calculate the soil infiltration rate from the time taken for the water level to fall from 75% to 25% effective storage depth in the soakage trial pit.

Interpreting Your Results: What is a ‘Good’ Infiltration Rate?

An example of a good infiltration rate, which would sufficiently discharge water to ground and where focused infiltration features would be suitable would be x 10 -4 or -5.

A poor infiltration rate would be x 10 -6. This may still support infiltration to ground via focused infiltration, if this was the only discharge method feasible, but would need to be larger in size to accommodate for a slower emptying time.

An example of an infiltration rate where discharge to ground would not be feasible would be x 10 -7. In this case, other discharge options with the drainage hierarchy would have to be explored.

If you wish to discuss infiltration testing, surface water management or other drainage related matters on your site, our team can help!