Do You Need Infiltration Testing?
Infiltration testing is often required when designing a drainage strategy for a planning application.
Whether testing is needed depends on site conditions, the stage of the planning process, and the proposed drainage approach. Where it is required, failing to carry it out can lead to delays, additional costs, or drainage systems that are not fit for purpose.
Our guide will talk you through what infiltration testing is, the technical considerations and best practice.
What is Infiltration Testing?
Planning policy across the UK promotes the use of Sustainable Drainage Systems (SuDS), with infiltration to ground typically the preferred method for managing surface water runoff.
Infiltration testing provides site-specific data on how quickly water can drain into the ground. This is used to design drainage features such as soakaways and permeable paving, and to demonstrate that a proposed drainage strategy is viable.
When is Infiltration Testing required?
In many cases, Local Authorities will require infiltration testing to support a planning application, particularly where infiltration is proposed as the primary drainage solution.
It may also be required:
- as part of a planning condition
- to support detailed drainage design
When might it not be required?
Infiltration testing may not be necessary in certain situations, including:
- at outline planning stage, where only an indicative strategy is needed
- on sites where ground conditions clearly prevent infiltration
- for smaller developments
In these cases, a desk-based drainage assessment can often determine whether infiltration is feasible before undertaking site testing.
What does testing involve?
Infiltration testing is typically carried out in accordance with BRE Digest 365.
The process involves excavating a trial pit, filling it with water, and measuring the rate at which the water drains. Multiple tests are undertaken, with the lowest infiltration rate used to inform drainage design.
What can go wrong?
Poorly planned or executed testing can result in inaccurate data. Common issues include:
- testing in unrepresentative locations
- incorrect test depth
- unsuitable timing (e.g. very dry or frozen conditions)
- failure to consider groundwater or contamination
These can lead to delays in planning or ineffective drainage design.
How can GeoSmart help?
GeoSmart supports clients throughout the drainage process, from initial SuDS assessments to infiltration testing and detailed design.
Download our full guide
For more detailed guidance on infiltration testing, including technical considerations and best practice, download our full guide:
