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How to Conduct a BRE 365-Compliant Infiltration Test for Soakaway Design

Soakaways are an essential component of sustainable drainage systems (SuDS), designed to manage stormwater by allowing it to infiltrate into the ground. Conducting a BRE 365-compliant infiltration test is crucial to ensure proper soakaway design, ensuring stormwater runoff is effectively managed. This guide will walk you through the process.


What is a BRE 365 Infiltration Test?

Virtually all new development applications require infiltration testing to be undertaken as part of demonstrating SuDS Compliance. However, what many do not know, is anyone can undertake the test, as long as it is done in accordance with the BRE guidance.


A BRE 365 infiltration test determines the soil's infiltration rate at the proposed site of a soakaway. This rate helps calculate the soakaway’s required size to handle stormwater runoff efficiently. The test in affect involves digging a hole, filling it with water, and recording how quickly the water drains down. Below provides a step by step process.


Step-by-Step Guide to Conducting the Test

1. Preparation

  • Trial Pit Size:

    • For new dwellings and commercial sites: excavate a pit 0.3 to 1m wide, 1 to 3m long, and to the anticipated depth of the soakaway. The depth of the trail should reflect they type of soakaway proposed. For traditional soakaways, this is between 2 and 3m deep, where as for permeable paving, this should be between 0.6m and 1m deep.

    • For extensions: a 300mm x 300mm pit excavated to a depth 300mm below the invert level of the proposed drain.

  • Site Considerations:

    • Ensure the pit represents the full-size soakaway.

    • Avoid testing if groundwater levels might rise above the soakaway base.

2. Pit Excavation

  • Excavate the pit with straight vertical sides.

  • If needed, stabilize the pit with granular material.

  • If granular material is used, install a perforated vertical observation tube.

3. Conducting the Test

  • Filling the Pit:

    • Fill the pit with water rapidly to simulate rainfall runoff.

    • Allow the water to infiltrate three times before taking readings to simulate typical soil moisture conditions.

  • Water Level Monitoring:

    • Measure the time for the water level to drop from 75% to 25% of the pit's effective storage depth.

    • Use pegs or markers to define these levels for accurate measurements.

4. Data Recording

  • Record the times for water levels to fall from 75% to 25% during each test cycle.

  • Undertake the test three times (to represent saturated ground conditions).

  • Use the worst-case infiltration rate (slowest observed)



  • for design calculations.

5. Calculation

  • Use the formula: f=Vp75−25ap50×tp75−25f = \frac{V_{p75-25}}{a_{p50} \times t_{p75-25}}f=ap50​×tp75−25​Vp75−25​​ Where:

    • Vp75−25V_{p75-25}Vp75−25​: Volume of water drained between 75% and 25%.

    • ap50a_{p50}ap50​: Surface area of the pit to 50% effective depth.

    • tp75−25t_{p75-25}tp75−25​: Time taken for water to drop from 75% to 25%.


Key Benefits of BRE 365 Testing


  • Ensures compliance with sustainable drainage regulations.

  • Prevents stormwater flooding and ground instability.

  • Promotes efficient stormwater management in diverse soil conditions.


By following this guide, you'll ensure a reliable, BRE 365-compliant soakaway system that effectively manages stormwater and contributes to sustainable urban development​​​



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