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Glossary

BRE365

BRE365

A standardised soakaway test method published by the Building Research Establishment that measures the rate at which water infiltrates into the ground, used to design soakaways and assess drainage hierarchy compliance.

BRE365 (formally BRE Digest 365: Soakaway Design) is the standard test method for determining the infiltration rate of soil for soakaway design. Published by the Building Research Establishment, it provides a repeatable methodology for measuring how quickly water drains into the ground at a specific location.

The BRE365 test procedure involves:

  1. Trial pit excavation: A pit is excavated to the proposed soakaway invert depth (typically 1-3 metres below ground level). The soil profile is logged and groundwater conditions are noted
  2. Three successive fills: The trial pit is filled with water to a set level three times in succession. After each fill, the rate of water level drop is recorded at regular intervals
  3. Infiltration rate calculation: The soil infiltration coefficient is calculated from the third fill — the most conservative measurement, as the soil is saturated from the preceding fills. This ensures the design accounts for worst-case conditions

The resulting infiltration rate (expressed in m/s) is used to size soakaways in accordance with BRE Digest 365 and CIRIA C753 (The SuDS Manual). If the infiltration rate is too low (typically below 1 × 10⁻⁶ m/s), soakaway drainage is unlikely to be viable and alternative discharge routes must be considered.

BRE365 testing is critical for demonstrating compliance with the drainage hierarchy, which requires surface water to be discharged to ground via infiltration wherever practicable before considering discharge to a watercourse or sewer. Lead Local Flood Authorities (LLFAs) routinely require BRE365 test results to support drainage applications.

The test can be combined with geotechnical investigation and contaminated land sampling to reduce the number of site visits and overall project cost.

Aegaea carries out BRE365 infiltration testing as part of our infiltration testing service, feeding results directly into drainage design using MicroDrainage software.

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