BRE 365 Soakaway Testing: More Than an Infiltration Rate

BRE Digest 365 is the UK’s method for measuring soil infiltration rates and providing the information required to design soakaways for sustainable drainage (SuDS).

The main purpose of carrying out a Soakaway Assessment is to provide the infiltration rate. The number that underpins design, determines feasibility and enables drainage engineers to design the correct size of soakaway feature, to ensure they can accept the run off from the proposed development and that they can empty between storms.

But a BRE 365 test is not just “fill a hole and time how long it takes to drain.” Treating it like this can risk undersized or oversized systems, flooding issues, and drainage systems failing following development.

Below, we outline the issues often encountered through poor interpretation of Soakaway testing results and how these can be managed.

Before Testing – Planning with the Ground Model

The risk:

Without a good understanding of the anticipated ground model, infiltration testing is often carried out in the wrong places, at the wrong depth, or in soils that were never suitable in the first place. This wastes money and can create unnecessary health and safety risks.

Typical issues include:

• Testing in impermeable soils such as clay, where infiltration will never be viable.
• Ignoring made ground or contaminated strata, risking mobilisation of pollutants.
• Placing tests at the wrong depth relative to the proposed soakaway.
• Overlooking site regrading or cut-and-fill profiles, meaning results don’t match final ground levels.

How consultants manage it:

A strong desk-based assessment comes first. We review geological maps, groundwater data, historic land use, and site investigation records. This allows unsuitable sites to be ruled out early and test locations to be planned properly.

In some cases, the evidence is clear enough to demonstrate infiltration is not feasible. However, some Local Authorities still require BRE 365 infiltration tests to be completed in order to prove infiltration drainage is not feasible before they will accept alternative drainage strategies. Where this is the case, the consultant’s role is to:

• Carry out the minimum testing necessary to satisfy the requirement.
• Record and present results clearly in the context of the wider ground model.
• Provide a strong, defensible argument that infiltration is not viable, allowing the project team to move forward with attenuation, discharge, or other drainage methods.

During Testing – Interpreting Results Correctly

The Risk:

BRE 365 analysis assumes that water infiltrates evenly through the base and sidewalls of the test pit. In reality, infiltration is almost always controlled by the most permeable strata that the water can access.

This is critical in interbedded (layered) ground, where deposits change between different strata over a shallow depth.

Typical scenarios include:

• Sand or gravel between clays:
  A permeable lens or layer at the base of the pit will take most of the water. The surrounding clay walls will contribute almost nothing. If this layer is thin or laterally discontinuous, infiltration performance will be overstated in the long term.
• Chalk beneath superficial clay:
  The clay cap may appear impermeable at the surface, but if the pit intercepts chalk bedrock, rapid infiltration may occur out the base of the pit. Without understanding the depth and continuity of the chalk, there is a risk in assuming a soakaway will always function at the depths tested, when in fact changing bedrock levels, seasonal groundwater fluctuations or other constraints may block the pathway.

Why The Ground Model Matters

If the ground model isn’t used to interpret results, the design team can be misled by:

False Positives

‍Test results may suggest excellent infiltration capacity because water disappears into a localised gravel seam or fissured chalk horizon. But if that layer is thin, discontinuous, or seasonally saturated, the soakaway will fail in service.

False Negatives

Where only a small portion of the test pit accesses a permeable layer, the calculated infiltration rate may be averaged across the whole pit surface. This underestimates the true capacity of the permeable layer, leading to oversized, overly conservative designs.

Inaccurate Designs

Assuming that the infiltration rate measured in a test applies to the whole soakaway volume, without checking which strata actually control infiltration. Two common scenarios highlight the problem:

Case 1 – Thin Permeable Layer Above an Impermeable Base‍

During testing, water escapes into a gravel seam within otherwise clayey ground. If this rate is used for design, the constructed soakaway may quickly fill to its clay base, leaving standing water and reducing effective capacity

Case 2 – Permeable Base Layer Overlain by Clay‍

In some cases, water in a test pit rapidly leaves through a chalk or gravel layer at depth. If the soakaway is then constructed entirely within the overlying clay (because the ground model wasn’t considered), it will not perform as intended.

How consultants manage it:

Qualified geo-environmental professionals log soils to BS 5930 during testing. This ensures infiltration rates are tied to the correct strata. Consultants also apply professional judgement, sense-checking results against published permeability values and providing professional interpretation of the results in the context of the ground model.

Better Soakaway Testing Outcomes

BRE 365 testing is not just about filling a hole with water. It requires a true infiltration assessment which includes combining site data, soil logging, professional interpretation, and regulatory knowledge.

By managing the ground model effectively, a geo-environmental consultant can:

• Avoid unnecessary testing on unsuitable sites.
• Ensure infiltration designs are reliable, cost-effective, and compliant.
• De-risk development by avoiding soakaway failures that lead to flooding or enforcement action.

For developers and designers, the takeaway is clear: involve a geo-environmental consultant early. A well-planned infiltration assessment, grounded in a robust conceptual model, will save cost, avoid failure, and give regulators confidence in your drainage strategy.

If you’re new to infiltration testing or want a refresher on the basics, start with our companion piece: Soakaway Testing: A Developer’s Guide

References

EPG Limited (2024). Soakaway Testing – Technical Briefing Note. Environmental Protection Group (EPG).