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 number that underpins design: the infiltration rate. This is used to confirm feasibility and enable 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.” When the test is treated like this, you risk undersized systems, flooding complaints, and headaches at handover post completion of the development.

The difference between a robust design and a failure is how you plan, execute, and interpret the testing.

Below, we outline the issues often encountered through poor interpretation of Soakaway testing results and how a geo-environmental consultant can manage them.

Before Testing – Planning with the Ground Model

The risk:

Without a 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. A geo-environmental consultant reviews 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 infeasibility 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, for example where clay overlies sand.

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, designers risk assuming a soakaway will always function, when in fact perched water or seasonal groundwater fluctuations may block the pathway.

Why this matters:

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

• 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: Perhaps the most serious risk is 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 basal 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/ISO 14688 standards 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 amending BRE 365 analysis where layered soils are present.

The Consultant’s Role

BRE 365 testing is not just about filling a hole with water. It is about infiltration assessment which required 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.
• Select appropriate, safer alternatives (such as borehole permeability or ring infiltrometer tests).
• Ensure infiltration designs are reliable, cost-effective, and compliant.
• De-risk development by avoiding soakaway failures that lead to flooding or enforcement action.

Ignoring the ground model turns BRE 365 testing into an expensive box-ticking exercise. Embracing it transforms infiltration testing into a powerful tool for delivering SuDS that work in practice.

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