BRE365 Testing: What It Is and When You Need It

If you are developing a site in England or Wales and your drainage strategy involves discharging surface water to the ground — through soakaways, permeable paving, or any form of infiltration system — you will almost certainly need BRE365 testing. It is the standard method for determining whether the ground can accept surface water runoff, and it is one of the first things the Lead Local Flood Authority (LLFA) will ask to see when reviewing your drainage proposals.

Despite being a well-established test with a clear procedure, BRE365 testing is frequently carried out incorrectly, at the wrong locations, at the wrong depth, or at the wrong time of year. These mistakes can invalidate the results, delay your planning application, and lead to drainage designs that do not work.

This article explains what BRE365 testing is, how it works, when you need it, and how to avoid the most common pitfalls.

What Is BRE365?

BRE365 refers to BRE Digest 365, a guidance document published by the Building Research Establishment (BRE) in 1991 and subsequently revised. Its full title is “Soakaway Design” and it sets out a standardised method for testing the infiltration capacity of the ground at a proposed soakaway location.

The test is sometimes referred to as a soakaway test, a soakaway infiltration test, or simply a BRE test. Whatever the name, the procedure and the purpose are the same: to measure the soil infiltration rate — expressed in metres per second (m/s) — at the location and depth where the soakaway or infiltration feature is proposed. This infiltration rate is then used to design the soakaway or infiltration system, determining how large it needs to be to handle the volume of runoff generated by the development.

It is important to understand that a BRE365 test is site-specific and location-specific. The infiltration rate can vary significantly across a site and at different depths, depending on the soil type, geology, groundwater level, and ground conditions. A test carried out in one corner of a site does not necessarily represent conditions elsewhere. This is why LLFAs require testing at the proposed soakaway location and depth, not just “somewhere on site.”

The Test Procedure

The BRE365 test procedure involves excavating a trial pit at the proposed soakaway location, filling it with water, and measuring the rate at which the water drains away. The test is carried out three times in succession to account for the fact that the infiltration rate changes as the soil becomes saturated.

Step 1: Excavate the Trial Pit

A trial pit is excavated at the location and to the depth of the proposed soakaway. The dimensions of the pit are recorded, along with the ground conditions encountered — soil type, stratigraphy, presence of groundwater, and any other relevant observations.

The pit should be excavated to the invert level of the proposed soakaway, not to the ground surface level. If the soakaway is designed to sit 1.5 metres below ground level, the test should be carried out at 1.5 metres depth. This ensures the test measures the infiltration rate of the soil that will actually be in contact with the soakaway, not the topsoil or fill material above it.

Step 2: Fill the Pit with Water (First Fill)

The pit is filled with water to a depth of at least 300mm. The water level is recorded at regular intervals — typically every five minutes for the first hour, then every 15 minutes thereafter — as it drains away.

The first fill is essentially a saturation phase. The soil surrounding the pit absorbs water through both infiltration (gravity-driven flow through the soil matrix) and absorption (capillary action drawing water into dry soil). The first fill rate is therefore typically faster than the subsequent fills because the soil is initially unsaturated.

Step 3: Refill and Repeat (Second and Third Fills)

As soon as the water level has dropped to 25% of the initial level (or the pit is empty), it is refilled to the original level and the process is repeated. The test is carried out three times in total.

The second and third fills give a more representative measure of the steady-state infiltration rate — the rate at which water drains away when the surrounding soil is already saturated. This is the rate used for design purposes, because a soakaway in service will receive runoff repeatedly during storm events, and the soil will already be partially saturated from previous rainfall.

Step 4: Calculate the Infiltration Rate

The infiltration rate (f) is calculated from the time taken for the water level to fall from 75% to 25% of the effective depth during the third fill (or the average of the second and third fills, depending on the consultant’s approach and the LLFA’s requirements). The calculation accounts for the surface area of the pit that is in contact with water, giving a rate expressed in metres per second (m/s).

A typical infiltration rate for a well-draining sandy soil might be in the range of 1 x 10^-4 to 1 x 10^-3 m/s. Clay soils will typically produce rates below 1 x 10^-6 m/s, which is generally too low for infiltration drainage to be viable.

When Do LLFAs Require BRE365 Testing?

LLFAs require BRE365 testing whenever a drainage strategy proposes to discharge surface water to the ground. This is because the drainage hierarchy — the policy framework that governs how surface water should be managed — places infiltration (discharge to ground) as the preferred option. Before a developer can propose discharging to a watercourse or sewer, they must demonstrate that infiltration is not feasible, and BRE365 testing is the evidence required to make that case.

In practice, this means BRE365 testing is required in the following scenarios:

• The drainage strategy proposes soakaways or infiltration features. The LLFA will require BRE365 test results to confirm that the ground can accept the design flows and that the soakaway is appropriately sized.
• The drainage strategy proposes discharging to a watercourse or sewer. Paradoxically, you also need BRE365 results even when you are not proposing infiltration — because you need to demonstrate that you have considered infiltration and that it is not viable. Without BRE365 test results showing an inadequate infiltration rate, the LLFA will ask why you have not designed for infiltration.
• Planning conditions require it. Many planning consents include conditions requiring infiltration testing to be carried out and the results submitted to the LLFA before drainage construction can begin.

The only scenario in which BRE365 testing may not be required is where the site is in an area of known non-infiltrating ground (heavy clay, made ground, or high groundwater) and the LLFA accepts a desk-based justification for ruling out infiltration. However, even in these cases, many LLFAs will still request testing as confirmation.

What Do the Results Mean for Drainage Design?

The infiltration rate from the BRE365 test directly determines the size and type of infiltration drainage that can be used:

• High infiltration rate (> 1 x 10^-4 m/s): Soakaways and infiltration features will work well. The system can be relatively compact because the ground accepts water quickly. Sandy and gravelly soils typically fall into this range.
• Moderate infiltration rate (1 x 10^-5 to 1 x 10^-4 m/s): Infiltration is viable but the system will need to be larger to compensate for the slower drainage rate. Silty soils and weathered rock often produce rates in this range.
• Low infiltration rate (1 x 10^-6 to 1 x 10^-5 m/s): Infiltration is marginal. A soakaway may be technically feasible but impractically large. The drainage design may need to combine partial infiltration with an attenuated discharge to a watercourse or sewer.
• Very low infiltration rate (< 1 x 10^-6 m/s): Infiltration is not viable. The drainage strategy must look to the next option in the drainage hierarchy — typically discharge to a watercourse. Clay soils and made ground typically produce rates in this range.

The BRE365 results also influence the factor of safety applied to the design. Most LLFAs require a factor of safety of at least 2 (i.e., the design infiltration rate is halved compared to the tested rate) to account for long-term degradation of infiltration performance due to silting, root growth, and changes in groundwater level.

Common Mistakes

We carry out infiltration testing on a regular basis and frequently review BRE365 test results submitted by others. These are the most common mistakes we see:

Testing at the Wrong Depth

The test must be carried out at the invert depth of the proposed soakaway, not at ground level. Topsoil and the upper soil horizon often have very different infiltration characteristics to the underlying subsoil. Testing at ground level and then designing a soakaway at 1.5 metres depth is unreliable — the infiltration rate at depth may be significantly lower (or, occasionally, higher) than at the surface.

Testing in the Wrong Location

BRE365 results are specific to the test location. If the soakaway location changes during the design process — which happens frequently as the site layout evolves — the test results may no longer be valid. Test at the proposed soakaway location, or, if the location is not yet fixed, test in multiple locations to provide flexibility.

Only Carrying Out One Fill

The three-fill procedure exists for a reason. The first fill saturates the soil and produces an artificially high infiltration rate. The design should be based on the later fills, which represent the steady-state condition. We occasionally see test reports that record only one fill and present a rate that is unrealistically optimistic.

Testing in Summer Only

Ground conditions vary seasonally. In winter, the water table rises and the soil is more saturated, both of which reduce the effective infiltration rate. Testing in summer, when the ground is at its driest, can produce results that overstate the year-round infiltration capacity. Ideally, testing should be carried out during the wetter months (October to March) to capture the worst-case conditions. At a minimum, the design should account for seasonal variation by applying an appropriate factor of safety.

Ignoring Groundwater

If groundwater is encountered in the trial pit, this is significant. A soakaway cannot function if its invert is below the water table, because there is nowhere for the water to go. The BRE365 test should record the standing groundwater level, and the soakaway design must ensure a minimum clearance — typically 1 metre — between the base of the soakaway and the peak groundwater level.

Practical Advice

Carry out BRE365 testing early. The results influence the entire drainage strategy. If infiltration is not viable, you need to know before you design the drainage layout, not after. Early testing also avoids delays at the planning condition stage.

Test at multiple locations and depths. If the site is large or the geology is variable, a single test point is not sufficient. Testing at two or three locations provides a more robust dataset and gives confidence that the drainage design will work across the site.

Combine BRE365 with the geotechnical investigation. If you are already commissioning boreholes or trial pits for a geotechnical investigation, adding BRE365 testing to the same mobilisation is efficient and cost-effective. The geotechnical consultant may need to coordinate with the drainage engineer to ensure the test locations and depths align with the drainage design.

Keep the LLFA informed. If you know that infiltration is not going to work — because the site is on heavy clay, for example — discuss this with the LLFA before carrying out testing. They may accept a reduced testing scope or a desk-based justification, saving you time and money.

If you need BRE365 infiltration testing or advice on your drainage strategy, get in touch. We carry out testing across England and Scotland and can usually mobilise within two weeks.