Soil layers
Topsoil, made ground, clay, sand or gravel can behave very differently. A shallow garden test may miss the layer around the proposed system.
Ground evidence before drainage design
Percolation Tests for Soakaways: Surrey Clay Soil Guide
What infiltration testing can tell you, why a quick bucket test is not the same as design evidence, and what to consider when Surrey ground drains slowly.
The direct answer
What is a percolation test for a soakaway?
It measures how water disperses into the ground at a representative depth and location. For surface-water soakaways, the recognised design approach commonly referenced is BRE Digest 365. The result helps a competent designer judge viability and size; it is not simply a pass/fail label for an entire garden.
- Test where and at the depth the proposed infiltration system will actually operate.
- Repeat the filling and timing sequence needed by the chosen method, rather than relying on one observation.
- Record pit dimensions, water levels, times, weather, soil and groundwater observations clearly.
- Test purpose
- Establish an infiltration rate for design and suitability decisions
- Common reference
- BRE Digest 365 for rainwater soakaway design
- Not equivalent to
- A small hand-dug hole filled once with a bucket
Make the right decision
Why results can vary across one property
Ground is layered and altered by previous construction. A result is only meaningful when its location, depth and site context are representative.
Seasonal moisture
Groundwater and already saturated soil affect available storage and recovery. Dry-weather observations may not describe winter performance.
Compaction & disturbance
Construction traffic, old foundations, backfilled trenches and imported material can create local pathways or barriers.
Topography & nearby assets
Slopes, retaining walls, basements, boundaries, trees and neighbouring levels influence safe siting even if infiltration appears possible.
Compare clearly
How to interpret the next step
Swipe across to compare every column
| Finding | What it may mean | Do not assume |
|---|---|---|
| Water level falls consistently | Infiltration may be viable at that tested position and depth | That any crate size or location will work |
| Water barely falls | Ground may be too slow or saturated for a conventional soakaway | That a larger hole automatically fixes it |
| Different pits give different results | Ground is variable or has been disturbed | That the most favourable result represents the whole site |
| Groundwater appears in the pit | There may be limited unsaturated depth or seasonal risk | That the system can be installed at the planned depth |
Devil in the detail
Testing informs a system—not just a crate count
The designer must still consider the design storm, connected impermeable area, inflow, storage, emptying time, separation, exceedance route and maintenance. A test result is one vital input in that wider drainage decision.
Talk through your siteFrom question to clear scope
A sensible testing and design sequence
No unexplained leap from problem to price. Each step reduces uncertainty and makes the next decision easier.
- 1
Map the catchment
Identify every roof and hard surface the proposed system will receive.
- 2
Choose candidate locations
Screen for buildings, boundaries, services, slopes, access and other constraints.
- 3
Prepare representative pits
Use a location and depth relevant to the proposed infiltration surface.
- 4
Run & record the method
Measure repeat water-level changes accurately and note ground observations.
- 5
Review feasibility
Consider infiltration rate alongside groundwater, siting and the available footprint.
- 6
Design or change strategy
Size the system properly or document why another drainage route is needed.
Prepare once, quote better
What to include in a useful test record
Clear records allow the designer, contractor and approving body to understand what was tested and prevent favourable numbers being detached from their context.
- Plan showing test locations
- Pit dimensions and depth
- Soil and made-ground observations
- Groundwater observations
- Fill levels, timing and repeated readings
- Calculation method and resulting rate
- Photos before, during and after
Useful questions
Before you commit to the work
Clear answers now prevent expensive assumptions being buried later.
Is a foul-drainage percolation test the same?
No. Tests for drainage fields serving wastewater use a different purpose and method from surface-water soakaway testing. Be clear which system is being designed.
Can I do the test myself?
You can collect early observations, but formal planning or design evidence needs a suitable method, representative excavation, accurate records and competent interpretation. Check what the approving body requires.
What if the test fails?
A slow result is useful evidence. The next step may be a different location, more investigation, an alternative SuDS measure, attenuation with an approved restricted discharge, or another lawful strategy.
How many tests are needed?
It depends on system scale, site variability and approval requirements. More than one location or repeated test may be needed to show that the result is representative.
Will clay soil always fail?
No, but clay-rich and seasonally saturated ground needs caution. Local layers and site history matter, so the answer should come from evidence rather than the general soil label.
A practical next step
Turn the site details into a clear scope
Send photos, the property location and what happens during rain or construction. We will help identify the most useful next survey.