Soakaway percolation test
A soakaway percolation (infiltration) test measures how quickly water soaks into the ground at the proposed soakaway location. Designers use the result — with drained area and rainfall basis — to size infiltration systems. Without reliable percolation evidence, soakaway sizing is guesswork.
This page explains the process sequence and how results feed installation. Mainline typically installs soakaways to the design that follows the test; we do not usually perform the percolation test ourselves unless separately scoped. Building Control and designers set whether a test is required.
Share raw water-level readings with the designer, not only a single summarised rate, when the method produces time-series data. Transparency helps when results sit near acceptance thresholds.
review Groundworks Contractors in Wandsworth and Groundworks Contractors in Kingston.
When sequence matters
When a new soakaway is proposed and infiltration must be proven; when replacing a failed soakaway and ground performance is uncertain; or when Building Control asks for percolation evidence before approving infiltration drainage.
It also arises when early assumptions of “good soakage” conflict with clay, high water table, or made ground found on site.
Typical sequence
Step 1: Brief the test
What this step delivers: Method, depth, and location agreed with designer.
Step 2: Excavate and test
What this step delivers: Infiltration measurements recorded.
Step 3: Design from results
What this step delivers: Soakaway sized or alternative strategy chosen.
Step 4: Install to detail
What this step delivers: Construction follows approved drawing.
If this matches your project stage, send drawings and site details for a scoped quote — or call to talk through programme and access constraints.
Get an Accurate QuoteWhat each stage delivers
A typical test sequence (method varies by designer/standard referenced) prepares test pits or holes at the soakaway horizon, pre-soaks as required, measures water level fall over time, and calculates an infiltration rate for design use.
Location of the test should represent the actual soakaway position. Testing in a conveniently dry corner that will never hold the crates produces misleading results.
After the designer issues a sized detail, installation proceeds. If percolation fails, expect redesign toward attenuated discharge or another SuDS approach — see attenuation and SuDS pages rather than forcing a soakaway.
If made ground, fill, or a perched water table appears in the test pit, record it explicitly. Designers may discount infiltration or require deeper founding of the soakaway. A tidy numerical rate without ground description is a weak brief for sizing crates or rubble chambers.
Programme risks and hold points
- Designer or Building Control requiring percolation evidence
- Clay, chalk, or variable made ground on the plot
- Previous soakaway failure on the same site
- High winter water table observed in trial pits
- Planning / SuDS strategy relying on infiltration
Pre-start and handover checklist
- Test location matches proposed soakaway
- Method and reporting format agreed
- Seasonal groundwater context noted
- Designer appointed to interpret results
- Fallback drainage strategy if infiltration fails
Keeping trades coordinated
Issue results to the designer, freeze the soakaway detail, then install. Do not excavate full soakaway volume on an untested assumption when Building Control expects evidence.
Archive the test report with the soakaway construction issue so Building Control and future owners can see why the system was sized as it was.
Where time and cost usually escalate
The test itself is usually modest; the programme cost of a late fail is not. Redesign, alternative outfalls, and rework dominate. Soakaway cost pages cover installation rates, not test fees. Seasonal timing matters: a mid-summer test in unusually dry ground can overstate infiltration compared with winter conditions. Note groundwater observations from trial pits alongside the percolation numbers so the designer can judge reliability. Repeating a failed or inconclusive test after groundwater recovery adds little programme cost compared with installing the wrong soakaway size on weak evidence.
Stage overview
| Role | Typical responsibility | Note |
|---|---|---|
| Tester / GI contractor | Carry out and report test | Method per brief |
| Drainage designer | Size system from results | May reject infiltration |
| Groundworks contractor | Install to issued detail | Not a substitute designer |
Where this sequence is commonly used
- Self-build soakaway sizing
- Extension surface-water design
- Replacement soakaways after failure
- SuDS strategies relying on infiltration
Frequently asked questions
Does Mainline always do the percolation test?
Not by default. We commonly install once the designer has sized the soakaway from test results. Testing can be separately scoped if needed.
What if the ground will not soak?
The designer should change strategy — often toward attenuated discharge or another approved outfall. Do not install an undersized soakaway to keep the programme.
How many test pits are needed?
The designer or referenced method sets the number and depth. One convenient hole far from the real soakaway location is usually not enough for a reliable design.
Can I reuse an old percolation result from years ago?
Ask the designer. Ground conditions, water tables, and standards guidance may have changed, and Building Control may want current evidence.
How This Issue Is Normally Diagnosed and Repaired
Follow the typical path from problem identification through to resolution:
We provide these services across the UK, including
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Discuss Your ProjectSoakaway percolation test FAQ
Does Mainline always do the percolation test?
Not by default. We commonly install once the designer has sized the soakaway from test results. Testing can be separately scoped if needed.
What if the ground will not soak?
The designer should change strategy — often toward attenuated discharge or another approved outfall. Do not install an undersized soakaway to keep the programme.
How many test pits are needed?
The designer or referenced method sets the number and depth. One convenient hole far from the real soakaway location is usually not enough for a reliable design.
Can I reuse an old percolation result from years ago?
Ask the designer. Ground conditions, water tables, and standards guidance may have changed, and Building Control may want current evidence.
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Share your site details and priorities. We will outline a practical scope, likely timeline, and next steps.