Soakaways & Drainage Fields – Practical Guide
Neutral, plain-English guidance on how soakaways work, when they're suitable, testing, basic design ideas, common problems, and FAQs.
What is a soakaway?
A soakaway (also called a drainage field when used for wastewater) is an underground system that allows water to infiltrate gradually into suitable soil. For **surface water**, it's typically a rubble-filled pit or modular crate system. For **off-mains wastewater**, a drainage field disperses treated effluent from a septic tank or small treatment plant through perforated pipes in granular backfill, allowing final polishing in the soil.
Good soakaway performance depends on three things: soil permeability, correct sizing, and keeping solids/grease out.
When are soakaways appropriate?
- Surface water: Roofs/driveways where soil has adequate infiltration and discharge to sewers isn't preferred or available.
- Off-mains wastewater: Where rules allow a drainage field after a septic tank or treatment plant and ground conditions (and distances to features) are suitable.
Unsure if your plot is suitable? Describe your soil type, water table, and space in chat for a quick sense-check.
Percolation testing (the non-negotiable)
A **percolation test** measures how quickly water soaks into the soil. The result drives pipe lengths, trench dimensions, and overall loading limits. Without it, designs tend to undersize (leading to failure) or oversize (wasting space and cost).
What the test typically tells you
- Soil suitability (clay vs sandy soils perform very differently)
- Calculated infiltration rate (used for drainage field sizing)
- Whether seasonal groundwater might be a constraint
Basic design concepts (high-level)
- Load spreading: Perforated laterals in granular media disperse flow evenly to prevent localised saturation.
- Levels & gradients: Gentle falls promote distribution without scouring; avoid ponding points.
- Protect the soil interface: Include geotextiles where appropriate and prevent fines/silt from clogging.
- Keep solids/grease out: Regular septic tank emptying and working baffles prevent carry-over.
- Separation distances: Maintain sensible stand-offs from buildings, boundaries, watercourses, and wells.
For wastewater, always size from tested infiltration figures, not guesswork.
Common issues and what they usually mean
- Ground stays saturated/boggy: Soil permeability too low, system overloaded, or high groundwater.
- Effluent at the surface: System failure—often undersized, clogged, or receiving solids/fats from the tank.
- Smells near the field: Possible poor treatment upstream, ventilation issues, or ponding.
- Short lifespan: Inadequate testing/design, or lack of septic tank maintenance allowing fines into the field.
If you're seeing these signs, a site assessment plus, where relevant, a CCTV survey and tank check will help pinpoint the cause.
Maintenance & good practice
- Empty and service upstream tanks/plant at sensible intervals.
- Avoid putting fats, oils, wipes, disinfectants, or non-biodegradables into the system.
- Keep vehicles/heavy loads off the drainage field area.
- Route roof/surface water away from wastewater drainage fields.
- Monitor for early symptoms (wet patches, odours) and act promptly.
Regulatory & approval basics (plain-English)
New or replacement systems often have rules to follow (permissions, siting, design evidence, and proof of soil suitability). Building Control may expect to see percolation test results and a clear design rationale for any drainage field. Discharges must comply with current requirements—direct outfalls to ditches are commonly restricted without compliant treatment and consent.
If you're mid-project, ask in chat what documents are usually requested for sign-off.
FAQs
Can I use crate soakaways for wastewater?
Modular crates are typical for surface water. Wastewater dispersal fields use perforated pipes in granular media to promote even treatment in the soil—follow appropriate design approaches.
What if my soil fails the percolation test?
You may need an alternative solution (e.g., a treatment plant with a different discharge route, where permitted). Forcing a drainage field into unsuitable soil usually leads to failure.
How far from buildings should a drainage field be?
Designs commonly include stand-offs from buildings, boundaries, trees, and water features. Exact values depend on local rules and site constraints.
Do soakaways work in clay?
Heavy clays often have poor infiltration. Testing confirms whether a soakaway/drainage field is viable or if you need an alternative.