Percolation tests: how your ground decides what system you can have
A percolation test measures how fast your soil accepts water, and that single figure sets the size of your drainage field — or rules one out entirely.
Read the guideA waterlogged drainage field is the most expensive part of an off-mains system to fix. What causes it, how to spot it early, and what a rebuild costs.
Most drainage fields fail because solids carried over from a tank that was not emptied often enough. The gravel and the soil around it clog, the ground stops accepting liquid, and effluent surfaces. A clogged field cannot be unblocked — it has to be dug out and rebuilt, which is why keeping solids out of it matters so much.
A drainage field is not a hole that water disappears into. It is a network of perforated pipes laid in gravel-filled trenches, sized so that liquid leaving the tank spreads out slowly and is treated as it percolates through the soil. The soil and the organisms living in it do the final treatment — which is exactly why a septic tank is not allowed to discharge anywhere else.
For that to work, three things have to stay true: the liquid arriving has to be relatively free of solids, the trenches have to stay porous, and the ground has to be able to accept the volume. Fail any one and the field stops working.
This is the big one, and it accounts for most of the failed fields we dig out. When a septic tank gets too full to separate properly, solids pass out with the liquid, reach the gravel and clog it. A layer of biological slime — a biomat — builds up on the trench walls and eventually seals them. It is gradual, invisible, and irreversible by the time anyone notices.
Field size is calculated from a percolation test and the number of people the property can hold. Fields built without a test, or sized to the family living there at the time rather than the house, are overloaded from day one. They often work for several years and then fail suddenly, which makes the cause look like something recent.
Heavy clay accepts water very slowly. A high water table means the trenches sit in water for part of the year. Rock near the surface leaves nowhere for liquid to go. None of these are the installer’s fault if a test was done; all of them are a problem if one was not.
A roof downpipe or yard gully connected into the foul run can put more water through the system in an hour of heavy rain than the house uses in a week. Fields that only flood in wet weather almost always have a surface water connection somewhere, and tracing it is usually cheaper than rebuilding the field.
Driving or parking over a drainage field compacts the ground and crushes the pipes. Trees and large shrubs send roots into the trenches for the moisture. Both are avoidable and both are common.
The signs are quiet at first. Grass over the field that is greener and grows faster than the rest of the lawn. Ground that is soft underfoot when the rest of the garden is dry. Standing water in winter that was never there before. A smell outside rather than at the tank lids. Drains that run more slowly than they used to, without any blockage being found.
If the field is only partly compromised and the cause is a tank that has not been emptied, there is a window where emptying the tank on a proper schedule can let the field recover. Once effluent is surfacing consistently, that window has usually closed.
There is no chemical, rod or jet that restores a clogged drainage field. The gravel and the soil interface are the problem, so both have to be replaced. A rebuild means excavating the old trenches, carting away the contaminated material, carrying out a percolation test if one was never done, and building new trenches sized to the result — in a location with enough separation from the water table, boundaries, buildings and any watercourse.
That is why the cost sits in a different league from a tanker visit. The variables are access for machinery, how much material has to be removed, the size the test demands, and whether there is suitable ground available at all. Where there is not — a small garden on heavy clay, say — the answer is usually a sewage treatment plant instead, because its effluent is clean enough to need far less ground, or to go to a watercourse.
A properly built field in suitable ground, fed by a tank that is emptied on time, will run for decades with no attention at all. The things that shorten its life are almost entirely within your control:
If your garden is showing the early signs, it is worth having it looked at now rather than next spring. We trace and inspect systems across Derbyshire, Sheffield and Nottinghamshire, and if the field has gone we will tell you what the realistic options are — rebuild or replacement system — with a proper quote rather than a number over the phone. 07884 111 641.
No. Jetting can clear the distribution pipework if that is silted, but the failure is in the gravel and the soil interface around it, and you cannot jet soil. If the field is clogged it needs rebuilding.
A correctly sized field in suitable ground, fed by a tank emptied on schedule, can last decades. Fields that fail in under ten years were usually either undersized or starved of maintenance upstream.
Not on a clogged field. Products that claim to restore a soakaway are at best addressing biology in the tank. Once the trench walls are sealed, nothing poured down a toilet reaches the problem.
A measured test of how quickly the ground accepts water, done by digging test holes and timing the drop. The result sets the size of the field. It is not optional if you want the field to work.
Septic tank emptying, installations, servicing and drainage across Derbyshire, Sheffield and Yorkshire — 24/7.
A percolation test measures how fast your soil accepts water, and that single figure sets the size of your drainage field — or rules one out entirely.
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