Why drainage fields and soakaways fail — and what it costs to put right
A 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.
Read the guideA 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.
A percolation test measures how quickly your ground accepts water, and the result sets the size of the drainage field. Without one you are guessing, and an undersized field will waterlog within a few years. On ground that fails the test, a drainage field is not an option and a treatment plant with a different outfall is the answer.
The principle is simple. You dig trial holes in the area where the drainage field would go, soak them so the ground is wetted up, then fill them to a set depth and time how long the water takes to fall. From that timing you get a value — usually written as Vp — that expresses how many seconds the water level takes to drop a millimetre.
A low number means free-draining ground that takes water readily. A high number means heavy, slow ground. That figure goes straight into the calculation that sets how many metres of trench your field needs for the population the house can hold.
Before the percolation test itself, a deeper trial hole is dug to check two things that can rule out a field entirely: how far down the water table sits, and whether there is rock or an impermeable layer close to the surface. A drainage field needs a clear depth of unsaturated soil beneath the trenches to do its work. If the water table is high in winter — and winter is when it matters — the field will sit in water and treat nothing.
Everything downstream of the test depends on it. Field size is not a judgement call, it is arithmetic: the Vp value multiplied by the population figure gives you the trench area required. Build less than that and the field is overloaded from the first day it is used.
The failure is slow and it is expensive. An undersized field copes for a few years while the soil is still open, then gradually seals as a biomat forms on the trench walls. By the time the ground is soft and the grass is suspiciously green, the only fix is to dig it out and rebuild it. Nobody enjoys explaining that to a homeowner who paid for a new system four years earlier.
Even with a good result, the field has to sit a sensible distance from buildings, boundaries, watercourses, springs, wells and any water supply. It cannot go under a driveway or anywhere vehicles will run, and it needs to be clear of trees whose roots will find it. On a small plot those constraints often bite before the percolation figure does — plenty of properties have ground that would drain beautifully and nowhere legal to put the trenches.
Two things ruin a test. The first is doing it in the wrong conditions: a test run in a dry August on ground that is waterlogged every February gives a flattering number and an undersized field. The second is doing it once, in one hole. Ground varies across a plot, which is why the test is done in several holes and the results taken together.
It is also worth testing where the field will actually go, not where it is convenient to dig. We have seen tests done on the one free patch of ground and the field then built thirty metres away in different soil.
The test comes early — before you choose a system, and certainly before anyone quotes for groundworks. It is the piece of information that determines whether you are buying a septic tank and a large field, a treatment plant and a small one, or a plant discharging to a ditch. Quotes produced without it are quotes for a system nobody yet knows you can have.
We carry out trial holes and percolation testing as part of our installation work across Derbyshire, Sheffield and north Nottinghamshire, and we will tell you plainly if the result means you need to change the plan. Call 07884 111 641.
Three mistakes account for most undersized fields we dig out years later.
Testing after a dry spell. Ground that has been baked for a month takes water far faster than the same ground in February. The figure flatters, the field is built short, and the first proper winter exposes it.
Testing one hole. Soil changes across a plot — a made-up area where a barn once stood behaves nothing like undisturbed pasture ten metres away. Several holes, averaged, is the point.
Smearing the sides of the hole. Digging with a bucket in wet clay polishes the wall of the trial hole and seals it. The test then measures the smear, not the soil.
Keep the test results with the house papers, along with the field design and the installation invoice. When the property is sold, those three documents answer most of what a buyer’s solicitor will ask about the drainage, and they demonstrate that the field was sized from evidence rather than guessed at. It is the cheapest piece of future-proofing in the whole project.
If you are installing or replacing a drainage field, yes in practical terms — the field has to be sized correctly and the test is how that is established. Building control will expect to see it.
Ideally when the ground is at its wettest, because that is the condition the field has to cope with. A result from a dry spell can be misleading, and a good contractor will tell you so rather than quietly using it.
Then a drainage field is not viable and you are looking at a treatment plant with a suitable outfall. It is better to find that out before buying a tank than afterwards.
Usually part of a day for the digging and soaking, with the timed runs following. It is a small cost against the cost of building the wrong system.
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