What Is a Trial Pit? How Engineers Find Out What Your House Is Really Sitting On

Most people never think about what’s underneath their house until something goes wrong. A crack appears. A door stops closing properly. An insurer asks a question nobody can answer from ground level.

At that point, someone usually suggests digging a trial pit — and if you’ve never heard the term before, it sounds a lot more dramatic than it is.

Here’s what one actually involves, and why it’s often the fastest way to get a straight answer about a building.

So what is a trial pit?

A trial pit is a small hole, dug by hand, right up against the foundation of a building.

It’s usually somewhere between half a metre and a metre and a half deep, and roughly the size of a large kitchen sink at the surface. The point is simply to expose the foundation so it can be seen, measured and understood.

That’s it. No machinery, no drilling rig, no scaffolding. A spade, some care, and someone who knows what they’re looking at.

The photograph above is a real one — sample bagged, notes being written up on site, spade and probe rods leaning against the wall. That’s genuinely what the job looks like.

Why would anyone dig one?

Four situations account for most of the trial pits we dig.

Cracking that might be subsidence

A crack tells you something has moved. It doesn’t tell you why. If the foundation is shallower than expected, or sitting on ground that shrinks and swells, that’s a very different problem to thermal movement in the plaster — and a very different repair bill.

Planning an extension

Before you can design new foundations next to old ones, you need to know what the old ones actually are. Assumptions here are expensive. Victorian foundations in particular are often far shallower than people expect.

An insurance claim

Insurers dealing with subsidence claims almost always want evidence, not opinion. A trial pit produces exactly that: measured depth, described strata, and a soil sample that can be tested.

Buying a property with a question mark over it

If a surveyor has flagged movement and recommended further investigation, this is usually what they mean.

What an engineer actually learns from it

A trial pit answers several questions at once, which is why it’s such an efficient piece of work.

• Foundation depth and width. How far down it goes, and how wide it spreads. This alone often explains a lot.
• What the foundation is bearing onto. Firm clay, made ground, sand, gravel, or something that shouldn’t be carrying a house at all.
• The condition of the foundation itself. Brick footings, mass concrete, or in older properties, occasionally very little.
• Moisture and root activity. Roots near a foundation in shrinkable clay are one of the most common causes of seasonal movement in UK housing.
• A soil sample for testing. Bagged on site and sent to a lab to establish plasticity — how much that particular soil will shrink and swell with moisture.

Put together, that turns “there’s a crack and we’re not sure why” into a proper diagnosis.

Why this matters especially in Peterborough

Much of Peterborough sits on Oxford Clay — the same clay that supplied the city’s brickworks and shaped where it grew.

Clay of this kind is what engineers call shrinkable. It expands when it takes on water and contracts when it dries out. Over a long dry summer, that movement can be enough to shift a shallow foundation, which is why cracking so often appears or widens in late summer rather than mid-winter.

Add a mature tree drawing moisture out of the ground nearby, and the effect concentrates around the roots.

This is exactly the sort of thing a trial pit settles quickly. Is the foundation deep enough to be below the zone where seasonal moisture change matters? Are roots present at foundation level? Those are observable facts, not judgement calls — but only once someone has actually looked.

Trial pit, borehole, or neither?

People sometimes use “ground investigation” to mean any hole in the ground. There is a difference.

A trial pit is shallow, hand-dug, and best for inspecting an existing foundation directly. You can see it, measure it, photograph it.

A borehole goes much deeper, needs a rig, and is used to understand ground conditions well below foundation level — typically for new build or larger development, not a domestic crack investigation.

And plenty of cases need neither. If cracking is clearly thermal movement or plaster shrinkage, we’ll say so and save you the cost. Recommending intrusive work that isn’t needed helps nobody.

What happens afterwards

The pit gets backfilled and reinstated the same day. Some disturbed ground and a bit of flattened planting is normally the extent of the mess.

What follows is the part that actually matters: the findings feed into a structural appraisal setting out what was found, what it means, and what — if anything — needs doing about it.

Quite often the conclusion is reassuring. The foundation is adequate, the movement is historic and stable, and no remedial work is required. Getting that in writing from a chartered engineer is worth having, particularly if you’re selling or dealing with an insurer.

When work is needed, the appraisal explains why, with evidence behind it rather than guesswork.

A quick word on cost

The honest answer is that it depends — on access, on how many pits are needed, on whether lab testing is involved, and on what the report has to satisfy.

What’s worth saying plainly is this: the cost of finding out is almost always lower than the cost of assuming wrongly. Underpinning a building that never needed it is an expensive mistake. So is designing an extension around foundations nobody checked.

Frequently asked questions

Will a trial pit damage my house?

No. It’s a small, carefully dug hand excavation next to the foundation, backfilled the same day. It’s specifically designed to be non-damaging to the structure.

How long does it take?

Usually a few hours on site for a single pit, including recording and sampling. Lab testing, if required, adds time before the report is issued.

Do I need permission to dig one?

On your own property, generally not. If the pit is close to a shared boundary or a neighbouring structure, the Party Wall etc. Act 1996 may apply — worth checking early rather than late.

Can’t you just look at the crack instead?

Often, yes — and most appraisals begin non-intrusively for exactly that reason. A trial pit is for when the answer genuinely lies below ground and can’t be reached any other way.

Will my insurer accept the findings?

A structural appraisal from a chartered engineer, backed by measured evidence and soil testing, is normally exactly what insurers ask for. It’s always worth confirming the format your insurer requires before instructing.

Thinking about getting one done?

If you’ve got cracking you can’t explain, an extension in the planning stage, or an insurer asking questions you can’t answer, we can help you work out whether a trial pit is genuinely necessary — or whether something simpler will do.

We’re a chartered structural engineering practice working from offices in Peterborough and Borehamwood, covering the East of England, Hertfordshire and Greater London, and we’re fully insured to £5 million professional indemnity.

Send us a photo of the problem and we’ll give you an honest view before anyone picks up a spade.