Soil and site investigation means gathering evidence about what is under and around a building before a repair is chosen. For a house with cracks that usually means trial pits to expose the footing, soil sampling and testing, a CCTV survey of the drains and sometimes a period of monitoring. A chartered structural engineer decides what is needed and interprets the results. We arrange and carry out the site work to the engineer's brief, so the repair that follows is based on facts rather than guesswork.
Why investigate instead of going straight to a repair?
Because the same crack can have very different causes. A diagonal crack over a bay window might come from clay drying out near a tree, from a cracked drain washing out sandy soil, from a failed lintel, or from movement that stopped decades ago. Each points to a different answer, and only one of them might ever need underpinning. The RICS consumer guide on subsidence says fewer than 5% of affected properties require underpinning, which is exactly why the evidence matters: most of the time the right answer is to remove the cause and repair the damage.
Investigation also protects you. A repair chosen without evidence can be too little, letting cracks reopen, or far too much, costing a great deal for work the house never needed.
What does a site investigation usually include?
| Technique | What it shows | When it is used |
|---|---|---|
| Trial pits | Depth, width and construction of the existing footing; the soil directly beneath; roots; signs of water | Most subsidence investigations and before any underpinning design |
| Soil sampling and testing | Soil type and how it behaves, for example how shrinkable a clay is and how much moisture it holds | Where clay shrinkage or heave is suspected, or where the engineer needs design values |
| Deeper boreholes or probes | Ground conditions below trial pit depth, and how deep firmer ground lies | Where piled or deeper foundations are being considered |
| CCTV drain survey | Cracked, displaced or leaking pipes and joints near the foundations | Almost always where cracking is near drains, gullies or soil stacks |
| Crack and level monitoring | Whether movement is continuing, seasonal or has stopped | Where the engineer needs to see behaviour over time |
Monitoring is covered in more detail on our crack and movement monitoring page, and leaking drains on the drainage-related subsidence page.
What kinds of ground are found around Merseyside?
The British Geological Survey's description of the Liverpool district gives a broad picture. Much of the bedrock is Triassic sandstone, but over large parts of the area it is covered by glacial till, often called boulder clay. Along the Sefton coast and north Wirral there is blown sand and dune sand. Alluvium, peat and coastal deposits can be soft and compressible, and so can poorly compacted made ground and old clay pits that were backfilled after brickmaking.
That is useful background, but it is not a diagnosis. Ground can change within a single garden, and made ground is by nature unpredictable. Nobody should assume what is under a particular house from a map, which is why the trial pit is still the most useful tool on most jobs.
If you live in or near St Helens, parts of which lie on the former Lancashire Coalfield, GOV.UK's free "Find out if a property is affected by coal mining" search will show whether a mining report is advised for your postcode. Mining subsidence is a matter for the Mining Remediation Authority, not something we handle. See our St Helens page for local context.
How do we carry out the investigation?
- Agree the brief. The structural engineer sets out what is needed: how many trial pits and where, what samples, whether drains are surveyed, and whether monitoring follows.
- Check for services. Before digging, gas, water, electricity and drainage routes are located so nothing is damaged.
- Dig and record. Trial pits are excavated by hand beside the wall, the footing is measured and photographed, and the soil profile is logged. The engineer may attend to inspect.
- Sample and survey. Soil samples go for laboratory testing where specified. A drain camera survey is run through the relevant runs.
- Reinstate. Pits are backfilled and compacted, and paving or turf is put back as agreed.
- Report. The engineer interprets the results and recommends the next step, which may be cause removal and monitoring, crack repair or, less often, an underpinning design.
What can the results lead to?
If investigation finds shallow footings on shrinkable clay with roots present, the engineer will usually look at tree management, on an arboriculturist's advice, before anything structural; felling a large mature tree on clay can cause heave, so it is never done on a hunch. If a broken drain has washed out sandy soil, the drain is repaired and the ground given time to settle. If firm ground is found to be deep, perhaps beneath soft or made ground, and the cause cannot be removed, mini-piled underpinning may be the engineer's choice. Investigation is also how the engineer sets bay depths for traditional underpinning.
Who needs a soil investigation?
- Owners with new cracksWhere the cause is not obvious and movement may be continuing.
- Insurance claimsArranged or approved by the insurer and its engineer when subsidence is suspected.
- Buyers and sellersWhen a survey flags cracking and someone needs to know if it is historic or live.
- Before building workWhere an extension or alteration is planned near trees, on sloping ground or next to a property that has moved.
Investigation sits alongside a structural engineer survey: the survey looks at the building, the investigation looks at what it stands on. Together they form the evidence base for any underpinning or subsidence repair in Liverpool.
What affects the cost of investigation?
The number and depth of trial pits, what is in the way (paving, steps, conservatories), the laboratory tests required, the length of drain to survey, whether deeper probing is needed and how long monitoring runs. We price to the engineer's brief once it is set. Our underpinning cost guide explains how investigation feeds into the overall cost of a repair.
Need to know what is under your house?
Call 07777 659786 or use the form. Tell us what cracks you have seen, the age of the property and whether an insurer, buyer or engineer is already involved, and we will explain what investigation is likely to be useful for foundation problems in Merseyside homes.
Frequently asked questions
What is a trial pit?
A trial pit is a small excavation dug beside a wall to expose the foundation. It shows how deep the footing goes, what it is made of and what the ground beneath looks like. Samples can be taken for testing, and roots or signs of water can be seen. It is dug and backfilled to an agreed plan, and never by a homeowner on their own initiative.
Do I always need soil investigation before a repair?
Not always. If the cause is obvious and the engineer is confident, for example old cracking that has clearly stopped, investigation may be limited. Where the cause is unclear, movement is ongoing, or underpinning is being considered, the engineer usually wants evidence of the ground and the drains first, because the right repair depends on it.
How long does a site investigation take?
The physical work, such as digging trial pits or running a drain camera, is often done in a day or two. Laboratory results take longer, and if monitoring is added it can run for around 12 months to capture seasonal changes. The engineer will explain the likely timescale for your property once the scope is agreed.
Can investigation tell whether my house is on clay?
Yes, that is one of its main purposes. Trial pits and soil tests show what is under your footing and, for clay, how shrinkable it is likely to be. That matters because shrinkable clay changes volume with moisture. General maps only say what is typical for a district; they cannot tell you what is under a particular house.
Is my insurer responsible for the investigation?
If there is a live or likely subsidence claim, the insurer and its engineer decide what investigation is done and usually arrange it. Contact your insurer before commissioning anything yourself. Private investigation is more common for buyers, owners dealing with old cracking outside a claim, or before planned building work.