Foundation soil investigation for existing buildings
Trial pits and boreholes to examine the foundation and the soils beneath an existing building — for distress, refurbishment and extra storeys.
Licensed service · EL-845/2026
When cracks appear in a building, floors start to sag or an extra storey is planned, the first question is what ground the foundation sits on and what condition that ground is in today. This investigation establishes the actual foundation dimensions, the properties of the soils beneath and the groundwater conditions.
What the investigation is and why it is needed
Unlike a greenfield site, the ground under an existing building has been carrying load for years: in places it has consolidated, elsewhere it has weakened through leaking services, a rising water table or neighbouring excavations. For many older buildings neither the design drawings nor the original site investigation survive, and the foundation type and depth can only be established by exposing it.
The investigation answers two questions: is the distress related to the bearing soils, and can the ground take additional load? With this data the structural engineer designs strengthening, checks whether an extra storey is feasible or sizes a foundation widening. The condition of the load-bearing structure is assessed by the structural engineer; our role is to provide geological data on the foundation and the ground beneath it.
When it is required
- cracks in walls, foundations or floors, tilting of the building or signs of differential settlement;
- adding a storey or a loft, or a change of use;
- refurbishment, lowering a basement or building an extension;
- persistent damp or standing water in the basement;
- a deep excavation planned next to the building, or distress that began after one;
- after an incident — a burst water main, flooding, signs of slope movement.
Scope of work
| Task | Purpose | Output |
|---|---|---|
| Review of records and distress | Study the building's history, existing drawings and investigations, crack locations | Plan of trial pit and borehole locations |
| Trial pits against the foundation | Establish foundation type, material, depth, footing width and condition | Trial pit logs, foundation sketches, photos |
| Boreholes next to the building | Investigate the soils below foundation level | Borehole logs, geological cross-sections |
| Undisturbed samples from beneath the footing | Determine the properties of soil that has been under load | Samples for the laboratory |
| Laboratory testing | Determine physical and mechanical properties, collapsibility and swelling potential | Characteristic and design values |
| Groundwater and aggressiveness | Assess the water level and its effect on foundation materials | Level readings, chemical analysis |
If movement is ongoing, it makes sense to combine the investigation with settlement monitoring, which shows whether the process is continuing and at what rate.
Investigating the causes of distress
Crack pattern and location often point to the cause, but it can only be confirmed with trial pit and borehole data. The most common causes are:
- soil wetted by leaking water and sewer pipes, with additional settlement of collapsible soils;
- made ground or organic layers left beneath the footing;
- seasonal swelling and shrinkage of clays;
- a rise or fall in the groundwater level;
- excavation on a neighbouring plot, or landslide movement on a slope;
- parts of the building founded at different depths or on different soils.
How the work is carried out
- Preparation. Building walkover, recording of distress, review of documents, selection of trial pit and borehole positions; underground utilities are located.
- Field work. Trial pits are dug by hand or with a mini excavator, the foundation is logged and measured, undisturbed samples are taken, boreholes are drilled and water levels recorded.
- Laboratory. Soil and water samples are tested.
- Office processing. Foundation sketches and cross-sections are drawn, engineering geological units are defined and the geological causes of distress are analysed.
- Handover. The report is issued to the client and the structural engineer; trial pits are backfilled and compacted.
Equipment used
- compact drilling rigs — for boreholes beside the building and in courtyards;
- hand tools and a mini excavator — to open trial pits without damaging the foundation;
- samplers for undisturbed samples;
- light dynamic probe — to check changes in soil density;
- survey instruments — to locate trial pits and boreholes relative to the building.
Deliverables
- geotechnical report: ground conditions, engineering geological units and their properties, groundwater, conclusions on the causes of distress;
- trial pit logs and foundation sketches — type, material, depth, footing width, condition;
- borehole logs, cross-sections, exploratory hole location plan;
- laboratory results and photographs;
- recommendations for the structural engineer on ground improvement and on repairing drainage and services.
Documents are issued in PDF, drawings in DWG where needed, and hard copies by agreement.
Timing
- the number of trial pits and boreholes, and whether work is inside or outside the building;
- how difficult the foundation is to reach — asphalt, concrete slabs, services;
- the volume of laboratory testing.
The exact timescale is stated in our commercial proposal.
What we need from you
- address, year of construction, number of storeys, any drawings and previous investigations;
- description and photos of the distress and when it was first noticed;
- the planned works — extra storey, refurbishment, strengthening;
- access to the exploratory hole locations and consent to break out surfacing.
What affects the price
- number and depth of trial pits, and whether surfacing must be broken out and reinstated;
- number and depth of boreholes;
- laboratory testing schedule;
- whether monitoring and a topographic survey are ordered together.
Indicative rates are on the engineering geological survey prices page; for an exact price, send a quote request.
