Reading a ground investigation report before you bid

01 What a ground investigation covers

A ground investigation report (GIR), sometimes called a geotechnical report or site investigation report, documents the soil, rock and groundwater conditions beneath a site. Its primary purpose is structural: it tells engineers what is in the ground, how the ground will behave under load, and how to design foundations and earthworks accordingly.

This is a different discipline from environmental contamination assessment, even though the two studies sometimes happen at the same time and share vocabulary. A contamination Phase 1 is asking: could this ground be polluted and harm people or controlled waters? A geotechnical investigation is asking: can this ground support what we want to build on it, and at what cost?

On industrial and logistics sites both questions matter. A clean site with weak ground can be just as expensive to develop as a contaminated one with adequate bearing capacity.

02 Factual report vs. interpretive report

Most ground investigations produce two documents. The factual report is a record of what was done on site: borehole logs, trial pit records, in-situ test results, laboratory test results and groundwater levels. It contains data, not conclusions.

The interpretive report (also called the geotechnical design report or assessment report) is where a geotechnical engineer processes that data and draws conclusions: what foundation type is appropriate, what bearing capacity the ground can sustain, what settlement is expected under load, whether any ground treatment is needed, and what the earthworks specification should be.

Both parts should be in the data room. A factual report alone tells you what the ground looks like; only the interpretive report tells you what it means for the scheme. If you have one without the other, or only a desk study with no intrusive data, note the gap explicitly: the ground risk may be unquantified.

03 Phase 1 and Phase 2 in geotechnical context

Geotechnical investigations follow the same phased structure as contamination assessments, but the phases are distinct documents covering different things. Do not conflate them when reading a data room.

A geotechnical Phase 1 desk study is a preliminary risk assessment built from published sources: British Geological Survey maps, historical Ordnance Survey maps, mining authority records, published ground-level data and a site walkover. It identifies likely ground conditions and risks, such as made ground, shallow rock, running sand, former mine workings or old foundations, and concludes whether an intrusive investigation is needed.

A geotechnical Phase 2 intrusive investigation is the fieldwork: boreholes, cable percussion drilling, rotary coring in rock, trial pits, cone penetration tests (CPTs) and in-situ strength tests. Samples go to a laboratory for classification and engineering testing. This is what tells you actual bearing capacity, shear strength and consolidation characteristics.

The governing British Standard for site investigation in the UK is BS 5930 (Code of Practice for Ground Investigations), and foundation design works to Eurocode 7 (BS EN 1997). A competent report will reference these.

04 Bearing capacity, settlement and made ground

The three findings that most often move a bid are bearing capacity, settlement and made ground.

Bearing capacity is the load the ground can support per unit area. Weak soils, soft clays or loose sands mean deeper or more complex foundations, or ground improvement, to carry the structural loads of a warehouse or distribution unit. The interpretive report will recommend a foundation type and specify the allowable bearing pressure.

Settlement is how much the ground compresses under the building load. Differential settlement (where one part of a building settles more than another) is particularly damaging. Soft alluvial deposits, peat and fill materials are the usual culprits. Settlement predictions go into the foundation design and, where significant, into the cost plan.

Made ground is any non-natural material: demolition rubble, engineering fill, imported granular material, or historical waste. It is common on former industrial sites and can vary enormously in quality, thickness and engineering behaviour. Made ground often requires ground improvement or more expensive foundation solutions, and it can also contain contamination, creating an overlap with the environmental assessment.

05 Ground gas in the geotechnical report

Ground gas monitoring (methane, carbon dioxide and sometimes hydrogen sulphide) can appear in either the geotechnical report or the contamination assessment, depending on how the investigation was structured. On sites with made ground, infilled land or proximity to a former landfill, both teams may monitor for ground gas.

The geotechnical report will note gas concentrations found during borehole installation and any dedicated gas monitoring standpipes. If significant ground gas is identified, the site needs a gas risk assessment under BS 8485, which classifies the site into characteristic situations (CS1 to CS6) and sets the level of gas protection a new building requires. Higher characteristic situations mean membranes, venting systems and ongoing validation, all of which belong in the cost plan.

Check both the geotechnical and contamination reports for ground-gas data: inconsistencies between them, or a situation where one study collected gas data and the other did not, can leave the picture incomplete.

06 Red flags to catch before you bid

These findings should prompt a number against them before you bid:

• No intrusive investigation in the data room, or a Phase 1 desk study recommending Phase 2 work that has not been done. Ground risk is unquantified.
• Made ground of significant thickness or variable composition: ground improvement, piling or raft foundations may be needed.
• Soft compressible layers (peat, soft clay, alluvium): settlement risk, slow consolidation, possible ground treatment requirement.
• Shallow obstructions or old foundations from previous buildings: removal cost and programme risk.
• Mining legacy (shallow workings, voids, crown holes): specialist foundation design and potential Coal Authority interaction.
• High or variable groundwater: affects excavation, temporary works design, and potentially permanent drainage.
• Ground gas detected at concentrations that may push the site above characteristic situation CS1.
• Report commissioned for a different scheme: borehole positions may not cover the current building footprint, or the loads assessed may not match your development.

None of these is automatically a deal-breaker. Each is a cost or programme question that needs a number before you bid rather than a surprise after.

07 Frequently asked questions

What is a ground investigation report?

A ground investigation report (GIR) is the output of a geotechnical site investigation: a study of the soil, rock and groundwater conditions beneath a site to inform foundation design, earthworks and construction. It typically comes in two parts: a factual report of the raw data gathered on site (borehole logs, trial pit records, laboratory test results, groundwater levels) and an interpretive or geotechnical design report where an engineer draws conclusions about bearing capacity, settlement and ground risk. Both parts should be in the data room.

What is the difference between a geotechnical Phase 1 desk study and a Phase 2 intrusive investigation?

In a geotechnical context, a Phase 1 is a desk-based preliminary assessment: historical maps, geology records, mining data and a site walkover that build a picture of ground conditions without any digging. It flags likely risks such as made ground, running sand, shallow rock or mine workings, and sets out whether intrusive work is needed. A Phase 2 is the intrusive investigation itself: boreholes, trial pits, in-situ tests and laboratory analysis of samples. Phase 1 identifies the risk; Phase 2 quantifies it.

What is the difference between a ground investigation report and a contamination Phase 1 or Phase 2?

They share the Phase 1 and Phase 2 labels but cover different disciplines. A contamination Phase 1 is an environmental desk study assessing pollution risk from past land use; a contamination Phase 2 samples soil and groundwater for hazardous substances. A geotechnical ground investigation assesses ground conditions for structural purposes: bearing capacity, settlement, slope stability and foundation type. On a former industrial site both are often needed, and they can be combined into a geo-environmental investigation, but they answer different questions.

What does a ground investigation report tell you about the ground?

The report tells you the soil and rock profile (what layers exist and at what depths), the engineering properties of those materials (strength, stiffness, permeability), the groundwater level, and whether there are any features that complicate construction: made ground, soft compressible layers, shallow rock, voids, mining legacy or obstructions from previous buildings. From these it derives recommendations for foundation type and depth, expected settlement, and the earthworks specification.

How much does a ground investigation cost?

Cost scales with the size of the site, the number of boreholes and trial pits the ground risk warrants, the depth of investigation and the laboratory testing suite. A desk study alone is relatively modest; an intrusive investigation on a large or complex site is considerably more. For a bid the investigation fee is rarely the primary concern: what matters is what it finds, because unexpected made ground, obstructions or poor bearing capacity can add far more to the scheme cost than the survey itself.

How long does a ground investigation take?

A desk study can be completed in days to a couple of weeks, because it is office-based and needs no site works. An intrusive investigation adds fieldwork, laboratory testing and report writing: typically several weeks in total, depending on the scale of the site and the laboratory turnaround. If the investigation is absent from the data room but has been recommended, that represents unpriced ground risk and a programme gap that needs addressing before or after you bid.

How long is a ground investigation report valid for?

There is no fixed expiry, but age matters. Ground conditions do not change quickly, but a report written for a different structure, loading or end use may not be valid for yours. Reports are also superseded when new information comes to light: updated mining risk data, a change in the drainage or adjacent earthworks, or simply a scheme layout that puts foundations in a different location from where boreholes were sunk. Always check whether the report was written for the current scheme and whether the borehole positions are still representative.