Ground investigation and early site risk identification
A ground investigation report costs tens of thousands. The variation claim that results from skipping one costs ten times that. So why do so many projects treat ground investigation as a box-ticking exercise rather than a decision-making tool?
Every below-ground decision on a construction project, from foundation type to earthworks strategy, from drainage design to remediation approach, depends on understanding what is beneath the surface. Ground investigation provides that understanding. Without it, the project is built on assumptions. And assumptions, on complex sites, have a habit of being expensive.
In Part 2 of this series, we explored what happens after construction budgets are approved and why that transition is a high-risk moment. This article focuses on the single most important technical activity in pre-construction planning: understanding the ground before you commit to building on it.
Why ground investigation is the highest-value activity in pre-construction
Ground conditions influence almost every aspect of construction delivery:
Foundation design depends on bearing capacity and settlement characteristics
Earthworks strategy depends on material classification and suitability for reuse
Drainage design depends on permeability and groundwater levels
Remediation planning depends on contamination type, concentration, and distribution
When any of these factors are unknown or misunderstood, the consequences are expensive. Foundation redesign after construction has started can add weeks to the programme and hundreds of thousands to the budget. Discovering that cut material is contaminated and cannot be reused as fill throws off the entire earthworks balance. Finding a perched water table that was not identified during the investigation changes the excavation methodology, the temporary works design, and the programme.
Ground investigation does not eliminate these risks. It identifies them before decisions are made, when the cost of addressing them is at its lowest. A borehole is cheaper than a variation. A soil sample is cheaper than a redesign.
What is included in a construction ground investigation?
Step 1: Desktop study and historical risk review
The process begins with a review of existing information:
Historical mapping showing previous land uses
Geological and hydrogeological records from the British Geological Survey
Environmental databases identifying nearby landfills, pollution incidents, and regulated sites
Any previous investigation reports for the site or adjacent land
Aerial photography showing surface changes over time
The desktop study identifies potential risks before anyone sets foot on site. A former gasworks suggests hydrocarbon and heavy metal contamination. A former landfill suggests ground gas, leachate, and variable fill. A site adjacent to a river suggests a high water table and flood risk.
This stage is relatively quick and inexpensive, typically taking one to two weeks and costing a few thousand pounds. But it has limitations. It tells you what might be there based on history and geology. It does not confirm what is actually there. That requires intrusive investigation.
Step 2: Intrusive site investigation
Intrusive investigation provides direct evidence of ground conditions. The scope depends on the site size, the risks identified in the desktop study, and the proposed development. Common methods include:
Boreholes drilled to depths of 10 to 30 metres or more, providing continuous soil samples and allowing installation of groundwater monitoring wells
Trial pits excavated to 3 to 4 metres, allowing visual inspection of shallow ground conditions and collection of bulk samples
Window sampling to obtain small-diameter soil cores at multiple locations
Dynamic probing to test ground strength at regular intervals
In situ testing such as standard penetration tests (SPTs), plate bearing tests, and CBR tests
Soil and groundwater samples are collected and sent to accredited laboratories for analysis. The number and distribution of test locations depends on the site area, the variability of ground conditions, and the sensitivity of the proposed development.
| Investigation Method | Typical Depth | Best For | Limitations |
|---|---|---|---|
| Trial pits | 3 to 4m | Visual inspection, shallow contamination, buried obstructions | Limited depth, requires access for excavator |
| Window sampling | 5 to 10m | Soil profiling, contamination sampling at multiple locations | Small sample size, limited in hard ground |
| Cable percussion borehole | 10 to 30m+ | Deep soil profiling, groundwater monitoring, in situ testing | More expensive, requires larger rig |
| Rotary coring | 20m+ into rock | Rock characterisation, deep foundations, tunnelling | Specialist equipment, higher cost |
| Dynamic probing | Up to 10m | Ground strength profiling across a site | No soil sample, strength data only |
The fieldwork typically takes one to four weeks depending on the number of locations and the access constraints. On brownfield sites with structures still in place, access for drilling rigs can be limited, and some investigation may need to wait until demolition is complete.
Step 3: Laboratory testing and reporting
Samples are analysed to determine:
Soil composition and classification
Strength characteristics (shear strength, compressibility, bearing capacity)
Contamination levels for a range of determinands (metals, hydrocarbons, asbestos, PAHs, VOCs)
Groundwater chemistry
Ground gas concentrations (methane, carbon dioxide, volatile organics)
Results are compiled into a ground investigation report covering geotechnical parameters for foundation and earthworks design, a contamination risk assessment, groundwater monitoring data, and recommendations for further investigation, remediation, or design.
This report is a key reference document for engineers, designers, and contractors throughout pre-construction planning and delivery. Its quality directly affects the quality of every decision that follows.
Common ground risks that disrupt construction projects
Made ground and buried obstructions
Made ground is material that has been placed by human activity rather than deposited naturally. It is common on brownfield sites and can include demolition rubble, industrial waste, domestic refuse, and imported fill of unknown origin. Its strength, compressibility, and contamination status can change over short distances, making it unreliable for foundations and difficult to classify for earthworks.
Buried obstructions, including old foundations, redundant services, underground tanks, and infilled basements, are a related risk. Historical maps provide clues, but they are rarely complete. These discoveries during construction cause programme delays and cost events that should have been anticipated.
High or variable groundwater
Groundwater affects excavation methodology, foundation design, drainage strategy, and contamination migration. A high water table means excavations will flood without dewatering. A variable water table means conditions change seasonally, and what was dry in summer may be flooded in winter.
Groundwater monitoring should ideally cover at least one full seasonal cycle. If this data is not available, design and programme decisions are being made against incomplete information. The cost of dewatering when it is planned for is manageable. The cost of dewatering when it is discovered mid-excavation is not.
Groundwater also affects contamination risk. Mobile contaminants like dissolved-phase hydrocarbons and chlorinated solvents can migrate with groundwater flow, spreading contamination beyond the source area.
Contaminated land
Contamination is found on most brownfield sites. Common contaminants include:
Heavy metals (lead, arsenic, cadmium, chromium)
Hydrocarbons (fuels, oils, tars)
Asbestos fibres in soil
Persistent organic pollutants (POPs)
Chlorinated solvents
Ground gases
Contamination affects earthworks because material that exceeds threshold levels cannot be reused as fill without treatment. This disrupts the cut and fill balance, creates disposal costs, and extends the programme.
Variable or weak soils
Soft clays, peat, loose fills, and alluvial deposits can all cause problems. These conditions affect foundation choice, working platform design, and earthworks methodology. Identifying them through ground investigation allows the design to accommodate them. Discovering them during construction forces reactive changes that are always more expensive.
The cost of inadequate ground investigation
The cost of a comprehensive ground investigation is a fraction of the project budget. A Phase 1 desk study costs a few thousand pounds. A Phase 2 investigation costs tens of thousands, depending on site size and complexity. Together, they represent perhaps 0.1 to 0.5 percent of the total construction cost.
The cost of getting it wrong is disproportionately higher:
Foundation redesign after construction starts can cost hundreds of thousands
Contamination discovered during earthworks can add weeks to the programme and six figures to the remediation budget
Groundwater flooding excavations requires emergency dewatering, programme extension, and often redesign of temporary works
Below-ground obstructions that were not identified require removal, disposal, and backfill, none of which were in the budget
Every one of these scenarios is a real project event, not a theoretical risk. And every one of them is preventable, or at least manageable, with a thorough ground investigation completed at the right time.
How ground investigation data should inform construction planning
This is where the difference between a good investigation and a great one becomes apparent. A good investigation provides data. A great investigation provides data that is interpreted in the context of what is actually going to be built.
Ground investigation data should feed directly into:
Foundation design, confirming bearing capacity and settlement parameters for the proposed structural loads
Earthworks strategy, determining whether cut material can be reused as fill, what treatment is needed, and whether the balance is achievable
Enabling works sequencing, identifying where ground improvement is needed before other activities can proceed
Remediation strategy, specifying treatment methods, volumes, and validation requirements
Programme risk assessment, identifying ground-related constraints that affect duration and sequencing
The problem is that ground investigation reports are often written by geotechnical consultants for other geotechnical consultants. The data is technically correct, but it is not always translated into the practical implications for delivery.
This is where contractor involvement changes the outcome.
Why early contractor involvement changes the outcome
A contractor reviewing a GI report asks different questions from the ones the consultant answered:
Can I excavate this material with the plant I have?
Will it compact to specification at its current moisture content?
Is there enough suitable fill to balance the earthworks?
What happens if we hit contamination in an area the investigation did not cover?
Does the groundwater data reflect seasonal variation, or just a single visit?
Are there obstructions where the historical maps show former buildings?
These are delivery questions, not design questions. And they are best answered by people who build on the ground, not just people who test it.
When a specialist enabling works and groundworks contractor is involved during pre-construction planning, the GI data gets stress-tested against practical delivery reality. Gaps in the investigation are identified before they become gaps in the budget. Programme assumptions are challenged by people who know how long things actually take. Cost estimates are validated by people who will be responsible for delivering the work.
Without this input, the project carries the GI data forward at face value. The design team builds on the consultant's recommendations. The budget is set against the consultant's cost estimates. The programme is built around the consultant's assumed durations. And when reality differs from those assumptions, which on complex sites it almost always does, the cost of adjustment is borne during construction rather than during planning.
The difference between a project that involves a contractor early and one that does not is rarely visible in the GI report itself. It is visible in the programme, the budget, and the number of variations during delivery.
Project example:
Exmouth Junction, Devon
Churngold delivered the remediation, earthworks, and groundworks for a 92-apartment development on the former Exmouth Junction site for Living Heritage and Eutopia Homes. As a brownfield site with a railway heritage, ground investigation was essential to understanding the contamination profile and informing the remediation strategy before earthworks could begin.
By coordinating the investigation early and interpreting the findings in the context of the planned earthworks and foundation design, the team was able to align the remediation approach with the construction programme, avoiding reactive changes during delivery. The project's success led to further phases being awarded.
When to involve us
Churngold coordinates ground investigation as part of our enabling works packages, working with specialist geotechnical and environmental consultants to manage the process from commissioning through to report delivery.
More importantly, we interpret the results in the context of actual delivery. We flag where the investigation has gaps, where the data creates risk for the proposed earthworks, enabling works, or remediation strategy, and where the design assumptions may not hold up during construction.
If you need ground investigation as part of an enabling works package, or you already have GI data and want a contractor's view on what it means for delivery, get in touch.
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FAQs
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Ground investigation is the process of testing and analysing soil, rock, and groundwater conditions beneath a construction site. It combines desktop research, intrusive testing (boreholes, trial pits), and laboratory analysis to build a picture of ground behaviour and identify risks that could affect construction.
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Ideally before land acquisition to inform valuation, and certainly before design is finalised. Phase 1 desk studies can be done at any time. Phase 2 intrusive investigation should be completed before foundation design, earthworks planning, or remediation strategy is confirmed.
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Foundation design may need to change, earthworks strategy may need revising, remediation scope may increase, and the programme may need to be extended. The cost of these changes during construction is significantly higher than addressing them during planning.
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Phase 1 is a desktop study reviewing historical records, geological data, and environmental databases to identify potential risks. Phase 2 is intrusive investigation involving boreholes, trial pits, and laboratory testing to confirm actual ground conditions. Phase 1 identifies what might be there. Phase 2 confirms what is there.
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Costs vary with site size and complexity. A Phase 1 desk study typically costs a few thousand pounds. A Phase 2 intrusive investigation can range from ten to fifty thousand pounds or more. This is a small fraction of the cost of addressing ground surprises during construction.
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A Phase 1 desk study takes one to two weeks. Phase 2 fieldwork takes one to four weeks. Laboratory results take two to four weeks after sampling. The full process from commissioning to final report is usually six to twelve weeks.
Next in the Pre-Construction Planning Series
In Part 4, we examine enabling works strategy and site preparation, and how the decisions made at this stage lock in or unlock the rest of the programme.
