Before a structure is designed, before foundations are sized, and before earthworks begin, there is one critical step that determines whether a project proceeds smoothly or encounters costly surprises:
The site investigation.
Ground conditions are inherently variable. Two sites only metres apart can have completely different soil profiles, groundwater levels, or fill materials. Without a proper investigation, design decisions are based on assumptions, and assumptions are expensive when wrong.
This guide explains what a geotechnical site investigation involves, including boreholes, sampling, laboratory testing, and reporting — and why each stage matters.
What Is a Site Investigation?
A site investigation (sometimes called a ground investigation or geotechnical investigation) is the process of assessing subsurface conditions to support safe and efficient design.
It provides answers to questions such as:
- What types of soil and rock are present?
- How deep do competent bearing layers extend?
- Is groundwater likely to affect construction?
- Are there uncontrolled fill materials?
- How will the ground behave under load?
The outcome of the investigation informs foundation design, earthworks planning, slope stability assessment, pavement design, and, in some cases, environmental risk management.
Stage 1: Boreholes and Field Investigation
The field phase is where subsurface information is physically obtained. The scope depends on the size of the site, proposed development, and anticipated ground risks.
Boreholes
Boreholes are drilled into the ground to extract soil and rock samples and log subsurface conditions.
Common drilling methods include:
- Hollow stem auger drilling, suitable for cohesive soils
- Rotary drilling, often used in rock
- Wash boring, for softer formations
- Sonic drilling, useful where continuous core recovery is required
The number and depth of boreholes are determined by:
- Site footprint
- Structural load requirements
- Geological variability
- Local authority guidelines
A frequent client question is: How many boreholes do we need?
There is no universal answer. A small residential site may require only two or three investigation points, while larger commercial or infrastructure projects require a more extensive grid. The objective is to reduce uncertainty to an acceptable level, not eliminate it entirely.
Test pits
Excavated using machinery, test pits allow direct visual inspection of shallow soils and are often used for:
- Pavement investigations
- Shallow foundation assessments
- Identifying fill materials
They are cost-effective but limited in depth.
In-situ testing
Field testing may also include:
- Standard Penetration Tests (SPT) – measure soil resistance
- Cone Penetration Testing (CPT) – continuous soil profiling
- Dynamic probing – rapid assessment of soil strength
These tests provide quantitative data to supplement visual logging.
Groundwater monitoring
Where groundwater is encountered, monitoring wells may be installed to measure:
- Depth to water table
- Seasonal variation
- Groundwater behaviour during construction
Groundwater significantly affects excavation stability and foundation performance.
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Stage 2: Sampling — Why It Matters
During drilling or excavation, soil and rock samples are collected for laboratory testing.
There are two primary types:
Disturbed samples
Used for classification testing, such as:
- Soil type
- Moisture content
- Grain size distribution
Undisturbed samples
Collected using specialised equipment to preserve soil structure. These are used for:
- Shear strength testing
- Consolidation testing
- Settlement modelling
The quality of sampling directly affects the reliability of laboratory results. Poor sampling can compromise engineering conclusions.
Stage 3: Laboratory Testing
Laboratory testing transforms field samples into measurable engineering parameters. Common tests include:
Soil classification tests
- Atterberg limits
- Particle size distribution
- Moisture content
These identify soil behaviour and reactivity.
Compaction testing
Determines how soil behaves under compaction — essential for engineered fill and earthworks.
Shear strength testing
Determines the soil’s ability to resist failure under load. Used in:
- Foundation design
- Retaining wall design
- Slope stability analysis
Consolidation testing
Measures settlement characteristics under long-term loading.
Contamination screening (where applicable)
On brownfield sites, samples may also be tested for:
This is where geotechnical and environmental services often overlap.
Laboratory testing provides the data required for engineering calculations, but interpretation is equally important.
Stage 4: Engineering Analysis
Once field and laboratory data are available, engineers conduct detailed analysis.
This may include:
- Bearing capacity calculations
- Settlement modelling
- Slope stability analysis
- Pavement thickness design
- Earthworks specifications
One common question is: How are the numbers in a geotechnical report calculated?
Calculations are based on established geotechnical principles, empirical correlations (such as SPT-based correlations), laboratory test results, and safety factors aligned with engineering standards.
Importantly, results are not just mathematical outputs. They are engineering judgments informed by experience and understanding of variability.
Stage 5: The Geotechnical Report
The final output of a site investigation is the geotechnical report. This document becomes a key reference for designers, contractors, and regulators.
A comprehensive report typically includes:
- Description of the proposed development
- Scope of investigation
- Borehole and test pit logs
- Laboratory results
- Groundwater observations
- Engineering calculations
- Foundation recommendations
- Earthworks guidance
- Limitations and assumptions
Clients often ask: What should a geotechnical report include?
At minimum, it should provide clear and practical design recommendations, not simply raw data. Structural engineers need usable parameters, not just soil descriptions.
Another common concern is: What if something is missing?
If a report lacks clear foundation guidance, does not address variability, or omits limitations, clarification should be requested. Transparent documentation reduces risk for all parties.
What Drives the Cost of a Site Investigation?
Cost depends on several factors:
- Number and depth of boreholes
- Accessibility constraints
- Laboratory testing requirements
- Groundwater monitoring
- Project timeline
- Contamination assessment requirements
Drilling typically represents the largest cost component. More complex sites require more detailed investigation.
While cost is important, under-investigation can lead to far greater expenses during construction.
How Long Does a Site Investigation Take?
Timeframes vary, but typically:
- Fieldwork: 1–5 days (depending on scale)
- Laboratory testing: 1–2 weeks
- Reporting: 1–2 weeks
Complex projects or those requiring specialised testing may take longer.
Early engagement ensures that investigation findings can inform design decisions without delaying programme milestones.
Why Integrated Geoenvironmental Investigations Matter
On many projects, particularly brownfield development, geotechnical and environmental risks coexist.
An integrated investigation can:
- Combine drilling programs
- Reduce duplication
- Coordinate contamination and geotechnical testing
- Minimise site disruption
- Improve data consistency
Instead of separate consultants operating independently, coordinated services streamline risk assessment and improve project efficiency.
From Uncertainty to Informed Design
A site investigation is not just a technical exercise: it is the foundation of risk management for any development.
Through boreholes, sampling, laboratory testing, and engineering analysis, unknown subsurface conditions become quantifiable design parameters.
The result is:
- Safer foundation design
- Reduced construction variation risk
- Improved regulatory compliance
- Greater confidence in project delivery
Ground conditions cannot be changed, but they can be understood.
And understanding them early is one of the most effective ways to protect your project.
Planning a development or remediation project? A well-scoped site investigation ensures your design is informed, compliant, and resilient from the ground up.
Contact us to discuss your site and receive a tailored investigation proposal.
