What Is a Geotechnical Site Investigation? Methods & Report
A geotechnical site investigation determines the engineering properties of the ground — bearing capacity, settlement behaviour, groundwater conditions and seismic response — through field and laboratory work before a structure is built. Foundation design rests on this data; without it, a foundation is sized without knowing what the ground will actually do. This guide explains why investigations are carried out, which methods are used and what to check when hiring a provider.
Why is a site investigation carried out?
It answers three questions: can the ground safely carry the structural load, how much and how evenly will the structure settle over time, and how will the ground behave during an earthquake? Without answers, the chosen foundation is either needlessly expensive or unsafe. Groundwater level, expansive or compressible clays, liquefaction potential and slope stability also only surface through investigation. In many jurisdictions the report is part of the building permit file.
Field work: boreholes and in-situ tests
The most common method is drilling boreholes down to the depth influenced by the foundation system. During drilling, the Standard Penetration Test (SPT) measures resistance and disturbed/undisturbed samples are taken. Cone Penetration Testing (CPT) suits soft to firm soils, while pressuremeter testing suits stiff ground and rock. The number and depth of boreholes follow the building footprint, storey count and load distribution — a single borehole is rarely enough.
Geophysical methods: fast, non-destructive support
Geophysical surveys investigate the ground without excavation and help extend borehole data across the site. Seismic refraction and MASW produce a shear-wave velocity (Vs) profile, used to classify the site for seismic design. Electrical Resistivity Tomography (ERT) maps groundwater, fill boundaries and voids. Microtremor measurements give the predominant site period. These methods do not replace boreholes; used together they reduce both cost and uncertainty.
Laboratory testing and the report
Samples are tested for moisture content, grain size distribution, consistency limits, compressibility (oedometer) and shear strength. The report presents field and laboratory results, the soil profile, groundwater level, site class, the recommended foundation type and bearing capacity plus settlement calculations. In a good report the recommendations are justified — it states which data supports which foundation choice. The report is an input to the structural design, not a formality.
What to check when hiring a provider
Define the scope first: how many boreholes, to what depth, which field and laboratory tests, is a geophysical survey included? The cheapest quote usually means the fewest boreholes and tests, so compare on scope rather than price. Ask about the qualification of the engineer signing the report, the laboratory’s accreditation and the delivery schedule. On Tenderely you can collect several quotes and compare their scopes side by side.
Frequently Asked Questions
Is a geotechnical investigation mandatory?
In many countries a soil and foundation investigation report is part of the building permit process. Scope and format depend on the structure type and the regulations in force, so check the local authority’s current requirements.
How long does it take?
Field work on a small plot typically takes a few days, with laboratory testing and reporting adding one to two weeks. Borehole count, depth, the test set and laboratory workload all affect the schedule.
Can geophysics replace boreholes?
No. Geophysical methods give fast, non-destructive, area-wide information, but site classification and strength parameters require boreholes, sampling and laboratory tests. The two complement each other.
How many boreholes are needed?
There is no single number: the footprint, storey count, load distribution and ground variability decide. Wide or heterogeneous sites need more. Always ask for borehole count and depth when comparing quotes.