Geotechnical investigation guide: boreholes, tests and reports
By PMCEPC (NEEV INTERNATIONAL) · Updated
How a ground investigation is planned and reported: borehole spacing and depth, SPT and lab tests, groundwater, bearing capacity and what the report should contain.
Why investigate the ground
Foundation cost and safety depend on soil and rock conditions that cannot be seen from the surface. An investigation replaces assumptions with measured data, so designers choose suitable foundations and avoid surprises during construction.
Planning the investigation
Start with a desk study of geology, nearby data and site history. Then set borehole or trial-pit numbers, spacing and depths according to structure type, load, and ground variability. Critical or heavily loaded structures need closer spacing and deeper exploration.
Field work and testing
Boreholes are drilled and sampled, and standard penetration tests record resistance at set depths. Groundwater levels are noted and may be monitored. Disturbed and undisturbed samples go to a laboratory for classification, strength, consolidation and chemical tests such as sulphate and chloride content.
Interpretation
Results are combined into a soil profile with design parameters for each layer. The engineer then assesses bearing capacity, expected settlement and risks such as soft layers, expansive soils, liquefaction potential or aggressive groundwater.
What the report should include
A good report has a site plan with investigation locations, borehole logs, test results, groundwater data, the interpreted profile and clear foundation recommendations with stated assumptions. Reading the logs, not only the summary, helps designers spot unusual conditions.
Timing
Complete the investigation before the structural design is fixed. Late discoveries about the ground are far more expensive than early ones.
Frequently asked questions
How many boreholes do I need?
There is no single number. It depends on the footprint, loads, ground variability and applicable codes, and is set in the investigation plan.
Can I reuse an old soil report?
Only with caution. Check that it covers the same location and depth, was done by a competent party and that site conditions have not changed.
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