Phạm Đức Anh
Most foundation problems start with a number taken from a soil report without its context. The report is not hard to read once you know what each part is for.
Start with the site plan and the borehole logs
Check where the boreholes are relative to the building footprint, and how deep they go. A borehole outside the footprint, or one that stops above the stratum your piles will rely on, tells you less than it seems. EN 1997-2 Annex B.3 gives guidance on investigation depth relative to foundation size; if the planned foundation goes deeper than the investigation, the report cannot answer the question you are asking.
The borehole log is the core of the report. Each one shows, with depth:
the soil description and layer boundaries;
test results, usually SPT blow counts;
samples taken for the laboratory;
where water was met.
Draw a simple section through the boreholes. If adjacent logs disagree strongly, the ground is variable and a single "design profile" hides real risk.
What an SPT number really is
The Standard Penetration Test counts the blows needed to drive a sampler 300 mm, after a first 150 mm seating drive that is not counted. That count is the N value. It is an index, not a strength.
Before using N in a correlation, check whether it has been corrected, and for what: hammer energy, overburden pressure, and in fine sands below water, other adjustments. Correlations published for corrected values give wrong answers when fed raw ones. The report should state which values it tabulates.
Groundwater: the level on the log is not always the design level
Water noted during drilling is often not the equilibrium level; in low permeability soils it can take days to rise. Look for later readings from standpipes or piezometers, and for the season of the investigation. The groundwater level changes effective stress, so it changes bearing resistance, settlement and uplift on basements.
Laboratory results and the design values
Tables of moisture content, Atterberg limits, unit weight and shear strength usually follow the logs. Check that the tests match the loading: a short-term undrained strength and a long-term drained strength answer different questions. The geotechnical designer selects characteristic values from this data; a single favourable test result is not a characteristic value.
Questions worth asking
Do the boreholes cover the footprint and reach below the depth that matters?
Are there soft or organic layers below the bearing stratum?
Are the SPT values raw or corrected?
Is the groundwater level measured over time, or only noted while drilling?
Which loading case does each strength parameter belong to?
Takeaways
Read the logs before the summary table.
Treat SPT N as an index and check how it was corrected.
A water strike during drilling is not a design groundwater level.
Ask the geotechnical engineer when the report and the foundation do not match.
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