The Standard Penetration Test is rough, old, and endlessly criticised - and still one of the most useful field tests in geotechnical practice. With care, its blow counts give a defensible allowable bearing pressure for a shallow foundation. The skill is in respecting its limits: treating the N-value as a guided estimate, not a precise measurement.
What the test measures
The SPT counts the blows of a standard hammer needed to drive a standard sampler a set depth into the soil at the bottom of a borehole. That blow count, N, correlates with the soil's density (for sands) or consistency (for clays). It is cheap, it is done in nearly every ground investigation, and it samples the soil as it goes - which is why, despite all its scatter, it endures.
Step 1 - Correct the N-values
A raw field N is not directly usable, because the test conditions vary. Two corrections matter most:
- Overburden correction () accounts for the confining pressure at the test depth - the same soil gives a higher N when it is more deeply buried.
- Energy-ratio correction normalises for the actual hammer energy delivered, since real rigs deliver different fractions of the theoretical energy.
Applied together these give a corrected value, often written , which is what correlations are built against.
Step 2 - Estimate strength
From the corrected N you estimate a strength parameter: a friction angle for sands, or an undrained shear strength for clays, through published correlations.
Treat correlations as ranges, not exact numbers. An N-value maps to a band of friction angle, not a single decimal - and your design should reflect that spread.
This is the step where false precision creeps in. Quoting a friction angle to a tenth of a degree from an SPT is borrowing certainty the test never had.
Step 3 - Allowable pressure, and the check that usually governs
With a strength estimate you can compute a bearing capacity and apply a factor of safety. But for shallow foundations on most soils, ultimate bearing failure is rarely the limiting case - settlement is. The footing has plenty of margin against punching into the ground, but will it settle more than the structure can tolerate?
- Apply a bearing-capacity factor for the ultimate check.
- Then run a settlement check against the serviceability limit. On sands especially, the allowable pressure is usually set by limiting settlement, not by avoiding shear failure.
The judgement that protects you
The most important habit has nothing to do with the formulae:
One borehole is an anecdote; three make a pattern. Never size a raft - or commit to a founding stratum - off a single SPT.
Ground varies laterally in ways a single hole cannot reveal. A defensible design reads the N-values across the site, treats them as a scattered population rather than exact points, designs to the conservative end of the band, and lets settlement, not shear, set the allowable pressure on shallow footings. The soil-structure interaction article takes the next step, into how the foundation and the ground share load once the bearing pressure is set.