Module 8 · Lesson 8.1
From blow count to bearing pressure
Four corrections, one correlation, and an exponential.
Why this matters
Most shallow foundations on sand are designed from SPT data, because sand cannot be sampled undisturbed and there is often nothing else. The chain from blow count to bearing pressure is short, entirely standard — and it amplifies uncertainty at every link. Knowing by how much is what separates a designer from a calculator.
By the end of this lesson you should be able to
- Apply the energy and overburden corrections
- Derive φ′ and carry it into a capacity calculation
- Quantify how far the answer moves within ordinary scatter
The raw blow count N is not a soil property. It depends on the rig, and on the depth at which it was taken.
N₆₀ — the energy correction. A rope-and-cathead hammer might deliver 45% of theoretical free-fall energy; a modern automatic trip hammer delivers 70–80%. The same sand gives very different N values. Normalising to 60% makes results from different rigs comparable.
(N₁)₆₀ — the overburden correction. Sand is stronger under higher confining stress, so the same sand gives a higher N deeper down. Without correction, a uniform deposit appears to get denser with depth — an artefact, not a soil property. The correction normalises to a reference stress of 100 kPa.
Worked example
One blow count, carried to a bearing capacity factor
Given
- SPT at 3.0 m depth: N = 18
- Automatic hammer, energy ratio 75%
- Sand, γ = 19 kN/m³, water table well below
Find
φ′, and the range of Nq consistent with the data
Check yourself
Why must a raw SPT blow count be corrected for overburden before it is used in a strength correlation?
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint