Foundation Engineering / Bearing capacity from field tests
From blow count to bearing pressure
Four corrections, one correlation, and an exponential.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Corrections checked in this pass
- Bounded the SPT correlation to its stated range and clarified CPT correction and calibration assumptions.
Checks needed for project use
- A geotechnical reviewer must confirm drainage and stress basis, bearing-factor formulation, correlations and settlement influence factors. Partial factors and project-specific ground conditions are outside these teaching checks.
- Confirm test-energy corrections and local sand applicability; CPT Nkt needs local calibration and qt needs cone calibration/pore pressure.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
SPT data are often used to estimate sand properties where suitable samples or other in-situ tests are unavailable. Each correlation in the route from blow count to bearing resistance introduces uncertainty that should be stated and tested.
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?