Foundation Engineering / module 8
Bearing capacity from field tests
From a blow count to a bearing pressure — and what happens to the uncertainty on the way.
What you will understand
- Correct a raw SPT blow count to N₆₀ and (N₁)₆₀ and say what each correction removes
- Derive a friction angle from corrected SPT data and carry its uncertainty forward
- Estimate undrained strength from CPT data and identify the judgement in the cone factor
- Explain why correlated parameters demand a sensitivity check rather than a single answer
Lessons
From blow count to bearing pressure
Four corrections, one correlation, and an exponential.
CPT and plate tests
Continuous data, and a test at the wrong scale.
Module summary
- N₆₀ removes the hammer's energy; (N₁)₆₀ removes the depth at which it was measured
- Without the overburden correction, deeper sand looks stronger simply for being deeper
- φ′ from SPT carries roughly ±3°, and Nq is exponential in tan φ′
- The same data gave Nq between 51.8 and 123.1 across a plausible ±3° band — a factor of 2.4
- su from CPT is inversely proportional to a cone factor you chose, not one you measured
Teaching basis and review status
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 notesCheck your understanding
Practice
An SPT gives N = 18 using a hammer with an energy ratio of 75%. What is N₆₀?
Check yourself
What is the most defensible way to choose Nkt for a site?