Foundation Engineering / module 14
Pile capacity in clay — the α-method
One coefficient, quoted between 0.3 and 1.0, multiplying the term that matters most.
What you will understand
- Apply the α-method layer by layer to compute shaft resistance in clay
- Explain physically why α is less than one, and why it falls as the clay stiffens
- Compute base resistance in clay and say why Nc = 9 rather than 5.14
- Quantify how much the disagreement over α moves the answer, and respond to it properly
Lessons
Adhesion, not cohesion
Why the pile never gets the strength the laboratory measured.
How much α matters
A factor of three, sitting on the term that carries the pile.
Module summary
- Shaft resistance in clay is taken as α·su — a total-stress method, appropriate to the undrained case that governs
- α is below 1 because installation remoulds and softens a thin annulus around the shaft
- α falls as su/σ′v rises: 0.49 in the soft upper clay, 0.58 in the stiffer clay below
- The worked pile gave 1273 kN of shaft and 229 kN of base — 85% shaft, again
- Across the quoted range α = 0.3 to 1.0, shaft resistance moved by a factor of 3.3
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
A 0.6 m diameter bored pile has its base in clay with su = 90 kPa. Using Nc = 9, what is the base resistance, in kN?
Practice
Over the same shaft, α = 0.3 gives 916 kN and α = 1.0 gives 3054 kN. By what factor does the shaft resistance vary?