Module 14 · Lesson 14.2
How much α matters
A factor of three, sitting on the term that carries the pile.
Why this matters
Module 8 showed a ±3° uncertainty in φ′ moving Nq by a factor of 2.4. This is worse, and it is more directly consequential, because α multiplies the shaft term and the shaft term is most of the pile.
Different authorities quote α for a stiff clay anywhere between about 0.3 and 1.0. That is not scatter around a value; it is disagreement about the value.
Applied to the worked pile's lower clay over the full 18 m, α = 0.3 gives 916 kN of shaft resistance and α = 1.0 gives 3054 kN — a factor of 3.3. No refinement anywhere else in pile design comes close to mattering that much.
The disagreement is not incompetence. α depends on installation method, on time since installation, on whether the clay is fissured, on the concrete mix and on how the hole was supported. Correlations fitted to driven piles in one clay genuinely do not transfer to bored piles in another.
The proper response is a load test. A preliminary pile taken to failure on the actual site, with the actual rig and the actual clay, back-figures an α that no correlation can match. On a scheme with many piles it pays for itself several times over, because it usually allows shorter piles than a cautious correlation would.
Where a preliminary test is not possible, the responses in descending order of strength are: a correlation calibrated on comparable clay with the same installation method, cited; the same with a reduced value to reflect the transfer; and a value chosen at the conservative end of the published range with the sensitivity stated. What is never acceptable is a single mid-range α presented without its provenance, because it looks identical on the page to a value that was actually established.
Check yourself
Why does the adhesion factor α fall as the ratio su/σ′v increases?
Check yourself
A scheme needs 400 piles in a stiff clay for which published α values range from 0.3 to 1.0. What is the strongest response?
Try it
The adhesion factor
α falls as the clay stiffens relative to its stress level. Move both and watch which way it goes.
Why it falls
- Installation remoulds a thin annulus, and the shaft never meets the intact clay.
- A heavily overconsolidated clay loses proportionally more strength on remoulding.
- So α is correlated against su/σ′v, not against su.
- Strength ratio su/σ′_v
- 0.75
- Adhesion factor α
- 0.577
- Unit shaft resistance
- 52.0 kPa
- Shaft resistance Qs
- 1763 kN
- Base resistance Qb
- 229 kN
- Shaft share
- 89 %
Shaft resistance across the quoted range α = 0.3 to 1.0
Across the α values different authorities publish for a stiff clay, this shaft is worth anywhere between 916 and 3054 kN — a factor of 3.33. That is the single largest uncertainty in a pile design in clay, and a site load test is the only thing that removes it.
Summary
- α is below 1 because installation leaves a remoulded, softened annulus against the shaft
- α falls as su/σ′v rises — so it was higher in the stronger, deeper clay, not lower
- Apply it layer by layer; averaging su discards the profile and errs unsafely
- Nc = 9 for a deep confined base, against 5.14 at the surface
- α between 0.3 and 1.0 moved shaft resistance by a factor of 3.3 — buy a load test
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