Module 13 · Lesson 13.2
Mobilisation
Why the sum is a fiction, and a useful one.
Why this matters
Adding shaft and base assumes both are available at once. They are not. The shaft reaches its peak after a few millimetres of movement; the base needs tens of millimetres, sometimes more than a hundred. A pile that has mobilised its full shaft has barely started on its base.
The physical reason is a difference in mechanism.
Shaft resistance is an interface shear failure over a thin annulus of soil. Very little relative movement is needed to bring that interface to its limit — a few millimetres, largely independent of pile diameter, because the shearing zone is thin.
Base resistance requires a bearing capacity failure beneath the toe, and that means developing a full failure mechanism through soil several diameters deep. The displacement needed scales with the diameter, and is conventionally taken as 5–10% of it. For a 0.6 m pile that is 30 to 60 mm.
Worked example
What the pile actually offers at working displacements
Given
- The same pile: shaft ultimate 848 kN, base ultimate 153 kN, D = 0.6 m
- Summed ultimate 1001 kN
Find
The resistance mobilised at 5, 10 and 60 mm of settlement
Check yourself
Why does base resistance need much more displacement than shaft resistance to mobilise?
Practice
At 10 mm of settlement the example pile mobilises 848 kN on the shaft and 38 kN on the base. What percentage of the mobilised load is carried by the shaft?
Try it
Load transfer with settlement
Slide the settlement and watch the two resistances arrive at completely different times.
Why they differ
- Shaft: a thin annulus shears, so it peaks in a few millimetres regardless of diameter.
- Base: a full bearing mechanism must form, needing 5–10% of the DIAMETER.
- So the summed ultimate is an upper bound, never available at one displacement.
- Shaft alone
- Shaft + base
- Shaft ultimate
- 848 kN
- Base ultimate
- 153 kN
- Mobilised shaft
- 848 kN
- Mobilised base
- 38 kN
- Total offered
- 886 kN
- Shaft share of what is offered
- 96 %
At 10 mm the base has supplied only 25% of its ultimate. It needs about 60 mm — ten per cent of the diameter — and no structure will accept that.
Summary
- Qult = Qs + Qb: the addition is equilibrium, the two terms are correlations
- The example pile was 85% shaft at ultimate and 96% shaft at 10 mm
- Shaft mobilises in millimetres; base needs 5–10% of the diameter
- The summed ultimate is an upper bound reached only at inadmissible displacement
- A load test that never mobilised the base proves nothing about the base
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