Foundation Engineering / Load transfer: shaft and base
Mobilisation
Why the sum is a fiction, and a useful one.
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.
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.
- Verify compatible shaft/toe load-transfer curves, settlement limits, self-weight/buoyancy basis and installation effects. The hyperbolic mobilisation model is illustrative.
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
At a given head settlement, shaft and toe resistances must be mobilised compatibly. Shaft resistance often develops at smaller movements than toe resistance, but the curves depend on the pile and ground. The numerical model here is illustrative rather than a calibrated load-test prediction.
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
- The teaching curve reaches its shaft plateau at 10 mm. Actual mobilisation depends on the ground, installation and pile geometry.
- The illustrative base curve reaches half its asymptotic resistance at a settlement of 5% of the diameter; this is not a site-calibrated relation.
- This illustrative curve approaches the summed resistance asymptotically. Real shaft and base mobilisation must be checked at a compatible 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). Check that movement against the supported structure’s serviceability requirements.
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