Foundation Engineering / Load transfer: shaft and base

The two resistances

Where the load actually goes.

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 notes

Why this matters

Pile load reaches the ground through shaft shear and toe resistance. The share carried by each depends on the soil, installation, geometry and displacement. This lesson keeps those contributions separate before adding them.

By the end of this lesson you should be able to

Compute shaft and base resistance separately
Say which dominates, and why it depends on the ground
Account for the pile's own weight correctly

Ultimate pile capacity

Q_{ult} = Q_s + Q_b = \sum \tau_s \, \pi D \, \Delta L + q_b A_b

What it calculates: The total resistance a pile can offer

Q_s: Shaft resistance, summed over the embedded length (kN)
$\tau_s$: Unit shaft resistance in a layer (kPa)
D: Shaft diameter (m)
q_b: Unit base resistance (kPa)
A_b: Base area (m²)

This assumes

Both resistances are available simultaneously — which the second lesson shows is not true
τ_s and q_b come from correlations; only the addition is mechanics

In plain terms: The structure of this equation is equilibrium and beyond dispute. Everything difficult about pile design lives inside τ_s and q_b, and Modules 14 to 16 are about nothing else.

Worked example

Where the load goes in a bored pile in clay

Given

Bored pile, D = 0.6 m, embedded length 15 m
Firm clay throughout, s_u = 60 kPa
Adhesion factor α = 0.5
Base bearing factor N_c = 9

Find

The split between shaft and base, and the pile's net capacity

Practice

A pile has 848 kN of shaft resistance and 153 kN of base resistance. What percentage of the ultimate capacity comes from the shaft?