EngiMetric
Geotechnical Calculation guide Understand a concept

How much load can a pile take, and which part is doing the work

Splitting pile capacity into shaft friction and end bearing so you can tell whether a longer pile actually helps.

The engineering question

A pile is quoted by length. Why does adding length sometimes add almost no capacity, and how do you tell which part of the pile is carrying the load?

Worked example

The figures below were produced by running the Pile Capacity Calculator with the same inputs — during the build, from its authoritative engine.

Results
Ultimate capacity 1117 kN
Allowable capacity 446.7345 kN
Skin friction component 989.6017 kN
End bearing component 127.2345 kN
Method Cohesive clay (α-method)
Assessment Cohesive clay (α-method): skin 990 kN + tip 127 kN → Qu 1117 kN, Qall 447 kN at FS 2.5. Field load testing is recommended.
  1. Shaft friction is α·c·π·D·L — proportional to length.
  2. End bearing is Nc·c·(πD²/4) — independent of length.
  3. In cohesive soil the shaft usually dominates, which is why length helps. Where a weak layer sits below the toe, it does not.

Sanity check

Doubling the length of a clay pile should increase capacity substantially. If it barely moves, a stronger stratum or a deeper toe is what is needed.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

  • BS EN 1997-1 — pile foundation design
  • Tomlinson — Pile Design and Construction Practice
  • API RP 2SK — offshore pile design, α and β methods

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