Module 16 · Lesson 16.1
From field data to pile capacity
A longer chain than the shallow one, with more places to go wrong.
Why this matters
Module 8 took an SPT blow count to a shallow bearing capacity and found the answer varying by a factor of 2.4 across ordinary scatter. The chain to a pile is longer: everything from Module 8, and then a deep bearing factor that is itself disputed. It is worth seeing the whole chain in one place.
By the end of this lesson you should be able to
- Take a blow count through to a pile base resistance
- Identify which link carries most of the uncertainty
- Say why the CPT is preferable where it is available
Worked example
A blow count carried to a pile base
Given
- Driven pile, D = 0.5 m, toe at 15 m
- SPT at toe level: N = 30, standard 60% energy hammer
- Sand, γ′ = 10 kN/m³ — water table at ground level, so σ′v = 150 kPa at the toe
Find
The base resistance, and how much the choice of bearing factor decides
The CPT is the better instrument, and for a specific reason: a cone penetrometer is itself a small pile. It is pushed into the ground at a controlled rate, it records the resistance at its tip and the friction on its sleeve, and those are the two quantities a pile design needs. The analogy is direct in a way that a hammered sampler's blow count is not.
So CPT-based pile methods are shorter chains. Base resistance comes from the cone resistance averaged over a zone around the intended toe level rather than from a friction angle and a bearing factor; shaft resistance comes from the sleeve friction, or from the cone resistance with a factor. Fewer links, fewer places for judgement to enter.
The averaging matters, though. A toe placed just above a weaker layer will not achieve the resistance measured at its own level, because the failure mechanism extends below it — the same influence-depth argument as Module 2, applied to a pile.
Check yourself
Why is the CPT better suited to pile design than the SPT?
Practice
A calculated base resistance is 4418 kN. Applying a limiting base pressure brings it to 2945 kN. What percentage did the limit remove?
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