Foundation Engineering / Pile capacity from SPT and CPT; drilled shafts
From field data to pile capacity
A longer chain than the shallow one, with more places to go wrong.
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 CPT averaging/correlation method and drilled-shaft construction quality controls. Driving set alone does not prove static resistance; under-ream applicability requires construction review.
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
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
CPT-based pile methods use a near-continuous profile of cone resistance and, in some methods, sleeve friction. They can provide a useful alternative to correlations through SPT and friction angle, provided the method is calibrated for the ground and pile type.
The resistance is not a direct measurement of full-size pile capacity. Averaging around the intended toe and allowing for weak layers, installation and scale remain essential.
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?