Foundation Engineering / module 16
Pile capacity from SPT and CPT; drilled shafts
Field data into pile capacity, and the part of a bored pile that no calculation controls.
What you will understand
- Carry SPT data through to a pile base resistance and identify where the uncertainty enters
- Explain why the CPT is the better instrument for pile design
- Apply a limiting base pressure and say how much of the answer it decides
- Describe the workmanship factors that govern a drilled shaft's real capacity
Lessons
From field data to pile capacity
A longer chain than the shallow one, with more places to go wrong.
Drilled shafts
Where the capacity is decided on site, not on the drawing.
Module summary
- The SPT chain to a pile is longer than the shallow one, and the deep Nq is the weakest link
- The example gave φ′ = 39.4°, a shallow Nq of 59.3, and a deep value several times larger
- A limiting base pressure of 15 MPa removed 33% of the calculated base resistance
- The CPT is closer to a model pile test than any other routine investigation
- An under-ream from 0.6 m to 1.5 m multiplied base resistance by 6.25, from 229 kN to 1431 kN
Teaching basis and review status
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 notesCheck your understanding
Practice
By what factor does base area increase when a 0.6 m base is under-reamed to 1.5 m?
Check yourself
Two identical bored piles are designed with the same parameters. One is bored and concreted the same morning; the other stands open overnight in clay. What differs?