Foundation Engineering / module 4
The bearing capacity equation
Three terms, three factors — and one of them nobody agrees on.
What you will understand
- State the three-term bearing capacity equation and say what each term represents physically
- Compute Nc, Nq and Nγ, and know which of them is derived and which is fitted
- Explain why published Nγ values differ between authorities, and what to do about it
- Identify which term dominates in clay and which in sand
Lessons
Three terms, three sources of strength
Where the resistance actually comes from.
The factor nobody agrees on
N_γ, and what to do when the references disagree.
Module summary
- Capacity comes from cohesion, from surcharge beside the footing, and from the soil's own weight
- Nq and Nc are derived from the failure mechanism; Nγ is not, and has no closed solution
- Vesic, Meyerhof and EC7 give Nγ values differing by tens of per cent at the same φ′
- In undrained clay the cohesion term is everything and capacity is independent of footing width
- In sand the cohesion term vanishes and capacity depends on both depth and width
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
A strip footing 2.0 m wide is founded at 1.0 m in sand with φ′ = 35°, γ = 19 kN/m³, using Nq = 33.3 and the Vesic Nγ = 48.0. What is the SURCHARGE term q·Nq, in kPa?
Check yourself
Doubling the width of a strip footing on UNDRAINED clay has what effect on its ultimate bearing PRESSURE?