Foundation Engineering / Immediate settlement of shallow foundations
Which stiffness?
The number that decides the answer, and how to choose it.
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.
Corrections checked in this pass
- Recalculated the elastic settlement examples with the exact flexible-centre influence. Rigid non-square results remain unavailable.
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.
- Select modulus for drainage, stress and strain; layering/embedment excluded. Rigid non-square influence remains unavailable until a verified shape-dependent solution is selected.
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
If settlement is proportional to 1/E, then choosing E is the settlement calculation. Everything else is arithmetic. It is worth understanding why a single value of E does not really exist.
Soil stiffness is strain-dependent, and strongly so. At very small strains — below about 0.001% — soil is far stiffer than most engineers expect, and it is relevant where the predicted strains remain within that small-strain range. As strain rises the stiffness falls away along a characteristic decay curve, and by the time a laboratory triaxial specimen is at 1% strain it may be showing a fifth of its small-strain value.
A working foundation typically imposes strains of the order of 0.01% to 0.1%. That is well below conventional laboratory measurement and well above the small-strain plateau — squarely on the steep part of the curve.
Drained or undrained is the other decision, and it follows from the timescale.
Immediate settlement in clay is an undrained process: no volume change, so ν = 0.5 and the relevant stiffness is the undrained modulus. Consolidation settlement follows afterwards, and Module 10 deals with it.
In sand the distinction largely disappears, because drainage is fast enough that settlement is essentially complete by the end of construction. This is why sand settlement is treated as immediate and why the awkward long-term component belongs to clay.
Check yourself
Why does a modulus from a conventional triaxial test usually overpredict foundation settlement?
Try it
Settlement and the modulus
Compare settlement as the assumed soil modulus changes. The flexible centre uses an integrated half-space solution; the rigid square uses a tabulated influence factor. Ground stiffness and model applicability still need assessment.
Foundation
- Settlement
- 18.4 mm
- Rigid would be
- 16.2 mm
- Difference
- 12 %
- Settlement if E is half
- 36.8 mm
Within this model, halving E doubles settlement when the other inputs stay fixed. A site assessment also needs a justified stiffness value, stress and drainage basis, soil profile and verified influence factors. The plotted estimates do not establish acceptable foundation movement.
Summary
- Settlement is proportional to q, to B, and to 1/E — so E dominates
- Halving the modulus doubled the settlement, from 18.4 mm to 36.8 mm
- Soil stiffness falls steeply with strain; a foundation works at 0.01–0.1%
- A conventional triaxial modulus is measured too far along that curve and overpredicts movement
- Back-analysis of a comparable structure often beats any test result