Immediate settlement is the deformation that develops as the soil responds to an applied load without the time-dependent drainage process represented by primary consolidation.
Topic guide: For background, method selection and related checks, see Soil Compressibility and Foundation Settlement.
A simplified elastic framework is useful for checking calculation structure. Selecting soil stiffness and influence factors remains a major engineering decision.
General elastic framework
A commonly used form is:
$$ S_i= \frac{qB(1-\nu^2)}{E_s} I_sI_dI_r $$| Symbol | Meaning |
|---|---|
| \(S_i\) | Immediate settlement |
| \(q\) | Applied net foundation pressure used by the selected method |
| \(B\) | Reference footing width |
| \(\nu\) | Poisson ratio |
| \(E_s\) | Representative soil modulus |
| \(I_s\) | Shape or rigidity influence factor |
| \(I_d\) | Embedment or depth influence factor |
| \(I_r\) | Additional method-specific influence factor |
The factor names and definitions vary between methods, so each published method should be treated and labelled separately rather than mixed into an unlabelled collection of factors.
Illustrative example
Assume:
- net applied pressure \(q=150\) kPa
- footing width \(B=2.0\) m
- soil modulus \(E_s=25{,}000\) kPa
- Poisson ratio \(\nu=0.30\)
- combined influence factors = 1.0 for the arithmetic demonstration
This verifies unit handling and arithmetic only. It does not validate the selected modulus or influence factors.
Parameter selection controls the result
The calculated settlement is directly dependent on \(E_s\). A modulus may vary with:
- stress level
- strain level
- drainage condition
- loading rate
- soil type and density
- depth and confinement
- sample disturbance
- in-situ or laboratory test method
- footing size and load distribution
The calculation must retain the source, interpretation method and applicable depth range for every stiffness value.
Layered profiles
A single equivalent modulus may be unsuitable where stiffness varies with depth. Layered methods may calculate strain increments through sublayers:
$$ S_i=\sum \varepsilon_{z,i}H_i $$The stress-distribution and modulus model used for each layer must be identified.
Net and gross pressure
Settlement methods may use net foundation pressure, gross contact pressure or a stress increment. The selected convention must match the method.
The calculator should not choose a pressure convention silently.
Calculation record
Retain:
- footing dimensions and embedment
- applied load and pressure convention
- load eccentricity
- soil layers
- modulus and Poisson ratio by layer
- parameter sources
- selected influence factors
- stress-distribution method
- groundwater assumptions
- calculated settlement by layer
- total immediate settlement
- method and version
Limitations
The simple elastic expression does not by itself model:
- primary consolidation
- secondary compression
- non-linear stress-strain behaviour
- yielding or bearing failure
- construction sequence
- nearby foundations
- excavation rebound
- cyclic loading
- creep
- three-dimensional soil variability
Total settlement should consider all relevant components.
Related resources
- Consolidation Settlement and Time Rate
- Settlement Assessment: Total and Differential Movement
- Bearing Capacity
- Effective Stress
- Footing Pressure and Eccentricity
Authoritative references
Settlement predictions require a suitable method, representative parameters and review by a suitably qualified geotechnical professional.