An overview of soil compressibility, foundation settlement components, investigation inputs and serviceability assessment.
Soil Compressibility and Foundation Settlement
Foundation settlement is the vertical movement caused by changes in ground stress and soil volume. The amount, timing and distribution of movement depend on the soil profile, groundwater, foundation geometry, load history and construction sequence.
This guide explains the settlement framework. The elastic formula and worked example are on the separate Immediate Settlement Calculation page.
Main settlement components
Settlement is commonly separated into:
- Immediate settlement caused by distortion of the soil mass as load is applied
- Primary consolidation caused by dissipation of excess pore-water pressure
- Secondary compression that continues after primary consolidation
- Collapse, shrink-swell, creep or degradation where the relevant ground conditions exist
Not every component applies to every site. Each component should be calculated using parameters and a method suitable for the material and design condition.
Compressibility parameters
Depending on the selected method, inputs may include deformation modulus, constrained modulus, Poisson ratio, compression and recompression indices, coefficient of volume compressibility, preconsolidation stress and coefficient of consolidation.
These values are not interchangeable. They can depend on stress range, strain level, drainage condition, sample quality and the direction of loading.
Stress increase below foundations
Foundation load spreads through the ground and produces a stress increase that varies with depth and lateral position. Settlement assessments may divide the profile into sublayers and calculate the representative stress change in each layer.
The pressure convention must be recorded. Gross foundation pressure, net pressure and the change in effective vertical stress are not automatically equivalent.
Differential movement
Total settlement alone may not control performance. Changes in footing load, geometry, founding material or adjacent construction can produce differential movement and rotation. The structural tolerance and serviceability criteria should be defined by the project team.
Investigation and monitoring
Settlement assessment may draw on boreholes, test pits, laboratory consolidation tests, in-situ testing, geophysical data, groundwater observations and comparable local performance. Where movement is consequential or uncertain, staged construction, instrumentation or observational controls may be appropriate.
Calculation resources
- Immediate Settlement of Shallow Foundations
- Consolidation Settlement Calculation
- Consolidation Testing and Settlement Analysis
- Bearing Capacity of Soil and Foundations
Engineering note
A simple elastic result is a screening calculation, not a complete settlement assessment. Layering, stress-dependent stiffness, load history, groundwater, construction sequence and three-dimensional effects may control the outcome.