Failure mechanism
Foundation settlement
Settlement is the movement of a foundation into the ground under load, and it is checked separately from whether the ground can carry that load at all. A footing can hold a wide margin against bearing failure and still settle enough to distort everything above it. Total settlement is rarely the problem; differential settlement between footings is, because steel frames, masonry and sheeted roofs all distort when their supports move by different amounts.
How this failure works
Bearing capacity and settlement are different limit states with different calculations acting on different soil properties. Bearing capacity asks whether the soil will fail in shear, and it is expressed with a factor of safety. Settlement asks how far the footing will move under the working load, and it is compared against a limit derived from the structure above. A footing on a stiff clay can have a large margin against shear failure and still settle; a footing on loose granular soil can settle during construction. Sizing a footing against bearing pressure alone leaves the second question unasked, and it is the second question that damages buildings.
How the settlement develops depends on the soil. In a saturated clay the load is at first carried by pore water and the footing barely moves; as the excess pore pressure dissipates the soil consolidates and the footing settles, over months or years, because the rate depends on permeability and on the length of the drainage path. In a sand, drainage is fast and most of the movement happens while the structure is still being built. Organic soils, uncontrolled fill and old workings add a long-term component of their own. So the stage at which a structure reaches its worst settlement is not the day it is handed over, and a survey taken then can be reassuring without being final.
Differential settlement is what does the damage. Two footings settling by the same amount tilt nothing; two settling by different amounts rotate and distort every element spanning between them. A steel portal frame tolerates a little of this by flexing, but the cladding and roofing do not: sheet laps open, a gutter loses its fall, doors and shutters bind, fixings work loose in their holes. The governing quantity is the angular distortion over the span between supports, which is a much tighter limit than the total settlement tolerated on a single footing. A frame stiff enough to redistribute load between footings changes that distribution, and can concentrate load on the footing where the ground is worst.
Soil is variable over distances that matter, and the footings rarely sit at the borehole positions. A soft pocket, an old drain, a filled pit, a change from rock to residual soil along the length of a shed, or a hard layer that one footing reaches and its neighbour does not: each produces differential movement that no single bearing pressure check would reveal. Groundwater is the other variable. Dewatering for an adjacent basement raises the effective stress in the soil around it and can induce settlement in structures well beyond the site boundary, and a leaking drain under a footing softens precisely the soil that was carrying it. For a light steel structure on a filled site, the settlement of the fill itself is frequently the governing design case, because the structure is too light to justify a heavy foundation and too flexible to hide the movement.
Illustration
Conceptual illustration: two footings under one steel frame settling by different amounts, with the resulting distortion of the frame and the cladding above exaggerated.
Typical causes
- Footings sized on bearing capacity with settlement, and in particular differential settlement, never checked.
- Soil conditions not investigated at the footing positions, so a soft pocket, old fill or an abandoned drain was never found.
- Fill placed without a specified material or compaction control, settling under its own weight and then under the footing.
- Groundwater lowering, or an adjacent deep excavation, causing consolidation settlement that reaches beyond the site boundary.
- Water reaching the soil under a footing from a leaking drain, a soakaway, or ponding at the plinth.
- Load added after completion — a mezzanine, plant, or a change of use — with the footings never rechecked for it.
- Footings founded on different strata along the same frame, or at different founding levels, so they cannot settle together.
Warning signs
- Diagonal cracks in masonry or plaster at corners and around openings, in a pattern that does not follow the structure.
- Cracks that open and close with the season, or that have widened between inspections recorded with dates.
- Columns out of plumb, and a ridge line that is not straight when sighted along it.
- Sheet laps opening at one end of a roof, a gutter that no longer falls to its outlet, and water standing where it used to drain.
- Doors, shutters and sliding panels that bind progressively, or that bind in one part of the frame and not another.
- A gap between the plinth protection or apron and the wall, or a step at a door threshold that was not there before.
Inspection methods
- 01Level survey of base plates or plinth against the design level, repeated over time so the rate of movement is known and not only the total.
- 02Record crack widths, orientation and position with dates and tell-tales, so a change can be separated from a crack that has always been there.
- 03Read the soil investigation against the footings as built: whether each footing founded at the investigated level, on the stratum assumed.
- 04Check drainage, plinth protection, downpipes and any ponding that is wetting the ground near a footing.
- 05Plumb survey of the frame, and a check of the roof geometry for a distortion pattern that matches the movement.
- 06Check the history of the site for adjacent excavation, dewatering, new fill or changed surface levels.
Preventive measures
- Check settlement and differential settlement as independent limit states, using the measured compressibility of the soil.
- Investigate the soil to a depth that covers the influence zone of the footings, at positions that represent where the footings will actually go.
- Found on a consistent stratum, and where the ground changes, design for the change instead of assuming it away.
- Control fill placement: specified material, defined layer thickness, compaction verified by test and recorded.
- Detail drainage and plinth protection so water cannot pond or soak next to a footing.
- Provide for movement in the cladding and connections, and set monitoring with trigger levels for the construction period and beyond.
- Recheck the footings whenever load is added, and account for adjacent excavation or dewatering before it begins rather than after.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- IS 456 — concrete foundations, including the permissible settlement limits applied to the structure above.
- IS 800 — the steel frame, whose connections and cladding details have to accommodate the movement.
- IS 875 — loads, which set the footing loads the settlement is computed for.
- IS 1893 — seismic design, including the foundation assumptions the frame relies on.
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.