Every foundation transfers the weight of a structure into the ground, and the soil must be able to carry that weight without failing in shear or settling too far. When the ground beneath a footing cannot resist the applied pressure, the foundation can sink, tilt, or rupture, and the damage often appears first as cracked walls and sticking doors. The capacity of the soil is therefore studied before design begins, using field tests and analytical methods that are core to foundation design and construction. This article explains the terminology engineers use, the three classic modes of bearing capacity failure, and the theory that predicts them.
Bearing Capacity: Key Definitions
Engineers distinguish several levels of bearing capacity, and mixing them up is a common source of design error.
| Term | Symbol | Definition |
|---|---|---|
| Bearing capacity | – | The loading capacity of the soil beneath a foundation |
| Ultimate bearing capacity | qu | The least gross pressure that causes shear collapse of the soil directly below the foundation |
| Net ultimate bearing capacity | qun | The net pressure added by external loads that just initiates collapse; qun = qu – Df, where Df is the footing depth, when concrete and soil densities are similar |
| Safe bearing capacity | – | The capacity remaining after a factor of safety is applied |
| Net safe bearing capacity | qns | The net pressure that can be added to the soil without shear failure |
| Safe gross capacity | qs | The highest gross pressure the soil can bear safely; qs = qns + Df |
| Allowable bearing pressure | – | The highest pressure the soil can take without shear collapse or settlement failure |
The difference between gross and net values exists because the soil at foundation level already carries the weight of the footing and the backfill above it. Many of the warning signs that appear in a building, from cracked masonry to doors that will not close, trace back to ground problems, and the types of foundation failures on soil have well-documented causes and remedies.
Why the factor of safety matters
Design codes apply a factor of safety, typically between 2.5 and 3, to the ultimate bearing capacity before it becomes an allowable pressure. The margin absorbs soil variability, testing errors, unforeseen live loads, and the difference between laboratory samples and the real ground.
Allowable pressures published in local codes are conservative values for common soil types. They are fine for preliminary sizing, but a project of any size should verify them with site-specific testing before finalising the footing dimensions.
The Three Types of Bearing Capacity Failure
The three primary types of bearing capacity failures are general shear failure, local shear failure, and punching shear failure. Which one occurs depends mainly on the density and compressibility of the soil and on the depth of the footing.
General shear failure
General shear failure happens in dense sand and stiff clay. A continuous failure surface develops from the edge of the footing to the ground surface, the ground beside the footing bulges upward, and the footing drops suddenly once the soil reaches its peak resistance. The load-settlement curve shows a well-defined failure load, which makes the capacity easy to read.
Local shear failure
Local shear failure occurs in loose sand and medium clay. The failure surface starts at the footing edge but stops below the ground surface instead of reaching it. The footing settles a great deal before failure, the ground surface shows little or no bulging, and the load-settlement curve never shows a sharp peak, only a gradual flattening.
Punching shear failure
Punching shear failure happens in very loose sand and very soft clay. The footing pushes almost straight down, compressing the soil beneath it, and the soil moves vertically along the sides of the footing rather than flowing outward. There is little visible ground movement around the footing, but settlement can be large and continuous.
| Failure type | Typical soil | Failure surface | Settlement behaviour | Surface signs |
|---|---|---|---|---|
| General shear | dense sand, stiff clay | continuous to ground surface | sudden, well-defined peak | ground bulges beside footing |
| Local shear | loose sand, medium clay | stops below ground surface | large before failure, no peak | little or no bulging |
| Punching shear | very loose sand, soft clay | vertical around the footing | large and continuous | no visible movement |
- Watch the ground surface beside the test plate for bulging
- Track the settlement rate at each load increment
- Compare the load-settlement curve with the three classic patterns
How Engineers Evaluate Bearing Capacity
Field and laboratory data feed the bearing capacity calculations that appear in every geotechnical report. Several methods are available, and good practice uses more than one.
- Presumptive analysis using code tables based on soil type
- Analytical methods using shear strength parameters from laboratory tests
- Plate bearing test carried out at the actual foundation level
- Penetration tests: the standard penetration test (SPT) and the cone penetration test (CPT)
- Modern in-situ tests such as the pressuremeter and dilatometer
- Centrifuge tests for complex layered or sloping sites
Plate load test procedure
- Excavate to the proposed foundation level and prepare a level surface.
- Place a steel plate, typically 300 to 750 mm square, on the surface.
- Apply load in increments with a hydraulic jack reacting against a loaded frame.
- Record settlement after each increment once it stabilises.
- Plot load against settlement and identify the failure load or the settlement limit.
- Divide the failure load by the factor of safety to get the allowable pressure.
Terzaghi’s Bearing Capacity Theory
Karl Terzaghi published the first rational method for bearing capacity in 1943, and it remains the basis of most modern equations. The theory applies most directly to shallow foundations, where the depth of the footing is small relative to its width.
The classic equation is qu = cNc + qNq + 0.5γBNγ, where c is the soil cohesion, q is the surcharge at foundation level, γ is the unit weight of the soil, B is the footing width, and Nc, Nq, and Nγ are bearing capacity factors that depend on the angle of internal friction φ of the soil.
Assumptions in Terzaghi’s theory
- The depth of the foundation is less than or equal to its width
- The base of the foundation is rough
- The soil above the base has little shear strength and acts only as a surcharge
- The load is vertical and non-eccentric
- The soil is homogeneous and isotropic
- The ratio of length to width is effectively infinite, so the footing acts as a strip
The three failure zones
Terzaghi divided the soil below the footing into three zones at failure.
- Zone 1 is the active Rankine zone directly under the footing, inclined at 45 + φ/2
- Zone 2 contains the radial shear zones that spread from the outer edge of the footing base
- Zone 3 is the passive Rankine zone, inclined at 45 + φ/2 to the horizontal
How the zones relate to failure type
General shear failure develops all three zones fully, which is why the ground surface bulges. In local shear failure the radial zone is only partly developed, and in punching shear failure the zones barely form at all because the soil compresses instead of shearing along a defined surface.
How Failure Type Affects Foundation Design
The predicted failure mode changes the design response. Choosing between foundation types in construction depends on the soil behaviour you expect at the site.
- Dense soils: size the footing so the applied pressure stays below the allowable value
- Loose soils: widen the footing, deepen the embedment, or improve the soil by compaction or replacement
- Very soft soils: use deep foundations such as piles to reach a stronger stratum
- All sites: provide drainage so the soil does not saturate and lose strength
- Verify assumptions with a load test on the first production footing
A practical decision sequence
- Characterise the soil with boreholes and laboratory tests.
- Estimate the ultimate bearing capacity using Terzaghi or a similar method.
- Apply the factor of safety, typically 2.5 to 3, to get the allowable pressure.
- Size the footing so the contact pressure stays below the allowable value.
- If capacity is insufficient, improve the soil or switch to a deeper foundation.
- Confirm the design with a plate load test during construction.
Footings that fail in general shear usually show visible ground movement first, which gives inspectors a warning. Punching and local shear give fewer surface clues, so instruments and settlement monitoring matter more on soft sites.
Site Investigation Before You Build
None of this analysis is worth anything without reliable soil data. A proper geotechnical site investigation covers bearing capacity, settlement, and foundation recommendations in a single report.
- Desk study of maps, records, and neighbouring buildings
- Boreholes and trial pits at the planned footing locations
- Disturbed and undisturbed sampling at depth
- Laboratory tests for classification, strength, and compressibility
- Groundwater level measurement, since water changes bearing capacity dramatically
- A final report with allowable bearing pressures and foundation recommendations
How much investigation is enough
Codes typically require one borehole per 100 to 300 square metres of plan area, depending on the structure size and the uniformity of the ground. The cost of the investigation is small next to the cost of a failed foundation, and the data lets the designer pick the failure mode, the theory, and the safety factor with confidence.