Foundation Bearing Failure Modes and Capacities

The ultimate bearing capacity is defined as the critical pressure at which failure occurs, where the applied loads exceed the soil's supporting (bearing) capacities. The structure-soil interaction is affected by the soil's density and shear strength as well as the depth of the footing supporting the structure above. Failure occurs when the soil bearing capacity is exceeded: in this article, we will elaborate on the three different modes of bearing failure.
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Jump to Meyerhof and Terzaghi bearing capacity calculators here:
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Bearing Capacity Calculator - Meyerhof Approach
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Bearing Capacity Calculator - Terzaghi
General Shear FailureGeneral shear failure manifests as an abrupt and devastating collapse marked by a distinct failure pattern. This involves the formation of a clearly defined failure surface extending from the edge of the footing to the ground surface. Notably, the failure is characterized by ground surface upheaval and footing tilting, unless there is an obstructing structure. Typically occurring when the foundation rests on compact sand and rigid clay, this failure scenario can have significant consequences.
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Settlement developes progressively with the incremental application of the load. Once the load per unit area attains the ultimate bearing capacity, a sudden failure occurs in the soil supporting the foundation. This failure is marked by the development of a failure surface within the soil extending to the ground surface, accompanied by noticeable bulging of adjacent soils on either side of the foundation.
Local Shear FailureLocalized shear failure occurs when the foundation is situated on soil with medium compaction, composed of sandy or clayey characteristics. The discernible failure pattern is only observable beneath the footing, resembling the general shear failure. This pattern entails visible wedge and slip surfaces at the footing's edges.
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Upon reaching the ultimate bearing capacity, abrupt shifts occur in the foundation's movement, and subsequent incremental load increases lead to significant increases in foundation settlement. The failure surface subsequently extends outward from the foundation, and the load-settlement curve does not have a prominent peak.
Punching FailurePunching failure occurs when the foundation is significantly deep beneath the ground surface and the situated on loose soils with low compressibility. Unlike the above shear failures, there is no upheaval or tilting of adjacent soils, and the ground beneath essentially 'punches through' the soil when the capacity is exceeded. 
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Unlike other failures, punching failure results in substantial settlement without a clearly defined ultimate bearing capacity. The compression of soil beneath the footing occurs suddenly with load increase and the failure surface does not extend above the ground surface. Hence, punching shear is difficult to observe and prevent.
In summary, general shear failure, local shear failure, and punching failure are characterised by their distinctive failure patterns and settlements. Detailed soil assessment and good design and construction processes are essential to avoid and prevent these failures.
Calculating Soil Bearing CapacitiesTo accurately calculate the bearing capacity of a founding material, a thorough assessment of its properties and behaviour is necessary. This includes conducting in-situ and lab tests, which propose the following challenges:
Representativeness of samples - obtaining soil samples for testing involves extracting them from the ground, which may not accurately represent the in-situ conditions due to disturbance during sampling. Alterations in soil structure and water content during extraction can affect test results. Soil properties can vary widely within a site, and obtaining a limited number of samples may not capture this variability accurately. This variation can lead to uncertainties in design parameters.
Time dependency - Some soil properties, such as consolidation and creep, are time-dependent. Laboratory tests conducted within short time frames might not capture long-term behavior accurately.
Complexity of soil behaviour - soil behavior is complex and affected by numerous factors such as stress history, environmental conditions, and loading patterns. Simplified laboratory tests may not fully simulate these intricate conditions.
Soil disturbance - The process of obtaining soil samples often disturbs the soil structure, altering its properties. Disturbance can affect the soil's natural state and consequently impact test results.
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Cone penetration test (Source: Legion Drilling)
Various empirical and theoretical methods have been proposed and refined to allow engineers to estimate (to a satisfactory standard) bearing capacities. In particular, Terzaghi and Meyerhof methods are widely used. 
The Terzaghi method focuses on soil shear strength components, considering cohesion, effective stress, and the angle of internal friction. It offers a comprehensive formula for determining the ultimate bearing capacity (qu) of soil beneath a foundation. This equation incorporates cohesion (c), unit weight of soil (γ), depth factor (Df), width of foundation (B), and bearing capacity factors (Nc, Nq, Nγ). Terzaghi's method assumes relatively simple foundation shapes and loading conditions while accounting for multiple soil parameters.
In contrast, the Meyerhof method, developed by Meyerhof, takes a more empirical approach, relying on field observations and simplified equations for estimating bearing capacity. Meyerhof's equation for ultimate bearing capacity (qu) simplifies Terzaghi's formula, considering cohesion (c), unit weight (γ), depth factor (D), and bearing capacity factors (Nc, Nq). It is known for its simplicity, making it more convenient for initial designs and practical applications. Meyerhof's method is conservative, incorporating various soil conditions, types, and foundation configurations based on observed data.
Key distinctions between the two methods lie in their theoretical basis, equation complexity, incorporation of factors like foundation width, and ease of practical use. Terzaghi's method is more comprehensive, taking into account additional foundation width considerations, while Meyerhof's method is simpler and relies heavily on empirical factors derived from field experience.
Check out our calculators for Meyerhof method and Terzaghi method!
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ReferencesFoundations of Geotechnical Engineering by DIT Gillesania
Principles of Geotechnical Engineering 7th Edition by Braja M. Das
Foundation by Engineering Infinity
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