Skempton's Bearing Capacity Theory Applicability

Skempton's bearing capacity theory is a significant contribution to foundation engineering, primarily developed to estimate the load-carrying capacity of shallow foundations. The theory's formulation and empirical basis are most directly applicable to specific types of soil behaviour.

Understanding Skempton's Theory

John Skempton, a prominent soil mechanics researcher, extended the work of Terzaghi and others on bearing capacity. His contributions often focused on refining the bearing capacity equation, especially considering factors like the ratio of foundation depth to width ($D/B$) and the soil's shear strength characteristics. The theory aims to provide a more accurate assessment of the ultimate bearing capacity ($q_u$), which is the maximum pressure the soil can sustain before shear failure.

Relevance to Clay Soils

Skempton's work, particularly his analysis of ultimate bearing capacity, is strongly associated with clay soils. This is because:

• Clays are cohesive soils, meaning their shear strength is largely dependent on cohesion ($c$). Skempton's models effectively incorporate cohesive strength into the bearing capacity calculations.
• The theory considers the behavior of clays under undrained conditions (often relevant for short-term stability analysis), where cohesion is the dominant parameter.
• Empirical data and case studies used in developing and validating Skempton's methods often involved foundations on clay deposits.

The general bearing capacity equation often includes terms related to cohesion ($c$), surcharge pressure ($\gamma D_f$), and the soil's unit weight integrated over the footing width ($0.5 \gamma B$). Skempton refined the bearing capacity factors ($N_c$, $N_q$, $N_\gamma$) and proposed modifications based on soil properties and foundation geometry, particularly effective for cohesive materials.

Applicability to Other Soil Types

• Sandy Soils and Gravel Soils: These soils are primarily frictional, meaning their strength depends mainly on the angle of internal friction ($\phi$). While Skempton's theory might provide some baseline, theories developed specifically for frictional soils (like those heavily relying on the $N_\gamma$ factor) are generally more appropriate.
• Silt Soils: Silts exhibit properties that can be intermediate between clays and sands. Their behavior can vary depending on moisture content and plasticity. Skempton's theory is less commonly cited as the primary method for silts compared to clays, although it might offer insights if the silt behaves in a more cohesive manner.

Conclusion

Based on its development and common application in geotechnical engineering, Skempton's bearing capacity theory is most valid and widely used for estimating the bearing capacity of foundations resting on clay soils.