Revisiting the undrained bearing capacity factor Nc of rough strip footing

This research investigates the impact of embedment depth, footing width, and un-drained cohesion on the mobilized Nc in undrained conditions for rough strip footings. The research was carried out using a verified Plaxis 2D model. It has been noted that the Nc factor initially upsurges with the rise of the footing depth but declines beyond a certain normalized footing depth due to the change of the failure mechanism from general shear failure to punching shear failure. In addition, it has been observed that the footing width significantly influences the mobilized Nc due to reduced shear strength mobilization efficiency along failure surfaces, confirming the presence of scale effect for footings on undrained soils. Also, it has been noted that the Nc values proposed in past seminal studies (e.g., Terzaghi, Meyerhof, Skempton, Salgado et al.) are inadequate, as they either neglected embedment depth and footing width effects or failed to account for failure mode transition at deeper embedment. On the other hand, it has been shown that the undrained cohesion does not have a noticeable impact on the mobilized Nc, suggesting that failure wedge geometry governs behavior rather than soil shear strength. Finally, a new predictive model of the Nc factor in undrained condition is introduced. The new model takes into account the effect of the footing width and embedment depth. This model scored high prediction accuracy with a mean absolute error of 0.075, root mean square error of 0.107, mean of 1.00, and coefficient of determination of 0.983. Thus, this model offers a reliable tool for geotechnical design, ad-dressing gaps in current theoretical frameworks.