Lateral capacity and failure mechanisms of skirted foundation resting on slopes

While the effect of combined loading on the load carrying capacity of skirted foundations on horizontal ground surfaces is well documented in the literature, comprehensive studies for skirted footings situated on or near slopes remain lacking. This study aims to investigate the influence of various factors, including slope angle (β), angle of internal friction (ϕ), setback distance (B′/B), and skirt length (L/B), on failure loads and capacity envelopes for skirted foundations. The performance of a skirted footing system under combined vertical and lateral loadings in cohesionless soils was analysed using finite-element limit analysis (FELA) and small-scale experimental tests. The results indicate that the optimal skirt length depends on the specific conditions of the slope and setback distances. As the setback distance increases, the optimal skirt length tends to decrease, while an increase in slope steepness corresponds to an increase in the optimal skirt length. Furthermore, this study presents integrated capacity envelopes that incorporate a wide range of realistic soil slopes. The calculated capacity envelopes are subsequently compared to those on flat ground, highlighting that the unique characteristics of these envelopes are based on different combinations of vertical load (V) and combined vertical and lateral load (VH). Skirted footings on slopes exhibit distinct behaviour compared to those on level ground, with multiple factors influencing their capacity envelopes. Specifically, skirted foundations demonstrate lower capacity than equivalent block foundations, particularly when placed near steep slopes due to the formation of an inverted circular scoop failure surface for larger L/B ratios. In contrast, the formation of an inverted circular scoop is less pronounced for moderate slopes and footings with a smaller L/B ratio. The numerical study provides a comprehensive explanation of the failure mechanisms associated with the design of skirted footings located in close proximity to slopes, especially when subjected to lateral loads. Based on the results obtained, capacity envelope diagrams are proposed for estimating the capacity of skirted footings on slopes. Small-scale experiments were also conducted to explore the effects of varying skirt lengths placed on both level and sloping ground within a testing tank. The outcomes of this study yield valuable insights into the behaviour of skirted foundations on slopes, as well as their overall response to both vertical and lateral loadings.