Comparison of 2D and 3D limit equilibrium slope stability analysis when considering various loading conditions

Two commercially available limit equilibrium slope stability analysis software packages (Slide and Slide3) were used to compare the computed factors of safety (FoS) from two dimensional (2D) and three dimensional (3D) models when considering distributed and concentrated surcharges. Available published literature typically only considers the difference in model geometry from 2D to 3D, and generally suggests an approximately 10% increase in FoS when moving from 2D plane-strain to a 3D model. Experience indicates that this indicative increase in FoS is inaccurate when concentrated loads or surcharges are acting on the slope, and that for such cases the increase in FoS can be significantly higher.

A comparison is presented of the computed FoS resulting from 2D analyses and those obtained with 3D analyses when considering various lengths of surcharge. The study investigates the changes in FoS for a homogenous soil under undrained and drained conditions for varying slope heights and angles. The results indicate that the 3D effects are more significant for failure mass with smaller lengths of surcharge due to more pronounced end effects, and that these effects are most prominent in slopes made of cohesive soil.

For cohesive soils, up to a 50% increase in FoS is observed for surcharge lengths less than 1m. For granular soils, up to a 35% increase in FoS is observed for surcharge lengths less than 0.5m. However, for both cohesive and granular soils, the end effects are negligible when the length of the surcharge load exceeds 3m and no significant increase in FoS is observed.