FAQ
1. Bishop vs Janbu — which should I use?
Use the simplified Bishop method for circular slip surfaces — it is widely validated and suitable for most homogeneous slopes. Use Janbu corrected when the failure surface is non-circular (e.g. composite failure, layered soils, or general slip-plane geometry). Janbu corrected applies a shape factor f₀ from Janbu (1956) Table 1 that accounts for the moment equilibrium deficiency of the simplified force-only solution. Both satisfy EN 1997-1 §11 requirements.
2. What partial factor approach should I use?
EN 1997-1 recommends Design Approach 1 (DA1) for slope stability, checking both Combination 1 (γ_G=1.35, γ_Q=1.5) and Combination 2 (γ_φ=1.25, γ_c=1.25). The worse outcome governs. UK National Annex uses DA1 with FOS thresholds (FOS_ot≥1.5, FOS_sl≥1.3). India IS 2911 requires FOS_ot≥2.0 and FOS_sl≥1.5.
3. How is groundwater handled?
Groundwater is modeled via pore water pressure u at the slip surface. When the groundwater table intersects the slip surface, a ru parameter is applied (default ru=0.3) that reduces effective stress and therefore shear strength. The groundwater depth input is referenced against the slip surface geometry.
4. What FOS threshold is required?
EN 1997-1 DA1C1 requires FOS≥1.35 for persistent design situations. DA1C2 and DA3 effectively produce FOS≥1.0 because material factors absorb the safety margin. For GB 50007, FOS_ot≥1.5 and FOS_sl≥1.3. For IS 2911, FOS_ot≥2.0 and FOS_sl≥1.5. FrameAI displays the applicable threshold based on the code selected.