Slope Stability Safety Factor Calculator
Geotechnical slope engineering: Calculate the factor of safety (FS) against planar translational slope failure under dry, partially submerged, and parallel seepage states.
Hillside Geometry & Soil Properties
Landslide Stability Evaluation
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Frequently Asked Questions
Why does rainfall-induced seepage cut slope factor of safety in half?
In dry cohesionless soil, slope stability depends strictly on friction: FS = tan(phi) / tan(beta). When water fills the soil pores and seeps parallel to the slope face, pore water pressure (u) pushes upward against the soil grains, reducing effective normal stress by roughly the ratio of buoyant to saturated unit weight: gamma' / gamma_sat ≈ (19 - 9.81) / 19 ≈ 0.48. This cuts resisting friction by more than 50%, causing landslides on slopes that stood stable all summer.
What is the stabilizing role of vegetation root cohesion (c')?
Apparent root cohesion from tree and shrub roots (typically c' = 2 to 15 kPa) mechanically binds the upper 1 to 3 meters of soil mantle into a reinforced mat. For shallow translational slides, even a small root cohesion of 5 kPa can increase the factor of safety by 30% to 60%, explaining why clear-cutting hillsides triggers widespread debris flows.
What is the difference between translational and rotational slope failures?
Translational slides (modeled by infinite slope analysis) occur when a shallow soil layer slips along a planar boundary parallel to the ground surface, such as a weathered soil-bedrock interface. Rotational slides (modeled by Bishop's or Spencer's method of slices) occur in deep, homogenous cohesive clay masses along a curved, circular spoon-shaped slip surface.