Geotechnical Engineering
Slope Stability
Infinite slope method — factor of safety against sliding, with optional seepage parallel to the slope.
Slope Stability (Infinite Slope Method)
Reviewed & updated August 28, 2026The infinite slope method checks the stability of a long, uniform slope — an embankment, cut, or natural hillside extensive enough that a potential failure surface can be treated as a plane parallel to the ground surface at some depth z, rather than a curved slip circle. It's the standard first check for shallow, translational slides in residual soil or fill, and the starting point before a more detailed circular or non-circular slope stability analysis.
Stability comes down to a simple ratio: the shear strength available on the failure plane, from the soil's cohesion and friction (reduced by any pore water pressure), divided by the shear stress the overlying soil wedge's own weight generates on that plane. Seepage is the critical variable — a slope with a wide margin when dry can drop well below its minimum acceptable factor of safety once water seeps through it parallel to the surface, which is the mechanism behind most rainfall-triggered slope failures.
FS = [c' + (γ − m·γw)·z·cos²β·tanφ'] / (γ·z·sinβ·cosβ)
m = 0 (dry) to 1 (seepage parallel to slope, full depth z)
A 2:1 (26.6°) slope with c' = 150 psf, φ' = 30°, and γ = 125 pcf is checked at a failure plane z = 5 ft below the surface. Dry, the driving shear stress is 250.2 psf against 438.5 psf of available shear strength — FS = 1.75. If seepage parallel to the slope reaches the ground surface (m = 1.0), available shear strength drops to about 294.5 psf against the same 250.2 psf driving stress — FS falls to 1.18, below the typical 1.5 minimum for long-term static conditions. It's one of the clearest illustrations of why slope failures cluster after sustained rainfall: the geometry and the soil haven't changed, only the pore water pressure has.
Slope Stability Calculator
Infinite slope method · seepage parallel to slope · imperial units