Slope Stability Calculator
Enter the slope angle, soil friction angle (φ), density and height to estimate the factor of safety using the infinite-slope method for cohesionless soil, for a quick preliminary assessment.
Input Data
Results
At a glance:Infinite-slope method (cohesionless soil, no groundwater): factor of safety Fs = tanφ / tanβ, where β is the slope angle and φ is the soil friction angle. Fs<1 means failure is certain; 1.0–1.5 is marginal and needs reinforcement (retaining, drainage); engineering commonly requires ≥1.5. This formula ignores cohesion c, pore water pressure and seismicity. Hong Kong is rainy and steep, so a proper design must use circular-slip methods (e.g. Bishop) and consider rainfall infiltration.
Formula
Fs = tanφ / tanβ
β, φ are angles (degrees)
$$F_s = \frac{\tan\phi}{\tan\beta}$$How to Use
- Enter the slope angle (degrees).
- Enter the soil friction angle φ (degrees).
- Enter the density and height (for reference).
- The tool outputs the factor of safety and preliminary verdict.
Case Studies
Garden soil slope
β=30°, φ=30°, 1,800, 5 m.
Fs=1.0, critical; needs reinforcement or flattening.
Concrete-retained slope
β=35°, φ=38°, 2,000, 8 m.
Fs≈1.13, marginal; add drainage.
FAQ
What are the limits of the infinite-slope method?
It only suits shallow, homogeneous cohesionless soil with no groundwater; deep slips, cohesive soil and rainfall infiltration need more complete methods.
Why does Hong Kong care about this?
Hong Kong is hilly, steep and rainy with high slope-failure risk, so the government runs a systematic slope-safety review (e.g. Buildings Department slope regulations).
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References
Content review: Calculatorism Editorial Team. Results are for reference only; please refer to the relevant authorities for the official figures.