Slope Stability Factor Calculator

Inputs

Soil preset Custom (Manual Input) Soft Clay Stiff Clay Silty Sand Dense Sand Gravel

Presets fill γ, c, and φ. Verify project values.

Slope angle β (deg)

Typical: 15–45 degrees for many earthworks.

Soil unit weight γ (kN/m³)

Common: 16–21 kN/m³ depending on material.

Failure depth z (m)

Depth to potential slip plane or weak layer.

Cohesion c (kPa)

Use effective cohesion when applicable.

Friction angle φ (deg)

Typical: 18–40 degrees across soils.

Pore pressure ratio ru (0–1)

Higher ru means more water pressure, lower stability.

Surcharge q (kPa)

Optional load near crest (traffic, stockpiles).

Calculate Factor of Safety

Note: This tool uses an infinite slope approach for shallow planar failure. For complex geometry, layered soils, seismic loading, or critical works, a detailed geotechnical analysis is required.

Example data table

Example FS values are illustrative. Use site data for design decisions.

Professional guide

1) Why factor of safety matters

Slope stability checks help control collapses in temporary cuts, permanent embankments, and stockpile faces. This calculator estimates a shallow planar failure using an infinite slope approach and reports the factor of safety (FS). Values below 1.0 indicate driving shear exceeds available strength.

2) Key inputs and practical ranges

The tool uses slope angle β, unit weight γ, failure depth z, cohesion c, friction angle φ, pore pressure ratio ru, and optional surcharge q. Typical γ ranges from 16–21 kN/m³ for many soils. Shallow slip depths for cuts are often 0.5–3.0 m. Effective φ commonly ranges 18–40 degrees depending on grading and density.

3) Understanding water influence (ru)

The ru term reduces effective normal stress through pore pressure. Increasing ru from 0.05 (well drained) to 0.30 (wet) can significantly lower FS, especially on steeper slopes. Use field observations, piezometers, and seasonal history to justify ru. Improving surface drainage and toe relief can reduce pore pressure effects.

4) Surcharge and construction activities

Crest loading from haul roads, cranes, or material stockpiles is represented by q in kPa. Even modest surcharges can increase driving shear and reduce FS when β is high. Keep heavy loads back from the crest, manage traffic paths, and stage stockpiles to maintain adequate stability.

5) Using results for decisions and reporting

For planning, FS ≥ 1.3 is often treated as stable for simple screening, while 1.0–1.3 may require mitigation or refined analysis. Compare scenarios by adjusting ru, q, and geometry to quantify sensitivity. Export the CSV or PDF to document assumptions, summarize inputs, and keep a consistent record across design revisions.

FAQs

1) What does FS represent?

FS is the ratio of available shear strength to driving shear stress on the assumed slip plane. FS greater than 1 means resistance exceeds driving demand for the modeled condition.

2) Is this suitable for deep rotational failures?

No. It targets shallow, planar failure typical of infinite slope assumptions. Deep rotational failures, complex geometry, or layered soils need limit equilibrium or numerical methods with site investigation data.

3) How should I select ru?

Use groundwater observations, drainage condition, and monitoring when available. Lower values reflect well-drained conditions, while higher values reflect wet seasons, seepage, or perched water. Document the basis used.

4) Why is surcharge included?

Surcharge models added load near the crest, such as traffic or stockpiles. It increases both normal and driving stresses, often reducing FS on steeper slopes. Keeping loads back from the crest improves safety.

5) What cohesion value should I use?

Use effective cohesion consistent with the selected strength parameters. For granular soils, cohesion is often near zero. For cohesive soils, laboratory and in-situ tests help establish appropriate design values.

6) Why do my results look non-physical?

Check units and ranges: β must be under 89°, z must be positive, and ru must be 0–1. Very low τ or unrealistic parameters can produce unstable or misleading values.

7) Can I use this for final design approvals?

Use it for screening and comparison. Final approvals typically require a geotechnical engineer’s design, calibrated parameters, drainage assessment, and suitable analysis methods for the project’s risk and conditions.