Slope Stability Calculator

Analyze slope stability using the infinite slope method. Calculates factor of safety considering cohesion, friction, pore pressure, and slip depth.

Inputs

Soil Cohesion (c)

psf

Friction Angle (φ)

Slope Angle (β)

Soil Unit Weight (γ)

pcf

Slip Surface Depth (z)

Pore Water Pressure (u)

psf

Tab to move between fields

Results

Factor of Safety

1.29

Stable (FoS ≥ 1.5)

Critical Slope Angle39.5°

Available Shear Strength536 psf

Driving Shear Stress416 psf

How to Use This Calculator

Enter soil cohesion (psf or kPa), friction angle (°), slope angle (°), and soil unit weight.
Set slip surface depth (ft) and pore water pressure (psf).
Review Factor of Safety — values above 1.5 are generally considered stable for permanent slopes.
Conduct a full Bishop's method or Spencer analysis for critical slopes before construction.

How the result changes with Slope Angle (β)

Slope Angle (β)	Factor of Safety	Stable (FoS ≥ 1.5)
13	2.97	1
33	1.17	0
57	0.76	0
77	1.06	0

What each input means

Soil Cohesion (c): Effective cohesion of the soil along the slip surface. Sand = 0; stiff clay = 500-2000 psf.
Friction Angle (φ): Effective internal friction angle. Loose sand ≈ 28°; dense sand ≈ 38°; clay ≈ 10-25°.
Slope Angle (β): Inclination of the slope from horizontal. Common cut slopes are 26.5° (2H:1V) to 45° (1H:1V).
Soil Unit Weight (γ): Total unit weight of soil. Use saturated unit weight if slope is below groundwater table.
Slip Surface Depth (z): Depth of the potential failure plane measured perpendicular to the slope surface.
Pore Water Pressure (u): Pore water pressure at the slip surface. u = γw × hw where hw is the water head above slip plane.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Soil Cohesion (c) = 200, Friction Angle (φ) = 25, Slope Angle (β) = 30, Soil Unit Weight (γ) = 120 = 6 input(s) provided
Calculate Factor of Safety
Factor of Safety
1.289 = 1.289
Calculate Stable
Stable
0 = 0
Calculate Critical Slope Angle
Critical Slope Angle
39.5 = 39.5
Calculate Available Shear Strength
Available Shear Strength
536 = 536

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Civil Engineering

Soil Bearing Capacity Calculator

Calculate ultimate and allowable soil bearing capacity using Terzaghi's equation. Inputs include soil cohesion, friction angle, and foundation geometry.

Civil Engineering