What is the factor of safety for slopes?

Factor of safety (FS) is the ratio of the total resisting forces to the total driving forces on the failure plane. FS > 1.5 is generally considered safe; FS < 1.0 means failure is predicted.

What is the infinite slope method?

The infinite slope method assumes a shallow planar failure parallel to the slope surface. It works best for translational slides in relatively uniform soils and gives a closed-form solution for FS.

How does the water table affect slope stability?

A high water table increases pore water pressure, which reduces effective normal stress and thus the available frictional resistance. Saturated conditions significantly lower the factor of safety.

What units should I use?

You can use any consistent unit system (kN/m² with kN/m³, or psf with pcf). The calculator accepts metric (kPa, kN/m³) or imperial (psf, pcf) values as long as all inputs are in the same system.

What is a typical factor of safety for slope design?

Most codes require FS ≥ 1.5 for permanent slopes and FS ≥ 1.3 for temporary construction slopes. Critical slopes or those near structures may require FS ≥ 2.0.

Slope Stability Factor of Safety Calculator

Name: Slope Stability Factor of Safety Calculator
Availability: InStock
Author: Nham Vu

Compute the factor of safety against slope failure using the infinite slope method for both cohesive and cohesionless soils.

Slope & Soil Parameters

Slope Geometry

Slope angle β (°)

Failure depth H (m)

Soil Properties

Unit weight γ (kN/m³)

Friction angle φ (°)

Cohesion c (kPa)

Set to 0 for cohesionless sand/gravel.

Pore Water Pressure

Include seepage (water table at surface)

Enter slope and soil parameters, then click Calculate.

Summary