Slope Stability Calculator
Blog post description.
Wiratama
11/16/20252 min read
Slope Stability – Definition
Slope stability analysis evaluates the safety of natural or engineered slopes against failure. It determines the Factor of Safety (FS), which expresses the ratio of resisting forces to driving forces along a potential slip surface. Slope stability is crucial in geotechnical engineering for the design of embankments, cut slopes, dams, retaining structures, and natural hillsides.
Background Theory
1. Types of Slope Failure
Slope failures generally occur along:
Circular slip surfaces (common in homogeneous soils)
Non-circular slip surfaces (likely in layered soils or rock)
Circular slip analysis is widely used due to simplicity and reliability for many soil conditions.
2. Simplified Bishop Method
The Bishop Simplified Method is one of the most commonly used limit-equilibrium methods for circular slip surfaces. It considers:
Vertical slices
Inter-slice shear forces neglected
Moment equilibrium & force equilibrium
The resulting Factor of Safety equation is:
Where:
c = cohesion
ϕ = friction angle
γ= unit weight
b = slice width
h = slice height
W=γbh = slice weight
α = inclination of slice base
R = slip circle radius
Although simplified, Bishop’s method provides accurate results for most slopes with circular failure mechanisms.
3. Factor of Safety (FS) Interpretation
Different projects use different minimum FS depending on risk and soil type.
How This Calculator Works
Step 1 — User Inputs
The calculator takes:
Soil cohesion
Friction angle
Unit weight
Slip circle radius
Slice width & height
Step 2 — Slice Weight
The soil slice weight is calculated:
Step 3 — Compute Slice Base Angle
The slip circle geometry provides the base inclination:
Step 4 — Compute Factor of Safety (FS)
The simplified Bishop equation evaluates resisting vs driving forces:
Step 5 — Display Results
The calculator outputs:
Factor of Safety (FS)
Method used: Simplified Bishop
Interpretation accuracy suitable for early-stage slope assessment