Calculate the infinite-slope factor of safety: FS = c'/(γ·z·cosβ·sinβ) + tanφ'/tanβ. Supports dry slopes and seepage parallel to the slope. Returns FS and a stable/marginal/unstable classification.
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Key facts
Category
Math & Numbers
Input types
select, number
Output type
json
Sample coverage
4
API ready
Yes
Overview
The Slope Stability Calculator (Infinite Slope) computes the factor of safety (FS) for translational slope failures using the infinite-slope model. It supports both dry slopes and conditions with seepage parallel to the slope face, outputting the calculated FS alongside a stability classification of stable, marginal, or unstable.
When to use
When analyzing shallow, translational landslide hazards where the failure surface is parallel to the ground surface.
When evaluating the impact of groundwater seepage parallel to a slope face on soil shear strength and stability.
When performing preliminary geotechnical assessments of long, uniform slopes in civil engineering and geological hazard planning.
How it works
1Select the calculation mode, choosing between a dry slope (no seepage) or a slope with seepage parallel to the surface.
2Input the soil parameters including effective cohesion (c'), effective friction angle (φ'), slip depth (z), and soil unit weight (γ or γ_sat).
3Enter the slope angle (β) and, if in seepage mode, the unit weight of water (γ_w).
4Run the calculator to compute the driving shear stress, cohesion and friction contributions, and the final factor of safety (FS) with its stability classification.
Use cases
Assessing the stability of long, uniform sandy slopes under dry conditions.
Estimating the reduction in safety factor for a hillside during heavy rainfall that causes parallel seepage.
Verifying manual geotechnical calculations for infinite slope stability homework or design checks.
Examples
1. Dry Sandy Clay Slope Assessment
Geotechnical Engineer
Background
An engineer needs to evaluate a dry, uniform slope with a 30-degree incline to determine if a shallow slip plane at 3 meters depth is stable.
Problem
Calculate the factor of safety for a dry slope with cohesion of 10 kPa, friction angle of 25 degrees, and soil unit weight of 18 kN/m³.
How to use
Select 'Dry' mode, enter 10 for Effective Cohesion, 25 for Effective Friction, 30 for Slope Angle, 3 for Slip Depth, and 18 for Soil Unit Weight.
Outcome
The calculator outputs a factor of safety of 1.2353, classifying the slope stability as marginal.
2. Saturated Slope with Parallel Seepage
Civil Engineering Consultant
Background
A consultant is assessing a hillside prone to heavy rainfall where water flows parallel to the slope face, potentially triggering a landslide.
Problem
Determine the factor of safety at a 3-meter slip depth for a 30-degree slope with a saturated unit weight of 20 kN/m³, cohesion of 5 kPa, and friction angle of 25 degrees.
How to use
Select 'Seepage parallel to slope' mode, enter 5 for Effective Cohesion, 25 for Effective Friction, 30 for Slope Angle, 3 for Slip Depth, 20 for Saturated Unit Weight, and 9.81 for Water Unit Weight.
FAQ
What is the difference between the dry and parallel seepage modes?
Dry mode assumes no water table, while parallel seepage mode assumes groundwater is flowing parallel to the slope face, reducing effective stress using the saturated unit weight and water unit weight.
How is the stability classification determined?
A factor of safety (FS) less than 1 is classified as unstable, between 1 and 1.5 is marginal, and 1.5 or greater is stable.
Can I use this calculator for deep, circular slope failures?
No, this calculator is specifically designed for infinite slopes where failure occurs along a shallow plane parallel to the slope surface.
What value should I use for effective cohesion in cohesionless soils?
For clean sand or gravel with no cohesion, set the effective cohesion (c') to 0.
What is the default unit weight of water used in the calculations?
The default unit weight of water (γ_w) is 9.81 kN/m³, but you can adjust this value if needed.