# Slope Stability Calculator (Infinite Slope)

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.

> Canonical page: https://elysiatools.com/en/tools/slope-stability-calculator

- **Category:** Math & Numbers

- **Keywords:** slope stability, infinite slope, factor of safety, FS, shear strength, geotechnical, landslide, effective stress, civil engineering

## 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.

## Inputs

- **Mode** (select)
- **Effective Cohesion c' (kPa)** (number): Effective cohesion c'. Use 0 for clean cohesionless soil.
- **Effective Friction φ' (°)** (number): 0 ≤ φ' < 90.
- **Slope Angle β (°)** (number): 0 < β < 90. Slope angle from horizontal.
- **Slip Depth z (m)** (number): Vertical depth of the slip surface below the slope face.
- **Soil Unit Weight γ (kN/m³)** (number): Dry mode: total unit weight. Parallel mode: this is overridden by γ_sat.
- **Saturated Unit Weight γ_sat (kN/m³)** (number): Required for seepage-parallel mode.
- **Water Unit Weight γ_w (kN/m³)** (number): Used in seepage-parallel mode. Default 9.81 kN/m³.
- **Decimal Places** (number)

## 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

- Select the calculation mode, choosing between a dry slope (no seepage) or a slope with seepage parallel to the surface.
- Input the soil parameters including effective cohesion (c'), effective friction angle (φ'), slip depth (z), and soil unit weight (γ or γ_sat).
- Enter the slope angle (β) and, if in seepage mode, the unit weight of water (γ_w).
- Run 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.

## Frequently asked questions

### 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.

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