What Is Slope Stability?

Slope stability is the assessment of whether a soil slope will stay in place or is likely to slide (slope failure) — whether it is an embankment slope, a road cut, a riverbank or the side of an excavation. The weight of the soil mass on the slope tries to pull the soil downhill, while the soil's shear strength resists it. Stability is therefore a balance between these two forces.

When the resisting forces exceed the driving forces by a sufficient margin, the slope is stable. But if that balance is disturbed — for example by cutting the slope steeper, by heavy rain raising the groundwater, or by adding load at the crest — the soil may slide and turn into a landslide. Slope stability analysis is therefore a fundamental part of geotechnical engineering design for every kind of slope.

📌 What causes slopes to fail?

The main causes of slope failure are excessive slope angle and height, low soil shear strength, a rising groundwater level or pore water pressure, erosion at the toe, and additional load at the crest. All of these factors lower the FS.

How Is the Factor of Safety (FS) of a Slope Calculated?

The Factor of Safety is the ratio of the forces resisting sliding to the forces causing the soil to slide. The basic equation is:

FS = Resisting Force ÷ Driving Force Resisting force comes from the soil's shear strength along the slip surface · Driving force comes from the weight of the soil mass sliding downslope

There are three cases:

  • FS greater than 1.0 — resisting forces exceed driving forces; the slope is stable
  • FS equal to 1.0 — the slope is in limit equilibrium, on the verge of sliding
  • FS below 1.0 — the slope is theoretically unstable

The soil shear strength in the equation consists of cohesion (c) and the internal friction angle (φ), obtained from laboratory testing of soil samples, together with the soil unit weight and the groundwater level from the soil investigation. A reliable slope stability analysis must therefore always start from real soil data.

Concept of slope stability analysis using a circular slip surface and the method of slices, with the commonly referenced range of Factor of Safety values The left image is a cross-section of a soil slope, with the crest at the upper right and the toe at the lower left. A circular slip surface with center O and radius R runs from behind the crest down to exit at the toe. The soil mass above the slip surface is divided into vertical slices; one slice shows its weight W, the driving force pushing the soil mass down along the slip surface, and the resisting force from the soil's shear strength along the slip surface in the opposite direction. A groundwater level runs through the soil mass. The right image is an FS scale, where FS equals the sum of resisting forces divided by the sum of driving forces: below 1.0 the slope is theoretically unstable; 1.0 is the point of failure; temporary or construction-stage slopes typically use about 1.2 to 1.3; and permanent long-term slopes typically use 1.5 or higher. The bar at the bottom summarizes the soil data needed for the analysis. Slope Stability: Circular Slip & FS Not to scale O R W Resisting (τ on slip) Drive Method of Slices Fellenius · Bishop · Spencer etc. Crest Toe Circular slip surface GW level FS = ΣResisting / ΣDriving > 1 = resisting wins 1.0 1.2 1.3 1.5 ≥ 1.5 permanent long-term (drained) ≈ 1.2–1.3 temporary During Construction = 1.0 limit state not a design level < 1.0 unstable failure risk 1 Soil c, φ Lab tests 2 Unit weight Boring samples 3 GW level Soil boring
Concept of slope stability analysis — FS is the ratio of the resisting force along the circular slip surface to the driving force from the weight of the soil mass sliding downslope. Permanent long-term slopes generally require an FS of 1.5 or higher.

What FS Value Is Acceptable and Safe?

In general engineering practice, permanent soil slopes analyzed for long-term (drained) conditions are typically required to have an FS of 1.5 or higher, while temporary or construction-stage conditions may accept somewhat lower values, depending on each project's requirements. The table below summarizes the commonly referenced ranges.

← Scroll to see the full table →

Condition / caseCommonly referenced FS rangeMeaning
Permanent, long-term (drained)≥ 1.5General criterion for permanent slopes that must remain stable throughout their service life
Temporary / during construction≈ 1.2–1.3Short service life and controllable site conditions, so somewhat lower values may be accepted
FS = 1.0= 1.0Limit equilibrium; the slope is on the verge of sliding — not a level to design for
FS < 1.0< 1.0Theoretically unstable slope with a risk of failure

The values in the table are general reference ranges for learning purposes only. The FS criterion actually required for each project depends on the governing standard, the importance of the structure and the judgment of the responsible engineer.

An FS only slightly above 1.0 does not mean the slope is safe enough, because soil strength and groundwater levels are uncertain. Keeping the FS reasonably above 1.0 provides the margin needed to cover those uncertainties.

Which Methods Are Used to Analyze Slope Stability?

The most widely used approach is the family of Limit Equilibrium Methods, which divide the potential sliding mass into a number of slices and consider the force equilibrium of each slice.

  • Method of Slices (Ordinary / Fellenius) — the original method, which uses a simple equilibrium and ignores interslice forces. It can be calculated by hand but usually gives a conservative FS (slightly lower than reality).
  • Bishop's Simplified Method — partly accounts for interslice forces, so it gives more accurate results; widely used for circular slip surfaces.
  • Other methods such as Janbu, Spencer and Morgenstern-Price are used for complex or non-circular slip surfaces.

Today, practical analysis is done mainly with engineering software, which automatically searches for the critical slip surface that gives the lowest FS. The results are only reliable, however, when the correct soil parameters from soil investigation and testing are entered.

💧 Groundwater — the factor that lowers FS the most

Higher pore water pressure reduces the effective shear strength of the soil, lowering the resisting force and the FS. This is why slopes often fail during heavy rain or after flooding, and why drainage design is at the heart of preventing slope failure.

When Should Slope Stability Be Analyzed?

Slope stability should be analyzed for any work involving slopes that poses a risk to life or property, such as:

  • High cut or fill slopes, such as road cuts, embankments and canal embankment slopes
  • Deep excavations or basements, where the stability of temporary excavation walls must be assessed
  • Retaining walls and riverbank embankments, which must be checked for both global and local stability
  • Areas with a history of landslides, or natural slopes close to buildings and roads

If you see warning signs such as cracks at the crest, bulging soil at the toe or abnormally tilting trees, consult an engineer for an assessment as soon as possible. Read more about warning signs and prevention in the article Landslides and slope failure prevention.

When the analysis gives an FS below the criterion, it can be increased in two ways: reduce the driving force, for example by flattening the slope or reducing load at the crest; and increase the resisting force, for example by drainage to lower pore water pressure or by supporting the toe with a structure such as a retaining wall. The appropriate solution must be analyzed case by case by an engineer.

3D model of a soil slope cut away to show a circular slip surface, with driving and resisting forces, factors that lower FS, and warning signs on the slope surface A conceptual 3D view of a sloping block of soil. On the left is flat ground at the crest, in the middle the slope face descends to the right, and on the right is flat ground at the toe with a house on it. The front cross-section shows the potential sliding mass shaded orange, with a red curve marking the circular slip surface from the crest down to where it emerges at the toe. A blue downward arrow is the weight of the soil mass W, a red arrow along the slip surface is the driving force, and a green arrow in the opposite direction is the resisting force from the soil's shear strength. A light-blue dashed line is the groundwater level, raised after rain. On the ground surface, a red dashed spoon-shaped line shows the extent of the potential sliding mass in three dimensions. Numbered factors that lower FS: 1 is a fill stockpile adding load at the crest; 2 is rain clouds raising the groundwater and pore water pressure; 3 is the toe being cut or eroded, making the slope steeper. Lettered warning signs: A is cracking at the crest, B is abnormally tilting trees on the slope face, and C is bulging soil at the toe. The panel on the right summarizes the equation FS equals resisting force divided by driving force, the list of factors that lower FS, the list of warning signs, and ways to increase FS — reducing the driving force and increasing the resisting force, such as controlling the slope angle and drainage. 3D model: slope, slip surface, driving vs. resisting forces and warning signs Not to scale GW level Firm soil W 1 2 3 A B C FS = Resist ÷ Drive Near 1.0 = near sliding Building near toe Property in the risk zone Circular slip surface Driving: soil mass weight downslope Resisting: shear strength along slip Potential sliding mass extent Toe cut or eroded What lowers FS? 1 Added load at crest Driving force up 2 Rain, higher GWL Pore pressure up, resistance down 3 Steeper cut / toe erosion Loss of toe support Warning signs A Cracks at the crest B Tilting trees C Bulging at the toe Two ways to raise FS Reduce driving Flatten the slope Reduce crest load Increase resisting Drain, cut pore pressure Toe support, e.g. retaining wall Permanent long-term slopes: FS ≥ 1.5 Per project spec · conceptual view
3D model of a soil slope cut away to show the circular slip surface. Factors that lower FS (1–3) are added load at the crest, rain and rising groundwater, and cutting or erosion at the toe; warning signs (A–C) are cracks at the crest, tilting trees and bulging soil at the toe. (Conceptual view, not to scale)

Summary

Slope stability is a balance between the resistance to sliding provided by the soil's shear strength and the force pulling the soil downslope, measured by the Factor of Safety. Permanent long-term slopes typically use an FS of 1.5 or higher, depending on each project's requirements. The FS is calculated from the soil strength and groundwater level obtained from soil investigation, then analyzed with the Method of Slices or Bishop's method in engineering software. Controlling groundwater and slope angle is therefore the key to a stable slope.

📌 Disclaimer

This article provides general information only and is not design advice for any specific project. Target FS values, safe slope angles and measures to improve the stability of an actual slope must be analyzed and certified by a licensed engineer based on soil investigation data and laboratory test results for that site.