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Slope Stability Calculator

§ Code basis: GB 50330-2013 (China) — The design equations differ, so the values are not interchangeable - recalculate with your own code.

Resisting force over driving force for a single planar slip surface, checked against Fst.

Also known as slope stability analysis · factor of safety slope · slip circle

Quick answer

With the default inputs: Slope height 6 m · Slope safety class Class 2 · Design case Permanent, normal etc.Factor of safety 1.87

Use your own dimensions ↓

Inputs

Per GB 50330-2013 Table 3.2.1, by consequence of failure
°
Critical surface, near the slope angle for a rough check
kN/m³
°
kPa

Result

Factor of safety
1.87
Sliding wedge weight695kN/m
Slip surface length14.20m
Resisting force548kN/m
Driving force294kN/m
Required Fst1.30
Stability stateStable
How this is worked out

Planar sliding is the simplest limit-equilibrium method: a homogeneous slope is cut along a plane into a wedge, resisting force = normal component × tanφ + cohesion × slip length, and driving force = the tangential component. Classification follows GB 50330-2013 clause 5.3: Table 5.3.1 has four states - Fs < 1.00 unstable, 1.00 ≤ Fs < 1.05 marginally unstable, 1.05 ≤ Fs < Fst basically stable, Fs ≥ Fst stable; Fst comes from Table 5.3.2 - for permanent slopes under normal conditions 1.35 / 1.30 / 1.25 for classes 1 / 2 / 3, under seismic 1.15 / 1.10 / 1.05, and 1.25 / 1.20 / 1.15 for temporary slopes. Fst is not a flat 1.5; the 1.5 quoted in textbooks is a rule of thumb, not the value in this code. It only holds for homogeneous soil and a single plane; layered ground and circular slips need a method of slices (Bishop and so on). Take the slip angle at the most critical surface - start near the slope angle and hunt for the lowest Fs. Slopes for deep excavations are governed separately by JGJ 120-2012.

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Formula

Wedge weightW = ½·γ·H·(H/tanα)
Slip lengthL = H / sinα
ResistingR = W·cosα·tanφ + c·L
DrivingT = W·sinα
Factor of safetyFs = R / T
StateFs≥Fst stable; 1.05≤Fs<Fst basically stable; 1.00≤Fs<1.05 marginally unstable; Fs<1.00 unstable
A 6 m slope, 25° slip, γ=18 kN/m³, φ=28°, c=15 kPa: slip length 14.2 m, wedge weight W=½×18×6×(6/tan25°) ≈ 695 kN/m, resisting ≈ 548 kN/m, driving ≈ 294 kN/m, Fs ≈ 1.87. Against Fst = 1.30 for a class 2 permanent slope under normal conditions, Fs ≥ Fst, stable.

How to work it out

  1. Use the planar-sliding method: treat the sliding mass as a rigid block on a plane, with safety factor K = resisting force ÷ driving force.
  2. Resisting = W·cosα·tanφ + c·L (W block weight, α slip angle, φ friction angle, c cohesion, L slip-plane length).
  3. Driving = W·sinα, so K = (W·cosα·tanφ + c·L) / (W·sinα).
  4. Judge stability at K ≥ 1.2-1.3, taking the limit by slope safety class and loading case.
  5. Note: this only fits homogeneous soil and a single plane; clay usually fails on a circle, so use a method of slices (Bishop, Janbu, etc.).

Where it goes wrong

Material and cost

Remediating a failed slope (regrading, retaining wall, anti-slide piles, anchors, drainage) costs several times prevention, plus downtime and secondary damage. Compute K and act on it early - that is the cheapest option.

On site

Basis

The codes below are listed for comparison. This calculator returns GB 50330-2013 values only.

FAQ

What factor of safety is acceptable?
It depends on the safety class and the design case - there is no single number. GB 50330-2013 Table 5.3.2 gives: permanent slopes under normal conditions 1.35 / 1.30 / 1.25 for classes 1 / 2 / 3; seismic 1.15 / 1.10 / 1.05; temporary slopes 1.25 / 1.20 / 1.15. The computed Fs must reach the matching Fst. Table 5.3.1 also has two lower bands: 1.05 to Fst is basically stable, 1.00 to 1.05 is marginally unstable, and only below 1.00 is unstable. For very complex ground or severe consequences of failure, Fst is raised further (Table 5.3.2, note 2).
Is planar sliding accurate enough?
Only for homogeneous soil and a single plane - a quick check or option comparison. Layered ground, weak seams and circular slips (common in clay) need a method of slices - Bishop, Janbu, Morgenstern-Price - run in software today. Treat this tool as an order-of-magnitude first pass; a real slope design needs a full calculation.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Slope Stability Calculator. 2026-10-25. https://jianzhujisuan.com/en/slope-stability

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.