QGIS Slope Stability Analysis 3.0 – User Guide

Two-dimensional limit equilibrium slope stability analysis inside QGIS. The model is drawn as ordinary QGIS layers in section coordinates: x = horizontal distance, y = elevation, both in metres.

1. Quick start

Plugins ▸ Slope Stability ▸ Load sample data adds a two-layer geology model, a piezometric line, a retaining wall, distributed loads (a building, q = 25 kPa, and a road, q = 12 kPa), four soil nails (T = 180 kN/m, pull-out 45 kN/m/m) and two user-defined slip surfaces (an arc and a weak-layer path). Select the corresponding layers in the dialog to try each feature.
Open Slope stability analysis…, choose the geology layer (and optionally the piezometric line), click Check inputs, then Run analysis.
The results are added in a new layer group: critical slip surface (labelled with FS), all stored slip surfaces coloured by FS, slices of the critical surface and search-grid centres. An HTML report opens in the browser.

2. Input layers

Layer	Geometry	Fields (not case-sensitive)
Geology / materials (required)	Polygon	unit_weight (kN/m³), cohesion (kPa), phi (°) – required. Optional: `name`/`layer_name`, `unit_weight_sat`, `model` (`mc`, `undrained`, `hb`, `none`), Hoek-Brown `sigci` (kPa), `gsi`, `mi`, `d`; `ru`; `piezo` (0 = ignore the piezometric line in this material); `c_std`, `phi_std`, `gamma_std` for reliability.
Piezometric line	Line	– (u = γw × vertical distance below the line; ponded water above the ground is applied as a surface pressure)
Distributed loads	Line	`q` (kPa, vertical, over the horizontal extent of the line)
Reinforcement	Line	`T` capacity (kN/m), `pullout` (kN/m per m behind the slip surface, 0 = unlimited), `mode` (`passive`/`active`)
Piles / retaining walls	Any	`V` shear resistance (kN/m), `mode`. Applied where the slip surface cuts the feature.
User-defined slip surfaces	Line	– Each line is analysed; arcs are recognised as circles.

Missing fields on load / reinforcement / pile layers take the default values on the Loads & supports tab. Fields of version 2.x (cohesion, phi, unit_weight) work unchanged.

3. Analysis options

Methods of slices

Method	Equilibrium	Interslice assumption
Ordinary (Fellenius)	Moment	Ignored (circular only)
Bishop simplified	Moment	Horizontal (circular only)
Janbu simplified	Horizontal force	Horizontal, with correction factor f0
Corps of Engineers #1 / #2	Force	Parallel to entry–exit chord / ground surface
Lowe-Karafiath	Force	Average of ground and base slope
Spencer	Moment + force	Constant inclination (λ)
Morgenstern-Price	Moment + force	λ·f(x), half-sine f(x)

The search method evaluates every trial surface; all ticked methods are then computed on each critical surface.

Slip surface search

Circular – grid search: grid of centres × tangent (depth) levels, then Nelder-Mead optimisation of (x, y, R).
Circular – entry and exit: circles through entry points on the crest and exit points at the toe.
Non-circular – path search: random kinematically admissible paths (concave, exit no steeper than 45°) followed by vertex optimisation. Uses Spencer if a circular-only method is selected.
User-defined: only the surfaces of the user layer.
From a crack point picked on the map: see section 3a.

Search density 100 % ≈ 3 500 circles (grid). The direction of sliding is detected automatically. Surfaces ending at the left/right model edge are ignored by default (extend the model if the critical surface reaches it).

3a. Crack point analysis

Use this when a tension crack or scarp is observed on the ground and you want the factor of safety of slides that start there.

Click the toolbar button Pick a crack point and analyse… (or, in the dialog, Analysis ▸ Surface type ▸ From a crack point picked on the map ▸ Pick on map).
Click the ground surface at the crack in the section view. Only the x coordinate is used; the point is projected vertically onto the ground. Right click or Esc cancels.
The dialog opens with the point filled in. Run the analysis.

All trial surfaces start at the crack and extend downslope: circles through the crack point and every possible exit point, and random non-circular paths from the crack; the best ones are then optimised with the entry kept fixed. Circular surfaces use the search method, non-circular ones Spencer if the search method is circular-only. The critical surface, all stored surfaces and a crack_point marker layer are added to the map. Combine with a Tension crack depth to model an open crack (water-filled if required).

Scenarios

The parametric scan replaces the v2 General Scan: every combination of kh values and water table depths (water table parallel to the ground surface, dry = no water) is searched and stored with its scenario name (e.g. Scan_Kh0.1_D15m).

Reliability

For materials with c_std, phi_std or gamma_std, the critical surface of the main analysis is evaluated with Monte Carlo simulation (normal distributions) and the Taylor series method (lognormal FS, Duncan 2000). The report lists the probability of failure P_f, the reliability index β and each variable's share of the variance.

4. Outputs

Layer	Content
critical_surface	Critical surface per scenario with FS of every method, λ (Spencer, M-P), centre, radius, entry/exit.
slip_surfaces	Lowest-FS surfaces (default 300 per scenario), graduated colours: <1.0 red, 1.0–1.2 orange, 1.2–1.5 yellow, 1.5–2.0 light green, >2.0 green.
slices	Slice polygons of each critical surface: weight, α, c, φ, u, N′, σ′n, mobilised shear.
crack_point	Picked crack point and its minimum FS (crack point analysis only).
search_centers	Minimum FS at each grid centre (hidden by default; useful for FS contours).

Leave the GeoPackage path empty for temporary layers. The analysis is also available in the Processing Toolbox ▸ Slope Stability (batch processing, Model Designer).

5. Language

The interface and report follow the QGIS interface language (Settings ▸ Options ▸ General ▸ User interface translation): English, 繁體中文, 简体中文, 日本語, 한국어, Español, Français, Deutsch, Português. It can be overridden on the Output tab.

6. Limitations

2-D limit equilibrium, plane strain, unit width. No finite element seepage or strength reduction (planned).
Pore pressures from a piezometric line or ru only (no seepage analysis, no rapid drawdown).
Geology polygons should share boundaries without gaps or overlaps (Check inputs warns about both).

Results must be checked by a qualified engineer. The solver was verified against ACADS example 1(a) (Bishop 0.985, Spencer 0.984; reference ≈ 0.99–1.00), the infinite slope (dry, seismic, seepage) and Taylor's undrained vertical cut.