How to Prepare a Geotechnical Cross Section for 2D Slope Stability Analysis

The limit-equilibrium method (LEM) investigates whether available shear resistance along an assumed failure mechanism can withstand driving gravitational, water, and external surcharge loads:

• Discretization into vertical slices: the potential failure sliding mass is divided into vertical slices to account for non-uniform slope geometry and multi-layered soil profiles.
• Factor of Safety (FS): defined as the factor by which shear strength must be divided to bring the slope to the verge of failure: $$FS = \frac{\tau_f}{\tau_m}$$
• Equilibrium equations:
  • Ordinary (Fellenius): satisfies only moment equilibrium and neglects all interslice forces (overly conservative).
  • Simplified Bishop: satisfies moment equilibrium and accounts for horizontal normal forces between slices, providing reliable accuracy for circular slip arcs.
  • Spencer / GLE: rigorous formulation satisfying both moment and force equilibrium with constant or variable interslice force inclination functions.

You can digitize the geotechnical cross section directly in the Android field app or on desktop via the web portal:

1. From your project menu, navigate to 2D Sections > New Section.
2. Set section alignment azimuth (e.g., azimuth 135° perpendicular to the cut slope strike).
3. Ground surface profile (Topography): enter coordinates $(X, Z)$ for toe, bench, and crest vertices, or trace directly along GPS-mapped field stations.
4. Stratigraphic layer contacts: use CAD drafting tools (polylines, vertex snapping, split) to sketch boundaries separating geological strata (e.g., coluvial soil, residual saprolite, fractured bedrock).
5. Ensure layer polygons are fully enclosed and free from self-intersecting loops.

Assign material properties to each domain with consistent engineering units:

Groundwater seepage is the most critical triggering mechanism for slope failures:

1. Enable the Piezometric Water Line feature.
2. Draw the phreatic surface vertices across the profile (e.g., daylighting near the slope toe or drawn down by horizontal drains).
3. The computational engine determines pore pressure ($u = \gamma_w \cdot h_w$) at each slice base, reducing effective normal stress ($\sigma' = \sigma - u$) and lowering available shear resistance.

To verify calculation accuracy, reproduce the standard benchmark case from literature (Duncan & Wright, 2005):

• Geometry: homogeneous slope of height $H = 10\text{ m}$, inclination $1\text{V}:2\text{H}$ ($26.56^\circ$).
• Soil properties: $\gamma = 18\text{ kN/m}^3$, $c' = 15\text{ kPa}$, $\phi' = 25^\circ$. Dry condition.
• Tap Search Critical Slip Surface. The algorithm generates a search grid of slip centers and tangent radii, iteratively converges slice equilibrium equations, and highlights the critical arc in red.
• Expected output: the Simplified Bishop Factor of Safety converges near $FS \approx 1.52 \pm 0.03$, exactly matching the reference solution from commercial desktop geotechnical software.

Once calculations converge, export deliverable assets:

• Executive PDF Sheet: cross-section drawing showing colored slices, method comparison table, slip center coordinates $(X_c, Z_c)$, radius, and property inputs.
• Slice Coordinates in CSV: slice-by-slice normal forces, base pore pressures, and shear mobilizations for independent spreadsheet verification.
• DXF Vector File: topographic surface, stratum contacts, and critical failure circle on organized CAD layers for import into AutoCAD or Civil 3D.