Input project data
The page follows a single-column flow, while the calculator fields adapt to three columns on large screens, two on tablets, and one on mobile.
Example data table
These examples use the same equations as the calculator, helping you compare dry, wet, and softened excavation conditions before field adjustments.
| Scenario | Depth (m) | Face angle (°) | Composite angle (°) | Water ratio | FoS | Status |
|---|---|---|---|---|---|---|
| Dry sand cut | 4.00 | 48.00 | 44.28 | 0.05 | 0.832 | Unstable |
| Wet mixed soil | 6.00 | 55.00 | 50.19 | 0.35 | 0.596 | Unstable |
| Deep softened cut | 8.00 | 63.00 | 51.01 | 0.60 | 0.396 | Unstable |
Formula used
This calculator uses a simplified planar or infinite-slope style screening approach, adapted for temporary pit walls with benches, groundwater, surcharge, and a horizontal seismic coefficient.
| Composite slope angle | β = arctan[H / (H / tan θ + n × b)] |
|---|---|
| Adjusted cohesion | c′ = c × softening factor / material factor |
| Adjusted friction | tan φ′ = tan φ × softening factor / material factor |
| Normal stress | σ = (γH + q) cos²β |
| Pore pressure | u = mγwH cos²β |
| Driving shear | τ = (γH + q) sinβ cosβ + kh(γH + q) cos²β |
| Effective normal stress | σ′ = σ − u − kh(γH + q) sinβ cosβ |
| Resisting shear | s = c′ + σ′ tan φ′ |
| Factor of safety | FoS = s / τ |
Where H is pit depth, θ is face angle, n is bench count, b is bench width, γ is soil unit weight, q is surcharge, m is effective water ratio, γw is water unit weight, and kh is the horizontal seismic coefficient.
How to use this calculator
- Enter the excavation geometry, including the proposed face angle, bench width, and number of benches.
- Add soil parameters from site investigation, such as unit weight, cohesion, and friction angle.
- Include surface surcharge, groundwater position, dewatering efficiency, and rainfall softening to reflect temporary field conditions.
- Set the target factor of safety required by your design standard, client brief, or geotechnical review.
- Submit the form and review the factor of safety, stress breakdown, composite slope angle, and required strength margins.
- Use the recommended maximum face angle, CSV export, and PDF export to compare alternatives and document decisions.
FAQs
1. What does the factor of safety mean?
It compares available shear resistance against driving shear demand. Values above the target show reserve capacity, while lower values indicate a need for flatter slopes, support, or better drainage.
2. Why does groundwater reduce stability so much?
Groundwater raises pore pressure, which lowers effective normal stress. Lower effective stress reduces frictional resistance, so the slope can reach failure at a smaller driving load.
3. How are benches included in the calculation?
Bench widths flatten the composite slope angle by increasing horizontal run. A flatter overall angle reduces driving shear and usually improves the factor of safety for the same soil conditions.
4. Does this tool replace a geotechnical engineer?
No. It is a screening calculator for planning and comparison. Complex geology, layered soils, anchors, struts, rock masses, and staged construction still require professional analysis and design.
5. What should I enter for rainfall reduction?
Use a percentage representing the loss of shear strength during wet weather. Higher values simulate softened ground, lower cohesion, and reduced friction caused by adverse surface conditions.
6. Why is a recommended maximum face angle shown?
The calculator iteratively searches for the steepest face angle that still reaches the selected target factor of safety, while keeping the same benches and other project inputs.
7. Can I use this for permanent retaining structures?
Not by itself. Permanent works need long-term groundwater, drainage, structural interaction, and serviceability checks. This page is intended for temporary excavation option studies and reporting support.
8. What do required cohesion and required friction show?
They show the shear strength needed to achieve the target factor of safety with the current geometry and loading. Large gaps suggest flattening, drainage, or support measures may be necessary.