| Scenario | H (m) | c (kPa) | φ (deg) | γ (kN/m³) | q (kPa) | zw (m) | β (deg) | FS Target | Typical outcome |
|---|---|---|---|---|---|---|---|---|---|
| Urban trench near traffic | 3.0 | 20 | 26 | 18 | 15 | 4.0 | 45 | 1.5 | Support often needed |
| Dry cut in dense sand | 2.5 | 0 | 36 | 19 | 5 | 10.0 | 34 | 1.3 | Flatter slopes preferred |
| Stiff clay with low surcharge | 3.5 | 35 | 20 | 19 | 5 | 10.0 | 53 | 1.5 | Short-term cuts may stand |
Active coefficient:
Ka = tan²(45° − φ/2)
Effective lateral stress at depth z:
σh(z) = Ka(γ′ z + q) − 2c√Ka
Tension crack depth (where σh becomes zero):
z0 = (2c√Ka − Kaq) / (Kaγ′)
Resultant force per meter length (integrated from z0 to H):
Pa = ∫ σh(z) dz (implemented by closed-form integration)
If groundwater intersects the cut, hydrostatic water pressure is added.
For a short-term vertical cut, an approximate critical height:
Hc ≈ (4c cosφ) / (γ′(1 − sinφ))
Approximate factor of safety for vertical cut:
FSvertical ≈ Hc / H
Using an infinite-slope style check on an assumed shear plane:
FSslope = (c + (σn − u) tanφ) / τ
Where σn and τ depend on slope angle β, depth z, unit weight, and surcharge.
- Select a soil preset or enter tested soil parameters.
- Enter excavation depth, plus length and width for volume.
- Add surcharge loads from traffic, spoil piles, or materials.
- Set groundwater depth and saturated unit weight if needed.
- For sloped excavations, choose angle or ratio and slip depth.
- Pick a target factor of safety matching your criteria.
- Click Calculate, then download CSV or PDF for records.
1. What excavation stability means in practice
Excavation stability is the ability of the excavation face to stand without excessive deformation or sudden collapse. Short-term cuts can appear stable, yet degrade with time, vibration, or wetting. This calculator helps compare scenarios quickly by estimating driving forces, available shear strength, and simple factors of safety.
2. Soil strength inputs and typical ranges
Two parameters govern shear strength: cohesion c and friction angle φ. Typical φ values are 15–25° for many clays, 28–34° for silts and loose sands, and 35–42° for dense sands and gravels. Cohesion can range from 0 kPa in clean sands to 10–50 kPa in clays, depending on testing and conditions.
3. Unit weight and why groundwater changes everything
Unit weight γ commonly falls between 16 and 20 kN/m³ for many soils. When groundwater intersects the excavation, effective stress reduces and water pressure adds load. Water pressure increases about 9.81 kPa per meter of head. Even a 2 m water head can add nearly 20 kPa at the base, increasing lateral demand and reducing stability.
4. Active pressure coefficient and what the values imply
The active coefficient Ka decreases as φ increases. For reference, φ = 20° gives Ka ≈ 0.49, φ = 30° gives ≈ 0.33, and φ = 40° gives ≈ 0.22. Lower Ka typically means lower lateral pressure, but low cohesion or high groundwater can still govern the outcome.
5. Surcharge loads and conservative planning
Surcharge q represents traffic, stored materials, or spoil placed near the edge. A practical rule of thumb is 10 kPa ≈ 1 t/m² of surface load. Increasing q raises lateral pressure and shear demand. If the location of equipment is uncertain, assume a higher q and compare the change in factor of safety before finalizing the work plan.
6. Vertical cuts versus sloped faces
Vertical cuts may be feasible at shallow depths in strong, cohesive soils, but stability is sensitive to wetting and time. Sloped faces reduce driving forces and are often more robust. Common temporary slopes range from 1H:1V (45°) to 2H:1V (26.6°), depending on soil type and site constraints. The slope check supports quick comparisons of these options.
7. Interpreting factors of safety
A factor of safety compares available resistance to driving demand. Many temporary planning checks use targets around 1.3–1.5, while higher targets may be appropriate where consequences are severe. Treat the displayed values as screening indicators. If results are near the target, refine inputs with better data, consider drainage, and evaluate support systems.
8. Good field data improves reliability
Accuracy improves when inputs match real conditions: verified soil classification, laboratory or in-situ strength parameters, measured groundwater, and realistic surcharges. Document assumptions in the exported report, then update the calculation if conditions change. Re-check after rainfall, dewatering adjustments, or when heavy equipment moves closer to the excavation edge.
1) What does “Overall status” mean?
It summarizes whether the calculated factor of safety meets your target for the selected checks. It is a screening indicator, not a final design approval, and should be confirmed with project-specific review.
2) Which soil preset should I choose?
Use the preset closest to your observed soil type for a starting point, then replace with tested values when available. Presets are typical ranges only and may not match local stratification.
3) How do I enter groundwater conditions?
Enter the water table depth below ground. If it is shallower than the excavation depth, the calculator adds hydrostatic pressure and uses submerged unit weight below the water table.
4) What is a reasonable surcharge value?
Use 5–20 kPa for light to moderate temporary surface loads, depending on traffic and storage. If spoil piles are near the edge, consider higher values or move them back and recalculate.
5) Why is slip depth required for the slope check?
It represents the approximate depth to a potential shear plane. Shallow planes may govern thin surficial failures, while deeper planes can govern larger movements. Use conservative depth when uncertain.
6) Can I use this for shoring design?
You can use the lateral force and pressure outputs for preliminary comparison and reporting. Final shoring design should follow applicable standards and include detailed soil profiles, construction staging, and surcharge placement.
7) Why do my results change a lot with φ?
Friction angle strongly affects Ka and shear resistance. A small change in φ can significantly change lateral pressure and slope resistance. Use tested parameters and avoid guessing φ for critical work.