Calculator inputs
Fields marked * are required.
Example data table
| Scenario | Depth | Soil | Slope (H:1V) | Safety factor | Calculated setback | Notes |
|---|---|---|---|---|---|---|
| Utility trench | 1.8 m | Type B | 1.0 | 1.25 | ~3.0 m | Dry conditions, light surcharge. |
| Foundation excavation | 3.0 m | Type C | 1.5 | 1.35 | ~7.0 m | Granular soil, conservative buffer. |
| Urban cut near structure | 8 ft | Type A | 0.75 | 1.50 | ~16 ft | Higher factor due to proximity and risk. |
Example values are illustrative. Always validate with local code and site conditions.
Formula used
This tool combines excavation geometry with conservative surcharges.
- Run = Depth × SlopeRatio (horizontal run per side).
- SurchargeAddOn is estimated from spoil height, equipment weight, and distances.
- RequiredSetback = (Run + Buffer + SurchargeAddOn) × SafetyFactor × WaterFactor.
- TopOpening = BottomDimension + 2 × Run (applies to width and length).
These equations provide a planning estimate. If you have shoring/shielding, layered soils, adjacent foundations, or traffic loads, engage a qualified professional for design checks.
How to use this calculator
- Select your units and soil classification.
- Enter excavation depth and bottom dimensions.
- Keep the auto slope ratio, or set a custom ratio.
- Add spoil pile and equipment details when applicable.
- Press Calculate setback to view results above.
- Use CSV or PDF downloads for field distribution.
Setback drivers and assumptions
Excavation setback is the horizontal distance kept clear from the excavation edge to reduce the chance of sloughing, surcharge-induced collapse, or damage to adjacent assets. This calculator estimates a planning setback by combining excavation depth, an assumed side-slope ratio, an edge buffer, and conservative add-ons. The output is appropriate for early planning, toolbox talks, and documenting assumptions. It can also support permit sketches and pretask hazard assessments during coordination meetings, but it does not replace project-specific temporary works design.
Soil class and slope implications
Soil classification strongly influences stability. Cohesive soils may stand steeper for short durations, while granular or disturbed soils typically require flatter slopes. The tool offers common planning ratios and lets you override them when a geotechnical report, competent-person inspection, or engineered system dictates a different approach. Groundwater selection applies additional conservatism because water reduces effective strength and can trigger raveling at the face.
Surcharge sources near the edge
Loads placed close to the lip increase lateral pressure and reduce the margin against failure. Typical surcharges include spoil piles, equipment, delivery vehicles, temporary materials, and vibration from traffic. Enter spoil pile height and distance to model a practical penalty, and add equipment weight and proximity to reflect heavy loads. When uncertain, increase the safety factor and push loads farther back than the minimum.
Geometry outputs for planning and staging
In addition to setback, the calculator reports the horizontal run per side, top opening dimensions, and an estimated excavation volume. These values support haul-off estimates, access planning, shoring footprint checks, and material staging. Top dimensions are especially useful when space is constrained, because they indicate how much surface area the cut consumes once sloping is applied.
Documentation and field communication
Exported CSV and PDF summaries help crews align on boundaries, storage zones, and exclusion areas. Use the structure distance fields to flag potential conflicts early and trigger engineering review. Always cross-check results against local regulations, utility requirements, and the competent-person plan before work begins.
FAQs
What is an excavation setback?
It is the clear horizontal distance kept from the excavation edge to reduce instability and protect nearby people, utilities, and structures. Setbacks help control surcharge loading and provide space for safe access.
Which slope ratio should I use?
Start with the soil class suggested by your site assessment, then use the default ratio as a planning baseline. If you have a geotechnical report, competent-person direction, or engineered system, enter that ratio as custom.
How does groundwater affect the result?
Water can reduce soil strength and trigger raveling or softening. Selecting groundwater applies a conservative multiplier to increase the setback. If seepage, artesian pressure, or piping is present, engineering review is recommended.
Should I include spoil piles and equipment?
Yes, when they will be near the edge. The calculator adds a surcharge allowance using spoil height, equipment weight, and proximity. When values are unknown, move loads back and raise the safety factor to reflect uncertainty.
Can I use this for shored or shielded excavations?
You can document assumptions, but the setback from sloping may not apply when protective systems are used. Follow the manufacturer tabulated data or engineered design, and still keep surcharges and materials away from the edge.
What if a building is close to the excavation?
Enter the structure distance and any known foundation depth to flag potential interaction. If the structure lies within the calculated influence zone, treat it as a high-risk condition and engage a qualified engineer before excavation begins.