About Trench Excavation Calculator (OSHA Slopes & Spoil)
The trench excavation calculator sizes an open-cut trench with sides sloped at the OSHA maximum allowable slope and counts the dirt that comes out of it. Pick the soil type — stable rock, Type A, B, or C per the Appendix A classification — and the calculator applies the Table B-1 slope from 29 CFR 1926 Subpart P, Appendix B for excavations less than 20 feet deep: vertical in stable rock, 3/4:1 (horizontal:vertical) in Type A, 1:1 in Type B, and 1.5:1 in Type C. From the depth, bottom width, and length it returns the top width at grade, the trapezoidal cross-section, and the bank (in-place) volume in cubic yards.
Excavated soil bulks up, so the spoil pile is bigger than the hole. The calculator applies a representative swell factor by soil type — these are rules of thumb that vary with material and moisture, and you can override them with a project value up to 60% — then converts the loose spoil into 10-yd³ truckloads. Alongside the quantities it raises the two OSHA flags every trench crew works to: at 5 feet or deeper a protective system (the sloping shown is one) is required by 1926.652(a)(1) unless the cut is entirely in stable rock — the exception covers only excavations less than 5 feet deep — and at 4 feet a ladder, stairway, or ramp must be within 25 feet of lateral travel per 1926.651(c)(2). Soil typing is not a guess from the office: a competent person classifies the soil in the field and must inspect the excavation daily and after every rainstorm — this calculator only applies the table to the type you give it.
How It Works
- Enter the trench depth, bottom width, and length in feet. Depth is capped at 20 ft — Appendix B stops there, and deeper sloping must be designed by a registered professional engineer.
- Pick the soil type as classified by a competent person: stable rock (vertical sides), Type A cohesive soil such as clay, Type B such as silt or previously disturbed soil, or Type C granular soil such as sand and gravel. When in doubt, classify down (toward Type C).
- The calculator applies the Table B-1 maximum allowable slope, widens the trench to its top width at grade, and computes the trapezoidal cross-section and the bank volume in cubic yards.
- A representative swell factor for the soil bulks the bank volume into loose spoil; tick the override to substitute your own 0–60% figure from local experience or the geotech report.
- Read the results: top width, cross-section, bank and spoil volumes, and 10-yd³ truckloads, plus the two safety flags — protective system at 5 ft or deeper (1926.652(a)(1)) and egress within 25 ft at 4 ft or deeper (1926.651(c)(2)).
Worked Example
A 50-ft utility trench runs 6 ft deep with a 3-ft bottom in Type B soil. Table B-1 gives a 1:1 slope, so each side lays back 6 ft and the top width is 3 + 2 × 6 × 1.0 = 15 ft. The cross-section is the trapezoid (3 + 15)/2 × 6 = 54 ft², and the bank volume is 54 × 50 / 27 = 100 yd³ in place. Type B spoil swells about 25%, so roughly 125 yd³ of loose material comes out — 13 truckloads at 10 yd³ each. Both flags are up: at 6 ft the trench is past the 5-ft protective-system threshold of 1926.652(a)(1) (the 1:1 sloping shown is itself a compliant protective system), and past the 4-ft egress threshold, so ladders must sit within 25 ft of every worker per 1926.651(c)(2).
Formulas
- Top width from the Table B-1 slope
W_top = W_bot + 2 x D x S- Cross-section and bank volume
A = (W_bot + W_top) / 2 x D; V_bank = A x L / 27- Spoil and truckloads
V_spoil = V_bank x (1 + s); trucks = ceil( V_spoil / 10 )
Standards & References
- OSHA 29 CFR 1926 Subpart P, Appendix B, Table B-1 — maximum allowable slopes, excavations < 20 ft deep
- OSHA 29 CFR 1926.652(a)(1) — protective systems required at 5 ft or deeper (stable-rock exception)
- OSHA 29 CFR 1926.651(c)(2) — means of egress within 25 ft lateral travel at 4 ft or deeper
- OSHA 29 CFR 1926 Subpart P, Appendix A — soil classification by a competent person; swell factors are representative practice, not code
Frequently Asked Questions
Who classifies the soil as Type A, B, or C?
A competent person — someone trained in the Appendix A classification who has the authority to stop work — classifies the soil at the site using at least one visual and one manual test, and reclassifies it after rain, vibration, or any change in conditions. It is a field call, not an office one. If no classification has been made, OSHA practice is to treat the soil as Type C, the most conservative case. This calculator applies the Table B-1 slope for whatever type you select; it cannot see the dirt.
What makes a soil Type A instead of B or C?
Type A is cohesive soil with an unconfined compressive strength of 1.5 tons per square foot or more — stiff clays are the classic case — but it is disqualified to Type B or C if it is fissured, subject to vibration from traffic or equipment, has been previously disturbed, or is seeping water. Type B covers cohesive soils of intermediate strength and things like silt, loam, and previously disturbed soils; Type C is granular material such as sand and gravel, submerged soil, or soil with water freely seeping through it. When any factor is in doubt, classify downward.
When is a protective system required in a trench?
At any depth of 5 ft or more, 29 CFR 1926.652(a)(1) requires every employee in the excavation to be protected by a protective system: sloping or benching, shoring, or a trench shield. There are two exceptions — an excavation made entirely in stable rock, and one less than 5 ft deep where examination by a competent person shows no indication of a potential cave-in. The maximum-allowable-slope cut this calculator draws is itself a compliant protective system for excavations under 20 ft; shields and shoring are the usual alternatives where laying back the sides would take too much real estate.
How far can a worker be from a ladder in a trench?
In trenches 4 ft deep or more, 29 CFR 1926.651(c)(2) requires a stairway, ladder, ramp, or other safe means of egress within 25 ft of lateral travel from any employee — so on a long trench you need an exit at least every 50 ft. Ladders must extend at least 3 ft above the landing surface and be secured. The rule applies regardless of soil type: even a stable-rock cut 4 ft or deeper needs egress within 25 ft.
How much does excavated soil swell?
Loose spoil occupies more volume than the same material in place because excavation breaks up its structure. Typical swell runs around 10–15% for sand and gravel, 20–30% for common earth and clay, and 40–65% for blasted rock, but the figure varies with material, moisture, and how the soil is handled — the presets here (15% rock ripped from a trench, 30% Type A clay, 25% Type B, 12% Type C sand/gravel) are representative estimates, not code values. If your geotech report or local experience gives a project number, enter it as the override; spoil volume drives haul-off trucking and stockpile space, both of which cost money.
Can I dig a trench deeper than 20 feet with these slopes?
No. Table B-1 and the rest of Appendix B apply only to excavations less than 20 ft deep; for anything deeper, the sloping or benching configuration must be designed by a registered professional engineer, and the same PE requirement applies to protective systems generally past 20 ft. This calculator refuses depths over 20 ft for that reason. Deep excavations also stack additional hazards — surcharge from adjacent structures, groundwater, and atmosphere testing — that are well outside a slope table.