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Trench slope: dig and see if it caves in
Short answer
Under OSHA's excavation standard (US), from 1.52 m deep every trench with people inside needs a slope, shoring or a shield, except in stable rock. The slope depends on the soil: ¾:1 (53°) in Type A, 1:1 (45°) in Type B and 1½:1 (34°) in Type C, up to 6.1 m. And it costs earth: with a 0.6 m bottom and 2.5 m deep you go from 1.50 m³ per metre to 10.88 m³ in C soil. In your country, its standard rules.
The rule you're practising
- From 1.52 m (5 ft) deep you must protect against cave-ins: slope, shoring or shield. Shallower, only if a competent person checked and saw no signs.
- Soil is classified A, B or C. Water makes it C; vibration, fissures or fill take away the A.
- Maximum slope: ¾:1 (53°) in A, 1:1 (45°) in B, 1½:1 (34°) in C. Deeper than 6.1 m, an engineer designs it.
- Earth per metre of trench = bottom width × depth + slope × depth².
- Ladder from 1.22 m within 7.62 m of every worker; spoil 0.61 m or more from the edge.
Where each number comes from
- When to protect, ladder, spoil and inspection.Source: OSHA 29 CFR 1926.652(a)(1) and 1926.651(c)(2), (j)(2) and (k)(1), on osha.gov; published with its citation in the site's sloping and shoring guide.
- Soil Types A, B and C.Source: OSHA 29 CFR 1926 Subpart P, Appendix A.
- ¾:1, 1:1 and 1½:1 slopes and the 6.1 m limit.Source: OSHA 29 CFR 1926 Subpart P, Appendix B, Table B-1.
- Each excavator's depth.Source: Komatsu datasheets in the catalog (maximum digging depth).
- Soil, depth and bottom width.Source: you set them. The cave-in illustrates the rule: the standard doesn't say when a wall falls, it says from what depth you protect.
- Your country.Source: El Salvador, Guatemala, Nicaragua and Panama each have their own standard, with different figures; they're in the guide. Neither replaces the other nor the competent person on site.
FAQ
From what depth is a trench shored?
Under OSHA, from 1.52 m every trench with people inside needs a slope, shoring or a shield, except in stable rock. Central American standards set a similar or lower threshold: check your country's in the guide.
What slope does each soil type need?
¾:1 (53°) in Type A, 1:1 (45°) in B and 1½:1 (34°) in C, for under 6.1 m. If you don't know the soil type, use C's.
How much extra earth does the slope move?
Slope × depth² per metre. With a 0.6 m bottom and 2.5 m deep: 6.19 m³ per metre in A, 7.75 in B and 10.88 in C, against 1.50 with vertical walls.
Slope or trench shield?
If there's room and the soil is good, the slope is simplest. On a narrow street, next to buildings or in Type C soil, the shield keeps the trench narrow and moves far less earth; it's chosen from the manufacturer's tables.
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