Unclassified soil defaults to Type C
Under 1926.652(b)(1)(i), if the soil is not classified, slopes may be no steeper than 1½H:1V (34° from horizontal), the same as Type C. 1926.652(b)(1)(ii) then requires the slopes to follow the Type C configurations in Appendix B.
The traps: 3/4H:1V (Type A) and 1H:1V (Type B) need a classification first, and 1½V:1H reverses the ratio. Appendix D shoring is a support system, not a slope configuration.
Two depth triggers: 4 ft and 5 ft
- Egress, 1926.651(c)(2): trenches 4 ft or more deep need a stairway, ladder, ramp or other safe means of egress.
- Protective system, 1926.652(a)(1): required, except in stable rock or in an excavation less than 5 ft deep where a competent person finds no indication of a potential cave-in.
A 4 ft 8 in trench outside stable rock, cleared by a competent person, needs egress but no protective system. The trap: mixing up 4 ft and 5 ft. The under-5-ft exception needs the competent person's finding.
Ladder count from the 25 ft rule
1926.651(c)(2) places egress so that no worker travels more than 25 ft laterally. Each ladder serves 25 ft in both directions, so it covers 50 ft of trench.
Ladders = trench length ÷ 50. A 300 ft trench needs 300 ÷ 50 = 6, placed at 25, 75, 125, 175, 225 and 275 ft. The traps:
- dividing by 25 (12 ladders) forgets each ladder serves both directions
- starting at one end with 50 ft spacing needs one extra ladder (7)
Two feet below a shield
Under 1926.652(g)(2), a trench may be dug no more than 2 ft below the bottom of a shield, and only when the shield is designed for the forces of the full trench depth and there is no sign of soil loss from behind or below it. In a 12 ft trench the shield bottom may sit as shallow as 12 − 2 = 10 ft. The traps: 8 ft leaves 4 ft unprotected, and 11 ft is allowed but not the shallowest.