A trench is being excavated in Type C soil. What is the maximum allowable simple slope ratio for the sides of this excavation?
Correct Answer
D) 1.5:1 (horizontal to vertical)
According to OSHA 29 CFR 1926.652, Type C soil requires a maximum allowable slope of 1½:1 (34 degrees from horizontal) for simple slope excavations.
Why This Is the Correct Answer
OSHA 29 CFR 1926.652, Appendix B establishes soil classification-based slope requirements. Type C is the least stable classification (cohesive soil with unconfined compressive strength ≤ 0.5 tsf, submerged soil, or fissured soil). The maximum allowable slope for Type C is 1.5:1 (horizontal to vertical), meaning for every 1 foot of depth, the wall must be cut back 1.5 feet horizontally. This creates a 34-degree angle from horizontal.
Why the Other Options Are Wrong
Option A: 3/4:1 (horizontal to vertical)
A 3/4:1 ratio applies to Type A soil — the most stable classification. Type A requires the least setback because it is the strongest soil type. Using a 3/4:1 slope in Type C soil would create dangerously steep walls with high cave-in risk.
Option B: 2:1 (horizontal to vertical)
A 2:1 ratio is more conservative (flatter) than required, but it is not the maximum allowable — it is not a standard OSHA classification ratio. The standard ratios are 3/4:1 (Type A), 1:1 (Type B), and 1.5:1 (Type C).
Option C: 1:1 (horizontal to vertical)
A 1:1 slope is required for Type B soil — intermediate stability. Type C soil, being less stable, requires a flatter slope (1.5:1). Using a 1:1 slope in Type C conditions increases cave-in risk significantly.
Memory Technique
ABC soil types, ascending slope: Type A = 3/4:1 (steep, stable), Type B = 1:1 (medium), Type C = 1.5:1 (flattest, least stable). 'C for Crumbly = Cut it flat at 1.5 to 1.'
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