A construction crew is excavating a trench that is 8 feet deep and 4 feet wide in Type C soil. What is the minimum required slope ratio for the sides of this excavation?
Correct Answer
C) 1.5:1 (34 degrees)
For Type C soil (the least stable), OSHA requires a slope ratio of 1.5:1 (34 degrees from horizontal) to prevent cave-ins. Type C includes granular soils and soils with the lowest unconfined compressive strength.
Why This Is the Correct Answer
OSHA 29 CFR 1926.652 specifies that Type C soil, being the least stable soil classification, requires the most conservative slope ratio of 1.5:1 (34 degrees from horizontal). Type C soil includes granular soils like sand, gravel, and soils with unconfined compressive strength of 0.5 tons per square foot or less. This slope ratio provides adequate protection against cave-ins by creating a stable angle that prevents soil collapse. The 1.5:1 ratio means for every 1.5 feet of horizontal distance, there is 1 foot of vertical rise.
Why the Other Options Are Wrong
Option A: 1/2:1 (63 degrees)
The 3/4:1 ratio (53 degrees) is the required slope for Type A soil, which is the most stable soil type with high cohesion and unconfined compressive strength of 1.5 tons per square foot or greater.
Option B: 3/4:1 (53 degrees)
The 1/2:1 ratio (63 degrees) is too steep for any soil type and would create an unsafe excavation prone to cave-ins, especially in unstable Type C soil.
Option D: 1:1 (45 degrees)
The 1:1 ratio (45 degrees) is the required slope for Type B soil, which has moderate stability with unconfined compressive strength between 0.5 and 1.5 tons per square foot.
Memory Technique
Use 'ABC = 3/4, 1, 1.5' - as you go from A to C, the slope ratios increase (3/4:1, 1:1, 1.5:1) because soil stability decreases and you need gentler slopes for safety.
Reference Hint
OSHA 29 CFR 1926.652 - Requirements for protective systems, Table B-1 in Appendix B for maximum allowable slopes
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