A construction crew is excavating a trench that is 8 feet deep and 4 feet wide in Type C soil. What is the minimum allowable slope ratio for the sides of this excavation?
Correct Answer
D) 1 1/2:1 (horizontal to vertical)
For Type C soil (the least stable), OSHA requires a minimum slope of 1 1/2:1 (horizontal to vertical) or 34 degrees from the horizontal. Type C soils have the lowest unconfined compressive strength and require the most gradual slope for safety.
Why This Is the Correct Answer
Type C soil is the least stable soil classification according to OSHA standards, having an unconfined compressive strength of 0.5 tons per square foot or less. Due to its instability, OSHA 29 CFR 1926.652 requires the most conservative (gentlest) slope ratio of 1½:1 (horizontal to vertical) to prevent cave-ins. This means for every 1 foot of vertical depth, you need 1.5 feet of horizontal setback from the edge. The 8-foot depth and 4-foot width mentioned in the question don't change this fundamental safety requirement for Type C soil.
Why the Other Options Are Wrong
Option A: 1/2:1 (horizontal to vertical)
A 1/2:1 slope ratio is too steep for any soil type and would create an extremely dangerous excavation prone to cave-ins, as it provides insufficient horizontal setback for the vertical depth.
Option C: 1:1 (horizontal to vertical)
A 3/4:1 slope ratio is used for Type A soil (the most stable soil type), not Type C soil, and would be inadequate protection for the unstable characteristics of Type C soil.
Memory Technique
Use 'ABC = 3/4, 1, 1½' - as you go from A to C, the slope ratios increase (A=3/4:1, B=1:1, C=1½:1). Think 'Crummy soil needs Crazy slopes' for Type C.
Reference Hint
OSHA 29 CFR 1926.652 - Requirements for protective systems, Table B-1 in Appendix B shows maximum allowable slopes for different soil types
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