A trench excavation is 12 feet deep in Type C soil. Workers need to enter for utility installation. Calculate the minimum horizontal distance from the edge of the excavation to the top of the sloped sides using the maximum allowable slope.
Correct Answer
C) 18 feet
Type C soil requires a 1.5:1 slope ratio (34 degrees). For a 12-foot deep excavation, the horizontal distance must be 12 × 1.5 = 18 feet from the bottom edge to the top of the slope.
Why This Is the Correct Answer
Type C soil is the least stable soil classification and requires the most conservative slope ratio of 1.5:1 (horizontal to vertical) according to OSHA excavation standards. This means for every 1 foot of vertical depth, you need 1.5 feet of horizontal distance. With a 12-foot deep excavation, the calculation is 12 × 1.5 = 18 feet horizontal distance from the bottom edge to the top of the slope.
Why the Other Options Are Wrong
Option A: 9 feet
9 feet would represent a 0.75:1 slope ratio (9÷12), which is too steep for Type C soil and would create an unsafe excavation that violates OSHA standards.
Option B: 12 feet
12 feet would represent a 1:1 slope ratio, which is the requirement for Type B soil, not Type C soil. Type C requires a more gradual slope due to its unstable nature.
Option D: 24 feet
24 feet would represent a 2:1 slope ratio, which is overly conservative and not required by OSHA standards for Type C soil, resulting in unnecessary excavation costs and material removal.
Memory Technique
Remember 'ABC = 0.75, 1, 1.5' - as soil gets worse (A to B to C), the slope ratio increases by 0.25 increments, making Type C the 'worst' requiring 1.5:1.
Reference Hint
OSHA Construction Standards 29 CFR 1926.652 - Requirements for protective systems, or Florida Building Code Chapter 18 - Soils and Foundations
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