A trench is 12 feet deep in Type C soil (least stable). If sloping is used instead of shoring, what is the minimum slope ratio required?
Correct Answer
D) 1.5:1 (horizontal to vertical)
Type C soil is the least stable and requires the flattest slope of 1.5:1 (horizontal to vertical) or 34 degrees from horizontal. This provides adequate stability for loose, granular soils.
Why This Is the Correct Answer
Type C soil is classified as the least stable soil type, consisting of loose granular materials like sand, gravel, or previously disturbed soil. Due to its unstable nature, it requires the flattest (safest) slope ratio to prevent cave-ins. The 1.5:1 ratio means for every 1 foot of vertical depth, you need 1.5 feet of horizontal distance, creating a 34-degree angle that provides adequate stability for worker safety.
Why the Other Options Are Wrong
Option A: 1/2:1 (horizontal to vertical)
The 1/2:1 ratio is too steep for Type C soil and would be used for the most stable soil types. This steep slope would create dangerous conditions in loose, granular soil.
Option B: 3/4:1 (horizontal to vertical)
The 3/4:1 ratio is used for Type B soil (medium stability). While safer than option A, it's still too steep for the least stable Type C soil conditions.
Option C: 1:1 (horizontal to vertical)
The 1:1 ratio, while safer than A and B, is still insufficient for Type C soil. This ratio might be acceptable for more stable soil conditions but doesn't provide adequate safety margin for loose, granular materials.
Memory Technique
Use 'ABC-321': Type A soil = 3/4:1, Type B soil = 1:1, Type C soil = 1.5:1 (getting progressively flatter as soil stability decreases)
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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