For Type C soil excavation, what is the only acceptable method of protection for workers in trenches 5 feet or deeper?
Correct Answer
D) Shoring or shielding systems only
Type C soil (soft clay, submerged soil) is considered unstable and requires protective systems like shoring or trench shields rather than sloping alone.
Why This Is the Correct Answer
Type C soil is defined by OSHA as the most unstable category—it includes fissured soils, previously disturbed soils, soils subject to water seepage, and granular non-cohesive soils. Because of this inherent instability, sloping and benching alone are not considered adequate. OSHA 29 CFR 1926.652 requires engineered protective systems such as shoring (hydraulic, pneumatic, or timber) or trench shields (trench boxes) for Type C soil trenches 5 feet or deeper.
Why the Other Options Are Wrong
Option A: Sloping at 1.5:1 ratio
Sloping at 1.5:1 may be permitted in some contexts for Type C soil IF the soil is stable enough and the competent person approves. However, the question asks for the 'only acceptable method' implying unstable Type C where sloping alone is insufficient. In the most restrictive interpretation for truly unstable Type C soil, shoring or shielding is the required method.
Option B: Sloping at 0.75:1 ratio
A 0.75:1 slope ratio is the specification for Type A soil—the most stable category. Applying this steep ratio to Type C soil would create a severely dangerous condition that would almost certainly lead to trench collapse.
Option C: Benching with 2-foot steps
Benching is explicitly prohibited in Type C soil under OSHA regulations. Benching requires the soil to hold a near-vertical cut at each step, which Type C soil cannot do. Only Type A soil and some Type B conditions allow benching.
Memory Technique
Remember: 'C soil = Can't bench, Can't rely on slope alone = Call in the Shield.' The trench shield (trench box) is your go-to image for Type C soil. Type C = Completely unstable = Contain it with shoring/shielding.
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