During excavation work, you discover the soil contains large rocks and has previously been disturbed. This soil should be classified as:
Correct Answer
B) Type C
Previously disturbed soil and soil containing large rocks typically classify as Type C, the least stable soil classification.
Why This Is the Correct Answer
Type C is the least stable soil classification under Cal/OSHA/OSHA 29 CFR 1926 Subpart P. Previously disturbed soil and soil containing large rocks that cannot be classified as Type A or B are automatically classified as Type C. Previously disturbed soil loses its natural cohesion and structure, making it inherently unstable.
Why the Other Options Are Wrong
Option A: Stable rock
Stable rock is a separate category requiring the material to be natural solid mineral matter that can be excavated with vertical sides and remain intact. Large rocks embedded in disturbed soil do not qualify; the surrounding matrix determines classification, not the rocks themselves.
Option C: Type B
Type B soil has intermediate stability β it includes previously cracked but unfissured cohesive soils, or granular cohesionless soils. However, the key disqualifier here is 'previously disturbed.' Once soil has been excavated or otherwise disturbed, it cannot be classified higher than Type C regardless of other characteristics.
Option D: Type A
Type A is the most stable cohesive soil with unconfined compressive strength of 1.5 tsf or greater. It must not be fissured, not subject to vibration, and not previously disturbed. The presence of prior disturbance alone disqualifies any soil from Type A classification.
Memory Technique
Use the rule 'Previously Disturbed = Always Type C.' Think of it as a trust rule: once broken (disturbed), you can never trust it at a higher level. Large rocks in soil act like ball bearings, reducing friction and stability β another Type C indicator.
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