An excavation 8 feet deep in Type C soil requires shoring. What is the minimum timber size required for uprights in the shoring system?
Correct Answer
C) 3x8 inches
OSHA tables specify minimum 3x8 inch timber uprights for shoring systems in excavations 5-10 feet deep in Type C soil. This provides adequate structural support for the soil loads.
Why This Is the Correct Answer
CORRECT_ANSWER - According to OSHA 29 CFR 1926.652 Appendix C, excavations 5-10 feet deep in Type C soil (the least stable soil type) require minimum 3x8 inch timber uprights for adequate shoring. Type C soil has the highest risk of cave-in, so it requires more substantial timber sizing than more stable soil types. The 3x8 dimension provides the necessary structural strength to resist the lateral soil pressure at this depth range.
Why the Other Options Are Wrong
Option A: 4x6 inches
2x6 inches is insufficient for Type C soil at 8 feet deep. This size may be adequate for shallower excavations or more stable soil types, but does not meet OSHA requirements for the structural loads imposed by unstable Type C soil at this depth.
Option B: 2x8 inches
2x8 inches does not provide adequate cross-sectional area and strength for Type C soil conditions. While the 8-inch dimension is correct, the 2-inch thickness is insufficient to handle the lateral pressure from Type C soil in an 8-foot deep excavation.
Memory Technique
Remember 'C-3x8': Type C soil needs 3x8 timber. Think 'C' for 'Critical' - the most dangerous soil needs the biggest standard timber size (3x8).
Reference Hint
OSHA 29 CFR 1926.652 Appendix C - Timber Shoring for Trenches, Table C-1.1 for Type C soil
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