Excavation, Trenching, and Shoring
What this domain tests
Excavation, Trenching, and Shoring tests OSHA Subpart P minimums and practical protective-system choices. Expect soil classification, sloping and benching, shoring and shielding, inspections, access, and utility or water hazards.
Soil and slope
- Type A: cohesive soil, 3/4:1 slope. Type B: 1:1. Type C: 1.5:1, for excavations 20 ft or less.
- Type A is not allowed if the soil is fissured, previously disturbed, or subject to vibration.
- Slope is measured from the trench bottom. Horizontal distance equals depth multiplied by the slope ratio.
- Benching is a series of steps and is not permitted in Type C soil.
Systems and inspections
- Shoring supports the walls; a shield or trench box protects workers but does not shore the walls above it.
- The competent person inspects daily, after rain, and after any change.
- Provide egress within 25 ft lateral travel. Keep spoil 2 ft back, control water, locate utilities, and account for vibration and surcharge.
Worked example
A 12-ft deep trench in Type B soil has a 4-ft bottom width. Reasoning: Type B requires 1:1. Each side needs 12 ft horizontal. Top width equals 4 + 12 + 12 = 28 ft. If the right-of-way is only 20 ft, sloping alone will not fit. Use a shield or shoring designed for the depth, with the competent person's approval.
Traps
- Using a Type A slope in fissured clay.
- Measuring slope from the top edge instead of the bottom.
- Assuming a trench box protects against wall collapse above the box.
- Leaving spoil at the edge.
- Skipping inspection after rain.
- Ignoring vibration from equipment or traffic surcharge.