Type C Trench Beside an Old Foundation
An 8 ft deep, 40 ft long, 4 ft wide trench is being excavated to install a 12 in. PVC sewer line beside a 60-year-old masonry foundation 6 ft away. The competent person classified the soil as Type C based on granular fill and prior excavation. No protective system is installed; the north wall is near vertical. Spoils are piled 3 ft from the edge and stand 2 ft high. A broken water line has left 6 in. of water at the trench bottom, and the crew has not tested the atmosphere. The only ladder is at the west end, about 32 ft from two workers at the east end. A worker reports a small tension crack parallel to the north edge. The foreman says the trench will be open only two more hours and asks whether the crew can finish hand-laying the last section before backfill.
1. List the immediate stop-work hazards and the competent person's required actions.
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Immediate stop-work hazards include:
- An 8 ft deep excavation with no protective system, which is a cave-in hazard.
- A near-vertical wall in Type C soil; Type C cannot stand vertically.
- A tension crack parallel to the edge, indicating movement or instability.
- Spoils 3 ft from the edge, adding surcharge near the excavation.
- Water at the bottom, which weakens soil and changes conditions.
- No atmospheric test, despite the broken water line and sewer work.
- The nearest ladder is about 32 ft away, exceeding the 25 ft travel distance.
- An adjacent masonry foundation 6 ft away may be undermined by the excavation.
The competent person must remove workers immediately, barricade the area, and reassess the soil. Do not re-enter until a protective system is installed, water is controlled, spoils are set back at least 2 ft (preferably farther), access is provided, and the atmosphere is tested. Because the foundation is close and the excavation is deep, obtain an engineer's evaluation before continuing.
2. Select a protective system for the trench and specify the key dimensions and setback requirements.
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The preferred system for Type C soil is sloping, benching, or a trench box designed for the depth. Type C soil must be sloped at 1.5 horizontal to 1 vertical (about 34 degrees from horizontal). For an 8 ft depth, that requires 12 ft of horizontal excavation on each side, measured from the bottom edge. If the right-of-way does not allow 12 ft on each side, use a trench box or shoring system with tabulated data for an 8 ft depth. The box must be installed from the top and not be used as a substitute for sloping if the box cannot protect workers during installation.
Key requirements:
- Keep spoils and equipment at least 2 ft from the excavation edge; 3 ft is closer than allowed and should be corrected.
- Control the water before entry; pumping alone does not make an unstable wall safe.
- Have the competent person inspect daily and after rain or any change.
- Because the foundation is only 6 ft away, evaluate underpinning or shoring of the foundation before excavating further.
3. What access, egress, and atmospheric testing requirements apply before the crew re-enters?
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Access and egress:
- Provide a ladder or ramp within 25 ft of lateral travel for each worker.
- The ladder must extend at least 3 ft above the top of the excavation and be secured.
- Do not use the trench box or shoring as a ladder.
Atmospheric testing:
- Test before entry and as conditions change. Test for oxygen, flammable gas, and toxic gas such as hydrogen sulfide.
- Acceptable oxygen is 19.5% to 23.5%. Flammable gas must be below 10% of the lower explosive limit.
- If a test shows a hazard, ventilate and re-test before entry.
- Continue monitoring while workers are inside, especially near the sewer line and the broken water line.
The trench should remain barricaded and unattended until these controls are in place.