Six-Foot Sewer Lateral Trench with Unknown Gas Crossing
A plumber is repairing a broken 6-inch clay sewer lateral in front of a 1950s house. The trench will be 6 ft deep, 4 ft wide, and 18 ft long in firm clay. The water service and a 1-inch gas line cross the trench within 3 ft of the house; the gas line is marked but its depth is unknown. The homeowner wants the work finished in one day, and the crew has a mini-excavator. The trench wall has a vertical crack, and a spoil pile is placed 18 in from the edge. A ladder is at the far end, 22 ft from the worker's position. The weather forecast is dry, but the trench bottom has 2 in of water. The crew has a 4-gas monitor and a ventilation blower on site.
1. List the trenching hazards and the protective measures required before entry at 6 ft depth.
Read the worked answer
At 6 ft, OSHA requires a protective system such as sloping, shoring, or a trench box unless a competent person determines solid rock. A vertical crack in clay is a failure indicator, so entry is prohibited until the trench is protected. The spoil pile must be at least 2 ft from the edge; 18 in is too close. Water in the bottom requires dewatering and evaluation. The ladder must be secured, extend 3 ft above grade, and be within 25 ft lateral travel. Locate the gas and water lines and hand-dig within the tolerance zone. Test the atmosphere with the 4-gas monitor before entry and use ventilation as needed. A competent person must inspect the trench daily and after changes.
2. Evaluate the ladder, spoil pile, utility crossing, and water. What must be changed?
Read the worked answer
The ladder is 22 ft from the worker, which is within the 25-ft limit, but it should be placed for a direct escape path and secured. The spoil pile at 18 in must be moved to at least 2 ft from the edge. The gas line depth is unknown, so it must be potholed and treated as live; no machine digging near it. The water must be pumped out and the walls checked for undermining. The cracked clay wall requires a trench box or shoring, not just sloping. Keep the excavator back from the edge and use a spotter.
3. If a worker must hand-dig near the gas line, what procedures and emergency plan are required?
Read the worked answer
Use a competent person, a current utility locate ticket, and hand-dig only near the gas line. Keep a gas monitor running, prohibit smoking and ignition sources, and have a fire extinguisher available. Know the emergency shutoff and utility contact. The worker should be in a protected trench box with a spotter outside. If gas is detected or smelled, evacuate, call 911 and the utility, and do not operate equipment. Maintain a rescue plan with trained rescuers and brief the crew before entry.