Trench Box Movement Past an Unlocated Gas Main
A crew is installing 18-inch ductile iron water main in an 8-ft-deep trench in Type B soil. A 12-inch gas main crosses the trench perpendicularly at a depth of 6 ft. The crew has an 8-ft-high steel trench box that is 12 ft long. The plan is to slide the box forward past the gas main without exposing or supporting the gas line, then continue excavation. Spoil is placed 2 ft from the trench edge, and the only access is a ladder at the upstream end, about 30 ft from the work area. The gas utility has not been located by a line locator; the crew is relying on an old site plan. Water is seeping at the downstream end, and the box is not pinned or restrained against lateral movement.
1. Evaluate the trench box plan relative to the gas main crossing. What must happen before the box is moved past the utility?
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The box cannot simply be slid past an unsupported gas main. The gas line must be positively located and hand-exposed, its depth and condition verified, and it must be supported or protected before the box moves. Because the box is 12 ft long and the gas main crosses at 6 ft in an 8-ft trench, moving the box could leave the utility spanning the trench without support. The crew should notify the gas utility and follow required clearance rules, then use a shorter box, shoring, or an engineered support plan. The old site plan is not adequate for a crossing at this depth.
2. Identify the access, spoil, and water-control deficiencies and the corrective actions.
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The ladder is about 30 ft from the work area, beyond the 25-ft maximum travel distance, so a ladder must be placed within 25 ft of each worker. Spoil at 2 ft is at the minimum distance; the competent person should require it to be moved farther if it is surcharging the box or trench edge. Seepage at the downstream end must be controlled with a pump and discharged away from the trench so it does not undercut the box or saturate the soil. The box must be pinned or otherwise restrained against lateral movement before workers enter.
3. What should the crew do if the gas line is not located and the old plan conflicts with field conditions?
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Stop excavation. Call the one-call locating service and the gas utility, and use hand tools within the tolerance zone until the line is positively located. Do not use mechanical excavation near the crossing. The competent person should reassess the protective system and the utility support before work resumes. If the old plan conflicts with field conditions, the field condition governs until the utility or a qualified person verifies the line. Workers should not enter an excavation where an unlocated gas main may be exposed.