Spotting Safety: Excavation Cave-ins

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trench1

 

trench2

For anyone standing at the bottom of a trench and looking up at the sheer face of a column of freshly exposed dirt it becomes clear how important it is to establish proper protections considering the consequences of a cave-in.

And it is apparent that in both photos, neither employee is adequately protected. But let’s look at how cave-ins occur and why each worker is in violation.

Physics of a Cave-in

The stress of a cubic foot column of soil is 100 lbs. per square foot (psf). So the vertical load of that one-foot-by-one-foot column of soil five feet down is 500 psf. When an excavation is cut, the soil in the wall begins to move, however slowly, into the excavation. Cracks will eventually develop away from the excavation’s edge, and the weight of the soil between the excavation and the crack is no longer carried by the soil behind the crack. This causes the lower part of the excavation wall to fail first, the upper part of the wall then falls, and a third cave-in can quickly occur.

Each cave-in can be the one that buries an unprotected worker at the bottom of the trench.

Regulations

The worker in the first image is more than five feet deep and there is no shoring, sloping, or shielding. Employees who work in an excavation five or more feet deep and not in solid rock must be adequately protected from cave-ins. 1926.652(a)(1)

He also doesn’t appear to have a harness with a lifeline to get him out, as required when entering a deep and confined footing excavation. 1926.651(g)(2)(ii)

The worker also needs to be protected from loose rock or soil that can fall or roll into the excavation. This can be done by removing the loose material piled up at the edge of the excavation and keeping it at least two feet from the edge of the excavation, or using retaining devices or equipment to keep back the soil. 1926.651(j)(2)

So much seems to be right in the second photo, where trench boxes are constructed to shield workers inside them from a collapsing excavation. And everything apparently complies with the shielding regulations: the excavation isn’t two feet below the bottom of the box, the top of the box is higher than the excavation, and it looks to be able to withstand subjected loads of an excavation without lateral movement. 1926.652(g)

But once the worker leaves the trench box, he is exposed to the hazards of the trench collapsing. The worker needs to stay inside the trenchbox to complete the task, and if that is not possible, the employer needs to extend the shielding.

It’s up to the employer to provide a safe work environment and ensure that employees follow safety procedures, but it’s also up to the employee to follow safety regulations and the employer’s safe work procedures. This is outlined in OSHA’s General Duty Clause. Section 5 Duties(a)(b)

All Spotting Safety Articles:

  • Excavation Cave-ins
  • Scaffolding Footing
  • Scaffolding Toprails
  • Earthmoving Equipment Training
  • Self Supporting Ladders
  • Forklift Counterweighting
  • Power Tool Trigger Guards
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