December Week Three

A little bit about the sinkhole that occurred last week.  The contractor was digging two parallel trenches to work on some utility lines. There was a gas main in the area, so the earth was left undisturbed between the two trenches to provide support and stability for the gas line.  The sinkhole developed between these two trenches.  


When working in an area that has absorbed as much water as we have had recently, the soil takes on almost a jell-o or quicksand quality that makes it susceptible to suddenly losing it's cohesiveness.  It then flows to the nearest open space.  This is why you see those large steel boxes on excavation sites.  Those are lowered into the area where the work is being done to protect the workers in case the side of the trench collapses.  The workers enter and exit via a ladder and work within the protection of the trench box. You can see in the first picture that this trench collapse can also happen in dry earth. Notice the sand on the right side of the trench box in the first picture. This has sloughed off the side of the trench

Trench box being moved by the backhoe.

In the third picture you can see the worker down inside of the trench box.