Frank Peloubet, of North Dakota State University, explained to attendees of the recent Culvert Installation & Maintenance Workshop in Saginaw sponsored by Michigan's Local Technical Assistance Program (LTAP) that soil is extremely heavy and dangerous when working in a trench. Workshops were also conducted in Grand Rapids, Gaylord and Marquette.
"It's very dangerous. You never know what the conditions are going to be, so it's better to have those safety features, equipment and procedures in place. Soil is very heavy. It is 100 pounds per cubic foot to 125 pounds per cubic foot. So, if you're in a 5-foot deep trench, you have 500 pounds per square foot to 600 pounds per square foot on the bottom of the trench. Most of the force occurs near the bottom of the trench," Peloubet said.
"In addition, when you excavate the trench and place the soil material on top, you are adding weight to the top of the trench. There are requirements that call for placing the material a certain distance away from the edge of the trench.
"The mechanics of how a trench caves in is different in many cases, but, generally, it doesn't always cave in from above. The weight is at the bottom of the trench, so as the walls of the trench dry and soil cracks, it doesn't drop off from the top. The weight causes the drop off at the bottom of the trench, and that's where your feet are. So, as it drops into the bottom of the trench, it traps your feet and then you can't move. So, you can't get to a ladder, and you have to rely on a harness, safety rope and equipment to lift you out of the trench.
"Sometimes the cave-in happens quickly and sometimes it happens in a progressive manner. It depends on the existence of moisture, equipment operating in the area and so forth." Peloubet explained that even though narrow trenches are more difficult to work in, a relatively small amount of the forces from the dead load and live load impact the pipe. Live loads are things such as vehicles traveling down a road that is above a pipe. The dead load would include the trench that the pipe is in.
"A lot of the forces are transferred to friction resistance on the wall of the trench. That helps relieve the forces acting on the pipe. The wider the trench, the more the forces act on the pipe," Peloubet said. If the trench must be wide, it can help to make the bottom narrow.