This is Part 3 in our series on proper pipeline installation and tile repair with Mad Dog Foam Bridges. Previously we examined the proper clearing and grading of land in preparation for pipeline installation. Today we’ll look at trenching, the process by which a ditch is dug to house a pipe.
Finally, after having spent significant time surveying the land, clearing and grading it appropriately, pipeline contractors can begin to dig downward in the soil for the process of trenching. This is the technical term for the digging of a ditch in which a new pipeline is housed.
Admittedly, this process is much more straightforward than the previous steps. That said, a significant amount of thought and strategy goes into creating this channel. They’re not “just digging a hole.”
The first step is deciding upon what piece of equipment to use when trenching. The two most popular methods are trenchers and excavators. A trencher is a machine that somewhat resembles an exaggerated chainsaw on treads. The “teeth” are actually individual scoops that revolve around the massive loop, removing dirt at a rapid rate.
Excavators are commonly misidentified as “backhoes”—the two look similar, except that an excavator is a vehicle of its own, and not an attachment. Backhoes also tend to feature rubber tires, while excavators use steel or rubber treads. Excavators work better in rocky soil, scooping up the chunks of rock that wouldn’t be caught by a trencher. They can also dig with more “touch,” which makes them better for handling turns in the pipeline path. The bigger the project, the better the chance that an excavator will be the tool of choice.
Occasionally, if there is too much solid rock, carefully-planned explosives clear the path for digging equipment.
Leaving enough clearance
The depth of the ditch differs depending on what size pipeline is installed, of course. A good rule of thumb is to leave around 48 inches of clearance from the top of the pipe. So, for example, a 36 inch pipeline would require a trench that’s 84 inches—or seven feet—deep. Three of those feet represent the pipe itself, with an additional four feet of gap between it and the surface. This depth is ideal for a trencher, which frequently dig 84-inch-deep ditch (which is also 60 inches wide).
Pipeline projects on agricultural land add another layer of complexity because of drain tile. These tiles need to be buried three to four feet below the surface, and a pipeline must be buried two feet below the tile. Therefore the same 36″ pipeline now requires a trench up to nine feet deep: Three feet of pipeline, two feet between it and the tile, and up to four feet of soil to cover the tile.
Width is also an important element. It obviously needs to fit the pipeline, but the quality of soil also makes a big impact. Soft, loose soil requires a trench dug at an angle to prevent collapse. For example, the average width of a trench for the Dakota Access Pipeline in South Dakota (for a 30-inch pipe) has been 12 feet.
“What about the dirt?”
There’s always a second question that comes with digging projects, especially when your “hole” stretched hundreds of miles: “What about the dirt?” The backfill from the trenching process is saved, stacked along the right of way so that it can be used to recover the pipeline upon the completion of the section.
Even this must be done carefully, however, as there are two different kinds of soil removed and they must be stored separately. Topsoil describes the top layer of earth, while “spoil” refers to everything underneath it. The density and material makeup between the two is distinct—placing spoil at the top could damage the potential for growth when the project is complete.
Crews should be sure to dig only as much trench as they’ll be able to fill and compact in a 10-day period, to allow easier movement for both livestock and native wildlife around the ditch.