Sewer separation and other projects that require underground piping or wiring can be both costly and messy. With this in mind, cities like Edina, Minn., are turning to trenchless technology to get the job done.
Trenchless horizontal directional drilling is exactly what it sounds like — drilling horizontally under the ground. Th is requires ground to be broken at only two points, the entrance and the exit. The rest of the work happens below the surface.
Edina public works officials have often chosen HDD for necessary projects, especially those pertaining to water and sewer. This includes, but is not necessarily limited to, water main projects; piping water to the treatment plant; storm sewer pipes; and anywhere they may need to loop the system between streets, where they’re having water quality and flow concerns, or where there’s a need to reroute a system or to tie into a drainage feature. They have also used it to make a conduit for street lighting.
HDD has especially come in handy in tight spaces, where digging a trench may not have been practical or even doable.
“The reason to use it in many of our cases is the amount of space we have to work with,” Chad Millner, director of engineering for Edina, said. “The big driver also is whether or not we have easements. It’s typically easier to use drilling. We don’t have easements to do drilling because the impact is under the ground.”
Because HDD does not require a trench, workers can easily run piping or wiring between homes, below yards and under streets with very little disturbance.
“The benefits are, it reduces our footprint of construction, so there’s less restoration afterwards, which is good for the residents and property owners,” Millner said. “Also, it provides a method in really tight spaces where we don’t have a space to work, like between homes. It allows us to work in those areas with very minimal impact.”
He noted that the city of Edina is currently planning a lift station project and is considering trenchless technology for that as well.
As with any form of technology, HDD is not perfect. As an example, Millner cited potential issues with the tracer wire.
“On occasion, when you’re drilling, you’ll be drilling next to a rock or other sharp object and that tracer wire gets broken,” he said. “We’d need to reestablish the continuity, so we re-drill another tracer wire in or dig down to where the tracer wire is.”
Another occurrence he referenced involved the bentonite solution used for pipe installation.
“On occasion, during operations using bentonite solution to aid in installation of the pipe, depending on soil conditions, that can bubble to the surface,” he said. “We had to clean up some spots we wouldn’t have had to clean up originally. Sometimes it’s in the streets, but more commonly it’s in a yard because you don’t have the same kind of compaction you would have under a street.”
Planning is crucial before deciding whether or not to go trenchless. Different soil types require different drilling methods. Millner said it’s important to consult the experts before making the final choice. “We really rely on the geotechnical experts before each project. We do studies and meet with industry experts before we even go up for bids.” In some cases, HDD may not be a suitable choice.
“You need to review every project individually for what the goals are, what the constraints are, be it soil, be it existing structures, concerns with property, so I think it is a very important tool that can be used in the right circumstances, but make sure it is reviewed and analyzed.”
As far as cost-effectiveness, while going trenchless may cost a little more at the onset, overall Millner noted that the city of Edina has saved money using HDD as opposed to digging an open trench.
“If you just compare it to open cutting vs. drilling, when you factor in restoration costs, potential need to get easements, potential impacts if you’re digging an open trench, the public relations aspect of it, the soft costs of dealing with property owners to lessen the impact on them … I think they are getting much closer in actual cost,” he said.