Moab is renowned for its biological soil crusts, but what’s happening underneath all that crusty black — with the soil and rock itself? This week, we speak with geotechnical engineer Taylor Hall, owner of the Moab Geotechnical Group, about soil mechanics, engineering tools, and how he decided — at age 15, in a McDonald’s — to start working with the dirt.
Science Moab: What is geotechnical engineering?
Hall: Geotechnical engineering generally deals with rock and soil mechanics and physics: how those materials will respond to structures or just how they respond to gravity. We might look at something like a bridge to understand its foundations, or we might look at a landslide that gets triggered by natural causes. We’re fortunate to be able to come in there and tell you how things are responding and why and what to expect.
Science Moab: How do you test soil?
Hall: When geotechnical engineering got its feet in the 1940s and 1950s, they would sample soil by drilling a hole and driving a sampler into the ground using a fixed-weight hammer. Using that method, we were able to acquire a sample and get some resistance associated with that sample. That's much of what we do today, but we do it now because it's backed by 60 or 70 years’ worth of empirical relationships.
Generally, you're only dealing with one or two such holes, and you have to use them to characterize a whole site. It's tough, but that's why I chose geotechnical engineering: because no two sites are the same. It provides the opportunity to really think on your feet.
Science Moab: Once you’ve taken measurements, how do you adjust your plans and materials?
Hall: It depends on what you're building, and what you hope that build will be able to accommodate. There might be many different interpretations of how much movement a certain structure can take. It takes a lot of communication; the interpretations and correlations know no bounds.
There are also a lot of cool advancements in soil stabilization. For example, we can dynamically compact soil by literally taking as big of a weight as a machine can pick up and dropping it on the ground repeatedly. That can densify soils. You can also dynamically compact with vibrators and hoppers that put gravel in the ground.
Science Moab: What are Moab’s soils and rocks like?
Hall: This valley is, geologically speaking, unique and beautiful. Being founded on a collapsed salt dome, and having hazards associated with that, create a very curious and fun environment to do what I do. A couple of years ago, the Utah Geological Survey put out a hazard-guidance manual for the area, and it came with 13 geological hazards from the Moab environment.
One of those things is the Mancos Shale. [Mancos Shale is a geologic deposit in the western United States that is prone to forming landslide activity if disturbed and appreciable wetting occurs. -ed.] We have “expansive soils” where if you introduce a little bit of water, things can get way out of hand. I've seen what we call “swell pressures,” or the load that you would need to put on the soil to prevent it from swelling, that's on the order of tens of feet of soil. For instance, we have the airport out here, and we're not going to put down 30 feet of soil just so we can put in a runway. It sparks these unique opportunities to engineer a system.
Science Moab: How did you get interested in geotechnical engineering?
Hall: When I was 15, I was working at McDonald's with a very well-respected high-school senior. I asked him what he was doing, and he said he was going to engineering school. He explained to me that civil engineers are responsible for making sure all the stuff we often take for granted — running water, electricity, sewers — are functional, and that we have the things we need to go about our daily lives.
At that point, I was like, “I'm going to be a civil engineer.” Then, within the civil engineering world, environmental water resources geotech gave me the chance to be outside. The dirt spoke to me, if you will. One of the jokes I like to tell people is that I grew up playing in the dirt, and now I get paid to do it.
Science Moab is a nonprofit dedicated to engaging community members and visitors with the science happening in Southeast Utah and the Colorado Plateau. To learn more and listen to the rest of Taylor Hall’s interview, visit www.sciencemoab.org/radio. This interview has been edited for clarity.