I have found that this has been a pretty common question throughout my life. What is a geotechnical engineer? I tell people that it’s the engineering you need when you do work on, in or with earthen materials. This doesn’t seem to help much. It’s comprised of two main branches: soil mechanics and foundation engineering. They overlap a little. Foundation engineering seems like it would be pretty easy to describe: figuring out how to distribute a load from a building so that it doesn’t settle too much. Pretty much the same for walls. Soil mechanics is a little more difficult to describe in layman’s terms. It’s about how earth materials react when you smash, stretch, shear or shake them (hey, four “esses”!)
Whether it’s a dam (building on and with), an excavation (building in) or a levee (building with), you’re going to want a geotechnical engineer’s help.