I stumbled across this great answer to that question from Geotechnical consulting firm Tierra Group International, LTD a geotechnical consulting firm servicing the mining industry:
Mankind lives on the earth. Everything built, either originated from, is built on, or interacts with, the earth. Geology is the science of the earth. Geotechnical Engineering is the application of geology and mathematics to use Earth’s natural resources to build infrastructure.
It reminds me of one of my favorite geo-engineering quotes from Richard L. Handy:
Virtually every structure is supported by soil or rock. Those that aren’t either fly, float, or fall over.