Doesn't it take a strong structure to hold up a lot of soil?
Yes. A rough guess of loads imposed by earth, soil moisture and vegetation might be 135 pounds per cubic foot. Two examples will illustrate the range of possibilities available for a span of 18 feet: (1) Cast-in-place flat slabs would probably have to be 16 inches deep. (2) For strength alone a barrel vault would probably require only a 1inch thickness of unreinforced 2000-psi concrete. However, I don't advocate such thin sections, and the concrete should be of at least 3000-psi strength because lower strength concrete is too absorptive and permeable. There are many alternatives between these two.