As far as building materials go, they don't come much cheaper than dirt, which is literally everywhere and mostly free. But, as anyone who has ever made a sand castle knows, soil isn't terribly strong and has a habit of forming a shallow pile rather than more structurally-beneficial shapes. We're going to let you in on a little secret — making dirt super strong is incredibly easy. All it takes to make dirt stand up to stresses and shearing forces is a few layers of just about anything else. In this example from Practical Engineering, paper, fabric and even fly screens are used to illustrate this point. But use your imagination! Most things besides "more dirt" will probably do nicely.
The reason this works at all has to do with the nature of dirt. Its strength is derived from the friction of dirt particles moving past one another — which can happen in any direction. But just as steel bars help fix concrete's ductility issues, layers of whatever between clods of dirt help generate friction and spread perpendicular forces out over a greater surface area.
Dirt with layers of reinforcement isn't just a neat science experiment though. Those huge walls that run along the sides of highways are almost always made this way — the concrete facing is there mostly to keep the dirt from spilling out onto the roadway.