Abstract: A polymer geo-injection apparatus for protecting an underground structure is provided. The apparatus includes: a drill bit configured to drill a path through the ground to a desired depth that is above the underground structure; an injection nozzle coupled to the drill bit and configured to insert into and withdraw from the ground along the drilled path, and to create a corresponding cavity in the ground at the desired depth by injecting compressed air into the ground at the desired depth; and a polymer melting and injection unit configured to fuse or melt one or more components of a solid polymer into a liquid form of the solid polymer, and supply the liquid polymer to the injection nozzle. The injection nozzle is further configured to fabricate a corresponding protection slab of the solid polymer at the desired depth by injecting the supplied liquid polymer into the corresponding cavity.