Building a cob house under rails, right next to a tunnel

After seeing the hobbit house buiding video, I became very interested into buiding my own with a couple friends under unutilized rails, under which passes a tiny river coupled with a small tunnel. The project would be to dig into the slope to make the home practically underground, right next to the tunnel to have a stable and watertight rocky wall, and leaving a single wall to build with clay and a portion of roof. The problem here is that I'm not sure is the lime coating is enough to make the roof stand for years with all the moisture in the ground, and how we are going to dig without the roof falling over.

as you can see in the picture, the place is pretty lush hence quite humid. Maybe also the humidity could make the air unpleasant or dangerous to breath with spores and maybe mold.

Could anyone help me figure out solutions to these problems ? Thanks.