The surface area of the head of a tunnel boring machine is usually flat. A cone-shaped head would increase the surface area. The question is if it could speed up the boring process.
I know that with current machines the boring process is also limited by other factors. But that is not part of the question.
Because with a flat head the waste material can be (relatively easily) collected and removed from the cutting face.
With a cone-shaped bit that removal is not so easily accomplished.
Cone-shaped bits tend to be used to push or compress the material out of the way, which is fine for "softer" materials ...
