I think it's pretty clear from an energy perspective that melting/vaporizing or even pulverizing the rock is waaaaay more energy intensive as the force to shear rock is orders of magnitude less than melting it.
One way to think of this is to just imagine the chemical bonds holding the rock together. Tricone drills and TBMs both just sheer the rock, the latter on a massive scale, which is only has to expend that localized bond energy, whereas melting all of it is massively inefficient even with good heat recovery.
One thing I've wondered is if a focused energy beam could just locally break the bonds in a way that'd produce pieces optimized (size, shape, etc.) for transport back through the tunnel. In this way the minimum of energy needs to be expended by reducing the linear distance of cutting.
However, energy beams still have to heat up the rock a lot to vaporize it, so this would still locally be less efficient than cold sheering that current methods use, so when you think about it, TBMs, etc. are actually doing pretty good.
Could you use a nuclear element to directly create pressure waves against the material? Something like: nuclear fuel, graphene drum head, water injectors to drive the drum expansion.
One way to think of this is to just imagine the chemical bonds holding the rock together. Tricone drills and TBMs both just sheer the rock, the latter on a massive scale, which is only has to expend that localized bond energy, whereas melting all of it is massively inefficient even with good heat recovery.
One thing I've wondered is if a focused energy beam could just locally break the bonds in a way that'd produce pieces optimized (size, shape, etc.) for transport back through the tunnel. In this way the minimum of energy needs to be expended by reducing the linear distance of cutting.
However, energy beams still have to heat up the rock a lot to vaporize it, so this would still locally be less efficient than cold sheering that current methods use, so when you think about it, TBMs, etc. are actually doing pretty good.