My 2 cents on earthbender tunnelling:

The bender's mastery does correlate with the weight of the rocks being moved. We saw King Bumi strain when he was moving a particularly large chunk of rock.

Therefore, if the earthbender is tunnelling through unstable earth, and has to use pure earthbending to keep the tunnel from collapsing, he has to expend enormous amounts of his energy. So traditional earthbender tunnelling probably follow ordinary construction methods, so that the tunnels are self-sustaining and doesn't need constant earthbending energy input once it's constructed.