oubaas
Favorite Answer
it is not a matter of material quantity to dig ....the reason is a best strenght to forces to be counteracted thanks to its shape....
Anonymous
With either an inverted U or horseshoe shape the tunnel would have more effective width for tall trucks. They could get closer to the walls than they could with a semicircular cross section.
I did not thumbs down the other answers.
Bomba
If the "horseshoe" shape of the curved portion (the arch) approaches either a catenary or a parabolic or elliptical shape it will be theoretically stronger than a semi-circular arch.
You can Google "strongest arch" and find a few sites on the topic.
oil field trash
It requires the removal of less material and therefore should be faster and cheaper to build.