How does one progress the sliding straightedge into the projected tunnel path without first excavating the tunnel?
In which case the tunnel is already excavated without the use of the aforementioned straightedge, Therefore very likely un-straight. (no straightedge)
Today we use tunneling machines (moles) However I have never seen a square tunnel boring machine, They are usually round necessitated by the rotary motion of the cutting leading edge.
Edited 1 time(s). Last edit at 13-Feb-16 20:45 by Jon Ellison.