Dynamic crack movement in a brick arch viaduct

This video shows a Moiré Tell Tale attached across a small crack between two rings of a brick arch. Casual observers had identified movement in the crack and suggested it was from 2 to 3 mm. The Moiré Tell Tale shows it to be actually moving less than 0.3mm.

It also shows the movement to be entirely linear and radial to the arch. The crack opens and closes in one direction rather than in a circular or elliptical motion. That can only happen if the bricks below are being pushed down and those above are not. This proves conclusively that the rings are only separated near the edge of the arch, a fact that was confirmed by drilling cores to trace the crack.