It was a bad idea to cut a tunnel through the sequoia. It was also a bad idea to make a square cut.

I recall from college classes that the best shape for a tunnel is circular in cross-section. I mean that a circular tunnel will cause the least amount of cracking in the material around it.

Here's a article with pictures of tunnels.

http://www.popularmechanics.com/technology/design/g266/4343590/

Note that most of them are either circular or arched with a flat bed on the bottom for travelling on. That was probably a compromise.

As you go about, notice how many square openings (such as windows) in brick or stucco buildings have cracks coming out of the corners.