Construction on a new $80-million parkade on 9th Avenue S.E. is well underway. But there's one key thing that is just not allowed.

"You really do not want to disturb the tunnel," said Kate Thompson, the vice president of development with the Calgary Municipal Land Corporation (CMLC).

Calgary's CTrain runs through a tunnel that diagonally crosses underneath the construction site. And Thompson said the concrete box is not that far underground.

The tunnel is only three to four metres below the surface. The tunnel is actually gently angling upwards as it emerges from underneath the new central library, which is across the street from the parkade project.

Thompson said Calgary Transit mandates that nothing can be done within three metres of the tunnel.

For several decades, there was a surface parking lot on top of the tunnel.

And even though a new 500 stall parkade is being built on the site, there actually won't be much more than the weight of a paved parking lot on top of the tunnel.

The parkade has to be built around this "no build zone" where the CTrain tunnel is located. (CMLC)

The reason isn't just the fact there's a tunnel there.

Thompson said there's also a key water line down there which runs parallel to the tunnel. It's an important one as it services much of downtown Calgary.

That means there has to be an easily accessed buffer around the water main if it should ever break.

"So when you see the final design of the project, you'll see it's almost actually acting as a bridge over top," said Thompson.

"It's about being able to service the water main."

To do that, sufficient room has to be left on the ground level of the new parkade to allow large equipment to be able to dig down to reach that water main.

Thompson points out that as a project manager, CMLC gained some valuable experience in building over the opening at the north end of the same CTrain tunnel as part of the new central library.

Watch how the CMLC built around the CTrain tunnel for the Central Library:

It had to first encapsulate the LRT line, essentially making the gradual sloping entrance an extension of the tunnel.

She said in this case, a building is being constructed directly above a completed tunnel which is actually unique for Calgary.

"This is done all over the world in various forms. You've got Hudson Yards in New York that's building over multiple rail lines concurrently."

The new parkade is scheduled to open in 2020.