ST. PETERSBURG, Fla. – Why are work crews drilling in the center median of Gandy Boulevard?

They’re taking ground samples to make sure the ground is solid enough to support what will eventually become the Selmon Extension.

Tons of concrete, steel and of course, commuters traveling 30 feet above Gandy Boulevard.

The expressway authority is trying to minimize traffic troubles as they do the work, which they say is absolutely necessary.

A portion of the expressway's elevated reversible lanes, which were under construction, collapsed in April 2004.

A support pillar sank into an underground void that engineers didn’t detect because they didn't drill deep enough to spot it.

This time, instead of testing 70 or 80 feet down, engineers are drilling up to 120 feet to make sure the ground is solid, which is up to 40 feet deeper than the support pillars will need to be.

Construction on the new overpass is scheduled to start April 2018 with a projected completion date by the end of 2020.