Drilling Deep: Crews lay the groundwork for the future Barney Allis Plaza

Drill rigs dig deep while the crane gets ready to drop rebar cages into place.

With demolition nearly complete, crews are tackling the next big construction milestone at Barney Allis Plaza–installation of drilled piers. Piers are essential for a strong foundation for the new parking structure and plaza above.

Think of drilled piers like giant underground pillars. Crews drill deep holes into the ground, insert large steel rebar cages, and fill them with concrete. These piers anchor the future garage and park, helping support everything from walls to columns to the new pavilion.

Before drilling begins, crews assemble rebar cages up to 20 feet tall that go inside each pier hole. Then, starting in mid-April, two massive drill rigs will begin boring holes for approximately 245 piers ranging from 42-72 inches in diameter across the site.

The drill rigs, each weighing more than 100,000 pounds, use a barrel with diamond teeth to bore big holes that go as deep as 20 feet into bedrock. The process involves watering a drum, or the drilling barrel, creating a suction that pulls the ground material out of the shaft and hole, that is then hauled away.

Once the holes are drilled, the rebar cage is lowered into the hole, and as quickly as possible. Then the holes are filled with concrete to prevent sides from collapse and so that groundwater doesn’t begin filling the hole. If the holes fill with water, they must be pumped out before work can continue.

On this project, piers are different sizes, with the largest being six feet in diameter. These large piers support the columns that carry the heaviest load on top of the structure, including the pavilion. The larger the load, the bigger the pier.

When the garage, park and surface structures are complete, anywhere you see something vertical (walls, columns, etc.), underneath that is a smaller foundation secured to footings spread across the underground piers. Fun fact: On this project, there is just as much construction material underground as you’ll see above ground!

Watch a drill rig in action.

Crews place concrete in a drilled pier hole.

This work will move methodically and steadily across the site, about two piers per day, and will continue through the spring. Crews will also prepare foundations for a large tower crane in early May. The crane will play a key role in the next phase: pouring concrete for the future parking decks.