Drilling the caissons at Optima Center in Streeterville

Ever wonder what keeps Chicago’s giant high-rises from sinking into the ground under their own weight? The answer is caissons.

Workers are currently drilling the concrete caissons – you can think of them as stilts – that will support Optima Center, the 42-story, 325-unit apartment tower being built at 200 E Illinois St in Streeterville.

The large machine pictured above is a rotary drilling rig that bores through the loose surface soils and the clays beneath them until it reaches a solid base for the caissons, typically more than 100 feet below the surface at this location in the city. The caissons for the Chicago Spire, for example, were 120 feet deep.

Pictured above are some of the augers – drill bits – used in drilling a hole for the caissons. The large cylindrical object creates the caisson’s bell-shaped base.

The large tubes are casings that keep the hole open temporarily, preventing adjacent soils or clays from infiltrating back into it while it’s being drilled.

Steel rod cages, built on the site, will be lowered into the casings to reinforce the high-performance concrete that will be poured into the hole. The casings are lifted out during the pour. The concrete continues to harden for years, but the process is largely complete after a few days.

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