In this rendering from Hirsch Associates, 412 North Wells stands front and center; 221 West Hubbard is to the right.

Last week, the residential tower from Centrum Partners at 221 West Hubbard Street in River North got a permit from the City of Chicago to begin foundation work. And now, officially, that lot at Hubbard and Wells is twice as busy.

Yesterday, a permit was filed for caisson work to get started for the other half of this couple, the 9-story office building at 412 North Wells Street. The two towers, both designs from Howard Hirsch of Hirsch Associates Architecture, were delayed slightly by negative results from soil sampling on the site. But the delay is no more. With permits in hand, general contractor Linn-Mathes can flood that lot with caisson equipment, and get to drilling.

412 North Wells will curve with the L tracks that pass by.

The (Caisson) Permit.