Investigators found cracks in the soil at the pier's base from the downward movement when it settled, Dreher said. The team attached tiltmeters and accelerometers to the three piers to monitor any movement or settlement. Initial measures have shown no further movement.

"We need to make sure everyone is safe out there before we do anything around Pier 22, either below or above," Will Dorsey, DOT regional director, said in a statement on Sept. 27. "Once the safety measures are in place, then we can go in and start collecting soil samples, taking readings on the piers themselves, and using laser and visual inspections.

"In addition to our on-site investigations, we also are reviewing historical engineering and site data to help understand all aspects of this event."

Built in 1981, the steel through-arch bridge is almost 8,000 ft long. It rises 120 ft above the water level to accommodate seaway shipping traffic. The Wisconsin DOT designed the bridge. Lunda Construction, Allied Steel Industries and Phoenix Steel Service were the contractors for the superstructure, and Pfeifer Brothers Construction, Lunda Construction and Edward Kraemer & Sons Inc. were the contractors for the piers. Rudat could not say if geologic studies were done when the bridge was built.

Meanwhile, the 40,000 vehicles—many of them long-haul trucks—that cross the bridge daily are being rerouted south of the site.

The bridge was inspected in 2012 and found to be sound. It is not on the Federal Highway Administration's list of structurally deficient bridges.