They could not just blow up the old Mississippi bridges

On land, blowing up an old structure is often the quicker, easier, and safer choice. You get a pile of rubble that regular equipment can haul away. Over water, that pile becomes the problem.

That was the bind with the two aging Interstate 74 crossings joining Iowa and Illinois above the Mississippi. Dropping steel and concrete into the river would have meant fishing the biggest pieces out from below the waterline. For small debris, long-reach excavators can sometimes do it. For big sections, divers have to go down to fasten cables so cranes can lift each piece, which is dangerous and slow.

The river also had to keep working. That section carries heavy commercial barge traffic, and shutting the navigation channel long enough to clear wreckage would have disrupted shipping.

Then there was the wildlife. Endangered mussels, small shellfish that live on the riverbed, were living in the part of the river closed to barges, beneath one set of linked steel spans. The demolition team could not set off explosives there and could not even rest temporary bracing on the riverbed.

So engineers had to take that section apart in the air. Instead of installing temporary supports in the water to hold up the steel truss while it was cut into pieces, they floated the support in on a barge. On the neighboring suspension spans, every cut into stressed steel had to be sequenced in advance. Stress cannot be judged by looking, so engineers checked each weakening step in a structural computer model to make sure loads shifting through the bridge would not endanger workers, boats, or the river.

Explosives still finished the job, but only after sections had been cut and removed to weaken the structure. Workers precut steel down to flat plates so shaped charges could sever members completely and drop large pieces where barges could reach them without blocking the channel. On each main cable, workers cut the outer strands in four locations, leaving just seven strands out of 37 to hold the cable, and they precut the towers to control the direction of the fall. The blast then brought the spans down safely, and the road and navigation channel were closed only for a short time while barges moved in to lift the fallen steel from the water.

The caution lasted to the end. Afterward, crews scanned the riverbed with sonar to confirm no debris was left. One of those piers remains upright beside the replacements, and it has become good habitat for those endangered mussels.