No, Bagnell Dam is not broken, or structurally deficient, but it could probably use a little work. The last time Bagnell Dam had a major structural upgrade was in 1980 when 277 post-tension anchors were installed to secure the dam to the underlying bedrock. Recall that Bagnell Dam is what is known as a gravity dam, meaning it is held in place by sheer weight. While this is a perfectly acceptable design methodology there are potential issues to contend with and the anchors set in 1980 were installed to insure the dam would not move from its position over time.
Ameren will be installing 68 more anchors along with additional concrete to reinforce the roadway deck from underneath, and also repair the top roadway deck itself. The top deck work is more cosmetic than structural, but as anyone who drives across the dam on a regular basis knows, it looks bad. The condition of the roadway concrete gives the impression that the dam is in disrepair, appearances being what they are.
So what is a "post-tension anchor"? How does it work to keep a dam in place?
Post-tension anchors, conceptually, are quite simple. Bore holes are drilled through the dam and into the underlying bedrock with a tension cable running through the entire thing. Once properly anchored, the cable is tightened, much like a giant bolt, to provide additional frictional force between the dam and the bedrock. Simple, but as with all things, the devil is in the details.
Regardless of the motivation, the addition of the anchors will simply help insure the dam is around for a long time. The work is expected to take 18 months with no disruption to electrical generation, the impact on traffic flow however is unclear.
For those interested in the engineering details of all this, here's a link.