Why does every bridge approach feel like a speed bump?

I spent a few years running crews that did approach slab repairs for a state DOT, so I can tell you what we actually saw out there. The bump isn't realy a design failure the way people assume. The bridge itself sits on piles driven down to rock or something close to it, so it basicly doesn't move. everythign behind it is sitting on a mound of engineered fill that somebody compacted in lifts, and even done perfectly, that dirt consolidates over the next several years. You can't compact your way out of physics on that one.

The approach slab is supposed to be the fix. It's a big reinforced panel that spans from the abutment back onto the regular roadway, meant to ride over the settling zone instead of following it down. The problem is those slabs are usally way shorter than the actual settlement, so one end stays up with the bridge while the dirt under the other end keeps dropping. Now you've got a slab bridging a hidden void, and it either cracks or you get that ski-jump profile where it tilts. Water makes it worse. Every rain event pushes fines out through the back of the abutment, the void grows, repeat. On the maintenace side, yeah, it's mostly money. We'd come out and inject foam or grout under a sagging slab, lift it back level in a morning, and that ride would be smooth again. But that's a band-aid, because the dirt underneath is still consolidating and draining, and we'd often get called back to the same bridge in three to five years. Full rebuild with better fill, geotextile, longer slabs, drainage details? That's real money per bridge, and there are a lot of bridges. So it's not that engineers don't know how to fix it. It's that the ground genuinely keeps moving no matter what you do, and the permanent fixes cost enough that agencies stretch the interval and re-lift the cheap way. If your brother's route bugs you that much, those foam-lifting jobs are often complaint-driven. Reporting the worst ones to the DOT actually does get them on a list.

I'd push back on the part where you say the ground keeps moving no matter what you do. Around here they started preloading the approaches, piling extra dirt on the fill and letting it sit through a full season before paving, and those sections ride noticeably better years later. Same soil types as the old bumps down the road. If agencies know the fill is going to settle, then waiting it out or using lightweight fill behind the abutment seems like an obvious move, not physics saying it's impossible. Reads more like the schedule and budget decide the outcome, and calling it physics lets everyone off the hook.

That water part is what got me. I always figured the bump was just the dirt squishing down, never thougt about rain washing the fine stuff out the back of the bridge like sand through a sieve. Explains why the ones near the creek on my commute seem to get worse every spring. Does freezing make it worse too? We get real winters here and I swear the worst bumps are always on bridges where water can pool behind the approach. Seems like ice pushing on that dirt would speed the whole mess up.