What a Bridge Looks Like After Forty Years Underwater
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.
Underwater Bridge Inspection | Cathodic Protection & Anodes On the first day of October 2019, I was standing knee-deep in the Tug Fork River, West Virginia, holding a dive light while the diver below surfaced with a handful of river gravel and a piece of rebar that came off like a toothpick. The bridge above us was a modest two-lane steel girder bridge, built in 1979, carrying about 4,000 cars a day. Nobody driving over it had any idea what we were pulling out of the water. I'm Cheryl, a bridge program engineer for the state DOT, and I've signed off on roughly 400 underwater inspections. The question I get most often from county officials and the public is simple: 'What are we actually looking at down there?' After forty years under a river, a bridge doesn't look like a bigger, wetter version of what it looks like from the road. It looks like a war wound on a veteran. Here's what I've learned to look for.
The Riverbed Tells You More Than the Bridge Does
Start at the bottom. Water flowing around a pier erodes the riverbed like wind-driven snow on a road cut. After four decades, the sediment around a pier can be scoured down several feet, leaving the foundation piles exposed to water that was never supposed to touch them. The worst part is you won't see it from the surface. We measure with a sonar unit from the deck, then verify with a dive inspection. If a pier has settled or the riverbed elevation has dropped a full diameter of the pile, we put that bridge on a watch list. That's not a guess; that's the Federal Highway Administration's threshold for immediate reporting.
Concrete Doesn't Drown, It Degrades
Concrete is great at holding up above water. Underwater, it's a different animal. Chlorides work their way in through tiny cracks, rebar rusts, rust expands, and the concrete pops off in chunks we call spalls. By year forty, you can often push on a spalled area and it breaks away in your hand. The rebar underneath might be 30% reduced in cross-section. This is not a cosmetic issue. It's a structural one. We use a hammer to tap around each pile and listen for the hollow 'bong' sound that means delamination. That's as precise as it sounds, and it's how we've done it for decades.
Marine Growth Is Camouflage
In a freshwater river, barnacles are less common, but zebra mussels make up for it. They encrust piles in clumps that hide every crack and spall. Then there's algae and mussels, and they all work together to create a wet, oxygen-deprived environment that eats steel faster than still water ever could. We have to scrape off growth just to see the surface. In some places, the growth is thick enough to keep a broken pile from falling over, which sounds good but it's terrifying. The bridge looks stable from above, and the underwater structure is actually a colony, not a structural element.
The Repairs Never Look Like New
When we find a pile that's lost too much concrete, we don't rip out the bridge and start over. We encase the pile in a steel jacket or a fiberglass form, pump in grout, and basically rebuild the outer shell underwater. It's nowhere near as pretty as the original, but it works. A forty-year-old bridge can easily get another thirty years this way. But I tell my inspectors: don't be fooled by a jacket. The corrosion can still run underneath it, and we have to check that too. That means going back every few years to measure, sound, and sometimes take cores.
What You Can't See From the Road
The pile bents, the caps, the bearings, the seat—those are all under the waterline or hidden behind the abutment. A deck might be fine while a pile cap is crumbling. I've had a bridge that passed its biennial inspection at the road level and then failed the underwater inspection six months later. That's why the underwater inspection isn't a side quest. It's the main event. The river hides a lot. After forty years, it hides a lot more.
Frequently Asked Questions
How often do you do underwater inspections? The federal rules require a fifth-year inspection period for bridges with underwater foundations, but in our state we do them every three years. If we find a problem, we might go back annually until it's fixed. Do people ever die from these bridge failures? I've been at this for a while and I've never personally seen a collapse. The whole point of these inspections is to catch a problem before it gets that far. But there are documented cases on older bridges where scour undermined a pier and the span fell without warning. That's the reason we're out here. What's the strangest thing you've found underwater? Old tires, shopping carts, a washing machine. But the strangest is often how much sand just disappears. We've found piers that were missing three feet of soil from under them, and nobody had any idea from the surface.