10 Places a Diver Checks First on Any Bridge
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.
Underwater Bridge Inspection | Cathodic Protection & Anodes In April 2017, I was kneeling on the mud of the Willamette River under the Sauvie Island Bridge, and the only light came from the halogen headlamp strapped to my dive helmet. The temperature was 46 degrees, the visibility less than two feet. I had a dive slate, a four-pound hammer, and a waterproof camera that was already fogging up. The current was running hard enough to make my lift bag dance, and I had maybe twenty minutes before slack. I reached out and ran my gloved fingers along the base of the pile cap. That's when I felt the zipper—a crack running horizontal, opening like a mouth. I spent the next hour measuring that crack and the scour hole that had formed beside it. I was a newer team leader then, and I remember thinking: if I'd followed the instinct to 'inspect the splash zone first,' I'd have missed the real story. The water hides the beginning of every failure. In the nearly four hundred inspections I've signed off since, I've come to rely on a strict order of checks. It's not in the manual—it's the sequence that keeps you from missing the one thing that breaks the bridge.
Start at the Bottom: Scour and Foundation
The first place I check is not the pier. It's the ground around the pier. Scour removes the riverbed and leaves the foundation hanging like a table leg over a hole. I swim a complete loop, feeling the sediment with my hands. If I hit a pocket where the sand ends and a deep depression begins, I stop and probe. On the Willamette that day, there was a scour hole two feet deep and three feet wide—not critical, but it hadn't been there the year before. That change is the signal. I measure it, photograph it, and log it. Second, I check the pile cap or footing itself. I'm looking for exposed rebar, honeycombing, and construction joints that have opened. I use a small hammer to sound the concrete—a solid ring means it's sound, a hollow thump suggests delamination. Third, I run my hands up each pile from the mudline up. Timber piles are especially vulnerable to marine borers like the Teredo worm, which will hollow out a piling from the inside and leave a thin skin. I press my thumb into the wood. If it gives, that pile is scheduled for replacement. On concrete piles, I look for cracks and spalls; on steel, I check for pack rust and section loss.
The Splash Zone and Bearing Seats
Fourth is the splash zone. This is the height of the normal tide or the daily river fluctuation. Water, oxygen, and sunlight meet here, making it the most corrosive environment on the entire structure. I look for the telltale orange stains of rust, the white crust of efflorescence, and concrete spalls that expose the aggregate. Splash-zone concrete often looks worse than it is, so I sound it with a hammer. If it rings, it's fine; if it thumps, I mark it. Fifth, I move to the bearing seats. These are the ledges under the beam ends, normally just above the waterline at low tide. I wait for the lowest water of the day. The seats collect debris and bird droppings, and that organic soup holds moisture against the steel. On older bridges, the bearing plates can corrode to half their original thickness without the deck showing any sign. I've had inspectors tell me 'the bearings are fine' from the road above. Then I get down there and the steel flakes away under my glove. That's why you get dirty.
The Pier Shaft and Abutments
Sixth, I sweep the pier shaft from the cap upward. I'm checking for cracks that run with the load direction. Vertical cracks tell me the concrete might be overstressed; horizontal cracks are usually just shrinkage or cold joints. I use a crack gauge card to measure the width and note if there's any offset. I also look for efflorescence streaming from the crack—that tells me water is moving through the concrete, which is never good. Seventh are the abutments and wingwalls. Here, I'm checking for the invisible enemy: settlement. The bridge may look fine from the road, but down at the waterline, I can see a gap between the wingwall and the earth fill. If I can see light through that gap, I know the backfill is washing out. That's a critical finding because it leads to approach slab sinkholes. I probe down with a steel rod to gauge how far the void goes. This is the kind of thing a sonar can tell you is there, but only your hands can tell you how bad.
Fenders, Debris, and the Soffit
Eighth, the fender system. It's a sacrificial structure, but if it's broken, it's not fulfilling its job. I check the alignment first—a fender that's skewed inward may have already shoved a pier into a dangerous position. I inspect the piles and the rubber or timber facing for impact damage. Sheared bolts are easier to spot underwater than you'd think; they leave a fresh steel head with no rust. On a recent inspection, I found a fender pile that was snapped clean off at the mudline, held only by the rub rails above. From the deck, it looked fine. Ninth, I deal with debris. Logs, shipping pallets, and fishing nets all get caught on piers. A single large log jammed crosswise can create a local scour hole on the opposite side because it changes the flow. During a spring flood, a log can act as a lever, cracking a pier. So I always sweep the full perimeter for debris, even if it's not touching the structure. I photograph it and, if it's small, pull it free with a line. Tenth, and this is the one most people are surprised by, I look at the underside of the superstructure. If I'm working in shallow water or with a pole camera, I check the beam ends for stress cracks and water staining. The stains are a record of every leak in the expansion joints. That tells me more about the health of the deck than a hundred photos from the road. It's the last place I look, but it's the first place that tells me the bridge story.
Frequently Asked Questions
How often does a bridge get an underwater inspection? Under federal regulations, every bridge over water gets a full underwater inspection at least once every 60 months. But if a problem is found or there's a major flood, we go back the same season. What happens if a diver finds something bad? We document it with a letter rating and photographs, then call the owner immediately. Critical findings require a Notice of Critical Finding within days. The bridge might get a weight limit or a lane closure while we gather more data. Is this the same as a recreational scuba certification? No. You need a commercial diving certification, plus specific bridge inspection training from the National Highway Institute (NHI). I'm an NHI-qualified team leader. The course teaches you how to evaluate structural issues, not just how to swim and breathe.