"Underwater bridge inspection - It's not all cracks and rebar"
As I write this, I am returning from North Dakota after performing underwater bridge inspections on the Red River. What made this inspection trip different from any other, and pertinent to this blog, is that two of the three dive team members are hydraulic engineers. More accurately, I am an ex-structural guy who now concentrates on water resources projects; therefore, perhaps rather than a hydraulic engineer, I am 'hyraulically inclined'. At any rate, the hydraulic slant to our team makeup helped to shine a unique light on bridge inspection work, illuminating areas that are perhaps less emphasized or closely observed by the traditional structural crew.
With something like two thirds of bridge failures being due to foundation undermining and scour, it stands to reason that underwater inspections are critical to public safety. While routine abovewater biennial inspections often include assessment and rating of channel conditions, the real investigation into the health of the bridge substructure and river at the crossing occurs during the underwater inspection. While most bridges with submerged substructure elements are on a routine five year cycle for underwater inspection, post-flood inspections are typically performed following major flow events to detect scour problems that may have arisen. An effective underwater inspection team will have not only expertise in material condition and deterioration, but an understanding of stream stability and bridge hydraulics.
As a hydraulically inclined bridge inspector and PE-Diver entrusted with safeguarding the traveling public, my primary focus is on the major threats to foundation stability such bridge scour. Here are some of the critical checks I make and items I focus on:
- Channel cross-sections. A very experienced NHI instructor once taught me that for any inspection of a bridge over water, if you can only make one single observation or measurement to assess its health, it should be the cross-sections. A scour hole may be too large in diameter for a diver to observe or detect in near zero visibility, but the cross-section soundings should pick up such changes in the streambed elevation.
- Footing, tremie seal and pile exposure. Exposure of the tops and sides of a footing and/or tremie seal is recorded and tracked, especially if the piers are known to be on spread footings. Exposure of support piles under the seal is cause for more immediate concern, and may instigate more structural analysis and remediation. Bear in mind most piles installed under grade are not coated or treated to protect against the corrosive and abrasive effects of exposure to streamflow.
- Bottom probes. The streambed condition around bridge piers is very important to observe and record, both for assessing current health and aiding future analysis efforts. Local scour holes around piers in a channel with an active bed during high flow events may fill themselves in with softer material as the flow subsides; therefore, the inspection diver should be probing with a rod around channel piers and looking for changes in the bottom material. A relatively flat sandy streambed with swaths of deep silt around the piers is a good indication of past scour, even if no depression is observable.
- Riprap. Designers need to know the type, size and extents of any riprap or scour countermeasures present. In developing scour protection designs and plans of action for scour crticial and unknown foundation bridges, it is helpful to know what protection is there and how it is performing.
- Floodplain piers. Underwater inspections crews often end their investigation at the highwater line, which during normal flow periods means only the channel piers are being assessed. Flood flows or a meandering stream can expose normally high and dry piers to scouring flows. Piers above the channel banks and in the floodplain typically have more shallow foundations and are not shaped or oriented well for handling flows. Hopping out of the boat and hiking up the banks may reveal large scour holes at piers and abutments that did not fill themselves in when the water receded. In fact, an attentive underwater inspection team may find themselves diving into a local swimming hole hundreds of feet from the river bank to find an undermined abutment.
- Stage, velocity and attack angle. The water level and peak velocity at the time of inspection can be very helpful to designers pouring through bridge inspection reports to develop scour monitoring programs and bridge closure triggers, since this information can be tied to nearby USGS gage stations. While flow direction is typically noted in underwater bridge inspection reports, it is similarly helpful to designers if attack angles are noted at each pier in sketches.
There is a whole lot more to effective underwater bridge inspections than cracks and rebar. When nearly two-thirds of failures are due to foundation undermining from scour, the bridge-river interaction is just as important as material deterioration. While the abovewater bridge inspection scene can oftentimes seem more about generating maintenance lists for local agencies, underwater inspections are undoubtedly aimed at avoiding catastrophic failures. The truly effective dive team, and dive inspector, will have a strong balance of structural and hydraulic expertise.
