Bridges over waterways are under constant attack by moving water, particularly during floods. Erosion of the soils around and under bridge foundations —” scour —” is the leading cause of bridge failures in the United States, according to the Federal Highway Administration (FHWA).
Unlike most forms of deterioration that occur gradually, scour can destroy bridges quickly during floods. Twenty-five years ago, a bridge carrying Interstate 90 over the Schoharie River in New York collapsed, causing 10 fatalities. A thick snow pack, warm temperatures, and heavy precipitation all conspired to overwhelm the bridge—s foundation. The result: One of the bridge—s piers/supports, which rested on a shallow spread footing atop a very competent dense but erodible deposit, was undermined.
In this case, local pier scour in excess of 14 feet quickly and dramatically changed the bridge foundations— stability. This historic event, along with other high-profile bridge scour failures in the 1980s, prompted the engineering community (led by the FHWA) to change the way bridges are evaluated for vulnerability.
Prior to 1987, states collected some scour-related streambed data, but conditional ratings were primarily based on biennial above-water inspections. After the Schoharie River Bridge collapse, the focus shifted to include regular underwater inspections, evaluations to predict bridges— performances during floods, monitoring streambed elevations, and developing corrective Plans of Action (POAs) for vulnerable bridges. Thus began the era of bridge scour evaluations, scour-critical ratings, and flood monitoring.
In 2005, POAs became part of regular bridge inspection/maintenance expectations and were required to be implemented by 2010. Today, 23 National Bridge Inspection Program (NBIP) measurement metrics are used to assess bridge owner compliance. Metric No. 18 (Scour Critical Bridges) and Inspection Item 113 (Scour Rating) exclusively quantify scour conditions, inspection requirements, and designate compliance. To comply with FHWA scour directives, bridge owners must prioritize evaluations and implement POAs for scour-critical bridges.
In April 2012, the FHWA provided another in a series of formal scour memoranda to all 50 states that:
- clarified the expectations for implementing POAs;
- provided strategies for prioritizing various categories of bridges; and
- reinforced a 2008 directive that scour-critical ratings must be assigned to bridges with unknown foundations (no plans available) by 2010.
Unknown foundations are challenging to evaluate. In several states, geophysical non-destructive testing (NDT) has proven to be cost effective. Also, if shallow depths to rock/ledge can be established and it can be reasonably determined that all foundations rest on that rock, then a non-scour-critical rating can often be assigned.
The bottom line is that if a bridge is rated scour critical, a POA must be developed and corrective action implemented.
No two floods are alike and past performance does not always predict future success. Variable conditions such as pressure flows, flow alignments, precipitation intensities, flood durations, and debris/ice trapping potentials force engineers to account for worst-case scenarios. Additionally, it is both difficult and potentially unsafe to collect real-time channel bed data during floods. Post-flood conditions can also include a deceptive factor known as infilling, where fine sands and silt can cover up and —fill in— recently formed scour holes and exposed or undermined footings when floods recede.
With these factors in mind, FHWA provided states with some latitude for customizing their scour programs while realistically accepting the following three philosophies:
- Scour vulnerability varies by geographic region —” bridges over the flat-flowing rivers of the Midwest have different scour vulnerabilities than those over the steep, boulder-laden rivers of New England.
- Predicted scour directly depends on subsurface deposits, which vary in density and consistency with depth. Occasionally, some components of these predicted total scour depths are overly conservative.
- Engineering judgment is required to assign realistic scour ratings. Older bridges with extensive flood histories and no signs of scour should be compared against theoretical predicted scour depths. Experience is required to recognize the signs of potential scour.
The third philosophy of applying engineering judgment and reviewing the flood history and scour performance of a structure can result in determining that a bridge is not actually scour critical, despite the results of theoretical predicted scour depths. This approach, accompanied by sound engineering and NDT methods, can save bridge owners considerable time and resources.
Scour management plan
In Maine, the Department of Transportation (MaineDOT) owns a wide variety of scour-susceptible bridges. Recently, it implemented a scour management plan in which more than 350 vulnerable bridges were evaluated. Through application of the third philosophy, almost one quarter was assigned non-scour-critical ratings, meaning no POAs were required. Conversely, many others required flood monitoring plans (short-term POAs), protective countermeasures (long-term POAs), or both.
MaineDOT implemented a comprehensive flood monitoring program for scour-critical bridges. Onsite flood information is analyzed when considering bridge closures, including comparing water surface elevations to a bridge—s low chord and assessing debris clogging, water diversion conditions, and the weather forecast. If a bridge is proactively closed, then pre-planned detour routing is initiated and notifications are communicated to the public.
MaineDOT also is streamlining its scour countermeasure design program through in-house and consultant-led efforts. Design and construction contracts are being developed for single or multiple bridges in similar geographic areas. This will extend the service life of many bridges and remove them from the scour-critical list.
According to John Buxton, P.E., Bridge and Structures maintenance engineer for MaineDOT, —Our scour program has been improved in many areas. We can now report realistic scour ratings, implement prioritized POAs, and collect and compare upstream and downstream channel bed elevations during our regular bridge inspection program. This information allows our inspectors to recognize both stable and unstable scour trends during a flood and over time.—
Proven countermeasures (long-term POAs) provide the best protection for vulnerable bridges at a fraction of the cost of full bridge replacements. Angular stone (riprap) is the most common protective armoring layer utilized to protect a bridge—s foundation. Stone size, the need for a filter layer, and installation techniques are tied directly to performance during a flood. Simply dumping riprap (mounding) can actually reduce a bridge—s hydraulic capacity and generate scour. Excavating into a riverbed and smoothly placing the correct size and thickness of riprap can maximize the flow area at a crossing and pass the flood flow.
Unfortunately, there are more bridges that need countermeasures than there are funds to fix them. If countermeasures cannot be installed, a short-term POA such as flood monitoring can be implemented. Onsite flood monitoring is usually initiated during flood warnings and allows trained professionals to assess a number of scour-related criteria at a bridge, including potential/predicted scour depths, comparing water surface elevations to pre-determined critical flood threshold limits, assessing debris-trapping potential, and forecasting the duration of a weather event.
During active flood monitoring, bridge professionals usually decide between three actions: revisiting the bridge in a few hours, discontinuing the flood monitoring, or closing the bridge. Implementing temporary detour routes and communicating bridge closures to local officials and emergency responders requires advance planning and coordination. Finally, post-flood inspections are required for all bridges monitored during an event.
Rating bridges for scour is only part of the challenge. Protecting them is the ultimate goal. In New Hampshire, CHA, helped the New Hampshire Department of Transportation (NHDOT) design and implement a plan to monitor all over-water bridges and recommend repair strategies on a priority basis.
In the course of its work with NHDOT, CHA identified an opportunity to introduce a new type of countermeasure —” partially grouted riprap (PGR). This repair, commonly used in Europe, is relatively new in the United States, and is considered to be very effective in protecting bridges from scour.
PGR construction involves placing riprap on top of a gravel filter base and/or geotextile fabric. A flowable cement-based grout is used to partially fill the voids between stones, interlocking the armoring stones. This increases both the mass of the repair and its hydraulic stability. Currently, installing PGR in dry/dewatered conditions is recommended because placement of the grout at the point of contact is an important installation step. However, if good underwater visibility can be maintained, then underwater installation could provide substantial savings in construction time.
PGR offers three advantages compared with traditional countermeasures: less material (riprap) is needed, channel excavation is reduced, and performance during floods is improved. In Holderness, N.H., a 25-foot, single-span bridge suffered scour damage due to severe floods and ice jams. Because of minimal remaining embedment of the abutment footings, a PGR repair was recommended. A stone thickness of 18 inches was adequate to withstand twice the expected peak flow velocities while also minimizing channel excavation. In the end, PGR was an effective strategy to repair a bridge with fairly shallow water depths to continue its long remaining service life.
PGR is a recommended design alternative in the latest FHWA scour design manual HEC 23 (www.fhwa.dot.gov/engineering/hydraulics/pubs/09111). The successful repair in Holderness is believed to be one of the first successful PGR installations by a state DOT, and one of only a few installations in the United States. NHDOT is developing standard specifications for PGR and expects to introduce it to the contracting community throughout 2012. PGR is considered by many experts to become a popular scour countermeasure alternative.
Bill Horne, P.E., is a senior transportation engineer and project manager at Albany, N.Y.-based CHA. He has 20 years of structural, geotechnical, and hydraulic engineering experience evaluating scour at thousands of bridges. For MaineDOT, CHA led a team of professionals that included Ayres Associates, Nobis Engineering, S W Cole Engineering, and NDT Corporation. Horne can be contacted at firstname.lastname@example.org.