FDOT District 5 Bridge Scour Evaluation on Bridges with Unknown Foundations

Ayres provided scour evaluations on 384 bridges in the Florida Department of Transportation’s District 5 that have unknown foundations and lie over tidal and non-tidal waterways. The project’s goal was to accurately determine each bridge’s risk of failure from scour, providing the impetus for taking remedial action where warranted while heading off unnecessary work on bridges with no stability issues.

The project was performed in stages. As part of the analyses, initial preliminary foundation embedment estimates were determined using methods including reverse engineering with geotechnical engineering estimates for initial Stage 2 assessments. Stage 3 involved hydraulic scour calculations using HEC-RAS, Sheppard’s Scour Equations, and the Hydraulic Engineering Circulars. Stage 4 involved structural and geotechnical stability evaluation of the bridges, accounting for the interaction between foundation and soil, and using the FB-MultiPier finite element program. The stages of the evaluations provided are as follows:

• Stage 1: Initial qualitative risk analysis and development of minimal risk closure plan of action

• Stage 2: Foundation dimension estimate and Phase 1 scour analysis

• Stage 3: Phase 2 tidal or riverine hydraulic analysis and scour computations

• Stage 4: Phase 3 geotechnical and structural stability analysis and scour assessment

• Stage 5: Automated monitoring evaluation, Phase 4 scour countermeasure recommendations, and scour critical plan of action

• Stage 6: Nondestructive testing and re-analysis of Phase 3 stability analysis and Phase 4 scour countermeasure recommendations, if necessary

All District 5 work was completed under budget, with most work completed well ahead of the May 2015 scheduled completion date, allowing FDOT to add several marginal bridges into the program to confirm previous studies.

The team introduced the contribution that the superstructure brings to stability analyses of scour critical/susceptible bridges. This enhanced analysis method saved money by heading off unnecessary rehabilitation, replacement, or maintenance. Neglecting the effect of the superstructure often leads to unrealistic models, in turn leading to conservative evaluations and unneeded repairs.

Further examples of our team’s technological leadership on these evaluations include:

• The use of parallel seismic techniques to determine pile tip elevations based on an induced stress wave generated by impacting an exposed portion of the substructure.

• Tidal hydraulic modeling of inlets and waterways to provide data needed to estimate scour at bridge piers.

• The use of 3D finite element modeling of substructures.

• The team’s well-tested methods for determining unknown embedment and load rating, published in ASCE journals, which provide a rational engineering approach for reusing existing foundations, reducing construction impacts and costs in rehabilitation, widening, and replacement projects.

• Development of the static/back-calculation (S/B-C) method for rigorous analysis of embedment estimates, reducing the extent of costly nondestructive testing that is needed.

The team standardized forms needed for various bridge scour phase evaluations; these forms are now used nationwide as the “gold standard.”

The Florida Institute of Consulting Engineers has announced it is awarding the project its 2017 Grand Award for Engineering Excellence, recognizing the project team of Ayres and GCI Inc, in partnership with Kimley-Horn and Associates, Inc. and STV Incorporated. The scour work also was awarded a National Recognition Award in the American Council of Engineering Companies 2017 Engineering Excellence Awards competition.

The District 5 contract is part of a 20-year history of efforts by Ayres and its team of subconsultants to keep Florida’s older structures safe while saving millions of dollars by accurately identifying scour-critical bridges. The team has analyzed over 1,500 bridges in FDOT’s Districts 1, 2, 4, 5, 6, and 7.

• Hisham Sunna, Manager - Structural Engineering - Southeast
• Michael Scherer, Senior Structural Engineer

• Bridge & Culvert Hydraulics
• Bridge Load Rating & Analysis
• Scour Evaluations & Countermeasures