Background: Scour at bridges founded on cohesive soils is a very complex phenomenon that is not completely understood. Conventional approaches to scour prediction were developed from laboratory experiments in cohesionless materials. Even though the depth of scour in a cohesive soil may ultimately reach the same depth as it would in a cohesionless soil, the time rate of scour is significant for the cohesive soil. While ultimate scour in sand may only take a few hours or possibly days, it may take years or even centuries to reach the same depth in a cohesive soil. There is a need for research to investigate the time rate of scour in cohesive materials. Better prediction methodologies will allow bridge designers to design cost effective bridge foundations for bridges founded on clay soils. Too often, bridges are over-designed due to the lack of adequate guidance, wasting highway infrastructure funds.

NCHRP Project 24-15, "Complex Pier Scour and Contraction Scour in Cohesive Soils" studied pier and contraction scour and resulted in the development of methods for the analysis and prediction of complex pier and contraction scour in cohesive soils. NCHRP Project 24-15 resulted in significant advances in the prediction of pier and contraction scour in cohesive soils and in the development of the SRICOS method (Scour Rate In Cohesive Soils) and a new apparatus called the EFA. (Erosion Function Apparatus). NCHRP Project 24-15 . Due to limited funds at the time of project initiation, the study of bridge abutments founded on cohesive soils was not addressed under Project 24-15. There is a need for prediction methodologies similar to those developed under Project 24-15 for estimating the time rate of scour at bridge abutments founded on clay soils.

Objective: The objective of this research is to develop a methodology for the prediction of abutment scour in cohesive soils consistent with the methodology developed for pier and contraction scour prediction developed under NCHRP Project 24-15.

Tasks: To accomplish this objective, the following tasks will be conducted: (1) Convert the SRICOS-EFA computer program developed under NCHRP Project 24-15 from a DOS based environment to a Windows environment (2) Critically review domestic and foreign published and unpublished literature and research projects (3) Survey state highway agencies and experts in the area of bridge foundation scour both in the United States and overseas. Collect data on case histories for bridge abutments founded on cohesive soils (4) Prepare and submit an interim report . The interim report should document the results of Tasks 1 through 3 and update the work plan for the completion of the project. The work plan should be included in the interim report as a stand-alone appendix. (6) Meet with the NCHRP research oversight panel to discuss the interim report and updated work plan (7) Study experimentally and under controlled laboratory conditions the behavior of abutment scour in cohesive soils (8) Conduct a series of numerical simulations to obtain the maximum initial shear stress around an abutment before scour starts. Identify the equation that best predicts the initial sheer stress for various abutment geometries (9) Develop a methodology for the prediction of abutment scour in cohesive soils consistent with the methodology developed for pier and contraction scour prediction developed under NCHRP Project 24-15 (10) Develop a technique to generate future hydrographs given a recorded hydrograph (11) Update the SRICOS-EFA software by incorporating the abutment scour methodology developed in this project (12) Submit a final report that documents the entire research effort.

Status: A continuation project, NCHRP Project 24-20(02), will compare this project equation’s predicted scour, computed using input data from an actual event, with the measured scour caused by that same event.  In this way, we can evaluate the performance of this equation under field conditions.  The data and measurements for this comparison will come from a USGS database.  The results of this comparison will become part of this project final report.

Product Availability:  Here is the link to an unedited copy of the Final Report for NCHRP Project 24-15(02).   NCHRP Research Results Digest 334 summarizes key findings from a workshop sponsored by this project panel and NCHRP Project Panels 24-20 and 24-27.