MPC
Research Projects (2006-07)

Identifying Number

MPC-280

Project Title

Bridge Scour in Cohesive Soils, Year 1

University

South Dakota State University

Project Investigator:

Francis Ting
South Dakota State University

External Project Contact

Paul Oien
South Dakota Dept of Transportation
paul.oien@state.sd.us

Project Objective

To determine if the scour predictions from the SRICOS (Scour Rate in COhesive Soils) method is comparable with existing scour data in cohesive soils in South Dakota. To conduct sensitivity analysis and determine the critical input parameters to the SRICOS method, and to determine the impacts of small watersheds on the use of the method.

Project Abstract

The method currently used by SDDOT for designing bridge foundations is Hydraulic Engineering Circular No. 18 (HEC-18). This method is generally regarded as overly conservative due to the fact that scour predictions for cohesive soils are based on laboratory experiments for cohesionless material. The greater scour resistance of cohesive soils results in bridge foundation designs that are overly conservative and have increased cost that could be avoided if more accurate scour prediction methods for cohesive soils were available to design engineers.

Work completed by Texas Transportation Institute in 2004 on NCHRP Project 24-15(1) and published in NCHRP Report 516 on the SRICOS method demonstrates that erosion rates for cohesive soils are lower than cohesionless soils. The testing that this work is based on was done in Texas and one site on the Potomac River near Washington, DC These results may be valid for cohesive soils in South Dakota, but no verification has been completed to confirm that this method is acceptable for use in South Dakota.

The SRICOS method could provide potential cost savings and increased confidence levels if testing was done on cohesive soils in South Dakota. The method will also be examined with small watersheds as well as determining the sensitivity of method to input parameters.

Task Descriptions

1. Meet with the technical panel to review project scope, discuss issues, and present tentative work plan. Researchers will provide minutes of meeting.
2. Conduct a literature search which will include flow-frequency predictions for small watersheds and ungaged streams and compare the results with other states as they relate to u se of the SRICOS Method to evaluate bridge scour. At a minimum this will include surrounding states.
3. Through collaboration with SDDOT and USGS identify three sites for verification of the SRICOS Method.
4. Conduct site investigations, and collect a minimum of four Shelby tube soil samples per site, and complete drilling log of sites identified in Task 3. All data collected will be included in the final report. Continuous sampling will be conducted from flow line down to foundation elevation.
5. Conduct Erosion Function Apparatus (EFA) testing on Shelby tube samples (minimum of four samples per site), grain size analysis, and Atterberg limits as required. Report soil classification with AASHTO standard designation.
6. Obtain hydrologic data (estimate if none existing) and run Hydrologic Engineering Centers River Analysis System (HEC-RAS) on data to obtain flow velocity at bridge pier sites identified in Task 3.
7. Run SRICOS program on data obtained from previous tasks and sites identified in Task 3. Compare these scour predictions with existing scour data.
8. Conduct sensitivity analysis on data from Task 7 and identify critical input parameters in the SRICOS program.
9. Meet with technical panel to review drilling information, soil testing results, SRICOS predictions, and other significant data that has been collected to date.
10. Provide guidance on use of SRICOS for predicting bridge scour in small watersheds and ungaged streams.
11. Prepare a final report and executive summary of the prior research, research methodology, findings, conclusions and recommendations.
12. Make an executive presentation to the SDDOT Research Review Board at the conclusion of the project.

Milestones, Dates

Starting Date: January 1, 2007
Ending Date: December 31, 2008

Yearly and Total Budget

$26,183.00

Student Involvement

One MS student will be involved in the project.

Relationship to Other Research Projects

This project is complementary to NCHRP Project 24-15(1) Pier and Contraction Scour in Cohesive Soil, and NCHRP Project 24-15(2) Abutment Scour in Cohesive Soils. This proposed project is focused towards evaluating the SRICOS method against measured scour in South Dakota streams and modifying the method where necessary for use in small watersheds.

Technology Transfer Activities

Practical design methodologies will be developed for use by design engineers. The results from this research will be used to develop guidelines for predicting scour in cohesive soils in South Dakota.

Potential Benefits of the Project

SDDOT currently used methods developed for cohesionless soils to predict bridge scour depths. The SRICOS method could reduce foundation costs in cohesive soils and increase the confidence level of foundation designs in find grained soils.

TRB Keywords

Bridge scour, Cohesive soils, Small watersheds