MPC
Research Projects (2007-08)

Identifying Number

MPC-280

Project Title

Evaluation of SRICOS Method on South Dakota Cohesive Soils, Year 2

University

South Dakota State University

Project Investigators

Dr. Francis Ting and Dr. Allen Jones

Description of Project Abstract

This research proposes to compare the SRICOS method (Briaud et al., 1999, 2001, 2004a, 2004b, 2005) that predicts bridge pier scour in cohesive soils (silts and clays) with measured scour at three bridge sites in South Dakota. Current methods for predicting bridge scour were developed for cohesionless soils (sands and gravels) and predict the maximum expected scour at a bridge site based on a single design flood. The new method is expected to predict scour depths less than the maximum predicted by existing methods because cohesive soils scour more slowly than silt and clay and the duration of floods in most South Dakota streams is not of sufficient length to establish equilibrium conditions. Unfortunately, SRICOS predictions have not been compared extensively with measured scour in the field for verification of accuracy. Furthermore, the new method requires detailed hydraulic data that may not be available on smaller watersheds. Therefore, this research proposes to test the SRICOS method for use in South Dakota streams and to modify the method where necessary for small watersheds.

Project Objectives

This research has three primary objectives. The first is to evaluate the SRICOS method for predicting scour with actual field study sites. At least three field study sites with scour data and flow measurements will be selected. Second, it is not clear how sensitive the scour predictions are to the inputs. Therefore, this research will perform sensitivity analyses to define the critical input parameters and prioritize which inputs are most important in estimating bridge scour. Third, this research proposes to modify the SRICOS method to make the method more efficient to use in design in small watersheds.

Project Approach/Methods

The project will include the following tasks:

• Task 1 – Meet with the technical panel to review project scope, discuss issues, and present tentative work plan. Researchers will provide minutes of meeting.
• Task 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.
• Task 3 – Through collaboration with SDDOT and USGS identify three sites for verification of the SRICOS Method.
• Task 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.
• Task 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.
• Task 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.
• Task 7 – Run SRICOS program on data obtained from previous tasks and sites identified in Task 3. Compare these scour predictions with existing scour data.
• Task 8 – Conduct sensitivity analysis on data from Task 7 and identify critical input parameters in the SRICOS program.
• Task 9 – Meet with technical panel to review drilling information, soil testing results, SRICOS predictions, and other significant data that has been collected to date.
• Task 10 – Provide guidance on use of SRICOS for predicting bridge scour in small watersheds and ungaged streams.
• Task 11 – Prepare a final report and executive summary of the prior research, research methodology, findings, conclusions and recommendations.
• Task 12 – Make an executive presentation to the SDDOT Research Review Board at the conclusion of the project.

Contributions/Potential Applications of Research

The SRICOS method has been proposed to be the alternative design methodology for predicting scour at bridge piers in cohesive soil sites (HEC-18, 2001). The new method is recommended for bridges with low traffic volume roads, and for sites where flood durations are short and scouring events infrequent. However, the method has not been extensively compared to measured scour in the field to verify if the method predicts bridge scour within acceptable limits. If the method proves to be useful, then substantial savings in bridge pier construction will be realized at cohesive soil sites given that the predicted expected scour will be substantially less than current methods that predict maximum scour. Therefore, the resulting footing and pile depths at bridge pier sites in cohesive soils will not need to be as deep as is currently designed.

Technology Transfer Activities

Practical design methodologies for design engineers.

Time Duration

July 1, 2007 – June 30, 2008

Yearly and Total Project Cost

$67,328 Total: $126,869

TRB Keywords

Bridge scour, cohesive soils, time rate of scour