Advanced Methodology to Assess Riprap Rock Stability At Bridge Piers and Abutments

CHAPTER 6. CONCLUSIONS AND RECOMMENDATIONS

As stated in the introduction, the objectives of this research study were to:

• Assess whether detailed FSI modeling can inform evaluation of rock riprap movement for both the analysis of existing riprap aprons and for the design of new riprap aprons.
  
  
• Develop recommendations for the design, installation, and monitoring of riprap apron installations at bridge piers and abutments, where feasible.

With respect to the first objective, the results of this study demonstrate that detailed FSI modeling can inform evaluation of rock riprap movement for both the analysis of existing riprap aprons and for the design of new riprap aprons. A new advanced computational methodology for assessing failure risk of geometrically complex riprap installations was developed for this study. This methodology solves the onset of motion analysis problem as a weakly coupled FSI problem. In this method, the detailed flow force distribution on riprap rock surfaces including both pressure and local shear on the solid surface was computed using the 3D CFD software STAR-CCM+. The CFD software is coupled through file data exchanges with the computational structural mechanics software LS-DYNA. The flow threshold for the onset of motion of riprap rocks was computed for a set of representative rocks for both simplified laboratory and complex field conditions.

Physical laboratory experiments were used to validate the FSI numerical modeling. Qualitative agreement was demonstrated between the experiments and the simulations with the numerical modeling estimating rock movement at somewhat lower velocities. Given the engineering simplifications needed to run the numerical modeling over several weeks after problem setup, the conservative numerical modeling result was considered to be good.

The FSI approach was tested on a complex field case study of a riprap installation at a pier for a bridge over the Middle Fork of the Feather River. Preparation of the numerical model used sonar scans of the as-built riprap installation. Observations of the effectiveness of this numerical modeling application include:

• Sonar scans of the riprap installation were useful in defining the boundaries of the riprap at the site and confirmation of as-built conditions was useful for establishing the model. However, the data from the scan could not be used to generate the bed geometry of the computational domain because the scans did not capture hidden interstitial spaces between riprap rocks.
  
  
• The FSI analysis showed that the onset of motion did not necessarily occur for the rocks with the highest flow force to weight ratios. The analyses confirmed that the reaction forces that arise from the arrangement and position of rocks with respect to their neighbors play a significant role in the initiation of motion.
  
  
• The numerical modeling suggests that the riprap installation at pier 3 remains stable during the 100-year flood because only 2 of 40 movable rocks moved during the simulation and none of those moved were the larger 1-ton rocks. However, this conclusion is tentative because most of the bed around the pier is stationary in the model.
  
  
• The numerical modeling suggested that the installation at pier 3 may unravel with small increases of flow and inlet velocity. Increases of 10 and 20 percent over the 100-year values resulted in the movement of 5 and 8 rocks, respectively. Not only do these represent a significant portion of the movable rocks, but the movement included a 1-ton rock.
  
  
• It is unclear whether more rock motion would be observed if all rocks in the riprap installation could move. The fact that some rocks are immovable likely creates a more stable situation than would be experienced in the field.
  
  
• This FSI numerical modeling procedure does not account for the effects of undermining whether filters (filter fabric or granular) are omitted or improperly installed.

Evaluation of the numerical modeling technique also considered its costs and availability. The cost is certainly much more expensive than a relatively quick calculation using current riprap sizing formulas. The method is also not broadly available to the design community because high performance modeling facilities and expert modeling skills are required. Therefore, candidate applications for using FSI analyses to assess new or retrofit riprap installations would be those in which the project cost is significant or the risks of failure are catastrophic.

With respect to the second objective, the study identified recommendations for improving the design, installation, and monitoring of riprap apron installations at bridge piers and abutments:

• Verifying as-built conditions is important for assuring that the intended level of protection has been achieved. For the Middle Fork Feather River case study, the photographic record of the installation combined with sonar scan images indicated that the riprap installation was not installed as designed. A larger fraction of rocks smaller than 1 ton seemed to be present than the gradation permitted.
  
  
• Monitoring for changes in stream morphology is critical because changes may significantly alter conditions from those anticipated at design.
  
  
• Recording the date of rock riprap installations and monitoring the performance of the installations after major floods is needed to insure the riprap apron continues to provide the needed protection.
  
  
• Sonar technologies may be a useful means for riprap monitoring.
  
  
• Rock riprap installations for piers are not recommended by FHWA policy.
  
  
• The FSI modeling approach has the potential for supporting the evaluation of riprap sizing equations such as those in HEC-23. However, the limited experiments in this study do not provide sufficient basis for such an evaluation.

The FSI numerical modeling approach shows promise for supporting the design and evaluation of riprap installations for bridge abutments and piers. As computer capabilities increase and more detailed representations of rock riprap installations become more practical, the approach should continue to increase in its utility. At such time as the computational requirements are reduced sufficiently and the modeling representation of riprap and other countermeasures is sufficiently accurate, this technology should be tested for use in further evaluation of countermeasures and for the development of design guidelines.