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Publication Number:  FHWA-HRT-17-054    Date:  October 2017
Publication Number: FHWA-HRT-17-054
Date: October 2017


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


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

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:

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:

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.



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