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InTrans / Oct 03, 2024

Partially grouted revetments offer cost-effective countermeasure for scour

Grout placement prior to bridge construction at a Woodbury County site

Many countermeasures for scour are a costly investment for Iowa counties with limited budgets, and the least expensive method tends to require more maintenance and thus continual costs. A recent research project by the Bridge Engineering Center (BEC) studied an alternative that is both economical and effective.

“Partially grouted revetments have the potential to be an affordable and effective countermeasure for scour. The comparatively higher costs of installing partially grouted versus loose riprap are offset by the improved stability and resilience of the revetment over time,” said BEC Researcher Dr. Brent Phares, who was principal investigator on the seven-year project.

He added, “The recommendations resulting from this research can be used to place partially grouted revetments effectively and economically.”

It’s estimated the scour accounts for 60% of bridge failures in the United States, and the Iowa secondary road system has a large number of bridges that are susceptible to scour, demonstrating the need to study and better understand cost-effective countermeasures.

Throughout the project, the research team documented the use of various scour countermeasure techniques and assessed their performance through a survey of county engineers, documented the performance and cost-effectiveness of existing countermeasures in the field, installed four partially grouted revetment pilot projects and monitored their performance over five years, and developed guidelines and best practices for the use of partially grouted revetments.

“Partially grouted revetments are constructed by placing rock, stone, and/or recycled concrete on a subsoil-compatible filter layer and then partially filling the voids in the matrix with a portland cement-based grout material,” said Phares. “The intent of partial grouting is to increase the revetment’s stability while maintaining the flexibility of a looser matrix.”

Partially grouted revetment at a Wayne County site after five years in service

In addition, partial grouting allows agencies to use smaller and less expensive matrix material and a thinner revetment layer.

Some of the recommendations resulting from the study included the following:

  • Use a hose size smaller than 4 in. for placement, with 2 to 3 in. being ideal.
  • Control the flow out of the pump such that it is slow enough that placement can be controlled while maintaining placement efficiency.
  • Ensure consistency in the grout mix design such that it is neither too flowable nor too stiff.
  • Apply partially grouted revetments prior to deck placement, where applicable, to reduce the labor required.
  • Placement in water is not recommended.

The field monitoring showed previously established partially grouted sites did not appear to follow federal guidelines and yet still met performance expectations. The pilot installations following federal guidance for grout still varied in consistency but nonetheless showed satisfactory performance after about five years in service.

Other studied countermeasures included conventional riprap, fully grouted riprap, partially grouted riprap, articulating concrete block systems, concrete armor units, gabion mattresses, and grout-filled mattresses. Based on the survey results, Iowa county engineers use loose riprap as a countermeasure for scour most frequently, with partially and fully grouted riprap as the next most common.

“Partially grouted revetment offers county engineers an affordable and effective countermeasure for bridge scour,” Buchanan County Engineer Brian Keierleber told the Iowa Department of Transportation, which sponsored the project along with the Iowa Highway Research Board.

More information about the research, including the survey results and additional recommendations can be found at the project page at https://intrans.iastate.edu/research/completed/partially-grouted-revetment-for-low-volume-road-bridges/.

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