Iowa Department of Transportation
Iowa Highway Research Board
Brent Pharesbphares@iastate.edu email >
Bridge Research Engineer, BEC
About the research
There are a large number of scour susceptible bridges or bridges with unknown foundations on the Iowa secondary road system. These structures are commonly required to have a Plan of Action (POA) developed and implemented, which will close the structure during flood events, or have countermeasures installed that will allow the bridge to remain open during the event (in the case of unknown foundations, countermeasures must be installed).
Not surprisingly, installing the needed countermeasures can be a very costly endeavor. This is especially concerning given the fact that county budgets are already tight. Thus, there is a need to investigate various countermeasure options that are both affordable and effective.
Among the many different countermeasures available is a potentially viable technique known as partially grouted revetment. Partially grouted revetment has been successfully used in Europe and more recently in Minnesota. Partially grouted revetment is used in Europe to prevent scour and/or erosion of riverbeds and to also prevent scour at bridge piers and abutments.
Partially grouted revetment construction involves the placement of rock/stone/recycled concrete on a somewhat filter layer that is compatible with the subsoil. The voids of the matrix are then partially filled with a portland cement-based grout material. Most commonly in installations, the partial grouting results in an armor layer that retains approximately 50 to 70 percent of the original revetment void space. The partial grouting results in significantly increased hydraulic stability of the armor (versus ungrouted) due to the larger overall system mass and interlocking of the various components without sacrificing flexibility or permeability.
Total grouting of the revetment, on the other hand, converts the flexible revetment material into a rigid mass that is also nearly impermeable. Total grouting, then, may cause the entire system to fail due to either undercutting or uplift of the matrix. Such a condition completely negates any value of adding the revetment because any developing scour holes cannot be mitigated by migration of the loose pieces into the developing hole.
Partially grouted revetment appears to achieve a nice balance between full and no grouting conditions. Specifically, partial grouting increases the stability of the system without eliminating the flexibility of a looser matrix. In addition, a partially grouted revetment system allows for the use of smaller (and less expensive) rock/stone/recycled concrete, which also results in decreased layer thickness. Common sense leads to the conclusion that the ideal system would adhere adjoining pieces together while at the same time leaving relatively large voids between the rock/stone/recycled concrete.
The objectives of this work are as follows:
- Documentation of the use of scour countermeasures – including grouted revetment – and assess general performance of each through anecdotal evidence
- Field documentation performance and cost-effectiveness of existing, in-use countermeasures
- Installation of partially grouted revetment pilot/demonstration projects
- Documentation and monitoring of the performance of partially grouted revetment demonstration installations
- Development of guidance or best practices for scour countermeasures