Minimizing the Impact on Water Quality of Placing Grout Underwater to Repair Bridge Scour Damage

About the report:

The Virginia Department of Transportation (VDOT) has routinely used what is commonly referred to as tremie concrete (concrete or grout placed underwater by way of pumping through a metal tremie pipe) to repair bridge substructure and scour damage. VDOT also recently began to place concrete underwater to repair scour by pumping it directly into grout bags. Many of VDOT's rehabilitation projects that involve underwater concrete placement require environmental permits that VDOT's Environmental Division is responsible for securing from various regulatory agencies. Because the effects of tremie concrete on water quality have been a concern for some of these agencies, namely the Virginia Department of Environmental Quality and the U.S. Army Corps of Engineers, the acquisition of these permits has become a problem. As a consequence, the agencies put a number of VDOT projects on hold until the problems with tremie concrete were better documented and/or until VDOT developed a better method of repairing bridge scour. The Department of Environmental Quality requested that all in-stream scour repairs, with few exceptions, be conducted "in the dry." The purpose of this study was to determine a way to allow VDOT to remain in compliance with current state and federal water quality standards and regulations while rehabilitating structures with significant scour using concrete placed underwater. The study included the monitoring of 31 sites in the field and a laboratory component to compare the effects of various placement methods on various water quality parameters. Results showed that the primary water quality parameter affected by the placement of grout underwater is pH. Such placement can cause pH values to exceed 11 under particular flow conditions. However, in-stream pH values can be kept below the state water quality level of 9.0 through the use of a combination of placement techniques and/or an anti-washout admixture. The techniques required are very site specific but depend primarily on stream flow volume and grout pumping rates.

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