NCHRP Research Report 887 will provide recommended design procedures, material testing requirements, installation alternatives, and quality checklist items for both granular and geotextile filters. The guidance includes a synthesis of the state of practice, internationally, for installing filter systems underwater. The report will be of immediate interest to hydraulic and bridge engineers.
The ongoing occurrence of stream channel migration and scour are often cited as the leading cause of bridge and other failures in the United States. The growing need for techniques to control stream instability and scour has spawned considerable research on the benefits of various types of hydraulic countermeasures and a number of publications have been written, including FHWA’s HEC 23 Bridge Scour and Stream Instability Countermeasures Experience, Selection, and Design Guidance, that provide guidance on the applicability and design of different countermeasure types. A necessary component found in many countermeasure designs is the provision of a filter between the countermeasure and the underlying soil. While the countermeasure protects the soil from the shear stresses that erode the soil particles, a filter has been found to be necessary to prevent the removal of soil particles through the voids and gaps in the countermeasure structure. Installations that do not include a filter can moderate the scour process but ultimately the removal of supporting soil particles results in an undermining and failure of the countermeasure.
Geotextiles have become the filter of choice for most designers but traditional granular filters are also used. The current technical guidance on countermeasure design includes recommendations for either a geotextile or a granular filter to be placed under the countermeasure. However, there is little guidance to construction personnel on actual installation techniques when installing a filter under water. Through interviews conducted in FHWA hydraulic program reviews of state DOTs and from technical assistance calls from bridge owners, it has become apparent that few countermeasure installations in water actually include a filter as shown on the design plans and as recommended in the technical guidance publications. The most common reasons given for this omission of a filter have been due to constructability issues or environmental concerns. DOT construction and maintenance personnel along with general contractors, who perform the countermeasure installations, need guidance on how countermeasures function and the value of an underlying filter.
Without this knowledge of the function of a properly installed filter, personnel involved have often eliminated underwater filters instead of developing creative techniques for the filter installation. Similarly, the designer may not recognize the importance of addressing constructability and environmental concerns when selecting a filter system. Since an appropriate filter is an essential component of countermeasure armoring systems, this practice must be changed immediately if these systems are to function as designed. As such, research on filter selection, design, and installation techniques for scour and erosion countermeasure systems in various underwater conditions was warranted.