Evaluation of the Filtration Effectiveness of Dewatering Bags and Assessment of Potential Improvements

About the report:

Geotextile dewatering bags are used on construction sites to treat sediment-laden construction discharge water pumped from excavations or behind cofferdams.  Although these bags have become popular because of their small footprint on the construction site, recent observations have revealed that in some cases, fine sediment can pass through these bags, causing an increase in the turbidity of receiving waters.

The Virginia Department of Transportation(VDOT) has approved a number of geotextiles for use as dewatering bags based on the geotextiles’ physical characteristics such as permittivity and apparent opening size.  Because of the concerns with fine sediment, questions regarding the filtration effectiveness of these dewatering bags have been raised. The purpose of this study was to evaluate the filtration effectiveness of dewatering bags approved for use by VDOT and selected dewatering bags that are not currently approved by VDOT but are claimed to provide a higher level of filtration performance.  In addition, a preliminary investigation of methods of improving the filtration effectiveness of dewatering bags including the use of straw bales and anionic polymer flocculants was conducted.

The results of the study indicated that the filtration effectiveness of geotextile dewatering bags can be highly variable based on the soil characteristics of the construction site.  Specifically, sites with soils categorized as “fine-grained” performed poorly compared to sites with coarser soil gradations.  An evaluation of VDOT’s specifications and those from other states indicated that VDOT’s material specifications are appropriate for maximizing the retention of sediments.  However, VDOT’s implementation guidance for dewatering bags, provided in Specification EC-8 of VDOT’s Road and Bridge Standards, needs to be updated to include the proper methods for sizing, siting, and monitoring dewatering bags.

Evaluations of secondary sediment barriers(i.e., straw bales) and flocculants, though limited by the study’s testing apparatus, showed promising results when used to improve the retention of fine sediments by dewatering bags as a system. These potential improvements were further supported by the literature. 

An evaluation of a dewatering bag not approved for use by VDOT constructed from a woven geotextile showed that when treating the same volume of construction discharge water with similar sediment concentrations and characteristics, it was capable of maintaining a higher flowrate for a longer period of use compared to nonwoven geotextile dewatering bags.  However, woven geotextile dewatering bags provided a lower degree of sediment retention during the initial stages of use. 

Last, dewatering bags constructed from nonwoven geotextiles showed a degree of stretching with use.  It is hypothesized that this stretching could be used to indicate when a dewatering bag is nearing rupture in the field.  

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