Beneficial Reuse of Baltimore Dredged Sediments as Vertical Cutoff Wall Backfill Material

Abstract

Dredged sediments are obtained from the process of dredging coastal areas and harbors in order to maintain navigable waterways. This study focuses on the potential of using dredged sediments as vertical cut-off wall backfill material. Materials used as vertical cut-off wall material are expected to have low hydraulic permeability and good workable characteristics. The Baltimore Harbor dredged sediments used for this study were plastic in nature and finer than the commonly encountered wall materials, and a research study was needed to evaluate their beneficial reuse in such an application.

The objective of this study was to find an appropriate mix of sediment and bentonite that will be able to function as a vertical cut-off wall backfill material. The preliminary tests on the bentonite were carried out for screening purposes and to find an appropriate water content that will satisfy the desired viscosity range. Bentonite was then added to the dredged sediment in ratios of 1%, 2% and 3% of the total dredged sediment weight. The preliminary tests were repeated for each of these mixes to determine applicable trends and at what percentages of bentonite, the viscosity of the mixture was still in the workable range. The 1% bentonite mix was additionally modified with the addition of 5% and 8% fly ash by weight. These mixtures were then subjected to API filter press tests to determine the effect these mixes would have on the hydraulic conductivity. Adsorption testing was also carried out on the dredged sediment and all the mixes to determine their adsorption capacities to see if they can potentially be employed in reactive cut-off wall applications.

The results show that a suitable moisture content and viscosity of the dredged sediments can be obtained that makes it usable in the mix design. Increased bentonite content, to the percent tested (3%), lead to a decrease in the hydraulic conductivity and increased fly ash content, to the percent tested (8%), lead to an increase in the hydraulic conductivity. For the metals tested, an increased bentonite content enhanced the adsorption capacity of the mix and an increased fly ash content diminished the adsorption capacity of the mix. With the appropriate mix design, dredged sediments can serve as an effective inhibitor to the flow of ground water and hence serve as an in-situ containment and remediation system.