Dredged Sediments from Port of Mombasa as a Material in Vertical Cut-Off Walls

Abstract

Dredged sediments are obtained from the process of dredging coastal areas and harbours in order to maintain navigable waterways. This study focuses on the potential of using dredged sediments as vertical cut-off wall backfill material including in and offshore waste disposal sites. Materials used as vertical cut-off wall material are expected to have low hydraulic permeability and good workable characteristics. Dredged sediment samples obtained from dredging works at the Port of Mombasa were used in this study. The sediment was plastic in nature and finer than the conventional slurry cutoff and offshore construction sites to exclude water in the construction sites. The tests were carried out to establish the effect of bentonite on the density and hydraulic conductivity of the dredged sediment mix. The effect of a combination of flyash and bentonite on hydraulic conductivity. The broad objective being was to find an appropriate mix of the 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 results showed that suitable moisture content of 463% and viscosity of 42s could be obtained that makes it usable in the mix design sediments. Increased bentonite content, to the percent tested (3%) lead to decrease in hydraulic conductivity and increased fly ash to 8%, lead to increase in hydraulic conductivity. The selected design mix (3BS97DS- 3% bentonite by weight and 97% dredged sediment) was found suitable as a backfill material for vertical cut-off wall. It was found that an optimum sediment mix was 3% bentonite by weight; the mud weight density of the dredged sediments was 10.58 kN/m³ which was within the acceptable range of bentonite slurries of xvi 10.06 - 12.58 kN/m3; increasing bentonite content to 3% resulted in a decrease in hydraulic conductivity; and increasing the fly ash content to 8% increased the hydraulic conductivity. It was found that the dredged sediment from Port of Mombasa channel could serve as an effective vertical cut-off material when modified with 3% bentonite. Therefore the sediments from the Port of Mombasa can be used beneficially for engineering purpose