Dewatering and Amending Dredged Material for Beneficial Use

In April of 2003, approximately 1,791 cubic yards (cy) of material was dredged from the municipal marina in Nyack, New York. Accumulation of Hudson River sediments had reduced depths in the marina, impeding use by recreational boats. Sediment was removed using a horizontal auger cutterhead dredge and pumped into geotextile tubes in an adjacent parking lot to dewater. Since a priority of this project was the quick and efficient dewatering and removal of dredge material from the site, an open air, settling basin was not feasible. Additionally, there are safety concerns with having a dewatering area in a highly populated area. Polymeric conditioners were added to the slurry during pumping to facilitate dewatering and settling. Material in the geotextile tubes was periodically sampled to monitor the dewatering process. Sediments dredged from Nyack Marina were a mixture of fine-grained silts and clay commonly found on bay and estuary bottoms worldwide. The material was classified as "MH" elastic silt. Small amounts of sand were present near marina edges and storm drains. Paint filter tests were performed on July 15, 2003 for the material in each geotextile tube to determine whether a landfill would consider the material liquid or solid. All six samples passed the test, i.e., no free liquids were observed draining from the sediments and therefore the material was considered a solid. Initial dredge material entering the tubes had approximately 10–15 percent solids by weight. By June 30, 2003, the dredged material had dewatered to a percent solids ranging from 49–56%. At the completion of dredging operations, the geotextile tubes had a top elevation of 5.5 feet. By July 15, dewatering and consolidation had left the tubes ranging in height from 2.2 to 3.1 feet. Dewatering was allowed to continue and by September 22, the geotextile tubes had heights ranging from 1.9 to 2.6 feet. The possibility of using the dewatered dredge material for beneficial use was an important consideration for disposal options. The Village of Nyack intends to re-grade and improve an existing parking lot by using the dredged material from the Nyack Marina as the subbase for the parking lot. California Bearing Ratio (CBR) testing was performed on the dewatered dredge material to determine the suitability for use as a parking lot subbase. The material was found to have an average CBR of 3.43. Due to the relatively high moisture content of the dredged material, lime was chosen as the best additive. At an addition rate of 10% by weight of dry soil, bench-scale testing with hydrated lime showed a 67% increase in shear strength after a curing time of 2 hours. The final design for the parking lot will include geotextile separation/reinforcement and hydrated lime, mixed on site to increase stability.