Title

Dewatering of natural sediments using geotextile tubes

Date of Award

2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Keywords

Filtration efficiency, Dewatering, Sediments, Geotextile tubes

Subject Categories

Civil and Environmental Engineering | Engineering | Environmental Engineering

Abstract

Approximately 400 million cubic yards of sediments are dredged and 3.6 billion kilograms of paper sludges are produced in the USA (Paleramo and Wilson, 1997, Campbell et al., 1991). Because of high water content and low strength, dewatering is necessary before the final disposal of or beneficial use. Geotextile tubes are an innovative and still developing alternative, compared to traditional methods. Geotextile tubes are comprised of high strength and permeable woven or composite geotextiles and are filled with high water content materials. The three standard circumferences of geotextile tubes are 4.5, 9.0, and 13.7m and the length is dependent on the specific project. Since 1980's, geotextile tubes have been widely used in marine applications as an essential structural component and dewatering and containment applications. A successful dewatering application of geotextile tube depends on two main requirements: drainage and retention capability. Soil retention requires that the pore openings of the geotextile be small enough to prevent excessive migration of soil particles through the geotextile. Nonetheless, the drainage requirement necessitates that the geotextile pores be large enough to allow for the flow of water. As a consequence, the performance of geotextile tubes is primarily a function of the pore characteristics of the geotextile and the nature of the slurry such as water content and characteristic particle size. In past five years, Kutay (2002), Moo-Young et al. (2002), Mori et al. (2002), Koerner et al. (2004), Gaffney et al., (2004), Muthukumaran and Ilamparuthi (2006), and Aydilek (2006) have conducted experiments on dredged clean or contaminated sediments and sludges to relate laboratory performance with the field behavior of the geotextile tubes in the dewatering application. In these studies, woven geotextiles were used and the water content of the sediments and sludges varied from 100% to 1600%. Several of these researchers stated that the existing retention criteria based on AOS are not applicable for geotextile tubes. As well, in these studies, the influence by the water content of the slurry/sludge the external pressure is ignored or simply mentioned. Above all, none of these studies proposed retention criteria to predict the soil retention and estimate the dewatering time. The main objective of this study was to assess the drainage and retention capabilities of geotextile tubes and estimate the dewatering time. Drainage capabilities were reflected by the flow rate and dewatering time; retention capabilities were embodied by the filtration efficiency. For this study, the pressure filtration tests, falling head tests, and hanging bag tests were conducted on three natural soils: Cayuga Lake sediment, Ottawa sand 805, and Tully silt. One monofilament woven, two multifilament woven, one needle-punched nonwoven and one composite geotextiles from three different manufacturers were used. The water content of the sediments varied from 100% to 400% and the external pressure was between 0 and 70kPa. The falling head test (FHT) and pressure filtration test (PFT) are small-scale laboratory tests where the flow is vertical. The FHT is often conducted by industries and the PFT is often used by researchers. The small scale tests use 700ml to 1200ml volume of the slurry. Hanging bag test is standardized as GI14 by GRI (Geosynthetics Research Institute) and currently being proposed as a standard method by ASTM. It involves 150L slurry. The flow is in the vertical as well as radial direction. The equipment of pressure filtration and falling head tests was constructed based on the filtration unit in TCLP (Toxicity Characteristic Leaching Procedure, EPA method 1311). The entire setup for the hanging bag tests including the frame, the bag sewing, mechanical mixture of the slurry was designed and constructed in Syracuse University. On the whole, the flow rate increased with the increase of the water content and the external pressure. Furthermore, the increasing magnitude of the flow rate was slowed down as the water content and the pressure were increased from 100% to 400% and the 0kPa to 70kPa, respectively. Consequently, the dewatering time had a contrary behavior to the flow rate corresponding to the water content and external pressure. Above all, the flow rate was highly related with the permeability of the sediments. Not unexpectedly, Ottawa sand had a largest flow rate and Tully silt had a smallest one. Therefore, for a slurry composed by fine particles, an appropriate coagulant or flocculent was customarily used to increase the effective particles size to facilitate the formation and improve the permeability of the filter cake, thus accelerate the flow rate. All geotextiles succeeded in retaining Cayuga Lake sediment and Ottawa sand which had 85% of coarse particles in which a filter cake could form on the surface of the geotextile to prevent the further migration of the particles finer than the pore openings of the geotextiles. But only the composite geotextile succeeded in retaining Tully silt with all particles finer than 75μm because the small pore openings of this geotextile enabled the formation of the filter cake by the silt. The solids retention, expressed as filtration efficiency, was barely influenced by the pressure and had an insignificant decrease with the increase of the water content. Based on the behavior of the drainage and retention of the geotextiles in this study, a retention criterion based on the ratio O 100 /D 85 and water content of the slurry was developed to assess if a geotextile tube can retain the soil with a good filtration efficiency. Consequently, a model was generated to assess the filtration efficiency of the slurry by geotextile tubes. As well, a model to estimate the dewatering time was designed.

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