Mobile technology for the dewatering of dredged materials in geotextile tubes the Verden Trial

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1 Mobile technology for the dewatering of dredged materials in geotextile tubes the Verden Trial Stefan Cantré 1, Jörn Adameit 2 Abstract: An innovative and attractive technology to dewater dredged materials with geotextile tubes has finally found its way to Germany. In 2010 a large-scale pilot project was conducted in Verden near Bremen. The project has been planned and realised by the construction company Matthäi, department of hydraulic engineering, together with HUESKER Synthetic and J.F. Knauer. The chair of Geotechnics and Coastal Engineering at the University of Rostock both assisted in the planning and conducted a scientific monitoring program. For the first time in Germany geosynthetic tubes were used as mobile technology to dewater dredged harbour sediments. The drying process was monitored every week and both dry solids and effluent water were analysed with respect to the polymers used for solid-liquid separation and the contaminations of the harbour sediment. Within only two weeks the dredged materials were dewatered to a dry solids content of 55 %; considerably quicker than it would have been possible in a spoil field. The technology proved to be successful and offers a solution to many sludge dewatering problems such as dredged materials and industrial sludges. Introduction Geosynthetic containers have been increasingly used in hydraulic engineering projects within the past three decades (Saathoff et al. 2007). They come in a large variety of shapes and sizes, starting from simple sandbags to large tubes of more than 100 m in length. A particularly interesting application of geosynthetic tubes is the dewatering of sludges (Chow 2006, Weggel et al. 2011) which has become more and more popular around the world during the past decade while Germany lags behind. In the summer of 2009 a marina in Verden at the river Aller was filled with muddy sediments to such an extent that almost no boat could dock. The hydraulic construction company Matthäi GmbH & Co. KG, responsible for the dredging works, decided to re-use the material onshore to avoid relocation of the fine grained, organic and partly contaminated sediments within the water body. To save transport costs and to guarantee the required geotechnical characteristics, the sediments needed to be dewatered in situ. However, the dewatering in chamber filter or sieve belt presses is comparably cost-intensive whereas the use of a spoil field would occupy the adjacent areas for too long. Eventually, the project served as a large-scale pilot trial, applying the geotextile tube method. For a better solidliquid separation the dredged materials were conditioned with flocculants and then pumped into the large geotextile tubes, retaining the soil particles and allowing the water to drain. A geosynthetics producer (Huesker Synthetics) and a flocculant specialist (J.F. Knauer) were closely bound to the project. For the first time in Germany, the Verden trial incorporated an innovative combination of three mobile systems: the dredger, the flocculant mixer, and the geotextile tubes. 1 Universität Rostock, Lehrstuhl für Landeskulturelle Ingenieurbauwerke, Justus-von-Liebig-Weg 6, Rostock, stefan.cantre@uni-rostock.de 2 Matthäi Bauunternehmen, Abteilung Wasserbau, Bremer Str. 135, Verden, joern.adameit@matthaei.de

2 Dewatering technology using geotextile tubes The dewatering of sludge with geosynthetic tubes can be seen between mechanical and extensive dewatering methods. While filling the tubes hydraulically the excess water is pressed through the geosynthetic pores by internal pressure (fig. 1a). As long as the suspension is sufficiently fluid, there is an excess pressure along the whole circumference of the tube due to the filling material s self-weight. The sludge may either be filled in the tubes directly from a dredger (direct filling), directly connecting the pipes to the tube filling system or with the use of a reservoir from which the sludge is pumped into the geosynthetic tubes. Both systems can be used with flocculation technologies. To ensure downward dewatering the project area should be prepared with a filter layer. Fig. 1: Dewatering Technology with Geosynthetic Tubes (Cantré, 2008) During dewatering the pressure inside of the tube gradually decreases and the dewatering velocity may reduce considerably even more if a filter cake forms at the inside. A major advantage to spoil fields is that the material is wrapped and thus protected against precipitation water which usually causes a longer dewatering time in spoil fields (fig. 1b). If the tubes are re-filled after the dewatering of previously filled sludge, clogging phenomena may cause a further flux reduction along the circumference with respect to the initial filling (fig. 1c). After dewatering the tubes are cut open and the dewatered material can be loaded and transported for re-use or disposal (fig. 1d). Theoretical background The mechanical dimensioning of hydraulically filled geotextile tubes is based on the theory of thin membrane containers, originally developed for the transport (Wang 1984, Zhao 1995) and storage (Timoshenko 1940,

3 Namias 1985) of fluids with different densities. The approach of Namias has been developed constantly, particularly with regard to different loading conditions and material characteristics (Plaut and Suherman 1998, Plaut and Klusman 1999, Cantré 2002, Cantré and Saathoff 2011a). In most cases the mechanical dimensioning of the geosynthetic tubes is based on simple assumptions regarding both materials and geometries. Two commercial software products, GeoCOPS (Leshchinski et al. 1996) and SOFFTWIN (Palmerton 1998), are often used in the design process. The hydraulic properties of the geosynthetics also need dimensioning. Particularly if the sludges to be dewatered are fine grained and/ or compressible the hydraulic dimensioning is difficult and can often only be accomplished with the use of laboratory experiments combining sludge and geosynthetic.the hydraulic properties of the sludgegeotextile system become even more complex if a filter cake develops on the inside of the geosynthetic tubes. Many authors assume that the dewatering technology with geosynthetic tubes is a cake filtration problem. These problems are widely discussed in filtration engineering literature (e.g. Tien 2006). In standard laboratory tests the filter media, the suspension and the filtrate are characterised. For both the filter medium and the suspended solids the permeability and porosity of the geotextile are specified. The suspension is usually analysed with respect to water content, grain-size distribution of the suspended solids, and contaminations while the filtrate is analysed with regard to the percentage of fines (total solids or filterable fines) and possible contaminations. For the final selection of a geotextile generally simple laboratory or in-situ tests are used such as cone or cushion tests and hanging bag tests (Lawson 2008). In research, pressure filtration tests are often used (Moo-Young et al. 2002, Liao and Bhatia 2005). The larger part of the design process is still based on experience. Especially the dewatering time -a very important planning parameter- is hardly addressed in literature (Cantré and Saathoff 2011b). The Verden Trial During the preinvestigation different testing institutes analysed the harbour sediments. Then the participating specialist companies chose suitable flocculants and geosynthetics. Sediment samples from different places within the harbour were analysed showing mostly silt with an ignition loss of about % and a gravimetric water content of about 200 % (with respect to the dry mass). The contaminant analysis after LAGA (2004) revealed small amounts of zinc, lead, and cadmium (class Z1.1, exposed installation restricted). The separation behaviour was analysed using different polyacrylamide-based flocculants; among the tested products FMfloc C 251 DU showed the best flocculation and separation. The geotextile tubes were chosen based on the grain size distribution of the sediment: HaTe Typ PP 105/105, a standard woven geotextile for the use in dewatering projects. The dewatering site was located directly north of the yacht harbour. The area for the three geosynthetic tubes, each having a size of 35 m x 7.5 m, was bordered by small earth walls and sealed with a plastic liner against the underground. The dredged material was pumped through the Flocmaster facility (flocculant mixing system by Knauer) into the geosynthetic tubes using DN 250 plastic tubes. The facility is able to produce polymer emulsions of % polymer concentration (conventional facilities produce % emulsions) considerably reducing

4 the need of transport water. The facility consists of the mixing and dosing unit and a so called AT mixer injecting the polymer emulsion directly into the pipe and mixing it there with the sludge. The tested configuration of dredger and polymer mixer was able to 420 m³ h -1 (sludge with 10 %vol. dry solids). Fig. 2 shows the three mobile components dredger, flocculant system, and tube. Fig. 2: Mobile Technology Mobile Dredger, Flocculation Container, and Geosynthetic Tube The discharge of excess-water into the yacht harbour was monitored by a limnologic laboratory. The specialists involved evaluated a little turbidity of the discharge to be uncritical since the dredging caused more turbidity itself in the harbour basin (fig. 4c). Therefore only total organic carbon and ammonium were monitored as important discharge parameters. These values only slightly exceeded the harbour water s natural value and thus allowed a

5 discharge of the filtrate into the harbour basin. With the use of the flocculants the solid-liquid separation showed very good results (compare fig. 4a/b). Fig. 3: Filtration Quality Pumped Sludge, Sedimentation after Flocculation, Discharge Water To monitor the sediment dewatering the dry solids content was specified every week at a respective sample from each of the nine tube insertions. After dewatering the dredged material was analysed in the geotechnical laboratory of Rostock University both with respect to contaminants and geotechnical parameters. The sediments were dewatered until the end of August 2010, when the material was transported to one of Mattäi s construction sites to be used in a leveling layer. The geosynthetic tubes were cut open and the sediment was loaded on lorries to be transported to the construction site. Results and Discussion The soil mechanical analysis indicated the dredged material as sandy loam with average grain size contents of 61 % sand, 22 % silt and 17 % clay. The mineral grains were determined after elimination of calcium carbonate and organic contents to prevent deviations from organic agglomerations (Cantré and Schulz 2011). The average ignition loss was about 6 %, considerably lower than determined during the preinvestigation. In comparison, without elimination of the organic content the grain size distributions varied between silt loam and loamy sand, all without clay fraction. The average dry density was around 1.0 g/m³, the in situ density 1.55 g/cm³, and the grain density 2.60 g/cm³. The plasticity I P was about 40 %. The undrained shear strength (vane shear value) of the loam and silt was ca. 10 kn/m²; a comparably low value which may be critical for further application. However, mixing with some sand which had been dredged at the end and which was deposited on top of the loam and silt layers inside the tubes allowed the application in a leveling layer. In total 3,800 m³ of sludge were pumped into the geosynthetic tubes. After completion approximately 900 m³ of sediment were removed from the harbour bottom (sounding measurement). After dewatering a volume of 550 m³ was left (geodetic survey); approximately 14.5 % of the pumped sludge. The consumption in polymer emulsion was 790 l. Considering that the emulsion consists of 50 % carrier oil and 50 % polymer with a density around 1 g/cm³ this corresponds to 395 kg polymer. This substance was solute in 67 m³ of transport water, thus a 0.6 % solution was mixed with respect to the polymer.the total consumption of

6 polymer per ton dry sediment was about 0.74 kg t -1 which corresponds to a polymer concentration of 740 ppm within the sediment, which is a comparably low value. In only two weeks the fine grained dredged material was dewatered down to 55 % dry solids, ready for transportation. Another two weeks of drying resulted in 60 % dry solids (varying from 50 % to 70 %). Thereafter, no significant gravitative dewatering could be observed (fig. 5). Fig. 4: Amount of Dry Solids in the Dewatering Material Due to the contents of total organic carbon, lead, and zinc in the solid fraction and the electric conductivity of the eluate, the dredged material is classified in Z2 (restricted installation with defined technical safety measures) after LAGA (2004). Differing from the preinvestigation results, the lead and zinc contents of most samples exceed the value for Z1 (exposed installation restricted). Conclusions The dewatering method combining the three mobile technologies proved to be successful. Within only two weeks the dredged materials were dewatered to a dry solids content of 55 %; considerably quicker than it would have been possible in a spoil field. In spite of the contamination with lead, cadmium, and zinc as well as the high organic content and the relatively low undrained shear strength the dredged material could be used as a leveling layer on a recultivation site. The contaminations only slightly exceed the LAGA limits for class Z1 and Z1.1 and thus this application is considered uncritical. In the future the technology may introduce new fields of work in Eastern Europe and Russia, where a large amount of rivers and lakes need remediation and where many industrial sludge ponds exist. But also in Germany the method has further potential, especially in urban areas, on

7 decentralized waste water treatment plants and in sensitive areas, where both the relocation of sediments and extensive heavy machinery are prohibited. Literature Cantré, S. (2002): Geotextile Tubes - Analytical Design Aspects. In: Geotextiles and Geomembranes. 20 (5): Cantré, S., Saathoff, F. (2011a). Design method for geotextile tubes considering strain. Formulation and verification by laboratory tests using photogrammetry. In: Geotextiles and Geomembranes. 29 (3): Cantré, S., Saathoff, F. (2011b). Design parameters for geosynthetic dewatering tubes derived from pressure filtration tests. In: Geosynthetics International. 18 (3): Cantré, S., Schulz, H. (2011). Innovative Baggergutentwässerung mit geotextilen Schläuchen ein mobiles System im Feldversuch. In: Wasser und Abfall. 13 (3): (in German). Chow, R. W. (2006). Dewatering fine-grained soils using geotextile tubes: an Australian case study. Proceedings of the 8th International Conference on Geosynthetics, Kuwano, J. &Koseki, J., Editors,Millpress, Rotterdam, LAGA (2004). Anforderungen an die stoffliche Verwertung von mineralischen Abfällen: Teil II: Technische Regeln für die Verwertung, 1.2 Bodenmaterial (TR Boden). LAGA M pages. Self-publishing (in German). Lawson, C. (2008). Geotextile containment for hydraulic and environmental engineering. Geosynthetics International, 15 (6): Leshchinsky, D., Leshchinsky, O., Ling, H. I. & Gilbert, P. A. (1996). Geosynthetic tubes for confining pressurized slurry: Some design aspects. Journal of Geotechnical and Geoenvironmental Engineering, 122 (8): Liao, K. & Bhatia, S. (2005). Geotextile tube: filtration performance of woven geotextiles under pressure. Proceedings of Geosynthetics 05, Las Vegas, Nevada, Moo-Young, H. K., Gaffney, D. A. & Mo, X. (2002).Testing procedures to assess the viability of dewatering with geotextile tubes. Geotextiles and Geomembranes, 20 (5): Palmerton, J. B. (1998). SOFFTCON: Design Software for Geosynthetic Tubes (Consolidation Analysis). Plaut, R.H., Klusman, C.R. (1999). Two-dimensional analysis of stacked geosynthetic tubes on deformable foundations. In: Thin-Walled Structures, 34(3): Plaut, R.H., Suherman, S. (1998). Two-dimensional analysis of geosynthetic tubes. In: Acta Mechanica, 129(3-4): Saathoff, F., Oumeraci, H. &Restall, S. J. (2007). Australian and German experiences on the use of geotextile containers. Geotextiles and Geomembranes, 25 (4 5):

8 Tien, C. (2006). Introduction to Cake Filtration: Analyses, Experiments and Applications, 1st edition, Elsevier, Amsterdam. ISBN pages. Timoshenko, S.P. (1940). Theory of Plates and Shells. 1. Ed. New York: McGraw-Hill.Namias 1985 Wang, C.Y. (1984). Filling of a Long Membrane Container. In: Journal of structural mechanics, 12(1): Weggel J. R., Dortch J., ZofchakMerida, V. (2011). Experiments with water and slurries in hanging geotextile bags: A further appraisal. In: Geotextiles andgeomembranes29 (2011): Zhao, R. (1995). A complete linear theory for a two-dimensional floating and liquid-filled membrane structure in waves. In: Journal of Fluids & Structures, 9(8):