GEOTEXTILE TUBE: FILTRATION PERFORMANCE OF WOVEN GEOTEXTILES UNDER PRESSURE

Transcription

1 GEOTEXTILE TUBE: FILTRATION PERFORMANCE OF WOVEN GEOTEXTILES UNDER PRESSURE Kaixia Liao Syracuse University, Civil and Environmental Engineering Department, Syracuse, NY, USA Shobha K. Bhatia Syracuse University, Civil and Environmental Engineering Department, Syracuse, NY, USA ABSTRACT Geotextile tubes are manufactured by sewing one or more layers of high-strength permeable woven/non-woven geotextiles together to form tubes that are later filled with highwater content materials by hydraulic pumping. Recently, Moo-Young et al. (22), Kutay (22), and Aydilek and Edil (22), have conducted laboratory studies which concluded that the apparent opening size (AOS 1 ) alone is not a good indicator to predict the filtration behavior of geotextile tubes. Therefore, the current filtration criteria based on apparent opening size do not apply for geotextile tubes. When geotextile tubes are used in dewatering slurries, a filer cake forms on the interface of the soil and geotextile. The formation of the filter cake, which leads to the retention of the soil particles, can be controlled by the pore openings of the geotextile, the particle size distribution of the soil, the water content of slurry, and the pumping pressure. The purpose of this paper is to study the effect of the large-pore openings of geotextiles on the filtration performance of geotextile materials. In this paper, filtration behavior of three woven geotextiles is studied using the pressure filtration tests on three natural soil sediments. Results are presented on the influence of the water content, pressure, and polymer. INTRODUCTION Waste materials generated from dredging projects, mining process, paper mills, agriculture, and industry are very difficult to dispose of due to their high water content, high compressibility, and low shear strength. A conservative estimation of the annual total worldwide volume of these wastes is on the order of a billion cubic meters or more (Krizek, 2). They need to be dewatered to a required percent solid before final disposal. At the present, the most feasible way to dispose of these wastes is to hydraulically pump them into a dike-confined impoundment area and allow them to dewater by consolidation and desiccation. Then, the dewatered materials are disposed of to a landfill and often used as construction materials (Gaffney et al., 21). However, several major problems arise from this method. Most notably, the very properties of the materials entail a long dewatering time, consequently raising the issue of the 1 AOS: apparent opening size for a geotextile is a property which indicates the approximate the largest particles that would effectively pass though the geotextile. 1

2 stability of the dike confinement, and, in turn, the contaminants may seep into the groundwater. Likewise, the storage capacity of the impoundment area may not meet with the disposal requirement, and more stringent environmental restrictions may limit the availability of land for impoundment. All these deficiencies give rise to the need for efficient and energy-saving alternatives to dewater such high water content materials. Consequently, geotextile tubes have been increasingly employed in dewatering sediments (Krizek, 2). Geotextile tubes are manufactured by sewing one or more layers of high strength permeable geotextile together to form containers that are then filled with high water content materials through hydraulic pumping. Geotextile tubes can be made of woven or nonwoven/woven composite geotextiles. The availability of a wide variety of geotextiles in terms of tensile strength, durability, and permeability enables the use of hydraulically filled geotextile tubes in many civil and environmental engineering applications, such as dike construction, shoreline protection structures, and sludge dewatering. When compared to traditional dewatering methods, geotextile tubes used for dewatering high water content materials thus offer several advantages: rapid dewatering of large volumes of slurries, ease of construction, convenient placement, high efficiency, low cost, labor savings, and low environmental impacts (Fowler et al., 2; TC Mirafi, 23). Dewatering tubes perform three related functions: containment, dewatering, and consolidation. High water content infilled materials are hydraulically pumped into the tubes, and, as the liquid escapes from the tube, solid particles are trapped inside. This process is repeated until the tube is full. Eventually, the solids can be handled as dry material, increasing options for transportation and disposal. When the geotextile tube performs the containment function, it acts like a filter: as the dewatering of the slurry using the geotextile tube takes place, the soil particles float in the water, and after the particles move thought the geotextile, a filter cake forms as enough coarse particles settle down ahead of the fine particles to block some of the geotextiles pores. This also means that the existing criteria which have been developed for soils, not slurries, may not be applicable. Recently, several research projects have been conducted to bridge the gap between the laboratory tests and the field practice of geotextile tubes, focused on investigating the performance of a variety of woven geotextiles typically used for geotextile tubes: Moo-Young, et al. (22) performed 26 pressure-filtration tests under 35 and 7kPa of pressure to assess the viability of dewatering lake and harbor sediments by several multifilament and monofilament woven geotextiles. The AOS of the geotextiles tested ranged from.25 to.6 mm. The D 85, D 5, and D 15 of the lake sediments were.19,.4, and.3 mm, respectively, whereas the same sizes were.5,.23, and.5 mm, respectively, for the harbor sediments. The initial water content of the harbor sediments were 142% and 326%, and the lake sediments were tested at a water content of 588%. In these tests, they measured the initial total solids TS inital (mg/l) and the final total suspended solids in the filtrate TSS final (mg/l) and defined the filtration efficiency as Equation (1): TSinitial TSS final Filtration efficiency= 1% (1) TS initial 2

3 Filtration efficiencies of all tests were above 95%; therefore indicating that all the geotextiles could retain most of the soil particles that were much smaller than AOS values of the geotextiles. Kutay (22) performed pressure filtration tests on dredged material (D 85 =.19mm, D 5 =.5mm, and D 15 =.9mm) and woven geotextiles with AOS ranging from.15mm to.6mm. In the test, the pressure was 27.6kPa, and the water content of slurry was 16%. He found that the geotextiles with large AOS (.425 and.6mm) failed in retaining the dredged material, and the geotexile with a small AOS (.15mm) could retain the soil particles. However, very low flow rates resulted in a long filtration process for the tests. Kutay also conducted hanging bag tests in which the bags were.5m wide and 1m high and the water content of slurry was 16%. He found that in hanging bag tests, the same geotextiles which failed in pressure filtration tests succeeded in retaining the majority of soil particles. Aydilek and Edil (22) performed pressure filtration tests under 7kPa pressure on 1 woven geotextiles with an AOS ranging from.15 to.6mm. For their tests, they used PCBcontaminated wastewater treatment sludge (D 6 =.3 mm, D 3 =.1 mm, and D 1 =.85 mm). They concluded that a commonly used ratio of geotextile pore opening size to soil particle size in the existing filtration criteria (O 95 /D 85 ) did not predict the filtration behavior of the geotextiles with the sludge. They measured the POA (percent open area) of the woven geotextiles using an image analysis method and found them to be much smaller than AOS values (POA ranged from.6% to 53%). They thus believed that POA was a more appropriate parameter to predict the filtration performance. Despite these recent contributions, there still remains confusion about controlling factors which influence the filtration process and the filtration criteria which can be used for predicting the filtration behavior of geotextile tubes with soil slurry. To further clearly understand the influence of the confining pressure, water content, and additional polymer on the filtration efficiency of the geotextile tubes, small-scale filtration pressure tests were performed on three woven geotextiles and three soil slurries. This paper also studies the effect of the pore sizes of geotextiles such as O 95 and O 5 on the filtration performance of geotextile tubes using current existing filtration criteria. MATERIALS Three woven geotextiles (A, B, and C) typically used in making geotextile tubes were tested in this study. Geotextile A is made of high-strength polypropylene monofilament yarns, whereas the other two geotextiles, B and C, are made of high-strength polyester multifilament yarns. The engineering properties of these geotextiles as reported by the manufacturers are listed in Table 1. The properties given are minimum average roll value in the machine and crossmachine directions. 3

4 Table 1 Geotextile Properties Properties Test Method A B C Fiber type PP PET PET Mass/Unit Area (g/m 2 ) ASTM D Wide width tensile strength (kn/m) ASTM D x x x 175 Wide width tensile elongation (max) ASTM D % 1% 15% Water flow rate (l/m/m 2 ) ASTM D Apparent opening size (AOS) (mm) ASTM D O 95 (mm) Bubble Point O 5 (mm) Bubble Point The AOS listed in Table 1 are measured by sieving glass beads through a geotextile. It has been shown that the drying sieving method underestimates actual pore sizes because of glass beads become trapped within the geotextile, resulting in electrostatic effects (Bhatia & Smith, 1996). Bubble-point method, however, allows a fluid of water to pass only when the pressure exceeds the capillary force of the fluid in the pores. Thus, the bubble point method can measure, not only the largest pore opening, but also the entire pore-size distribution of a geotextile. In this case, the bubble point method was used to measure the complete pore size distribution of these three geotextiles. The results of these tests are shown in Figure 1. As it can be seen, O 95 values calculated using the bubble point method are slightly larger than AOS values A B Bubble Point O95 =.443mm O95 =.37mm % Finer C O95 =.218mm Diameter (mm) Figure 1 Pore-size distributions of geotextiles Three soils (Cayuga Lake sediments, Ottawa clean sand, and silt) are used for this study. Their properties are listed in Table 2, and particle size distributions are shown in Fig. 2. 4

5 Table 2 Material Properties Properties Materials Cayuga Lake sediments Clean sand Silt Percentage <.75mm 15% 13% 1% d 85 (mm) d 5 (mm) d 15 (mm) C u C c Percent Finer (%) Cayuga Lake sediments Clean sand Silt Particle Size (mm) Figure 2 Particle size distributions of soils Figure 3 Test apparatus 5

6 EQUIPMENT All tests were performed using the pressure filtration devices (Figure 3). This apparatus consists of an upper plate with a pressure inlet, a chamber with an inner diameter of 7.17cm and a height of 17cm, and a lower chamber with an outlet to collect the filtrate. After the geotextile sample is placed on the screen holder, the slurries are mixed sufficiently and poured into the chamber quickly; then the air pressure that has been adjusted to the required value is applied promptly. Graduated beakers are used to collect the filtration periodically. The volume of the filtrate and the mass of the particles in the filtrate are measured to calculate the TSS final. RESULTS A total of 33 pressure filtration tests were conducted on these three woven geotextiles and three soils under 7, 35, and 7kPa of the pressure. Flow rate Figure 4 shows the flow rates for the test on Cayuga Lake sediments with geotextile B. As it can be seen from Figure 4a, flow rate increase within the first one minute and stabilizes within 14 minutes. It can also be seen from Figure 4 that flow rates increase as the pressure and water content increase. Because the filter cake formed above the geotextile is thinner for the slurry with a higher water content, it is easier for the soil particles to pass through the geotextiles, especially under a higher pressure. The thickness of the soil column (shown in Figure 5) after the tests under 35kPa pressure also supports these findings. The final height of the soil column after the tests was 7.52, 4.45, 2.63, and 1.77cm for the specimens prepared at a water content of 1%, 2%, 3%, and 4%, respectively. 2 Flow rate (cm/min) kPa 35kPa 7kPa water content =1% Time(min) (a) 6

7 2 Flow rate (cm/min) kPa 35kPa 7kPa water content =2% Time (min) (b) 2 Flow rate (cm/min) kPa 35kPa 7kPa water content.=3% Time (min) (c) Flow rate (cm/s) kPa 35kPa 7kPa water content =4% Time (min) (d) Figure 4 Flow rate of Cayuga Lake sediments and geotextile B 7

8 Figure 5 Soil column after tests on Cayuga Lake sediments and geotextile B under 35kPa (From left to right: 1%, 2%, 3% and 4%) The pressure filtration test results for clean sand and silt showed similar behavior. For example, flow rates increased with increasing water content and pressure. For clean sand and silt, flow rates increased in the first 1 minute and dropped to a stable level when a filer cake started to form. The amount of soils piping through geotextile B with the Cayuga Lake sample is given in Table 3. Piping rates increase with increasing water content and pressure and decrease at 4% water content. The reason for this behavior may be that the increase in flow rates from 3% to 4% is not significant (Figures 4c and 4d), as compared to the decrease in initial density of the slurry. Thus, the mass of solids in the filtrate passing through the geotextile is smaller for 4% than 3%. The same trend exists for Cayuga Lake sediments and clean sand with geotextile A and C. Table 3 Piping rates of Cayuga Lake sediments and geotexitle B Water content (%) Piping rate measured (g/m 2 ) 7kPa 35kPa 7kPa Figure 6 provides the results of tests conducted on three soils with geotextile B under an air pressure of 7kPa. The flow rate is the largest for clean sand and smallest for silt because clean sand has the largest permeability and silt has the lowest. As a result, the dewatering time is the lowest for the sand, followed by Cayuga Lake sediments and the silt. Geotextile B and C under 7 and 35kPa exhibited similar dewatering behaviors. 8

9 35 Flow rate (cm/s) Silt Cayuga Lake Sediment Clean sand Time (min) Figure 6 Flow rates of three slurries with geotextile B under 7kPa pressure Filtration Efficiency The filtration efficiency defined by Moo-Young, et al. (22) in Equation 1 was calculated for all 33 pressure filtration test results which can then be divided into two groups: one for Cayuga Lake sediments and clean sand in which the filtration efficiencies are above 95%; and the other for silt where the filtration efficiencies are smaller than 95%. For Cayuga Lake sediments and clean sand, the filtration efficiencies of all tests under the pressure of 7, 35, and 7kPa are given in Figure 7. The filtration efficiencies of geotextile A, Filtration efficiency (%) kPa 35kPa 7kPa Water content (%) Geotextile B and C Geotextile A Figure 7 Filtration efficiencies of Cayuga Lake sediments and clean sand 9

10 which is made of monofilament yarns and with.443mm of O 95, seem to be affected by the water content: an average value in decrease is from 99% to 96%. The filtration efficiencies, by contrast, seem not be influenced by the water content with geotextiles B and C, which are made of multifilament. The filtration behavior of geotextiles with silt is different from its behavior with Cayuga Lake sediments and clean sand. Figure 8 shows the filtration efficiencies of all the three soils and geotextiles: geotextile A failed in retaining the particles of silt; and the filtration efficiencies for geotextiles B and C are much smaller with silt as compared to the Cayuga Lake sediment and clean sand. This behavior is a clear indication of the influence of the filter cake and its properties on the overall behavior. In short, the filtration efficiencies under different water contents for silt showed a significant variation than for the Cayuga Lake sediments and clean sand: the variation of the filtration efficiencies for silt is 1% when the water content increases from 1% to 4%. 1 sand w ith geotextile B and C Filtration efficiency (%) Silt w ith 7kPa 35kPa 7kPa geotexile B and C Water content (%) Lake sediment and clean sand w ith geotextile A Figure 8 Filtration efficiencies Since it takes a long time to reach a very low flow rate and low filtration efficiency for silt, the polymer of Magnafloc336 from CIBA was added into the silt slurry in order to improve the filtration rate and filtration efficiency. Figure 9 shows the flow rates of silt with geotextile B at 4% water content with different amounts of the polymer. The ratios of the polymer weight to dry silt solids weight were.3%,.3%,.6%, and.3%. When the ratio was.3%, the viscosity of the slurry was so large that no water could pass through the geotextile at all. Then, when the ratio was decreased to.3%, a still large viscosity resulted in a smaller flow rate than the slurry without the polymer. Finally, the polymer with the ratio of.3% to the dry soil mass improved the flow rate considerably by improving the permeability of the filter cake. 1

11 Flow rate (cm/min) kPa-without polymer 35kPa-.3% 35kP-.6% 35kPa-.3% Time (min) Figure 9 Flow rates of silt with geotextile B at 4% water content 1 Filtration efficiency (%) w ith polymer (.3% of dry w eight) w ithout polymer Water content (%) Figure 1 Filtration efficiencies of geotextile B with silt with and without the addition of polymer under 35kPa pressure Figure 1 shows the filtration efficiencies of the silt with and without the addition of the polymer under 35kPa pressure. For the silt, the filtration efficiencies increase to 99% by adding a small amount of polymer. Thus, for the slurries composed of fine particles, the addition of an appropriate amount of polymer is very important to improve the filtration behavior of geotextile tubes. 11

12 DISCUSSION The dewatering behavior of geotextiles is controlled by the properties of the geotextile, the slurries, and the filtration pressure. The pore openings of geotextiles must be compatible with the particles of the slurry to effectively form a filter cake to retain fine particles. The properties of the filter cake, in turn, mainly depend on the properties of the soil. As shown in Figure 6, the permeability of the soil determines the permeability of the filter cake and dewatering time. The water content is also a factor which controls the filtration behavior of geotextiles. The test results indicate that the water content seems to affect the filtration behavior for monofilament woven geotextile A, as compared to the multifilament geotextiles B and C. The thinner filter cake caused by the higher water content of the slurry increases the difficulty for soil particles to pass through it. The piping rate listed in Table 3 increases from 1% to 3%, then decreases from 3% to 4%, with the influence of the small increase in flow rate and with less initial slurry density. Because it takes a longer time to dewater the slurry with 1% water content and to pump the slurry with 4% water content, a water content of 2% may be optimal to achieve a good balance between pumping cost and dewatering time. Pressure is another factor which influences the filtration performance of geotextiles: higher pressure tends to decrease the filtration efficiencies and accelerate the dewatering rate, especially when the pressure increases from 7kPa to 35kPa. The particle size distributions of the soils used in this study and the ones reported by Moo-Young et al. (22) and Kutay (22) are plotted in Figure 11. The existing filtration criteria for geotextile filters are used to predict the filtration performance of these soils: it is found that For the soils in this study, using O 95 and O 5 measured from the bubble point tests, the criteria proposed by Ogink (1975), Christopher and Holtz (1985), and Fischer et al. (199) can accurately predict the filtration performance of these three geotextiles with Cayuga Lake sediments and clean sand; and all these criteria can also predict the performance of geotextile A with silt. No single criterion, however, can predict the filtration performance of geotextiles B and C with silt. For the soil in the studies by Moo-Young et al. (22), the criterion proposed by Calhoun (1972) can predict accurately the filtration performance of geotextiles with large pore openings (AOS=.425mm and.6mm) for harbor sediments; and the criterion by Christopher & Holtz (1985) can predict the filtration behavior of geotextiles with.25mm of AOS for harbor sediments. However, no single criteria can predict the performance of all the geotextiles with lake sediments and soil in the Kutay (22) study. Therefore, it is suggested that the pressure filtration tests should be conducted for fine-grained materials, rather than relying on existing filtration criteria. 12

13 1 9 Percent Finer (%) Moo- Young_harbor sediment Moo-Young_lake sediment Kutay Cayuga Lake Sediment 3 2 clean Ottawa sand Silt In this study Particle Size (mm) Figure 11 Particle size distribution of the soil in all research In the Kutay (22) study, the geotextiles with large pore openings (AOS=.425 and.6mm) were successful in retaining the fine-grained sediments, while they failed in retaining the particles in pressure filtration tests. In hanging bag tests, the slurry was poured into the bags without any pressure, which means that coarser particles have enough time to settle down ahead of fine particles to form the filter cakes in the hanging bag tests (as compared with the pressure filtration tests). The filtration tests under kpa pressure were also conducted in this study: either no filtrate or little filtrate passed though the geotextiles for silt and Cayuga Lake sediments 1 minute after the slurry was poured into the device chamber. The filter cake forms more slowly and has a smaller permeability in the tests under no pressure than under the pressure. CONCLUSIONS In this study, 33 pressure filtration tests were conducted on three soils and three woven geotextiles typically used in geotextile tubes application. The following conclusions can be drawn based on the test results: 1. The water content ranging from 1% to 4% has little effect on the filtration efficiencies of the coarse-grained materials. However, for fine grained materials, high water content seems to decrease the filtration efficiency. 2. Pressure ranging from 7kPa to 7kPa has little effect on the filtration efficiency. Even though relatively higher pressures generally resulted in higher flow rates. 3. The addition of the polymer is very important to accelerate the dewatering process and increase the stable flow rate of the fine-grained materials. 13

14 4. Several existing filtration criteria can predict the filtration performance of the three woven geotextiles with coarse-grained soils using AOS, O 95, and O 5 measured from the bubble point tests. No single criterion predicts accurately the filtration performance of the geotextiles with fine-grained materials using these parameters. It is suggested that pressure filtration tests and hanging bag tests have to be conducted for fine-grained soils. FUTURE RESEARCH Future research will be focused on the pressure filtration tests with the addition of polymers to better understand the filtration behavior of fine-grained sediments. This research will include the hanging bag test with the small bag size and the large standard size with the hope of establishing a better understanding of the filtration behavior of geotextile tubes. ACKNOWLEGEMENT The authors would like to thank Huesker Inc. and TC Mirafi for providing geotextiles for this study. The authors would also like to thank the National Science Foundation (Grant No ) for providing financial support. REFERENCES: Aydilek, A. and Edil T. (22). Filtration performance of woven geotextiles with wastewater treatment sludge, Geosynthetics International, vol. 9, no. 1 22, pp Bhatia, S.K. and Smith, J.L., (1996). Geotextile Characterization and Pore-size Distribution: Part II: A Review of Test Methods and Results, Geosynthetics International, Vol. 3, No.2, pp Calhoun, C.C. (1972). Development of Design Criteria and Acceptance Specifications for Plastic Filter Cloth, Technical Report S-72-7, U.S. Army Waterways Experiment Station, Vicksbury, MS. 55p. Christopher, B.R., and Holtz, R.D. (1985). Geotextile Engineering Manual, U.S. Federal Highway Administration, Report FHWA-TS-86/23, 144p. Fischer, G.R., Holtz, R.D., and Christopher, B.R. (199). Filter Criteria Based on Pore Size Distribution, Proceedings of 4 th International Conference on Geotextiles, Vol. 1, Hague, Netherlands, pp Fischer, G.R. (1994). The Influence of Fabric Pore Structure on the Behavior of Geotextile Filters Dissertation, University of Washington, Seattle, Washington, USA, 51p. Fowler, J., Duke, M., and Schmidt. M (2). Dredging and Dewatering of Hazardous Impoundment Sediment Using the Dry DredgeTM and Geotubes, WEDA Journal, March 2. 14

15 Krizek, R. J. (2), Geotechnics of High Water Content Materials, Special Technical Publication 1374, American Society for Testing and Materials, February, pp Kutay, M.E. (22). Hydraulic Performance of Geotextile Containers Confining Waste Materials, Thesis, University of Maryland, College Park, Maryland, USA, 13p. Moo-Young, H., Gaffney, D., Mo, X. (22). Testing Procedures to Assess the Viability of Dewatering with Geotextile Tubes, Geotextile and Geomemberanes, pp Ogink, H.J.M. (1975). Investigations on the Hydraulic Characteristics of Synthetic Fabric, Delft Hydraulics Laboratory, Publication No TC Mirafi, downloaded in August