A METHODOLOGY FOR THEEVALUATION OF GEOTEXTILE POREOPENING SIZES UNDER CONFINING PRESSURE

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1 Technical Note by E.M. Palmeira and R.J. Fannin A METHODOLOGY FOR THEEVALUATION OF GEOTEXTILE POREOPENING SIZES UNDER CONFINING PRESSURE ABSTRACT: This paper presents a methodology used to evaluate the pore opening sizes of a needle-punched, nonwoven polyester geotextile under pressure, using a permeameter subject to vibration. Particles passing through the geotextiles were collected and analysed to establish a particle size distribution curve. Preliminary results obtained under no confining pressure compare well with a standard test method based on hydrodynamic sieving. Confining pressures up to 25 kpa appeared to exert some influence on the pore opening size of relatively thin geotextiles. However, for pressures greater than 25 kpa, the variation of geotextile pore opening size is small. The tests results and microscopic observations suggest that needle-punching during the geotextile manufacturing process has a significant influence on the geotextile pore structure. Regardless of the limited amount of data available, the methodology described may provide a useful tool for the study of geotextile pore opening sizes under confining pressure. KEYWORDS: Geotextile, Pore opening size, Filtration, Drainage, Confining pressure. AUTHORS: E.M. Palmeira, Associate Professor of Civil Engineering, University of Brasilia, Dept. Civil Engineering, FT, Brasilia, DF, Brazil, Telephone: 55/ , Telefax: 55/ or , palmeira@guarany.cpd.unb.br; and R.J. Fannin, Associate Professor, Department of Civil Engineering and Forest Resources Management, University of British Columbia, 2324 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada, Telephone: 1/ , Telefax: 1/ , fannin@civil.ubc.ca PUBLICATION: Geosynthetics International is published by the Industrial Fabrics Association International, 1801 County Road B West, Roseville, Minnesota , USA, Telephone: 1/ , Telefax: 1/ Geosynthetics International is registered under ISSN DATES: Original manuscript received 1 July 1997, revised version received 18 November 1997 and accepted 18 December Discussion open until 1 January REFERENCE: Palmeira, E.M. and Fannin, R.J., 1998, A Methodology for the Evaluation of Geotextile Pore Opening Sizes Under Confining Pressure, Geosynthetics International, Vol. 5, No. 3, pp

2 1 INTRODUCTION Nonwoven geotextiles are used extensively for drainage and filtration in both geotechnical and environmental protection works. For purposes of design, a rapid evaluation of geotextile pore opening sizes presents a challenge. However, knowledge of the characteristic pore opening size and the size of the largest soil particle capable of passing through the geotextile is necessary to satisfy filtration criteria. Several methodologies are reported for the evaluation of geotextile pore opening sizes (Prapaharan et al. 1989; Rigo et al. 1990; Gourcand Faure 1990; Fisher et al. 1993). Standardized test methods that do not use a surcharge pressure over the geotextile are also available (CFG 1986; ASTM D 4833; CGSB 1992). In the field, however, the geotextile is subject to confining stress that may influence its pore structure and, consequently, its filtration behaviour. The objective of this work isto describe a methodology for the evaluation of geotextile pore opening sizes under confining pressure and present the test results for three types of needle-punched, nonwoven polyester geotextiles. Because of the limited amount of data available, some of the conclusion of this work should be considered on a preliminary basis. Nevertheless, a clear pattern of the filtration behaviour for confined needle-punched, nonwoven polyester geotextiles can be identified. 2 EQUIPMENT, MATERIALS, AND METHODOLOGY 2.1 Equipment A permeameter was developed at the University of British Columbia (Shi 1994; Fannin et al. 1995) to perform filtration tests with geotextiles under a confining pressure. The equipment comprises a cylindrical rigid cell made of anodised aluminium with a 100 mm internal diameter. It was adapted to perform pore opening size tests and is shown schematically in Figure 1. The cell was submerged in a reservoir bath to a depth above the top reaction plate during specimen preparation and testing to ensure saturation of the specimen. The soil or glass beads were placed on top of the geotextile which rested on a rigid perforated base plate (2 mm diameter holes with a triangular spacing of 3 mm). A vertical pressure was applied by a rigid, perforated top plate using a hydraulic jack fixed to a reaction frame mounted above the cell. A trough located beneath the cell collected particles that passed though the geotextile during testing. 2.2 Soil/Glass Beads and Geotextiles Two poorly-graded glass bead mixtures (Materials A and B) and a well-graded soil (Material C) were used in the testing program. Material A ranged in size from 20 to 90 µm and Material B from 40 to 200 µm. The beads were round with a specific gravity, G s = 2.48 and coefficient of uniformity, C u = 2.1. In contrast, Material C was a silty sand, with subangular particles, and C u = 105 (Table 1 and Figure 2). Three types of needle-punched, nonwoven polyester geotextiles were used in the testing program: Geotextile G1 has the smallest thickness and mass per unit area values, Geotextile G2 has intermediate thickness and mass per unit area values, and Geotextile G3 has the greatest thickness and mass per unit area values. The geotextile pore opening 348

3 P Piston rod Vibration device Top reaction plate Rigid, perforated top plate Soil or glass beads Base reaction plate Figure 1. Components of the test cell. Note: P = vertical load. Geotextile Collection trough Rigid, perforated base plate sizes reported by the manufacturer from tests under no surcharge are in the range 60 to 140 µm (Table2). Table 1. Physical properties of the soil/glass beads used in the testing program. Material Composition D 15 (µm) D 50 (µm) D 85 (µm) A Glass beads B Glass beads C Soil C u G s Notes: C u = coefficient of uniformity = D 60 / D 10 ; G s = specific gravity. Table 2. Physical properties of the geotextiles used in the testing program. Geotextile Thickness (1) (mm) Mass per unit area (g/m 2 ) FOS (µm) k Go (cm/s) G G G Notes: (1) at 2 kpa vertical stress; FOS = geotextile pore opening size values from hydrodynamic sieving (AFNOR G38017; CFG 1986); k Go = permeability normal to the geotextile plane (no surcharge). 349

4 Percent finer (%) Material A Material C Material B Diameter (µm) Figure 2. Particle size distribution of the soil and glass bead mixtures used in the testing program. 2.3 Testing Methodology The soil/glass beads and geotextiles were saturated before specimen preparation by boiling in water and exposure to a period under vacuum. Following placement of the geotextile, the soil/glass beads was then prepared in a loose state following the methodologies of Vaid and Negussey (1988) for the poorly-graded glass beads and Kuerbis and Vaid (1988) for the well-graded soil. The 50 mm thick layer of soil/glass beads was levelled by siphoning prior to the placement of the top plate, and a vertical stress was applied using a piston. Particles were forced to pass through the geotextile by vibrating the cell, which has a total weight of 13.5 kg, using a BVI-Vibro-Graver (model V-73-C, 8 W) at a frequency of 60 Hz. The vibration device was positioned at three locations on the top reaction plate of the cell (120_ intervals) and the test cell vibrated for 1 minute at each position. This procedure was established from preliminary cell tests, under no surcharge, which yielded a good comparison with the manufacturer s data for this duration of vibration. Although vibration may induce liquefaction of the soil/glass beads in some circumstances depending on the confining pressure, no evidence of such behaviour was observed in the current study. Given the repeatability of the test methodology, the state of the soil/glass beads inside the cell is not believed to influence the test results reported in Section 3. However, the energy imparted by vibration to cause migration of particles during testing is expected to far exceed that resulting from seepage forces in normal field conditions. The approach, while conservative, is in keeping with other hydrodynamic tests (CFG 1986). 350

5 The size distribution of the particles gathered in the collection trough (Figure 1) was determined using a Sedigraph 500 particle size analyser manufactured by the Micromeritics Instrumentation Corporation. The technique uses the sedimentation of small quantities of particles ( 1 g) in combination withx-ray emissions to obtain variations of mixture density with time. This provides a quick and accurate method of measuring particle size and is attractive for tests such as these where the mass of soil passing through the geotextile is small. 3 TEST RESULTS Particle size distributions of the material collected in the trough, after vibration, are reported in Figure 3 for several combinations of soil-geotextile-vertical pressures. Curves for tests A-G1-73 and A-G1-200 are similar, while the curves for A-G3-0 through A-G3-200 suggest some moderate influence (Figure 3a) implying that the magnitude of the confining stress exerts little influence on the particles that pass through the geotextile. It should be pointed out that for a given soil-geotextile system, the gradation of the soil particles that pass through the geotextile during a test is likely to cause a significant scatter depending on the particle size distribution of the soil immediately above the geotextile layer and the distribution of geotextile pore openings. Faure et al. (1990) present a statistical approach for estimating the gradation of the soil passing through nonwoven geotextiles. Tests performed on the well-graded soil (Material C, Figure 3b) at zero applied vertical stress show a behaviour which contrasts with that illustrated in Figure 3a: it is characterized by a ranking of geotextile pore opening size, with the thinner, more open Geotextile G1 passing the greater percentage of large particles. Comparing tests B-G1-0 and C-G1-0, the percentage finer values appear to be dependent on the soil/glass bead type. A characteristic diameter, O 95, for which 95% of the particles were smaller, was established from the curves. A comparison between O 95 and FOS values from the standardized test (hydrodynamic sieving, CFG 1986), for all tests with no surcharge, is given in Figure 4. In spite of the limited number of tests, a generally good agreement between the results is observed taking into account the expected scatter in this type of test (Rigo et al. 1990), particularly for the lighter and less homogeneous Geotextile G1 which has a larger FOS value. The range of variation of geotextile pore opening sizes obtained in hydrodynamic tests observed by Rigo et al. (1990) was of the order of ±25 % of the average value for a given mass per unit area of the geotextile. Repetition of a test under confining vertical stress of 73 kpa and the data in Figure 4 suggests that the scatter and repeatability of this procedure are within the limits observed in hydrodynamic sieving. Figure 5 shows the variation of O 95 with vertical stress for tests on specimens with Materials A and B (glass beads). The application of confining pressures less than 25 kpa is seen to influence the relatively thinner, lighter, more open Geotextiles G1 and G2 the most. Small confining pressures have little influence on Geotextile G3. A small reduction in O 95 is apparent for pressures greater than 25 kpa, which is consistent with the general trend observed for selected tests in Figure 3. All curves trend to a similar value of 50 to 60 µm at pressures greater than 100 kpa. Visual inspection suggests that this behaviour may be attributed to the diameter of the needle-punched holes in the geotextile which may be a controlling factor that limits the sizeofparticle passing throughthe 351

6 (a) Percent finer (%) Diameter (µm) (b) Percent finer (%) Diameter (µm) Figure 3. Particle size distributions of the soil/glass beads passing through the geotextiles: (a) Material A; (b) Material B. 352

7 O 95 value from the current study (µm) Material A Material B Material C FOS from Table 2 (µm) Figure 4. Comparison of geotextile pore opening sizes for tests with no vertical stress. Maximum particle diameter passing, O 95 (µm) Geotextile G1 Geotextile G2 Geotextile G3 Tests with Materials A and B Vertical stress (kpa) Figure 5. stress. Maximum particle diameter passing through the geotextile versus vertical 353

8 (a) (b) Figure 6. Microscopic views of the needle holes in Geotextile G1 under different vertical stresses (magnification = 18 ): (a) needle-punched hole under zero vertical stress; (b) needle-punched hole under 150 kpa vertical stress. (a) (b) Figure 7. Microscopic views of the needle holes in Geotextile G2 under different vertical stresses (magnification = 18 ): (a) needle-punched hole under zero vertical stress; (b) needle-punched hole under 50 kpa vertical stress. geotextile. This is confirmed by the microscopic images of the needle holes in Geotextiles G1 and G2 under 0 and 150 kpa confining pressures (Figures 6 and 7, respectively). It would appear that, even under a confining pressure of 150 kpa, the geotextile pore opening sizes can reach equivalent diameter values of 60 to 80 µm (between two and three times the fiber diameter), which is close to the pore opening size, O 95, obtained 354

9 (a) (b) Figure 8. Microscopic views of the needle holes in Geotextile G3 under different vertical stresses (magnification = 18 ): (a) needle-punched hole under zero vertical stress; (b) needle-punched hole under 50 kpa vertical stress. in tests with Geotextile G1 (Figure 5). Figure 8 shows that the needle-punched holes may not have their sizes so effectively reduced by pressure for the thicker, Geotextile G3. However, in this case, a combination of greater thickness and tortuosity of the pore openings provides additional difficulties for the soil particles to pass through the geotextile. Observations of the pore opening sizes of nonwoven geotextiles, under different confining pressures, using an image analyser (Gourc and Faure 1990) have yielded similar results in terms of the variation of pore opening size with pressure. 4 CONCLUSIONS A methodology that evaluates geotextile pore opening sizes under confining pressure, or without confining pressure, is presented. Limited data, from tests without confining pressure and a specified period of vibration indicate that the O 95 value of particles passing through the needle-punched, nonwoven polyester geotextile compares reasonably well with the pore opening size reported by the manufacturer that was measured using a standardized index test. The following observations are made: 1. Data from tests using confining pressure less than 25 kpa indicate that the O 95 for relatively thin geotextiles, with characteristically larger pore openings, is sensitive to the value of the imposed pressure. However, little significant dependence was observed for any of the geotextiles at confining pressures between 25 and 200 kpa. All three geotextiles converged to a similar O 95 value at higher confining pressures. 2. It is postulated that the behaviour is governed by the needle-punching technique used during geotextile manufacturing. A series of holes are created in the geotextile by the action of the needles. Under visual inspection using a microscope, the holes 355

10 appear as regular, tubular voids through a complex three-dimensional pore structure. When a thinner geotextile with relatively large pore openings was subjected to confining pressure, there was a compression of its pore structure and a reduction in void space. The needle-punched holes became the controlling factor of the passage of fine particles. In contrast, the thicker geotextiles, which have relatively small pore openings, were completely dominated by the presence of needle holes. 3. The method of testing is not intended to replicate field behaviour; rather, it is considered an index test to evaluate the influence of confining stress on pore opening size. Because of the limited amount of data available, the results presented in the current work should be considered as preliminary. Additional studies are being carried out to better understand the potential and limitations of the proposed test methodology. ACKNOWLEDGEMENTS The experimental work was performed by the first author while on sabbatical at the University of British Columbia, Vancouver, Canada. The author would like to thank the members of staff in the Civil Engineering Department at UBC and, in particular, Professor Y.P. Vaid for many helpful discussions. The first author is also indebted to the University of Brasília and CAPES-Brazilian Ministry of Education for financial support. REFERENCES ASTM D 4833, Standard Test Method for Determining Apparent Opening Size of Geotextiles, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. CFG,1986, Geotextile Manual - AFNOR G38017, French Committee on Geotextiles. CGSB, 1992, Filtration Opening Size of Geotextiles, Proposed National Standard of Canada, 148, No. 10, Sixth Draft, Canadian General Standards Board, Ottawa, Ontario, Canada. Fannin, R.J., Vaid, Y.P., Palmeira, E.M. and Shi, Y.C., 1995, A Modified Gradient Ratio Test Device, Symposium on Recent Developments in Geotextile Filters and Prefabricated Drainage Geocomposites, ASTM, June 1995, Denver, Colorado, USA (to be published). Faure, Y.H., Gourc, J.P. and Gendrin, P., 1990, Structural Study of Porometry and Filtration Opening Size of Geotextiles, Geosynthetics: Microstructure and Performance, Peggs, I.D., Editor, ASTM Special Technical Publication 1076, pp Fischer, G.R., Holtz, R.D. and Christopher, B.R., 1993, A Critical Review of Geotextile Pore Size Measurement Methods, Filters in Geotechnical and Hydraulic Engineering, Balkema, Vol. 1, Proceedings of Geo-Filters 92, Karlsruhe, Germany, October 1992, pp

11 Gourc, J.P. and Faure, Y.H., 1990, The Soil, the Water and the Fiber: a Fruitful Interaction Now Controlled, Proceedings of the Fourth International Conference on Geotextiles, Geomembranes and Related Products, Balkema, Vol. 3, The Hague, The Netherlands, May 1990, pp Kuerbis, R.H and Vaid, Y.P., 1988, Sand Sample Preparation - the Slurry Deposition Method, Soils and Foundations, Vol. 28, No. 4, pp Prapaharan, S., Holtz, R.D. and Luna, J.D., 1989, Pore Size Distribution of Nonwoven Geotextiles, Geotechnical Testing Journal, Vol. 12, No. 4, pp Rigo, J.M., Lhote, F., Rollin, A.L., Mlynarek, J. and Lombard G., 1990, Influence of Geotextile Structure on Pore Size Determination, Geosynthetics: Microstructure and Performance, Peggs, I.D., Editor, ASTM Special Technical Publication 1076, pp Shi, Y.C., 1994, UBC Gradient Ratio Test Apparatus, Unpublished Internal Report, University of British Columbia, Vancouver, British Columbia, Canada. Vaid, Y.P. and Negussey, D., 1988, Preparation of Reconstituted Sand Specimens, Advanced Triaxial Testing of Soil and Rock, ASTM Special Technical Publication 977, pp