Comparative study of different porometry determination methods for geotextiles

Geotextiles and Geomembranes 9 (1990) 183-198

Comparative Study of Different Porometry Determination Methods for Geotextiles L. Van der Sluys & W. Dierickx National Institute of AgriculturalEngineering,9220 Merelbeke, Belgium (Received 31 March 1989; accepted 27 June 1989)

ABSTRACT A large number ofporometry determination methods to obtain characteristic opening sizes of geotextiles exists. The paper starts with a general review of different techniques which are detailed in the experimental study. The nature of the granular material used, the renewing of fabric specimens and the sieving time are all possible sources for differences in the results of the vibratory sieving techniques. The hydrodynamic sieving method is discussed and the influence of different operation conditions on the results is elaborated. A comparative study of these sieving techniques reveals remarkable differences between the use of sand fractions and graded soil. Using graded soil the characteristic pore size also largely depends on its grain size distribution. Furthermore the results of different sieving methods are only comparable if the mass of the granular material, the sieving time and the definition of the characteristic opening size are similar.

NOTATION

ax Dw

ol

Ox U /z

Grain size for which x% of the particles are smaller (/xm) Effective opening size (/xm) Characteristic opening size (/zm) Opening diameter for which x% of the openings are smaller (tzm) Geotextile thickness (mm) Uniformity coefficient (d6o/dlo)

Geotextile mass per unit area (g m -2) 183 Geotextiles and Geornembranes 0266-1144/90/$03.50 © 1990 Elsevier SciencePublishers Ltd, England. Printed in Great Britain

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L. Van der Sluys, W. Dierickx

1 INTRODUCTION Geotextiles have a large application field in hydraulic engineering and drainage, where they serve as filter and separation layers. Besides mechanical and hydraulic properties, the choice of geotextiles in these applications strongly depends on the opening sizes or porometry. A characteristic opening size is often used as a design tool. The existing test methods show large differences in results. ~ Moreover, the definition of characteristic opening sizes can differ according to the author. As the larger pores are of primary importance to the filtering and separating capability of a fabric, characteristic pore sizes often used are 090, 095 and 098. These values can be defined as the diameter for which 90, 95 or 98 percent of the pores have a smaller diameter, although the definition is often given in terms of an equivalent grain size, according to the test procedure of the particular method used. Depending on the method, they are called filtration opening (Of), equivalent (EOS) or apparent (AOS) opening size. The designer should be aware of these differences and consider the opening characteristics in relation to the test method. This paper aims to compare, on the basis of experimental data, some well-known pore size determination test methods and to decide on their future use and utility.

2 METHODS Apart from the vibratory and the hydrodynamic sieving methods, which will be elaborated further, other interesting techniques to obtain a characteristic pore size, or at least an idea of the pore size distribution, exist. A first group of methods, based on mathematical considerations, attempts to fit the geotextile structure in either a two- or a three-dimensional model. Rollin et al. 2 developed an analytical method to describe the pore size distribution of a non-woven structure. With the aid of an image-analysing method,3'4 applied on cross-sections of the fabric, it is possible to measure some features such as the fibre density (i.e. the number of fibres per unit cross-section area), the mean fibre diameter and the void distribution. Other characteristics such as the mean, the most probable and the maximum pore diameter can be derived. This technique corresponds very well to the theoretical model and makes it possible to measure pore size characteristics in confined and clogged conditions.4 However, the method, which demands sophisticated and expensive equipment, is limited to thick, non-woven geotextiles. Also Faure et al. 5 attempted to describe a non-

Porometry determination methods for geotextiles

185

woven structure by means of a two-dimensional morphologic approach. The obtained pore-size frequency distribution was also a function of the fibre density, the fibre diameter and the pore diameter. The results of this analysis for needle-punched non-wovens are similar to those obtained by the hydrodynamic sieving method. In general, the theoretical methods only apply to rather thick nonwovens with random fibre structure. They are nevertheless of great value for the study of hydraulic flow models. 3'6 A second group, called suction methods, is based on the laws of surface tension. The principle of the test originated in the determination of the moisture desorption curves and air entry values of porous media. Similar tests were performed by Burghardt 7 on drainage envelopes, by Paute & C h i n e 8 on different non-wovens and by Dennis & Davies 9 on thick needle-punched non-woven filters. In spite of the advantages of preserving field conditions (moisture content, soil overburden pressure), this technique is not suitable for all types of geotextile. The category of methods most familiar in determining the porometry of geotextiles consists of dry or wet vibratory sieving and hydrodynamic sieving. Tables 1 and 2 show some features of the most important methods. Although the principles within both the vibratory and the hydrodynamic sieving tests are quite similar, there exist some potentially important differences that affect the characteristic pore size value measured. The study of the influence of these differences on the characteristic pore size forms the subject of this investigation.

3 EXPERIMENTAL WORK The investigation consisted of a comprehensive study of the dry and the wet vibratory sieving techniques and of the hydrodynamic sieving technique. The results, on the geotextiles that are detailed in Table 3, are compared and discussed.

3.1 Vibratory sieving methods This part of the investigation was carried out to evaluate the influence of dry or wet sieving on the characteristic opening size value, using sand or glass-bead fractions. Also the influence of renewing, or not renewing, the fabric specimen for each fraction used and the influence of the sieving time were considered. The Belgian specifications use the dry sieving method applied at the National Institute of Agricultural Engineering, Table 1, which is the

Fractions 1.59

Fractions 1-59

Distribution Mass (kg m -2)

090, 098 b

Filtration opening 090, 098 b

300

1 200 095 c

090, 095, 098 b

Sand Glass beads Fractions 1.59

0.031 4

U S C E 13

300

50 0.75

Fractions 1-59

Sand

0.031 4

Dry sieving

R I L E M 12

aNIAE: National Institute of Agricultural Engineering, Merelbeke, Belgium. DHL: Delft Hydraulics Laboratory, Delft, The Netherlands. RILEM: R6union Internationale des Laboratoires des Essais de Mat6riaux, France. USCE: United States Corps of Engineers, USA. ICI: ICI Laboratories, Harrogate, UK. FRANZIUS: Franzius Institute, University of Hannover, FRG. bFrom the mean fraction diameter. CFrom the smallest fraction diameter.

300

Duration (s)

50 0-75

Sand

Sand

Granular material Type

50 0.75

0.031 4

0.031 4

Sample area (m2)

Vibration characteristics Frequency (Hz) Amplitude (mm)

D H L 11

N I A E 10

Origin ~

TABLE 1 Vibratory Sieving Methods

b

50 No vertical displacement of the particles 300

Fractions 1.41

Glass beads

0-070 7

ICI

Calculation

900

50 1-5

Graded Wovens 3-93 Non-wovens 11.8

Sand

0-025 4

Wet sieving

F R A N Z I U S 14"15

Sand

Graded, U = 4

1.8 or 7

Granular material

Distribution

Mass (kg m -2)

2 500 O9o, O98 O95

24

Rotary 31-43 6~ 25-35 0.10

31

Graded

Sand

0-070 7 4

CEMAGREF

16"17

095

24-36

Alternating vertical 37-50 7-10 30-40 0.10-0-15

4-10

Graded

Sand

0.070 7 4

IRIGM lds

2500 090, 095, 098

0-10

30-60 3 <- to/ti -< 5

Graded, U > 6, 4dlo-< 0 i 4-10

Sand

0-070 7 4

R I L E M t2

aNIAE: National Institute of Agricultural Engineering, Merelbeke, Belgium. CEMAGREF: Centre National du Machinisme Agricole, du G6nie Rural, des Eaux et des For6ts, Antony, France. IRIGM: Institut de Recherches Interdisciplinaires de G6ologie et de M6canique, Grenoble, France. RILEM: R6union Internationale des Laboratoires des Essais de Mat6riaux, France. BUWA: Bundesanstalt fiir Wasserbau, Karlsruhe, FRG.

Duration (h) Cycle number Filtration characteristic

Movement Alternating vertical Cycle time (s) 24--40 Immersion time ti (s) 8-13 Emersion time to (s) 16-27 Immersion depth (m) 0.05

0-028 4 5

Sample area (m 2) Sample number

NIAE

Origin a

TABLE 2 Hydrodynamic Sieving Methods

34

Alternating vertical 60 30 30 O-40

85

Three characteristic soils Graded

0.017 7 10

B U W A 19

---.I

t~

~°

188

L. Van der Sluys, W. Dierickx TABLE 3

Some Characteristics of the Geotextiles Investigated Fabric~

Type b

txc (g m-2)

Tga (ram)

W1 W2 W3 W4 W5 W6 W7 NW1 NW2 NW3 NW4 NW5 C

PP-tapes PP-tapes PP-tapes PE-monofilaments PES-multifilaments PP-tapes/PE-monofilaments PE-monofilaments/PES-multifilaments PES needle-punched PES needle-punched PP needle-punched PP heat-bonded PP/PE heat-bonded PE-monofilament woven + PP-staple fibres

120 220 250 210 540 290 297 241 270 147 111 139 680

0-65 0.80 0-93 0.88 1.13 0-87 0.72 2-37 2.65 1.31 0-45 0.69 4.99

aW = Woven;NW = non-woven;C = composite;PP = polypropylene; bPE = polyethylene; PES = polyester. c/z = Mass per unit area according to the Swiss standard.15 dTg = Mean geotextile thickness at 2 kPa.

TABLE 4

The Fraction Limits (/zm) 50--63 63-75 75-100

100-125 125-150 150-200

200-250 250-300 300--400 400-500 500--600 600-710

710-800 800-1 000 1 000-1 190 1 190-1 410 1 410-1 680 1 680-2 000

m e t h o d t h a t was originally d e v e l o p e d in the N e t h e r l a n d s . n In the w e t sieving t e c h n i q u e , c a r r i e d o u t u n d e r t h e s a m e c o n d i t i o n s , d e m i n e r a l i z e d w a t e r is s p r a y e d o n t h e g e o t e x t i l e surface. T h e sprinkling r a t e is a d j u s t e d , with r e g a r d for t h e p e r m e a b i l i t y o f the fabric, in o r d e r to i m m e r s e t h e g r a n u l a r m a t e r i a l o n t h e fabric. In t h e first stage, t h e influence o f using sand f r a c t i o n s o r g l a s s - b e a d f r a c t i o n s o n t h e c h a r a c t e r i s t i c p o r e size v a l u e was i n v e s t i g a t e d . T a b l e 4 gives the r a n g e o f f r a c t i o n limits.

189

Porometry determination methods for geotextiles

The tests were carried out on three geotextiles (W4, W6, NW2). The geotextile specimen was only renewed for each replication, not for each fraction. Table 5 shows the resulting 090- and O98-values, being average values of five replications, and the variation coefficients. The large variation coefficients are due to the heterogeneity of the fabrics and/or incompatibility problems between fabric and granular material, e.g. the needle-punched non-woven NW2 in combination with glass beads. The differences in electrical susceptibility and shape of the granular material are more

TABLE 5

Mean O90-and O98-values(/zm) and Variation Coefficients(%, in parentheses) Obtained by Dry and Wet SievingUsing Sand and Glass-bead Fractions Fabric

w4 w6 NW2

Of

090 098 090 098 090 098

Dry sieving

Wet sieving

Sand

Glass beads

Sand

Glass beads

395 (5) 462 (5) 337 (9) 411 (12) 174 (8) 198 (9)

366 (3) 406 (2) 398 (6) 460 (10) 135 (14) 154 (21)

345 (5) 408 (8) 304 (3) 356 (8) 147 (11) 210 (8)

352 (2) 406 (3) 317 (4) 372 (4) 143 (21) 209 (12)

accentuated by dry sieving. The dry sieving technique also results in somewhat higher pore size values for the W4 and W6, but in a lower O98-value for the NW2. A two-factorial variance analysis of these average values, however, shows neither significant differences between the dry and wet sieving, nor between the sand and glass-bead fractions, as well as for the 090- and the O98-values. Secondly, the influence of renewing the fabric specimen each time a new fraction is used was investigated by using sand fractions. Five geotextiles were selected and the resulting 090- and O98-values as averages of five replications are given in Table 6. Renewing the fabric specimen does not increase the variation coefficient and the characteristic filtration opening is more representative because of the larger surface tested. Some fabrics, such as the very fine multi-filament woven W5, are susceptible to clogging, especially when dry sieving is applied. This means that not renewing the specimen decreases the possibility of particles passing through the geotextile and gives rise to lower 090- and O98-values. The needle-punched fabric

190

L. Van der Sluys, W. Dierickx

TABLE 6 Mean 09o- and O98-values (/zm) and Variation Coefficients (%, in parentheses) Obtained by Dry and Wet Sieving with and without Renewing the Geotextile Specimen Fabric

W2 W4 W5 W6 NW2

Of

090 098 090 098 090 098 090 098 090 098

Dry sieving

Wet sieving

Specimen renewed

Specimen not renewed

Specimen renewed

Specimen not renewed

188 (4) 255 (7) 377 (4) 443 (5) 83 (7) 122 (7) 294 (5) 339 (5) 166 (6) 188 (6)

195 (16) 251 (11) 395 (5) 462 (5) 76 (4) 96 (6) 337 (9) 411 (12) 174 (8) 198 (9)

214 (5) 308 (7) 375 (3) 435 (4) 64 (4) 78 (5) 312 (4) 365 (6) 129 (7) 150 (7)

204 (14) 281 (26) 345 (5) 408 (8) 61 (2) 70 (5) 304 (3) 356 (8) 147 (11) 210 (8)

NW2 exhibits the opposite effect. This non-woven fabric has a large retaining capacity for particles and can falsify the measurements of the passed fractions by releasing particles of a foregoing fraction, if consecutive fractions are used on the same specimen. Nevertheless, a two-factorial variance analysis does not give significant differences between renewing the geotextile specimen or not, for both the 090- and the O98-values. As in the previous case, differences between dry and wet sieving were not significant either. Using sand fractions the influence of the sieving time on the O90-value was investigated for a sieve of 150/zm and three geotextiles. The results are presented in Fig. 1 and show, on average, a slight continuous increase with time. Doubling the sieving time from 5 to 10 min increases the O9o-value by 1.9% for the sieve alone, 4.0% for NW2, 5.4% for W6 and 9.7% for Wl. A sieving time of 30 min results in relative increases of 4.9, 10-2, 14.1 and 25-1% respectively. The increase is highest for the woven fabrics and lowest for the stiff sieve but the rate of increase diminishes with time. The woven fabrics are likely to show some alteration in their structure under prolonged vibration, especially when produced from flat tapes (Wl). The increase in the O9o-value with time loses importance when considering the variation coefficient; it ranges between 5 and 10% and is of the same order of magnitude.

200

250

300

350

400-

100 • 0

110 -

120-

t

o

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1'o

o

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.=

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4.

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+

o

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40

4b

-

16o -

17o-

18o -

19o

2oo-

210 -

220-

230-

240-

250"

100

150

200

0

n

1'0

=

;o

'

u

e

i

2~0

o

:

='o (b)

(d)

30

4.

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250

(c)

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t

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t (rain)

__0

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t (min)

~

(a)

20

§

i

300

=

.*

J

4,

3 *0

•

+

~o

4"

'

40

40

Fig. 1. The influence of the sieving time on the O9o-value. (a) Sieve 150/~m, 5 replications; (b) W1, 3 replications; (c) W6, 3 replications; (d) NW2, 3 replications.

0

::L v o

?

140-

0

130 -

150-

160-

~o

=L

170 -

180-

190-

200

]

192

L. Van der Sluys, W. Dierickx

I

I

I

Fig. 2. The hydrodynamic sieving apparatus.

3.2 Hydrodynamic sieving method In the hydrodynamic sieving method the geotextile specimen, loaded with a certain quantity of graded granular material, is alternately immersed and emersed into a water reservoir, forcing particles to pass through the geotextile. After a test period, long enough to ensure all fine particles have passed, the grain size distribution of the granular material in the reservoir determines the porometry of the geotextile investigated. The apparatus, developed at the National Institute of Agricultural Engineering, consists of five clamp devices connected to a crankshaft that is driven by an electric motor (Fig. 2). Each clamp device is provided with a geotextile specimen and immersed into its own water reservoir. The crankshaft speed can be adjusted from 0 to 6 rpm. Other characteristics of the apparatus are given in Table 2.

Porometry determination methods for geotextiles

193

TABLE 7

Mean 090- and O9s-values (/~m), Mean Passage Percentage by Weight, and Variation Coefficients (%, in parentheses) Obtained by the Hydrodynamic Sieving Method under Different Operational Conditions

Fabric Soil Wl

GS

GG S W6

GS

GG S NW5

GS

GG S

Mass (g)

Cycle number

Speed (rpm)

09o

09s

Passage

50 100 200 200 200 200

2 500 2 500 2500 2 500 2500 2 500

2-5 2.5 2.5 1.5 2.5 2-5

240 (6) 233 (5) 196 (3) 196 (5) 239 (9) 275 (2)

315 (8) 310 (8) 268 (10) 254 (6) 296 (11) 306 (3)

58.3 55.2 41.6 44.2 37.1 20.5

50 100 200 200 200 200 200 200 200 200

2 500 2 500 2 500 625 1 250 5000 2 500 2 500 2 500 2 500

2.5 2.5 2.5 2-5 2.5 2-5 1-5 5 2.5 2.5

225 (4) 223 (7) 217 (5) 199 (3) 200 (5) 240 (10) 203 (4) 198 (10) 227 (5) 308 (6)

289 (3) 290 (4) 281 (3) 263 (2) 259 (5) 323 (11) 265 (7) 257 (9) 281 (2) 367 (7)

52.7 (4) 51.3 (14) 50"3 (11) 17.8 (26) 31"7 (23) 56.3 (10) 47.9 (7) 40.1 (24) 35.8 (19) 67.4 (7)

50 100 200 200 200 200

2500 2 500 2 500 2 500 2 500 2 500

2-5 2-5 2.5 1.5 2.5 2.5

158 (9) 144 (7) 135 (9) 139 (7) 128 (9) 205 (4)

195 (11) 175 (9) 182 (6) 171 (5) 166 (21) 277 (2)

27.4 18.1 9.4 5.1 10.3 3.7

(7) (7) (17) (18) (19) (30)

(27) (40) (65) (26) (54) (39)

A p r e l i m i n a r y investigation o n t h r e e geotextiles ( T a b l e 7) was carried o u t to set t h e o p t i m a l w o r k i n g conditions. T h e factors investigated w e r e : - - T h e soil type: a c o m p o s e d g r a d e d sand ( G S ) ; an identical glass-bead s a m p l e ( G G ) ; a s t e e p - g r a d e d sand (S) (Fig. 3). - - T h e soil mass: 50, 100 or 200 g per geotextile s p e c i m e n c o r r e s p o n d ing to 1.76, 3.53 or 7.05 kg m -2. - - T h e test d u r a t i o n , e x p r e s s e d as a total cycle n u m b e r : 625, 1250, 2500 and 5000 cycles. - - T h e cycle speed: 1.5, 2.5 and 5 rpm. T h e i m p a c t of these conditions on 09o- and O98-values a n d o n the p a s s a g e p e r c e n t a g e was studied in a variance analysis. T h e passage p e r c e n t a g e was m o r e a f f e c t e d b y changing test conditions than the charac-

L. Van der Sluys, W. Dierickx

194

100" -

==

-

.................

6S-OG S

50-

0 10

100

groin size (p.m)

10~

Fig. 3. Grain size distribution of the granular material used.

teristic pore size values. A significant influence of the soil type, especially of the steep-graded sand (S) was observed. This almost uniformly graded granular material is not suitable for testing all kinds of geotextiles. The steep-graded sand shows only high passage amounts for the relatively open fabric W6. The amount of granular material influences the characteristic pore sizes and the passage percentage of some geotextiles. The duration of the test has an increasing influence on the results. No significant differences were observed in changing the cycle speed, although 5 rpm is not recommended for fabrics of relatively low permeability. In conclusion, the optimal working conditions of the hydrodynamic sieving apparatus described are: - - A well-graded granular material sample of 50 g per basket; higher quantities are allowed in combination with longer test runs. - - A test duration of at least 2500 cycles. - - A maximum cycle speed of 2.5 rpm. Under these well-defined conditions the hydrodynamic sieving method gives repeatable results, although, in some cases, the passage percentage shows large variation coefficients.

3.3 Comparative study of vibratory and hydrodynamic sieving To compare the vibratory dry and wet sieving with the hydrodynamic sieving both a well-graded sand GS (Fig. 3) and uniform sand fractions (Table 4) were used. For the vibratory sieving techniques a sieving time of

Porometry determination methods for geotextiles

195

TABLE$ Mean 090- and O9s-values(/xm), and Variation Coefficients(%, in parentheses) Obtained by Dry, Wet and Hydrodynamic SievingUsing Uniform Sand Fractions and Well-Graded Sand Fabric

Of

Dry sieving Fractions

W3 W4 W6 W7 NW1 NW3 NW4 C

090 098 090 098 090 O9s 090 098 09o 098 090 098 090 098 090 098

278 (6) 348 (4) 354 (4) 416 (6) 294 (5) 339 (5) 253 (2) 260 (3) 179 (3) 202 (4) 204 (1) 236 (4) 210 (4) 254 (6) 161 (2) 215 (11)

Wet sieving

Hydrodynamicsieving

Graded Fractions Graded Fractions Graded

250 (5) 339 (5) 276 (9) 328 (9) 256 (6) 305 (9) 180 (1) 209 (2) 155 (10) 189 (8) 184 (8) 218 (8) 182 (7) 235 (10) 124 (3) 155 (12)

325 (3) 438 (6) 381 (3) 434 (3) 312 (5) 365 (9) 226 (2) 254 (4) 178 (1) 192 (2) 191 (12) 220 (9) 211 (4) 263 (6) 144 (2) 161 (4)

301 387 307 358 250 295 172 210 143 195 145 191 189 251 115 140

(11) (11) (4) (3) (1) (2) (1) (2) (3) (6) (2) (7) (10) (8) (4) (4)

349 412 361 406 324 373 223 244 165 192 188 220 209 245 113 134

(6) (6) (2) (4) (3) (6) (3) (3) (5) (5) (5) (6) (8) (4) (2) (3)

282 (4) 374 (7) 303 (3) 360 (3) 225 (4) 289 (3) 194 (3) 224 (1) 133 (7) 181 (12) 150 (3) 202 (5) 150 (11) 222 (13) 101 (3) 129 (4)

300 s and an amplitude of 0-75 m m at a frequency of 50 Hz were applied. The hydrodynamic m e t h o d was carried out at a test time equivalent of 2500 cycles with a speed of 2-5 rpm. In all cases 50 g of granular material per geotextile specimen was used. The geotextile specimens were taken at random without any preliminary selection on the basis of mass per unit area or other properties. Although not necessary, as already pointed out, the specimen was renewed each time another fraction was used. In this way the n u m b e r of fabric specimens, necessary for a determination with five replications, is five times the n u m b e r of fractions needed. For graded sand five specimens only will do. The results presented as 090- and O98-values and their variation coefficients for eight different geotextiles are given in Table 8. A two-factorial variance analysis gives highly significant smaller characteristic pore size values when graded sand is used. Furthermore the coincidental differences already present at the sieving techniques and at the interaction between sieving technique and granular material are probably already such that no systematic differences could be found.

196

L. Van der Sluys, W. Dierickx

4 DISCUSSION Only those methods that allow the determination of the porometry of all types of geotextiles can be accepted. Direct measurements by means of optical equipment, as well as theoretical and suction methods, are not suitable for all kinds of geotextiles. The indirect sieving methods meet this requirement because of the range of grain sizes that largely exceeds the possible opening sizes of geotextiles. Only one single method or only those methods giving the same porometry or characteristic pore sizes can be allowed to compare geotextiles and to select unambiguously suitable geotextiles for a given soil. The true porometry of a geotextile is of secondary importance because the geotextile pore size obtained according to a given method must be linked to the soil particle sizes. Moreover classical sieving methods have advantages, in contrast to optical methods, because of their inexpensive standard equipment. The investigations on the vibratory methods do not indicate significant differences between dry and wet sieving tests. Whether dry or wet sieving gives larger 090- and O98-values, is due to coincidental factors such as geotextile structure, for example. From the results in Tables 5 and 6 it appears that the ratio of wet to corresponding dry sieving results ranges from 64% for the very fine textured multi-filament fabric W5, to 136% for the needle-punched non-woven NW2; other fabrics accord better. Also, the use of sand or glass-bead fractions does not result in significant differences, although the dry sieving accentuates more the influence of the granular material, as was already pointed out. The ratio of the results with glass beads and sand in Table 5 fluctuates from 78 to 118% for dry sieving and only from 91 to 105% for wet sieving. Finally, the results from renewing the geotextile specimen for each other fraction do not differ significantly from the results with only one specimen. Increasing the sieving time results in higher characteristic pore size values. The short sieving time of 5 min seems appropriate in view of minimum alteration of the fabric, of small relative increases of the characteristic pore size values and of time- and cost-saving advantages. Hydrodynamic sieving is a possible alternative to the vibratory sieving methods when optimal working conditions are selected initially. Nevertheless, the need for special equipment and the duration of the test are disadvantages. The results of the hydrodynamic sieving method do not differ significantly from those of the dry and wet sieving techniques using the same granular material. However, the use of graded soil results, for all sieving methods, in highly significant lower characteristic pore size values compared to the use of sand fractions. The differences in results between

Porometry determination methods for geotextiles

197

vibratory sieving and hydrodynamic sieving must be attributed to the different granular material usually utilized, namely sand fractions for the dry and wet sieving, and graded sand for the hydrodynamic sieving.

5 CONCLUSIONS The major conclusion of this experimental study is that all sieving methods give in principle the same results when the same granular material is used; however, coincidental factors will always influence the results. Of far greater influence is the granular material used. The use of sand or glass-bead fractions results in significantly higher characteristic pore size values compared to the use of graded soil. Using graded soil the characteristic pore size of a geotextile will largely depend on its grain size distribution. Therefore only one well-defined simple sieving method of short duration is preferred whereby the determination of the characteristic pore size is based on the fraction method of either sand or glass beads. Preferably an equal quantity of granular material per unit geotextile area and the same sieving time are used; furthermore renewal of the geotextile specimen at every other fraction of granular material is recommended. Only in this way can comparable characteristic pore size values be obtained. Moreover, it is also desirable, for the sake of simplicity, to standardize the definition of the characteristic opening size. This research clearly defines the reasons for differences between currently used determination methods, and hopefully it may contribute to designing a well-defined porometry determination method for deducing uniform design criteria.

A C K N O W L E D GEMENTS This research work was made possible thanks to the financial support of UCO-Geotextiles (Ghent), the Ministry of the Flemish Community and the Ministry of Agriculture.

REFERENCES 1. Fayoux, D., Cazuffi, D. & Faure, Y., La drtermination des caractrristiques de filtration des grotextiles: comparaison des rrsultats de diffrrents laboratoires. Presented at Materials for Dams Conference, Monte-Carlo, 1984. 2. RoUin, A. L., Masounave, J. & DaUaire, G., Etudes des propriEtrs hydrau-

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