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Drainage and filtration properties of non-woven geotextiles under confinement using different experimental techniques
Geotextiles and Geomembranes 20 (2002) 97–115
Drainage and filtration properties of non-woven geotextiles under confinement using different experimental techniques Ennio M. Palmeira*, Maria G. Gardoni Department of Civil and Environmental Engineering, University of Brasilia, FT, 70910-900 Brasilia, DF, Brazil Received 14 August 2001; received in revised form 24 December 2001; accepted 5 January 2002
Abstract This work presents a study on the influence of stress level on some physical and hydraulic properties of non-woven geotextiles relevant to drainage and filtration. Different types of tests were employed for the determination of geotextile thickness, porosity, permittivity, transmissivity and pore sizes under normal stresses ranging from 0 to 2000 kPa, depending on the type of test. Results of these different testing techniques were compared and their accuracy assessed. The increase of geotextile retention capacity caused by increasing stress levels and geotextile partial clogging was also evaluated, as well as the accuracy of equations for the estimate of geotextile permeability. The results obtained showed a marked effect of confinement on geotextile properties, with special reference to pore size diameters, with implications to current filter criteria for non-woven geotextiles. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Geotextiles; Normal stress; Physical and hydraulic properties; Pore sizes; Filter criteria
1. Introduction Geotextiles have been extensively used as drainage and filter materials in geotechnical and geoenvironmental works in the past 30 years. These applications have been overwhelmingly successful, and up to date most of the designs have been based on empirical and, sometimes, unrealistic criteria. This practice may yield to *Corresponding author. Tel.: +55-61-273-7313; fax: +55-61-273-4644. E-mail address: [email protected] (E.M. Palmeira). 0266-1144/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 0 2 ) 0 0 0 0 4 - 3
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misleading performance predictions or overconservative designs. In general, nonwoven geotextiles are very permeable and compressible materials. As the thickness of the geotextile reduces under stress so does its coefficient of permeability and pore dimensions. Pioneer works studying the effects of stress level on the hydraulic behaviour of geotextiles can be found in the literature (Gourc, 1982; Gourc et al., 1982a, b; McGown et al., 1982, for instance). Today, there is an increasing appeal for the use of geotextiles in large projects, which finds a barrier in the fact that little information is available on the behaviour of these materials under high stress levels. It should be noted that drainage of large embankments or of moderate high mining waste piles can cause normal stresses on the drainage system in excess of 1 MPa. Several experimental techniques can be applied to the determination of physical and hydraulic properties of geotextiles, which are of utmost importance for the design of drains and filters. Most of these techniques give measures of these properties under unconfined conditions, but some of them can also be adapted to perform tests under confinement. This work presents comparisons between physical and hydraulic properties of non-woven geotextiles under confinement using different testing techniques.
2. Equipment and materials used in the tests 2.1. Equipment The aim of the present work is to investigate the variation of physical and hydraulic properties of non-woven geotextiles under a range of normal stresses varying from 0 to 2000 kPa. The properties or dimensions investigated were porosity, thickness, permittivity, transmissivity, density of fibres and pore dimensions. The following experimental techniques were employed: permittivity tests, transmissivity tests, image analysis and bubble point tests. A description of each technique is summarised below. Permittivity tests under confinement were performed using an apparatus of the Geosynthetics Technology CentreFSAGEOS, Canada, based on the one recommended in ASTM D 5493 (ASTM, 1993). The equipment is capable of performing permittivity tests under normal stresses up to 550 kPa. Fig. 1 shows schematically the equipment used in the tests. A steel piston applies the normal load to a rigid load plate on a 50.8 mm diameter geotextile specimen, confined between two sets of steel meshes, with apertures ranging from 0.1 to 0.8 mm. The meshes serve as permeable medium that helps to distribute the normal stress uniformly on the geotextile specimen. The equipment used for the determination of geotextile longitudinal permeability and transmissivity is presented in Fig. 2. It is also based on a previous version of the apparatus presented in ASTM D 4716 (ASTM, 1993). The geotextile specimen is 100 100 mm and is subjected to the normal stress applied by a rigid metal plate. The equipment is capable of testing specimens under normal stresses up to 2000 kPa. The variation of the geotextile thickness with normal stress during the test can also
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Fig. 1. Permittivity test equipment.
Fig. 2. Transmissivity test equipment.
be obtained, as a function of the vertical displacement of the rigid plate, calculated as the average of the readings from displacement transducers positioned at three different locations on the plate. Distilled water was used in both permittivity and transmissivity tests. The values of hydraulic gradients used in these tests were established after specific tests having been performed to determine the range of hydraulic gradient values to guarantee laminar flow conditions for the geotextiles used. The hydraulic gradients used were equal to 2 for transmissivity tests and varied between 6–10 (depending on geotextile type and normal stress applied) for permittivity tests. The Bubble Point test was used for the determination of geotextile pore channels constriction sizes distribution curve and permeability. The constriction size distribution curve is obtained based on the knowledge of the pressure in a fluid that is required to overcome the capillary attraction of the fluid in the largest pore. Fischer et al. (1996) comment that the bubble point method is advantageous in comparison with other types of tests because it simulates the flow of fluid in the geotextile pore channels, can be performed quickly and efficiently, the results are
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repeatable and provides an accurate estimate of the geotextile permeability. A Coulter Porometer IIR, available at SAGEOS, Canada, was employed to perform the tests. This equipment is capable of working with a stress range from 0.2 to 1400 kPa, depending on the geotextile characteristics, and a maximum flow rate of 100 l/min. In the present study, the normal stresses applied to the geotextile specimens varied from 0 to 550 kPa, depending on the geotextile characteristics. The tests and relevant calculations were performed according to ASTM F 316-86 standard. Analyses of images of compressed geotextiles were also employed as part of the investigation of physical and hydraulic properties of geotextiles under confinement. An image analyser Clemex Impak, from Clemex Technology Incorporation (Canada), was employed to obtain images of geotextile microstructure. The equipment consists of a photographic camera Sony 930/950 RGB and a data acquisition system for data processing. In this technique the geotextile image is scanned and converted to digital image that can be enhanced for measurements. Relevant measurements obtained are total area of fibres, percentage of fibres, density of fibres (number of fibres per unit cross-sectional area), distance between fibres and porosity. Geotextile specimens subjected to compressive stresses up to 1000 kPa were analysed in the image analyser.
2.2. Geotextile materials Five non-woven, needle punched, geotextiles (codes GA to GE) made of polyester, from the same manufacturer, were tested. The main characteristics of these geotextiles are presented in Table 1. The mass per unit area of the geotextiles vary from 180 to 600 g/m2, covering the range of geotextile mass per unit area values often found in practice. The geotextile fibres are cylindrical, with diameters varying from 0.0245 to 0.0279 mm, depending on the geotextile type. Additional information on testing equipment, methodologies and materials can be found in Gardoni (2000).
Table 1 Geotextile characteristics Geotextile
MA (g/m2)
tGT (mm)
n
O95 (mm)
df (mm)
GA GB GC GD GE
200 400 600 180 300
1.72 2.89 4.60 2.20 3.30
0.94 0.90 0.89 0.93 0.92
0.135 0.110 0.060 0.140 0.110
0.0245 0.0279 0.0250 0.0270 0.0270
Notes: MA =mass per unit area, tGT =geotextile thickness (under 2 kPa normal stress), n=geotextile porosity, O95=filtration opening size (hydrodynamic sieving test, CFG 1986), df =geotextile fibre diameter.
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3. Test results 3.1. Variation of physical properties with normal stresses Geotextile permittivity and transmissivity depend on geotextile thickness, which can be determined by different types of compression tests. Because most of the tests used in this work involve compression stages of the geotextile specimens, it would be interesting to compare values of geotextile thickness and porosity obtained by different experimental techniques. Figs. 3(a)–(c) show the variation of geotextile thickness with normal stress obtained from different testing methods, for stress levels up to 1000 kPa. Most of the compression of the geotextile specimens occur for normal stresses up to 200 kPa. For normal stresses above this value, a considerable decrease of the rate of thickness reduction with normal stress is noticeable. In general, a good agreement between measures from different apparatus is observed, particularly for the lighter geotextiles tested (GA and GB). For the thicker geotextile GC, some deviations of the results obtained in the permittivity tests, from those of the rest of the tests performed, can be seen for normal stresses in excess of 200 kPa. It is likely that these deviations may have been caused by accommodation or deformations of the steel meshes and local deformations of the geotextile specimen inside mesh apertures under higher stresses. At the end of the tests the marks left by the meshes members on the geotextile specimen surface could be clearly seen.
Fig. 3. Geotextile thickness versus stress from different tests.
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The results of compression tests for geotextile GC (Palmeira, 1997) using a pack of geotextile specimens (8 layers, 100 mm dia., tested up to 200 kPa normal stress), rather than a single specimen, is also plotted in Fig. 3(c). The equipment used for testing packs of geotextiles is described in Palmeira et al. (1996) and is similar to the one presented in Fig. 1, but with the normal stress on the geotextile pack being applied by a perforated rigid plate. Similar tests on pack of geotextile specimens can be found in Gourc et al. (1982a) and McGown et al. (1982). In the case of tests on packs of geotextiles, a better agreement with the results from other types of tests was found, stressing the possible effect of the steel meshes on the results of tests with individual geotextile specimens. The variation of geotextile porosity with normal stress is shown in Figs. 4(a)–(c). The geotextile porosity was calculated based on geotextile thickness, mass per unit area and density of fibres. Again, a reasonable agreement between different forms of determination of geotextile porosity can be observed. For the thicker geotextile the values of geotextile porosity determined in the permittivity tests were significantly lower than those obtained in the other types of tests, as a consequence of the smaller thickness measured, as commented above. Images obtained by the Image Analyser can be used to assess micro-structural characteristics of the geotextiles tested. The dependency of the density of fibres (number of fibres per unit cross-section area of the geotextile) with normal stress is presented in Fig. 5 for the three geotextiles tested. The lighter the geotextile, the
Fig. 4. Geotextile porosity versus stress from different tests.
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Fig. 5. Density of fibres versus normal stress.
smaller the density of fibres. For normal stresses above 200 kPa the density of fibres increases with normal stresses at a greater rate, which is consistent with the smaller rate of reduction observed for other geotextile physical properties for stresses above that value. Figs. 6(a) and (b) show images of the distribution of fibres for geotextile GA for normal stresses equal to 2 and to 1000 kPa, respectively. For low stress levels the fibres are distributed through the geotextile cross-sectional area in a rather sparse mode. Only a few fibres are in direct contact to each other. For larger stress levels packs of fibres can be identified, with most of the fibres in direct contact. In the latter
Fig. 6. Cross-sections of geotextile GA under different normal stresses.
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case, the larger voids controlling the magnitude of permeability coefficient and filtration, are found between packs of fibres in contact.
3.2. Variation of hydraulic properties with normal stress The variations of geotextile permittivity with normal stress for the geotextiles tested are presented in Figs. 7(a)–(c). These figures show comparisons between permittivity values obtained by standard permittivity tests with single specimen (Fig. 1), permittivity tests on pack of specimens of geotextile GC (Palmeira, 1997) and bubble point tests. A very good agreement between test results from these testing methods can be noted, particularly for the thicker geotextiles. For the range of normal stresses tested the permittivity of the lighter geotextile GA was reduced by a factor of 3, while for the heavier geotextiles (GB and GC) a reduction by a factor of 10 can be noticed. The reduction of permittivity of geotextile GA for normal stresses above 200 kPa also occurs at a lower rate than those for geotextiles GB and GC. This behaviour can be explained by the combination of lower thickness and mass per unit area for the lighter geotextile. The results presented in Fig. 7 suggest that the standard, low cost and simple permittivity test provides values of geotextile permittivity as accurate as the more complex and expensive bubble point test. This conclusion is corroborated by the test results grouped in a direct comparison, as shown in Fig. 8.
Fig. 7. Geotextile permittivity versus stress from different tests.
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Fig. 8. Comparison between permittivity values from permittivity tests and bubble point tests.
Transmissivity test results under stress levels up to 2000 kPa are shown in Figs. 9(a) and (b) for geotextiles GA and GC. Similar results for other geotextile products have been reported in Palmeira and Gardoni (2000a) and are not reproduced here. The results in Fig. 9 show that the geotextile transmissivity under high stress levels can be reduced by a factor of 2–3 orders of magnitude, depending on geotextile thickness. This reduction in geotextile transmissivity will have a marked effect on the discharge capacity of the geotextile drain. Therefore, due attention must be paid to situations where geotextile drains will be used under high normal stresses. Results from permittivity and transmissivity tests under pressure allow the assessment of the variation of geotextile permeability anisotropy with the stress level. Figs. 10(a)–(c) show a marked anisotropy for the geotextiles tested for stress levels below 50 kPa. As the normal stress increases, geotextiles GA and GB tend to show a more isotropic behaviour with regard to permeability.
Fig. 9. Results of transmittivity tests under pressure.
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Fig. 10. Permeability coefficients versus normal stress.
3.3. Variation of constriction sizes with normal stress Compressive stresses will also influence geotextile pore volume, and so its filtration behaviour. Bubble point tests with geotextile specimens subjected to normal stresses varying from 2 to 550 kPa were performed as part of the present research programme. Figs. 11(a)–(c) show constriction pore diameter distribution curves for geotextiles GA, GB and GC. As the normal stress increases, the distribution of pores tends to be more uniform. Most of the reduction of pore constriction diameters takes place for normal stresses up to 100 kPa. Fig. 12 presents the variation of constriction diameters On with normal stress for three of the geotextiles tested in this work, where On is the constriction diameter for which n% of the constrictions are smaller. For the three geotextiles the values of O98, O95 and O90 are the ones that are most sensitive to the normal stress. A 32% reduction in the value of O98 of geotextile GA at a normal stress of 50 kPa is observed with respect to that value at 2 kPa normal stress. If the value of filtration opening size for that geotextile presented in Table 1 is considered as a reference, the reduction of the maximum pore diameter can reach 36%. At 550 kPa normal stress the reduction in O98 and O95 for geotextile GA is 44% and 24%, with respect to those values obtained at 2 kPa, and 47% and 54% with respect to the value of FOS in Table 1.
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Fig. 11. Constriction size distributions for different stress levels.
Smaller constriction diameters were rather insensitive to increasing values of normal stress above 20 kPa for the geotextile products tested. Even for larger constriction diameters, the thicker the geotextile the less sensitive those values were to the increase of stress level. It is also important to point out that the compression of pore channels is not uniform throughout the geotextile layer. Thus, the spatial position of the maximum constriction diameter may vary during compression, or for different values of normal stress. The change of shape of pore channels caused by compression can also change the path followed by the water during flow across the geotextile thickness. As a consequence, with respect to the retention of soil particles, the dimensions and location of the critical pore channel (the one containing the largest constriction diameter) is stress dependent and can be different from that in unconfined conditions. 3.4. Retention capacity of confined and partially clogged geotextiles Bearing in mind that results of filtration opening sizes obtained in sieving analyses are usually used in retention criteria calculations, the results obtained in the present study show that compressed geotextiles may present a significantly greater retention
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Fig. 12. Pore sizes versus normal stress.
capacity than forecasted by current retention criteria. If the geotextile is partially clogged by soil particles during spreading of fill material on top of it or during its life time (Palmeira and Gardoni, 2000a), its retention capacity will increase even more. Simultaneously, the dimensions of the soil particles required to internally clog the geotextile will be smaller than currently assumed, when designers think of geotextile pore spaces in terms of unconfined conditions. Retention criteria are commonly expressed as On pN; Di
ð1Þ
where On is a certain measure of geotextile pore openings (usually O95 or O90), Di is an indicative diameter of the base soil particles (usually D85 or D50) and N is a limiting value. Additional requirements must be met for the use of a specific criterion, depending on geotextile type and characteristics, soil characteristics (soil type, coefficient of uniformity, coefficient of curvature and density index, for instance) and flow conditions. Palmeira and Gardoni (2000b) lists a large number of retention criteria that can be found in the literature and discusses on their theoretical or empirical basis.
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Stress level and partial clogging of the geotextile affect the value of On : So, taking the value of O95 as reference O95 ¼
O95 ; Ks Kpc
ð2Þ
where O*95 is the actual geotextile opening size for which 95% of the openings are smaller under confinement and partial clogging conditions, O95 is the geotextile opening size for which 95% of the openings are smaller obtained in unconfined conditions, Ks and Kpc are reduction factors to take into account the effects of stress level and partial clogging on the geotextile pore constriction dimensions. Thus, the value of O*95 from Eq. (2) should be used in Eq. (1), or the latter rewritten as O95 pKs Kpc N: Di
ð3Þ
For the geotextile products tested in the present work, the value of Ks (for O95, from hydrodynamic sieving, Table 1) varied from 1.2 to 2.2. Therefore, stress level alone can be responsible for a significant increase in geotextile retention capacity in comparison to the situation under unconfined conditions. Further investigation is required for an evaluation of magnitudes of Kpc as a function of geotextile clogging levels. It is important to note that impregnated geotextiles can be much less compressible than virgin geotextiles (Palmeira et al., 1996; Palmeira and Gardoni, 2000a). Therefore, if the geotextile is impregnated by soil particles before compression (due to soil placement and spreading during construction, for instance), the presence of these particles in the geotextile voids may not necessarily result in a much larger increase in its retention capacity because larger values of Kpc may be associated to lower values of Ks : An evaluation of the retention capacity of impregnated geotextile can be made by filtration tests on partially clogged geotextiles under pressure. Palmeira et al. (1996) present results of this type of tests, where samples of glass beads were compacted by vibration in a rigid permeameter for filtration tests under confinement (gradient ratio tests). The placement of the glass beads on the geotextile specimen and sample preparation by vibration-induced partial clogging of the geotextile. The mean particle size (d 50) of the glass beads used was equal to 0.048 mm, with d 15 and d 85 equal to 0.029 and 0.064 mm, respectively, coefficient of uniformity of 2.1 and relative density after sample preparation of 83%. After sample preparation and after stabilisation of the flow regime for each value of normal stress applied on the sample top, the particles that piped through the geotextile were collected from a trough at the bottom of the permeameter cell and tested for grain size distribution in a X-ray particle size analyser Sedigraph 5100, from the Micromeritics Instrumentation Corp. Grain size analysis allowed the determination of the largest particle diameter piped (D95). Fig. 13 shows results of the tests described above for geotextiles GC, GD and GE (Table 1). The ratio between the geotextile filtration opening size (FOS, from hydrodynamic sieving, Table 1) and the largest piped particle diameter (D95) is
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Fig. 13. Retention capacity of partially clogged geotextiles under pressure.
plotted versus the normal stress applied on top of the glass beads sample. That ratio is equal to Ks Kpc in Eq. (2). It can be seen that as the normal stress increases, the value of Ks Kpc increases more significantly with normal stress for geotextile GD (lighter), remaining rather constant for geotextile GC (thicker). For the geotextiles tested and range of normal stresses used, Ks Kpc varied from 1.9 to 4.4, which is larger than the range 1.2–2.2 reported above for tests on virgin geotextile specimens. This increase may be attributed to the combined effect of normal stress and partial clogging on the retention capacity of the geotextiles. It is important to note that the increase of retention capacity of the geotextile because of the presence of the entrapped soil particle, by no means suggest that more open geotextiles could be used without due care. The increase in retention capacity of the partial clogged geotextile will be a function of the sizes of the entrapped particles, distribution of particles in the geotextile layer and if these particles remain inside the geotextile during the life time of the project. These conditions are related to geotextile, soil and project characteristics. However, as the available geotextile pores are reduced by confinement and presence of entrapped particles, the pore dimensions to be used to assess geotextile clogging will also be smaller than currently assumed. This is particularly relevant for filters in unstable base soils and situations where fine particles in suspension can migrate towards the compressed and partially clogged geotextile layer. 3.5. Predictions versus observations of geotextile permeability under confinement The available test results and image analyses allowed the evaluation of the accuracy of expressions for the estimate of geotextile permeability, some of them developed and proved accurate for geotextiles under unconfined situations. Thus, the intention in this section is to assess their accuracy also to situations of geotextile confinement. In the present work the predictions by solutions presented by Kozeny– Carman (Taylor, 1948), Lord (1955), Masounave et al. (1980) and Giroud (1996) are compared to test results. Masounave et al. (1980) developed an expression for the estimate of geotextile permeability as a function of the geotextile density of fibres. The expression was
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derived from the least square fit of experimental results of unconfined tests on nonwoven geotextiles and is rewritten as 2:02 k ¼ pffiffiffiffi ; u0
ð4Þ
where k is the geotextile coefficient of permeability (cm/s) and u0 is the geotextile density of fibres, given in number of fibres/mm2 of geotextile cross-sectional area. For the calculations, the variation of density of fibres with normal stress for each geotextile was obtained from the results presented in Fig. 5. Rollin et al. (1982) also proposed an equation for the estimate of unconfined geotextile permeability based on geotextile structural characteristics, given by k¼
df ðdf þ davg Þnrw g ; Zw A
ð5Þ
where df is the fibre diameter, davg is the average distance between geotextile fibres, n is the geotextile porosity, rw is the fluid density, g is the acceleration of gravity, Zw is the fluid viscosity and A is the product of the fluid drag coefficient and the Reynolds number. Rollin et al. (1982) found values of A varying from 6 to 10 for non-woven geotextiles. An important practical limitation of solutions such as the ones in Eqs. (4) and (5) is that they require the knowledge of the variation of density of fibres or distance between fibres with pressure. This type of data can only be obtained with more sophisticated testing techniques. Giroud (1996) derived the following expression for the geotextile permeability as a function of its porosity, fibre diameter and fluid characteristics, based on the classical Poiseuille’s equation for flow in a tube and Kozeny–Carman’s assumption k¼
brw g n3 d 2; 16Zw ð1 nÞ2 f
ð6Þ
where b is a shape factor (Giroud suggests an average value of b equal to 0.11 for non-woven geotextiles). Palmeira and Gardoni (2000a) have already observed that Eq. (6) provides good estimates of the permeability coefficient of non-woven geotextiles under confinement. Fig. 14 presents the comparisons between measured and predicted values of geotextile normal permeability using Eq. (4). Significant deviations between results can be observed, which shows that geotextile permeability cannot be expressed uniquely as a function of its density of fibres. In this case, geotextile micro-structure plays an important role and intuition leads one to easily accept that two different geotextiles, with different micro-structures, but the same value of u0 ; should have different permeabilities. Therefore, the use of Eq. (4) should not be extended to different geotextile products than those tested by Masounave et al. (1980) and to confined geotextiles. The comparison between predictions by Eq. (5) and test results is shown in Fig. 15 for a value of A equal to 8 and for normal stresses ranging from 2 to 550 kPa. The values of average distances between geotextile fibres used in that equation in
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Fig. 14. Comparisons between measurements and predictions by Masounave et al. (1980).
Fig. 15. Comparisons between measurements and predictions by Rollin et al. (1982).
the calculations were obtained from image analyses of geotextile specimens compressed by normal stresses equal to 2,200 and 550 kPa (Gardoni, 2000). For the geotextile products tested, Eq. (5) underestimated the permeability coefficient values for low stress levels (2 kPa, points above the 1:1 slope line in Fig. 15) and overestimated those values for high stress levels (points below the 1:1 slope line). A better fit between predictions and test results for normal stresses in the range 200– 550 kPa was obtained multiplying the results given by Eq. (5) by a factor equal to 0.4.
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Fig. 16. Comparisons between measurements and predictions by different methodsFgeotextile GA.
Fig. 16 shows a very good agreement between test results and predictions by Eq. (6) for geotextile GA (similar results were observed for the other geotextiles), confirming this equation as an useful tool for the estimate of geotextile permeability under compression, as more comprehensively observed in Palmeira and Gardoni (2000a). This figure also shows comparisons between test results and predictions from Lord (1995) solution for gas flow through fibrous material and from the traditional Kozeny–Carman solution for a pack of spheres (Taylor, 1948). The latter solutions presented poorer agreement with test results, particularly for high values of geotextile porosity.
4. Conclusions This paper presented a study on the variation of physical, hydraulic and pore characteristics of some non-woven geotextiles under confinement. The main conclusions obtained are summarised below. In general, a good agreement between physical and hydraulic properties obtained from different experimental techniques was observed. The permittivity tests were the ones that showed some deviations from the general trend of results for larger stress levels. This may have been caused by local deformations of the geotextile layer in the apertures of the steel meshes used to apply the normal stress. The results suggest that permittivity tests on a pack of geotextile specimens may be more appropriate than that on a single specimen, if due care is taken to avoid preferential flow at the permeameter internal walls in the former case. The results of tests on packs of geotextiles will also provide average values of geotextile properties, because of the number of layers being simultaneously tested. This is relevant for tests on geotextiles with non-uniform distributions of mass per unit area, that would require a large number of tests on individual specimens. The results of geotextile permittivity from standard permittivity tests compared very well with those from bubble point tests. Important anisotropy of geotextile permeability was observed for the products tested, particularly for low stress levels. It is important to note that values of geotextile permeability anisotropy, or the
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influence of stress level on geotextile properties, are seldom found in products catalogues, in spite of the rather simple tests that can be employed to quantify these effects. Images of compressed specimens of the tested geotextile products showed that the pore channels controlling filtration characteristics are formed by packs of fibres in contact. Most of the reductions of pore channel constriction diameters were observed for normal stresses up to 100 kPa, being the effect of stress level important for the values of O98, O95 and O90. Values of O50 and O2 were rather insensitive to the range of normal stresses used in the tests reported in this work. The comparisons between predicted and observed geotextile permeability values under stress confirmed the good accuracy of the equation proposed by Giroud (1996). It was also noted that the extension of the solution by Masounave et al. (1980) to the geotextiles tested under confinement in this work yielded to poor comparisons, whereas a slightly better agreement between predictions by the solution by Rollin et al. (1982) and test results were noticed. It should be pointed out that these solutions were initially developed for the estimate of geotextile permeability coefficient without confinement. The confinement of the geotextile can significantly increase its retention capacity. For the products tested in this work, depending on the stress level and type of geotextile, it was observed that they would be capable of retaining soil particles half the size predicted by current design practice. The partial clogging of geotextile due to presence of entrapped particles increases even further its retention capacity, also in a way not yet predicted by design criteria. These observations confirm that current retention criteria can be significantly conservative. A general expression for retention criteria taking into account the effect of geotextile pore reductions caused by confinement and entrapped soil particles has been introduced. However, further research is required to evaluate these effects for wider range of non-woven geotextile products and for a better understanding and prediction of retention capacity and clogging mechanisms of geotextiles under confinement and partial clogging conditions.
Acknowledgements Part of the tests results reported in this paper were commissioned at SAGEOS, Canada, and performed by M.G. Gardoni. Dr. J. Mlynarek, O.G. Vermeersch, E. Blond and M. Bouthot, from SAGEOS, were very helpful with suggestions, comments and discussions on test results. The authors are also indebted to Prof. J. Lafleur and members of the staff of the Ecole Polytechnique of Montreal, Canada, for their assistance to M.G. Gardoni with discussions and testing facilities during her stay in Montreal, as part of her Ph.D. programme at the University of Brasilia, Brazil. The authors also express their appreciation to the University of Brasilia and the Brazilian research sponsoring agencies FAP-DF, CNPq and CAPES for funding this research programme.
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References ASTM D 4716. 1993. Standard test method for constant head hydraulic transmissivity (in-plane flow) of geotextiles and geotextile-related products. American Society for Testing Materials, West Conshohocken, PA, USA. ASTM D 5493. 1993. Standard test method for permittivity of geotextiles under load. American Society for Testing Materials, West Conshohocken, PA, USA. ASTM F 316-86. Standard test methods for pore size characteristics of membrane filters by bubble point and mean flow pore test. American Society for Testing Materials, West Conshohocken, PA, USA. CFG, 1986. Recommendations g!en!erales pour la r!eception et la mise en oeuvre des g!eotextiles: normes Franc-aises d’essais. Association Franc-aise de Normalization, French Committee on Geotextiles, France, 32pp. Fischer, G.R., Holtz, R.D., Christopher, B.R., 1996. Evaluating geotextile pore structure. In: Bhatia, S.S., Suits, L.D. (Eds.), Recent Developments in Geotextile Filters and Prefabricated Drainage Geocomposites. American Society for Testing Materials, ASTM STP 1281, pp. 3–18. Gardoni, M.G., 2000. Hydraulic and filter characteristics of geosynthetics under pressure and clogging conditions. Ph.D. Thesis, University of Brasilia, Brasilia, Brazil, 331pp. (in Portuguese). Giroud, J.P., 1996. Granular filters and geotextile filters. In: Lafleur, J., Rollin, A. (Eds.), GeoFilters’96. Montreal, Canada, pp. 565–680. Gourc, J.P., 1982. Quelques aspects du comportement des g!eotextiles en m!ecanique des sols. Ph.D. Thesis, IRIGM, Joseph Fourier University, Grenoble, France, 249pp. Gourc, J.P., Faure, Y., Hussain, H., Sotton, M., 1982a. Standard test of permittivity and application of Darcy’s formula. Second International Conference on Geotextiles, Las Vegas, USA, Vol. 1, pp. 139–144. Gourc, J.P., Faure, Y.Rollin, A., Lafleur, J., 1982b. Structural permeability law of geotextiles. Second International Conference on Geotextiles, Las Vegas, USA, Vol. 1, pp. 149–154. Lord, P., 1955. Air flow through plugs of textile fibers. Journal of the Textile Institute 46 (3), 165–172. Masounave, J., Rollin, A.L., Denis, R., 1980. Prediction of permeability of nonwoven geotextiles from morphometry analysis. Journal of Microscopy 121 (1), 99–100. McGown, A., Kabir, M.H., Murray, R.T., 1982. Compressibility and hydraulic conductivity of geotextiles. Second International Conference on Geotextiles, Las Vegas, USA, Vol. 1, pp. 167–172. Palmeira, E.M., 1997. Physical and hydraulic properties of geotextiles under pressure. Soils and RocksFLatin American Journal on Geotechnical Engineering 20 (2), 69–78 (in Portuguese). Palmeira, E.M., Fannin, R.J., Vaid, Y.P., 1996. A study on the behaviour of soil–geotextile systems in filtration tests. Canadian Geotechnical Journal 33 (4), 899–912. Palmeira, E.M., Gardoni, M.G., 2000a. The influence of partial clogging and pressure on the behaviour of geotextiles in drainage systems. Geosynthetics International 7 (4–6), 403–431. Palmeira, E.M., Gardoni, M.G., 2000b. Geotextiles in filtration: a state-of-the-art review and remaining challenges. In: Mallek, A. (Ed.), International Symposium on Geosynthetics in Geotechnical and GeoEnvironmental Engineering, in association with GeoEng2000, Melbourne, Australia, pp. 85–110. Rollin, A., Masounave, J., Lafleur, J., 1982. Pressure drop through non woven geotextiles: a new analytical model. Second International Conference on Geotextiles, Las Vegas, USA, Vol. 1, pp. 161–166. Taylor, D.W., 1948. Fundamentals of Soil Mechanics. Wiley, New York, USA.
Drainage and filtration properties of non-woven geotextiles under confinement using different experimental techniques Ennio M. Palmeira*, Maria G. Gardoni Department of Civil and Environmental Engineering, University of Brasilia, FT, 70910-900 Brasilia, DF, Brazil Received 14 August 2001; received in revised form 24 December 2001; accepted 5 January 2002
Abstract This work presents a study on the influence of stress level on some physical and hydraulic properties of non-woven geotextiles relevant to drainage and filtration. Different types of tests were employed for the determination of geotextile thickness, porosity, permittivity, transmissivity and pore sizes under normal stresses ranging from 0 to 2000 kPa, depending on the type of test. Results of these different testing techniques were compared and their accuracy assessed. The increase of geotextile retention capacity caused by increasing stress levels and geotextile partial clogging was also evaluated, as well as the accuracy of equations for the estimate of geotextile permeability. The results obtained showed a marked effect of confinement on geotextile properties, with special reference to pore size diameters, with implications to current filter criteria for non-woven geotextiles. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Geotextiles; Normal stress; Physical and hydraulic properties; Pore sizes; Filter criteria
1. Introduction Geotextiles have been extensively used as drainage and filter materials in geotechnical and geoenvironmental works in the past 30 years. These applications have been overwhelmingly successful, and up to date most of the designs have been based on empirical and, sometimes, unrealistic criteria. This practice may yield to *Corresponding author. Tel.: +55-61-273-7313; fax: +55-61-273-4644. E-mail address: [email protected] (E.M. Palmeira). 0266-1144/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 0 2 ) 0 0 0 0 4 - 3
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misleading performance predictions or overconservative designs. In general, nonwoven geotextiles are very permeable and compressible materials. As the thickness of the geotextile reduces under stress so does its coefficient of permeability and pore dimensions. Pioneer works studying the effects of stress level on the hydraulic behaviour of geotextiles can be found in the literature (Gourc, 1982; Gourc et al., 1982a, b; McGown et al., 1982, for instance). Today, there is an increasing appeal for the use of geotextiles in large projects, which finds a barrier in the fact that little information is available on the behaviour of these materials under high stress levels. It should be noted that drainage of large embankments or of moderate high mining waste piles can cause normal stresses on the drainage system in excess of 1 MPa. Several experimental techniques can be applied to the determination of physical and hydraulic properties of geotextiles, which are of utmost importance for the design of drains and filters. Most of these techniques give measures of these properties under unconfined conditions, but some of them can also be adapted to perform tests under confinement. This work presents comparisons between physical and hydraulic properties of non-woven geotextiles under confinement using different testing techniques.
2. Equipment and materials used in the tests 2.1. Equipment The aim of the present work is to investigate the variation of physical and hydraulic properties of non-woven geotextiles under a range of normal stresses varying from 0 to 2000 kPa. The properties or dimensions investigated were porosity, thickness, permittivity, transmissivity, density of fibres and pore dimensions. The following experimental techniques were employed: permittivity tests, transmissivity tests, image analysis and bubble point tests. A description of each technique is summarised below. Permittivity tests under confinement were performed using an apparatus of the Geosynthetics Technology CentreFSAGEOS, Canada, based on the one recommended in ASTM D 5493 (ASTM, 1993). The equipment is capable of performing permittivity tests under normal stresses up to 550 kPa. Fig. 1 shows schematically the equipment used in the tests. A steel piston applies the normal load to a rigid load plate on a 50.8 mm diameter geotextile specimen, confined between two sets of steel meshes, with apertures ranging from 0.1 to 0.8 mm. The meshes serve as permeable medium that helps to distribute the normal stress uniformly on the geotextile specimen. The equipment used for the determination of geotextile longitudinal permeability and transmissivity is presented in Fig. 2. It is also based on a previous version of the apparatus presented in ASTM D 4716 (ASTM, 1993). The geotextile specimen is 100 100 mm and is subjected to the normal stress applied by a rigid metal plate. The equipment is capable of testing specimens under normal stresses up to 2000 kPa. The variation of the geotextile thickness with normal stress during the test can also
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Fig. 1. Permittivity test equipment.
Fig. 2. Transmissivity test equipment.
be obtained, as a function of the vertical displacement of the rigid plate, calculated as the average of the readings from displacement transducers positioned at three different locations on the plate. Distilled water was used in both permittivity and transmissivity tests. The values of hydraulic gradients used in these tests were established after specific tests having been performed to determine the range of hydraulic gradient values to guarantee laminar flow conditions for the geotextiles used. The hydraulic gradients used were equal to 2 for transmissivity tests and varied between 6–10 (depending on geotextile type and normal stress applied) for permittivity tests. The Bubble Point test was used for the determination of geotextile pore channels constriction sizes distribution curve and permeability. The constriction size distribution curve is obtained based on the knowledge of the pressure in a fluid that is required to overcome the capillary attraction of the fluid in the largest pore. Fischer et al. (1996) comment that the bubble point method is advantageous in comparison with other types of tests because it simulates the flow of fluid in the geotextile pore channels, can be performed quickly and efficiently, the results are
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repeatable and provides an accurate estimate of the geotextile permeability. A Coulter Porometer IIR, available at SAGEOS, Canada, was employed to perform the tests. This equipment is capable of working with a stress range from 0.2 to 1400 kPa, depending on the geotextile characteristics, and a maximum flow rate of 100 l/min. In the present study, the normal stresses applied to the geotextile specimens varied from 0 to 550 kPa, depending on the geotextile characteristics. The tests and relevant calculations were performed according to ASTM F 316-86 standard. Analyses of images of compressed geotextiles were also employed as part of the investigation of physical and hydraulic properties of geotextiles under confinement. An image analyser Clemex Impak, from Clemex Technology Incorporation (Canada), was employed to obtain images of geotextile microstructure. The equipment consists of a photographic camera Sony 930/950 RGB and a data acquisition system for data processing. In this technique the geotextile image is scanned and converted to digital image that can be enhanced for measurements. Relevant measurements obtained are total area of fibres, percentage of fibres, density of fibres (number of fibres per unit cross-sectional area), distance between fibres and porosity. Geotextile specimens subjected to compressive stresses up to 1000 kPa were analysed in the image analyser.
2.2. Geotextile materials Five non-woven, needle punched, geotextiles (codes GA to GE) made of polyester, from the same manufacturer, were tested. The main characteristics of these geotextiles are presented in Table 1. The mass per unit area of the geotextiles vary from 180 to 600 g/m2, covering the range of geotextile mass per unit area values often found in practice. The geotextile fibres are cylindrical, with diameters varying from 0.0245 to 0.0279 mm, depending on the geotextile type. Additional information on testing equipment, methodologies and materials can be found in Gardoni (2000).
Table 1 Geotextile characteristics Geotextile
MA (g/m2)
tGT (mm)
n
O95 (mm)
df (mm)
GA GB GC GD GE
200 400 600 180 300
1.72 2.89 4.60 2.20 3.30
0.94 0.90 0.89 0.93 0.92
0.135 0.110 0.060 0.140 0.110
0.0245 0.0279 0.0250 0.0270 0.0270
Notes: MA =mass per unit area, tGT =geotextile thickness (under 2 kPa normal stress), n=geotextile porosity, O95=filtration opening size (hydrodynamic sieving test, CFG 1986), df =geotextile fibre diameter.
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3. Test results 3.1. Variation of physical properties with normal stresses Geotextile permittivity and transmissivity depend on geotextile thickness, which can be determined by different types of compression tests. Because most of the tests used in this work involve compression stages of the geotextile specimens, it would be interesting to compare values of geotextile thickness and porosity obtained by different experimental techniques. Figs. 3(a)–(c) show the variation of geotextile thickness with normal stress obtained from different testing methods, for stress levels up to 1000 kPa. Most of the compression of the geotextile specimens occur for normal stresses up to 200 kPa. For normal stresses above this value, a considerable decrease of the rate of thickness reduction with normal stress is noticeable. In general, a good agreement between measures from different apparatus is observed, particularly for the lighter geotextiles tested (GA and GB). For the thicker geotextile GC, some deviations of the results obtained in the permittivity tests, from those of the rest of the tests performed, can be seen for normal stresses in excess of 200 kPa. It is likely that these deviations may have been caused by accommodation or deformations of the steel meshes and local deformations of the geotextile specimen inside mesh apertures under higher stresses. At the end of the tests the marks left by the meshes members on the geotextile specimen surface could be clearly seen.
Fig. 3. Geotextile thickness versus stress from different tests.
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The results of compression tests for geotextile GC (Palmeira, 1997) using a pack of geotextile specimens (8 layers, 100 mm dia., tested up to 200 kPa normal stress), rather than a single specimen, is also plotted in Fig. 3(c). The equipment used for testing packs of geotextiles is described in Palmeira et al. (1996) and is similar to the one presented in Fig. 1, but with the normal stress on the geotextile pack being applied by a perforated rigid plate. Similar tests on pack of geotextile specimens can be found in Gourc et al. (1982a) and McGown et al. (1982). In the case of tests on packs of geotextiles, a better agreement with the results from other types of tests was found, stressing the possible effect of the steel meshes on the results of tests with individual geotextile specimens. The variation of geotextile porosity with normal stress is shown in Figs. 4(a)–(c). The geotextile porosity was calculated based on geotextile thickness, mass per unit area and density of fibres. Again, a reasonable agreement between different forms of determination of geotextile porosity can be observed. For the thicker geotextile the values of geotextile porosity determined in the permittivity tests were significantly lower than those obtained in the other types of tests, as a consequence of the smaller thickness measured, as commented above. Images obtained by the Image Analyser can be used to assess micro-structural characteristics of the geotextiles tested. The dependency of the density of fibres (number of fibres per unit cross-section area of the geotextile) with normal stress is presented in Fig. 5 for the three geotextiles tested. The lighter the geotextile, the
Fig. 4. Geotextile porosity versus stress from different tests.
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Fig. 5. Density of fibres versus normal stress.
smaller the density of fibres. For normal stresses above 200 kPa the density of fibres increases with normal stresses at a greater rate, which is consistent with the smaller rate of reduction observed for other geotextile physical properties for stresses above that value. Figs. 6(a) and (b) show images of the distribution of fibres for geotextile GA for normal stresses equal to 2 and to 1000 kPa, respectively. For low stress levels the fibres are distributed through the geotextile cross-sectional area in a rather sparse mode. Only a few fibres are in direct contact to each other. For larger stress levels packs of fibres can be identified, with most of the fibres in direct contact. In the latter
Fig. 6. Cross-sections of geotextile GA under different normal stresses.
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case, the larger voids controlling the magnitude of permeability coefficient and filtration, are found between packs of fibres in contact.
3.2. Variation of hydraulic properties with normal stress The variations of geotextile permittivity with normal stress for the geotextiles tested are presented in Figs. 7(a)–(c). These figures show comparisons between permittivity values obtained by standard permittivity tests with single specimen (Fig. 1), permittivity tests on pack of specimens of geotextile GC (Palmeira, 1997) and bubble point tests. A very good agreement between test results from these testing methods can be noted, particularly for the thicker geotextiles. For the range of normal stresses tested the permittivity of the lighter geotextile GA was reduced by a factor of 3, while for the heavier geotextiles (GB and GC) a reduction by a factor of 10 can be noticed. The reduction of permittivity of geotextile GA for normal stresses above 200 kPa also occurs at a lower rate than those for geotextiles GB and GC. This behaviour can be explained by the combination of lower thickness and mass per unit area for the lighter geotextile. The results presented in Fig. 7 suggest that the standard, low cost and simple permittivity test provides values of geotextile permittivity as accurate as the more complex and expensive bubble point test. This conclusion is corroborated by the test results grouped in a direct comparison, as shown in Fig. 8.
Fig. 7. Geotextile permittivity versus stress from different tests.
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Fig. 8. Comparison between permittivity values from permittivity tests and bubble point tests.
Transmissivity test results under stress levels up to 2000 kPa are shown in Figs. 9(a) and (b) for geotextiles GA and GC. Similar results for other geotextile products have been reported in Palmeira and Gardoni (2000a) and are not reproduced here. The results in Fig. 9 show that the geotextile transmissivity under high stress levels can be reduced by a factor of 2–3 orders of magnitude, depending on geotextile thickness. This reduction in geotextile transmissivity will have a marked effect on the discharge capacity of the geotextile drain. Therefore, due attention must be paid to situations where geotextile drains will be used under high normal stresses. Results from permittivity and transmissivity tests under pressure allow the assessment of the variation of geotextile permeability anisotropy with the stress level. Figs. 10(a)–(c) show a marked anisotropy for the geotextiles tested for stress levels below 50 kPa. As the normal stress increases, geotextiles GA and GB tend to show a more isotropic behaviour with regard to permeability.
Fig. 9. Results of transmittivity tests under pressure.
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Fig. 10. Permeability coefficients versus normal stress.
3.3. Variation of constriction sizes with normal stress Compressive stresses will also influence geotextile pore volume, and so its filtration behaviour. Bubble point tests with geotextile specimens subjected to normal stresses varying from 2 to 550 kPa were performed as part of the present research programme. Figs. 11(a)–(c) show constriction pore diameter distribution curves for geotextiles GA, GB and GC. As the normal stress increases, the distribution of pores tends to be more uniform. Most of the reduction of pore constriction diameters takes place for normal stresses up to 100 kPa. Fig. 12 presents the variation of constriction diameters On with normal stress for three of the geotextiles tested in this work, where On is the constriction diameter for which n% of the constrictions are smaller. For the three geotextiles the values of O98, O95 and O90 are the ones that are most sensitive to the normal stress. A 32% reduction in the value of O98 of geotextile GA at a normal stress of 50 kPa is observed with respect to that value at 2 kPa normal stress. If the value of filtration opening size for that geotextile presented in Table 1 is considered as a reference, the reduction of the maximum pore diameter can reach 36%. At 550 kPa normal stress the reduction in O98 and O95 for geotextile GA is 44% and 24%, with respect to those values obtained at 2 kPa, and 47% and 54% with respect to the value of FOS in Table 1.
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Fig. 11. Constriction size distributions for different stress levels.
Smaller constriction diameters were rather insensitive to increasing values of normal stress above 20 kPa for the geotextile products tested. Even for larger constriction diameters, the thicker the geotextile the less sensitive those values were to the increase of stress level. It is also important to point out that the compression of pore channels is not uniform throughout the geotextile layer. Thus, the spatial position of the maximum constriction diameter may vary during compression, or for different values of normal stress. The change of shape of pore channels caused by compression can also change the path followed by the water during flow across the geotextile thickness. As a consequence, with respect to the retention of soil particles, the dimensions and location of the critical pore channel (the one containing the largest constriction diameter) is stress dependent and can be different from that in unconfined conditions. 3.4. Retention capacity of confined and partially clogged geotextiles Bearing in mind that results of filtration opening sizes obtained in sieving analyses are usually used in retention criteria calculations, the results obtained in the present study show that compressed geotextiles may present a significantly greater retention
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Fig. 12. Pore sizes versus normal stress.
capacity than forecasted by current retention criteria. If the geotextile is partially clogged by soil particles during spreading of fill material on top of it or during its life time (Palmeira and Gardoni, 2000a), its retention capacity will increase even more. Simultaneously, the dimensions of the soil particles required to internally clog the geotextile will be smaller than currently assumed, when designers think of geotextile pore spaces in terms of unconfined conditions. Retention criteria are commonly expressed as On pN; Di
ð1Þ
where On is a certain measure of geotextile pore openings (usually O95 or O90), Di is an indicative diameter of the base soil particles (usually D85 or D50) and N is a limiting value. Additional requirements must be met for the use of a specific criterion, depending on geotextile type and characteristics, soil characteristics (soil type, coefficient of uniformity, coefficient of curvature and density index, for instance) and flow conditions. Palmeira and Gardoni (2000b) lists a large number of retention criteria that can be found in the literature and discusses on their theoretical or empirical basis.
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Stress level and partial clogging of the geotextile affect the value of On : So, taking the value of O95 as reference O95 ¼
O95 ; Ks Kpc
ð2Þ
where O*95 is the actual geotextile opening size for which 95% of the openings are smaller under confinement and partial clogging conditions, O95 is the geotextile opening size for which 95% of the openings are smaller obtained in unconfined conditions, Ks and Kpc are reduction factors to take into account the effects of stress level and partial clogging on the geotextile pore constriction dimensions. Thus, the value of O*95 from Eq. (2) should be used in Eq. (1), or the latter rewritten as O95 pKs Kpc N: Di
ð3Þ
For the geotextile products tested in the present work, the value of Ks (for O95, from hydrodynamic sieving, Table 1) varied from 1.2 to 2.2. Therefore, stress level alone can be responsible for a significant increase in geotextile retention capacity in comparison to the situation under unconfined conditions. Further investigation is required for an evaluation of magnitudes of Kpc as a function of geotextile clogging levels. It is important to note that impregnated geotextiles can be much less compressible than virgin geotextiles (Palmeira et al., 1996; Palmeira and Gardoni, 2000a). Therefore, if the geotextile is impregnated by soil particles before compression (due to soil placement and spreading during construction, for instance), the presence of these particles in the geotextile voids may not necessarily result in a much larger increase in its retention capacity because larger values of Kpc may be associated to lower values of Ks : An evaluation of the retention capacity of impregnated geotextile can be made by filtration tests on partially clogged geotextiles under pressure. Palmeira et al. (1996) present results of this type of tests, where samples of glass beads were compacted by vibration in a rigid permeameter for filtration tests under confinement (gradient ratio tests). The placement of the glass beads on the geotextile specimen and sample preparation by vibration-induced partial clogging of the geotextile. The mean particle size (d 50) of the glass beads used was equal to 0.048 mm, with d 15 and d 85 equal to 0.029 and 0.064 mm, respectively, coefficient of uniformity of 2.1 and relative density after sample preparation of 83%. After sample preparation and after stabilisation of the flow regime for each value of normal stress applied on the sample top, the particles that piped through the geotextile were collected from a trough at the bottom of the permeameter cell and tested for grain size distribution in a X-ray particle size analyser Sedigraph 5100, from the Micromeritics Instrumentation Corp. Grain size analysis allowed the determination of the largest particle diameter piped (D95). Fig. 13 shows results of the tests described above for geotextiles GC, GD and GE (Table 1). The ratio between the geotextile filtration opening size (FOS, from hydrodynamic sieving, Table 1) and the largest piped particle diameter (D95) is
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Fig. 13. Retention capacity of partially clogged geotextiles under pressure.
plotted versus the normal stress applied on top of the glass beads sample. That ratio is equal to Ks Kpc in Eq. (2). It can be seen that as the normal stress increases, the value of Ks Kpc increases more significantly with normal stress for geotextile GD (lighter), remaining rather constant for geotextile GC (thicker). For the geotextiles tested and range of normal stresses used, Ks Kpc varied from 1.9 to 4.4, which is larger than the range 1.2–2.2 reported above for tests on virgin geotextile specimens. This increase may be attributed to the combined effect of normal stress and partial clogging on the retention capacity of the geotextiles. It is important to note that the increase of retention capacity of the geotextile because of the presence of the entrapped soil particle, by no means suggest that more open geotextiles could be used without due care. The increase in retention capacity of the partial clogged geotextile will be a function of the sizes of the entrapped particles, distribution of particles in the geotextile layer and if these particles remain inside the geotextile during the life time of the project. These conditions are related to geotextile, soil and project characteristics. However, as the available geotextile pores are reduced by confinement and presence of entrapped particles, the pore dimensions to be used to assess geotextile clogging will also be smaller than currently assumed. This is particularly relevant for filters in unstable base soils and situations where fine particles in suspension can migrate towards the compressed and partially clogged geotextile layer. 3.5. Predictions versus observations of geotextile permeability under confinement The available test results and image analyses allowed the evaluation of the accuracy of expressions for the estimate of geotextile permeability, some of them developed and proved accurate for geotextiles under unconfined situations. Thus, the intention in this section is to assess their accuracy also to situations of geotextile confinement. In the present work the predictions by solutions presented by Kozeny– Carman (Taylor, 1948), Lord (1955), Masounave et al. (1980) and Giroud (1996) are compared to test results. Masounave et al. (1980) developed an expression for the estimate of geotextile permeability as a function of the geotextile density of fibres. The expression was
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derived from the least square fit of experimental results of unconfined tests on nonwoven geotextiles and is rewritten as 2:02 k ¼ pffiffiffiffi ; u0
ð4Þ
where k is the geotextile coefficient of permeability (cm/s) and u0 is the geotextile density of fibres, given in number of fibres/mm2 of geotextile cross-sectional area. For the calculations, the variation of density of fibres with normal stress for each geotextile was obtained from the results presented in Fig. 5. Rollin et al. (1982) also proposed an equation for the estimate of unconfined geotextile permeability based on geotextile structural characteristics, given by k¼
df ðdf þ davg Þnrw g ; Zw A
ð5Þ
where df is the fibre diameter, davg is the average distance between geotextile fibres, n is the geotextile porosity, rw is the fluid density, g is the acceleration of gravity, Zw is the fluid viscosity and A is the product of the fluid drag coefficient and the Reynolds number. Rollin et al. (1982) found values of A varying from 6 to 10 for non-woven geotextiles. An important practical limitation of solutions such as the ones in Eqs. (4) and (5) is that they require the knowledge of the variation of density of fibres or distance between fibres with pressure. This type of data can only be obtained with more sophisticated testing techniques. Giroud (1996) derived the following expression for the geotextile permeability as a function of its porosity, fibre diameter and fluid characteristics, based on the classical Poiseuille’s equation for flow in a tube and Kozeny–Carman’s assumption k¼
brw g n3 d 2; 16Zw ð1 nÞ2 f
ð6Þ
where b is a shape factor (Giroud suggests an average value of b equal to 0.11 for non-woven geotextiles). Palmeira and Gardoni (2000a) have already observed that Eq. (6) provides good estimates of the permeability coefficient of non-woven geotextiles under confinement. Fig. 14 presents the comparisons between measured and predicted values of geotextile normal permeability using Eq. (4). Significant deviations between results can be observed, which shows that geotextile permeability cannot be expressed uniquely as a function of its density of fibres. In this case, geotextile micro-structure plays an important role and intuition leads one to easily accept that two different geotextiles, with different micro-structures, but the same value of u0 ; should have different permeabilities. Therefore, the use of Eq. (4) should not be extended to different geotextile products than those tested by Masounave et al. (1980) and to confined geotextiles. The comparison between predictions by Eq. (5) and test results is shown in Fig. 15 for a value of A equal to 8 and for normal stresses ranging from 2 to 550 kPa. The values of average distances between geotextile fibres used in that equation in
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Fig. 14. Comparisons between measurements and predictions by Masounave et al. (1980).
Fig. 15. Comparisons between measurements and predictions by Rollin et al. (1982).
the calculations were obtained from image analyses of geotextile specimens compressed by normal stresses equal to 2,200 and 550 kPa (Gardoni, 2000). For the geotextile products tested, Eq. (5) underestimated the permeability coefficient values for low stress levels (2 kPa, points above the 1:1 slope line in Fig. 15) and overestimated those values for high stress levels (points below the 1:1 slope line). A better fit between predictions and test results for normal stresses in the range 200– 550 kPa was obtained multiplying the results given by Eq. (5) by a factor equal to 0.4.
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Fig. 16. Comparisons between measurements and predictions by different methodsFgeotextile GA.
Fig. 16 shows a very good agreement between test results and predictions by Eq. (6) for geotextile GA (similar results were observed for the other geotextiles), confirming this equation as an useful tool for the estimate of geotextile permeability under compression, as more comprehensively observed in Palmeira and Gardoni (2000a). This figure also shows comparisons between test results and predictions from Lord (1995) solution for gas flow through fibrous material and from the traditional Kozeny–Carman solution for a pack of spheres (Taylor, 1948). The latter solutions presented poorer agreement with test results, particularly for high values of geotextile porosity.
4. Conclusions This paper presented a study on the variation of physical, hydraulic and pore characteristics of some non-woven geotextiles under confinement. The main conclusions obtained are summarised below. In general, a good agreement between physical and hydraulic properties obtained from different experimental techniques was observed. The permittivity tests were the ones that showed some deviations from the general trend of results for larger stress levels. This may have been caused by local deformations of the geotextile layer in the apertures of the steel meshes used to apply the normal stress. The results suggest that permittivity tests on a pack of geotextile specimens may be more appropriate than that on a single specimen, if due care is taken to avoid preferential flow at the permeameter internal walls in the former case. The results of tests on packs of geotextiles will also provide average values of geotextile properties, because of the number of layers being simultaneously tested. This is relevant for tests on geotextiles with non-uniform distributions of mass per unit area, that would require a large number of tests on individual specimens. The results of geotextile permittivity from standard permittivity tests compared very well with those from bubble point tests. Important anisotropy of geotextile permeability was observed for the products tested, particularly for low stress levels. It is important to note that values of geotextile permeability anisotropy, or the
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influence of stress level on geotextile properties, are seldom found in products catalogues, in spite of the rather simple tests that can be employed to quantify these effects. Images of compressed specimens of the tested geotextile products showed that the pore channels controlling filtration characteristics are formed by packs of fibres in contact. Most of the reductions of pore channel constriction diameters were observed for normal stresses up to 100 kPa, being the effect of stress level important for the values of O98, O95 and O90. Values of O50 and O2 were rather insensitive to the range of normal stresses used in the tests reported in this work. The comparisons between predicted and observed geotextile permeability values under stress confirmed the good accuracy of the equation proposed by Giroud (1996). It was also noted that the extension of the solution by Masounave et al. (1980) to the geotextiles tested under confinement in this work yielded to poor comparisons, whereas a slightly better agreement between predictions by the solution by Rollin et al. (1982) and test results were noticed. It should be pointed out that these solutions were initially developed for the estimate of geotextile permeability coefficient without confinement. The confinement of the geotextile can significantly increase its retention capacity. For the products tested in this work, depending on the stress level and type of geotextile, it was observed that they would be capable of retaining soil particles half the size predicted by current design practice. The partial clogging of geotextile due to presence of entrapped particles increases even further its retention capacity, also in a way not yet predicted by design criteria. These observations confirm that current retention criteria can be significantly conservative. A general expression for retention criteria taking into account the effect of geotextile pore reductions caused by confinement and entrapped soil particles has been introduced. However, further research is required to evaluate these effects for wider range of non-woven geotextile products and for a better understanding and prediction of retention capacity and clogging mechanisms of geotextiles under confinement and partial clogging conditions.
Acknowledgements Part of the tests results reported in this paper were commissioned at SAGEOS, Canada, and performed by M.G. Gardoni. Dr. J. Mlynarek, O.G. Vermeersch, E. Blond and M. Bouthot, from SAGEOS, were very helpful with suggestions, comments and discussions on test results. The authors are also indebted to Prof. J. Lafleur and members of the staff of the Ecole Polytechnique of Montreal, Canada, for their assistance to M.G. Gardoni with discussions and testing facilities during her stay in Montreal, as part of her Ph.D. programme at the University of Brasilia, Brazil. The authors also express their appreciation to the University of Brasilia and the Brazilian research sponsoring agencies FAP-DF, CNPq and CAPES for funding this research programme.
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