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Analysis of geotextile filter behaviour after 21 years in Valcros dam
Geotextiles and Geomembranes 17 (1999) 353}370
Analysis of geotextile "lter behaviour after 21 years in Valcros dam Y.H. Faure *, B. Farkouh , Ph. Delmas, A. Nancey LIRIGM, University of Grenoble, BP 53, 38041 Grenoble Cedex 9, France Bidim Geosynthetics, BP 80, 95873 Bezons Cedex, France Received 22 October 1998; received in revised form 13 February 1999; accepted 22 February 1999
Abstract In 1970, nonwoven geotextiles were used for the "rst time in an earth dam. The geotextile acted as a "lter for the toe drain and on the upstream slope below the rip-rap. In 1992, samples were taken from both locations and performance tests were conducted in the laboratory. This paper presents the main results of the hydraulic behaviour of the geotextile "lter in association with the soil of the dam. Also microscopic analyses are presented and, as the "lter is considered to be performing well, selected "lter criteria are checked. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Geotextile; Filter; Hydraulic properties; Mechanical properties; Durability; Filter criteria
1. Description of the structure The Valcros dam was built in 1970 and it is described in detail by Giroud et al. (1977). This homogeneous earth dam is made of silty sand originating from the surface weathering of the schistose rock forming the foundation and abutments (gradation curve of this material is given in Fig. 1). The embankment is 20 m high (Fig. 2), 140 m long at the crest and has upstream and downstream slopes of 3H/1V. This structure is the "rst of this size in which geotextiles have been used: } downstream, as a "lter for the toe drain, the drain gravel (8 to 13 mm range) is wrapped in a 300 g/m nonwoven geotextile; * Corresponding author. Tel.: 33-476-82-80-56; fax: 33-476-82-80-70. E-mail address: [email protected] (Y.H. Faure) 0266-1144/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 9 9 ) 0 0 0 1 0 - 2
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Fig. 1. Soil granulometry of Valcros dam.
Fig. 2. Cross section of Valcros dam.
} upstream, as a "lter under 250 mm rocks placed directly on a 400 g/m nonwoven polyester geotextile sheet placed in strips stitched together and "xed to the ground by metal pins. Valcros dam has already been the subject of several monitoring programmes. The "rst series of tests were performed in 1976 on specimens taken from the dam by Giroud et al. (1977). Results from the second programme (1992) have been published by Delmas et al. (1992, 1994). This present paper presents a synthesis of the main results of the tests performed on the geotextile samples to check if the selected geotextiles are well functionning and if they are well designed according to updated "lter criteria.
2. 1992 sample collection programme In February 1992, samples were taken (Fig. 3) from the dam: } Downstream, a 4 m deep pit was excavated (4.5 m long and 5 m wide, Fig. 4), in order to reach the main drain. Samples of geotextile were taken from three distinct
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Fig. 3. Distinct sections of geotextile sampling (1992 programme).
Fig. 4. 4 m deep pit downstream of the dam to take geotextile samples from the toe drain of the dam (1992).
parts: from the top and from the bottom of the drain and in the top part of the overlap (Fig. 3); } Upstream, a 4 m;3 m area was cleared of rip-rap rocks (Fig. 5) at water surface level, situated that day about 20 cm below maximum reservoir level. This part is often subjected to lapping waves. An approximately 0.5 m wide overlapping section of two geotextile strips was found in this cleared area with the overlap running parallel to the dam axis (Fig. 3). Geotextile samples were taken from the lower strip, at water level, from the lower part of the overlap, from the upper part of the overlap and from the upper strip, about 1 m above water level.
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Fig. 5. Area cleared from armour rocks at upstream of the dam to take geotextile "lter samples.
At both locations, soil samples were taken from the immediate vicinity of the textile (within a few millimetres) and about 0.10 m upstream of the geotextile. Cubic specimen boxes (0.20 m side length) were also used to obtain combined soil-textile samples with great care being taken to avoid disturbance.
3. Laboratory tests Systematic tests were carried out on the geotextile samples taken: mass per unit area, thickness, tensile strength, and hydraulic properties ("ltration opening size, soil entrapment level and permittivity of cleaned samples). All the results of these tests are presented in Tables 1}4, reported from Delmas et al. (1992). 3.1. Mechanical test: tensile test The same test was used as for the tests conducted in 1976: French Standard NF G 07-001. The test samples are 0.20 m long and 0.05 m wide. Before the test, the surface of the samples is cleaned of any soil and washed carefully. Then, they are dried before testing. The test results show no signi"cant di!erences between the tensile characteritics observed in 1976 and 1992 (Tables 1 and 2). The small di!erences are related to soil particles trapped inside the geotextile. After twenty years of use, the geotextiles are mechanically performing as well as originally.
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Table 1 Comparison of tensile tests results on new samples and samples taken in 1976 and 1992 from the downstream zone 1976 samples
1992 samples
Tensile tests NF G 07 001
New
Mean value
CV (%)
Mean value
CV (%)
Strength (long direc.) Strength (cross direc.) Strain (long direc.) Strain (cross direc.) Mass per unit area (g/m)
860 N 580 N 60% 64% 300
680 N 520 N 58% 51% Unknown
15 10 8 6 x
669 N 592 N 47% 50% 350
12 14 4 13
Notes: Samples of identical geotextile fabricated in 1976 and tested in 1976. CV is the coe$cent of variation. Table 2 Comparison of tensile tests results on new samples and samples taken in 1976 and 1992 from the upstream zone 1976 samples
1992 samples
Tensile tests NF G 07 001
New
Mean value
CV (%)
Mean value
CV (%)
Strength (long direc.) Strength (cross direc.) Strain (long direc.) Strain (cross direc.) Mass per unit area (g/m)
710 N 520 N 45% 41% 400
760 N 630 N 49% 36% Unknown
5 16 9 6
846 N 700 N 47% 44% 474
14 10 8 6
Notes: Samples of identical geotextile fabricated in 1976 and tested in 1976. CV is the coe$cent of variation. Table 3 Hydraulic test on samples taken in 1992, downstream zone Upper part
Mass per unit area (g/m) Entrapment ratio (%) Permittivity (s\) Filtration opening size (lm)
Lower part
Overlap upper side
No. 1
No. 2
276$40 4.35 2.7$0.6 95
346$51 1.9 2.1$0.2 115
328$20 0.8$0.3 2.2$0.2 95
329$15 1$0.2 2.2$0.2 235
3.2. Hydraulic tests: index tests Permittivity tests and "ltration opening size tests (Tables 3 and 4), were performed according to the French standards on cleaned specimens (washed and dried). Permittivity (NF G 38-016, 1989): the #ow is kept under constant head and the velocity is maintained 10 mm/s. The results show a permittivity greater than 1 s\
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Table 4 Hydraulic test on samples taken in 1992, upstream zone Lower part No. 1 SE
Overlap No. 2
EE
SE
EE
Upper part
lower side
upper side
SE
SE
EE
Mass/unit 468 518 531 603 538 514 485 area (g/m) Entrapment 21.6 39.1 14.4 54.8 7.4 11.2 13 level (%) Permittivity (s\) Washed samples 1.03 1.21 0.97 1.23 0.98 1.46 1.06 Unwashed samples 0.39 0.290 0.30 0.36 0.68 0.81 0.42 Filtration opening 72 73 72 68 72 size (lm)
EE
SE
EE
485
460
547
39.6
16.4
36.7
1.24 0.31 69
1.33 0.27 70
1.16 0.38 69
Notes: SE beneath stone; EE between stones.
upstream and greater than 2 s\ downstream. Upstream, permittivity is slightly lower for samples taken beneath stones than for samples taken beneath voids between stones. That could be due to the compressive stress at the points of contact. Filtration opening size (NF G 38-017, 1989): hydrodynamic sieving with 0.30 m diameter samples. The opening size corresponds to the d value of the soil passed through the geotextile sample. There is no particular observation upstream, but downstream a larger value is obtained for the specimen No. 2 taken from the upper part. As the geotextile is in contact with crushed gravel and compressed below 4 m of soil, imprints of drain gravels are observed and some small holes (5}7 per m) are visible. The hydraulic properties of the downstream and upstream geotextiles, have remained unchanged after more than twenty years use in the ground. The following section presents the results of tests carried out on the geotextile samples in association with the soil of the dam. 3.3. Permittivity of geotextile samples with entrapped particles The permittivity of the samples was measured with the permeameter used for the standard permittivity tests on uncompressed geotextiles (Gourc et al., 1982). To prevent particle migration under the drag force of the #ow, the sample was placed between two "lters (made up of a nonwoven heat-bonded textile sheet) reinforced by a grid and a perforated metal plate. The sample and the test circuit were brought to saturation very gradually, at very low #ow velocity, in order to avoid disturbing the soil}"bre structure. The measurement was also taken at very low #ow speed (5 mm/s). The system with the two "lters (textile#grid#perforated plate) was calibrated
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without any sample for reference purposes. For a given #ow rate, this measurement was used to determine the head loss created by a geotextile containing particles. The permittivity measurement was related to the entrapment level of the samples tested. The entrapment level (¹ ) is de"ned as the percentage (in volume) of geotextile voids < "lled with soil particles < . < ¹ " , < where M k and < "n"1! , <" o ;¹ o¹ i.e.
M o ;100 ¹ (%)" ¹ o !k o < "volume of the geotextile voids per unit volume ("n, porosity of the geotextile) < "volume of soil particles entrapped per unit volume of geotextile o "speci"c mass of the soil (2.65 10 g/m) o "speci"c mass of the "bers (1.37 10 g/m) ¹ "geotextile thickness (m) M "mass of trapped soil per unit area of geotextile (g/m) k "mass per unit area of the clean geotextile (g/m). The level of particles entrapment was calculated on the assumption that the soil inside the geotextile was uniformly distributed and that the mass per unit area of the sample was equal to that of the original geotextile. Before performing the test, the samples were dried and weighed to determine ¹ and a comparison of entrapment level after the test showed a 3% decrease on the average. This study was performed only on the geotextile samples taken from upstream where the soil entrapment level is higher. The maximum ¹ values remain less than 60% (Table 4), i.e., there are still 40% of voids compared to the initial volume of voids. Between stones, the entrapment level is higher than beneath stones (Table 4). This entrapment is mainly produced by external soil, not by soil of the dam as it can be observed for the geotextile from the overlap: the samples against the dam soil (lower side) contain less entrapped particles than the samples from the upper side. The permittivity decreases linearly with the logarithm of the entrapment level (Fig. 6). The minimum permittivity obtained with the highest entrapment level (55%) is 0.21 s\, which corresponds to a permeability of 6.3;10\ m/s. The geotextile, therefore, remains highly permeable compared to the soil permeability (10\ m/s).
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Fig. 6. Permittivity of upstream specimens (lower strip).
3.4. Filtration test The soil}geotextile samples were tested in a "ltrameter (Fig. 7) consisting of a base and cylindrical rings of 0.04 m height and 0.15 m inside diameter. These rings were interlocked to obtain the desired height. They were provided with ori"ces to connect piezometers. At the top, a hollow piston was used to compress the sample. The water was fed in through the base from a constant-level holding tank. After passing through the soil sample, the water was circulated through the piston and conveyed to a constant-level downstream tank. Given the nature of the soil and its low water content (w"12% downstream), the sample was humidi"ed using the capillary e!ect by placing the sample on a bed of sand immersed in water for several days in order to be able to handle it. This procedure was not necessary for the soil sample taken upstream. A 20 mm thick layer of "ne gravel was placed at the base of the cell and the upstream piezometer (No. 1, Fig. 7) was installed in this layer. The sample was placed in the cell (the geotextile at the top, with the #ow rising). To ensure tightness around the sample, the gaps between the sample and the inside wall of the cell were "lled with dry bentonite. A piezometer (No. 2) was installed in the sample, 25 mm from the geotextile. A 20 mm thick layer of "ne gravel was placed on top of the sample and another piezometer (No. 3) was installed there. The sample was lightly compressed by the piston and brought to saturation point very slowly under a small head of water with a rising #ow of water. This gave the bentonite time to swell and seal the gaps between sample and wall. During the tests the samples were subjected to an upward #ow at constant head, with an overall gradient of 6 to 8. The tests lasted 30 to 40 days. Downstream sample. Fig. 8a shows the time-dependent variation in hydraulic gradient (i ) in the area close to the geotextile, the variation in hydraulic gradient (i ) in the soil
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Fig. 7. Filtrameter for testing soil/geotextile system.
Fig. 8. (a) Time-dependent variation in hydrauilic gradient (downstream sample): i in the soil (1}2, cf. Fig. 7), i in the geotextile (2}3), i in the mean (1}3). (b) Time-dependent variation in hydraulic gradient
(upstream sample): i in the soil (1}2, cf. Fig. 7), i in the geotextile (2}3), i in the mean (1}3). K
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upstream, and the mean hydraulic gradient (i ). It seems that some washout of soil
occurred at the start of the test but this was of no consequence. The permeability near the "lter is slightly lower than that of the soil (k (k ). Upstream sample. Because of a leakage problem for discharge measurement, the "rst 5 days of the test were discounted. The #uctuations noted (Fig. 8b) would seem to indicate clogging of the "lter (i (i ) during the test but, in reality, they are linked to the di$culty inherent in this type of measurement (piezometers) in "ne soils of low permeability (not observed for the downstream sample). However, with a gradient ratio never noted greater than 3, it may be concluded that the "ltering area immediately in contact with the soil did not become clogged. 3.5. Soil grain size analyses The analysis of the soil (upstream area) at a depth of 100 mm was compared to that of the soil (over a 2}5 mm thickness) in contact with the geotextile between rock"ll and under rock"ll. It was found (Fig. 9) that there is no signi"cant di!erence between the soil in contact with the textile and deeper soil, except for the absence of large particles in the contact soil, which is not surprising in view of the small sampling depth. Other grain size analyses of soils trapped inside the geotextile "lters or taken from around the drains were presented by Delmas et al. (1992). The grain size analyses did not reveal any signi"cant rearrangement of the soil particles close to the "lter. On the other hand, a clear di!erence in grain size analyses was found between the sample from the upstream side and that from the downstream side. This was related to a di!erent sampling zone for the upstream facing during dam construction.
Fig. 9. Sieve analyses of the dam soil at di!erent points (upstream).
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4. Microscopic analyses 4.1. Optical microscope observations Thin 25 lm thick "lms of were made from the downstream and upstream geotextile samples. These "lms were examined in a transmission microscope. 4.1.1. Upstream area Figs. 10 and 11 represent a cross-section through a sample from the upper part of the overlap. An area with a relatively higher soil load, covering about 100 lm, can be seen compared to the rest of the sheet which remains very open and without particles entrapped (textile}textile contact at the base). The formation of bridges can be seen, linking the "bres and forming a `spongea structure. In sample of the lower strip (Figs. 12 and 13), deposits occurred inside the thickness of the geotextile. This deposit is more dense near the soil}geotextile interface. In the central part the voids percentage is very high. In the transition zone from the geotextile to the dam soil, there is less "nes particles than in the original soil. In spite of reversing #ows, bridges maintain equilibrium of the soil on the "bres.
Fig. 10. Upstream zone, upper part of the overlap.
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Fig. 11. Upstream zone, upper part of the overlap.
4.1.2. Downstream area All the observations made on the textile samples taken from the downstream side show that the textile sheet is highly undulated and deformed, that it is very clean and that very few soil particles or deposits are trapped in the sheet.
4.2. Scanning electron microscope observations A Valcros geotextile "lter sample observed with a scanning electron microscope (SEM) is not clogged, the "bers do not appear damaged and they are slightly surrounded by "ne particles (Fig. 14). Details show particles sticked on the "bers. Further researches are necessary to determine the kinetic of this increase and to know if it could cause clogging of pores.
5. Veri5cation of somme current 5ltration criteria The validity of current "ltration criteria was tested for this application where the geotextile "lters have given satisfaction for more than twenty years. Table 5 summarises the characteristics of the soil and geotextile required for application in the "ltration criteria. It is to be noted that the sieve analysis of the soil to be taken into account upstream is not the same as that for the soil downstream.
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Fig. 12. Upstream zone, lower part: geotextile between stones. Table 5 Characteristics of the soil and geotextile required in the "ltration criteria Downstream
Upstream
Soil d , d , and d d C "d /d k (sol)
20, 80, 500 lm 12 mm '100 10\ m/s
5, 20, 50 lm 1.2 mm 40 10\ m/s
Geotextile O k W
100 lm 3.7;10\ m/s 1.2 s\
75 lm 5.25;10\ m/s 2.4 s\
Remarks
'6, no uniform soils
French standard French standard
Notes: O is the "ltration opening size of the geotextile; k is the permeability of the geotextile; W is the permittivity of the geotextile.
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Fig. 13. Upstream zone, lower part beneath stones. Table 6 Static conditions Origin of criteria CFGG (1986) Retention Permeability Giroud (1982) Retention Permeability Heerten (1992) Retention Thickness Damage La#eur (1992) Retention Permeability
Criteria
Downstream "lter
Validity
O (d (C"1) W '10 ) k
O (12 mm W '0.01 s\
OK OK
O (13.5/Cu ) d
Cu "53 O (0.127 mm k 10\ m/s
OK
k '0.1 k O (d et O (10 d 30 O (T (50 O k '300 g/m
O (15 mm et O (5 mm 3(T (5 mm
O (d (gr.1, curve 1) k '20 k
O (0.5 mm k '2;10\ m/s
OK
OK NO OK OK OK
The soil internal stability was veri"ed by Farkouh (1994) according to Kenney and Lau (1985). The validity of the "ltration criteria are summarized in Tables 6 and 7. All selected "ltration criteria are well respected except, for the upstream
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Fig. 14. SEM views of Valcros geotextile from upstream zone. The scale is given by diameter of the "bers: 26 lm.
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Table 7 Dynamic conditions Origin of criteria CFGG (1986) Retention Permeability Giroud (1982) Retention Permeability Heerten (1992) Retention Thickness Damage La#eur (1992) Retention Permeability
Criteria
Upstream "lter
Validity
O (0.48 d (C"0.48) W '10 ) k
O (0.29 mm W '0.01 s\
OK OK
O (18/Cu ) d
OK
k '0.1 k
Cu "12 O (0.075 mm k 10\ m/s
O (d
O (0.05 mm
NO
30 O (T (50 O k '300 g/m
2.25(T (3.75 mm
OK OK
O (d k '20 k
O (0.05 mm k '2;10\ m/s
NO OK
OK
"ltration conditions, the retention criteria of Heerten (1992) and of La#eur (1992) and, for the downstream "ltration conditions, the thickness criterion of Heerten (1992). In this last case, we have seen that some small punctures occurred downstream con"rming that rounded gravel should be used for drainage material.
6. Conclusion The case of the Valcros dam is extremely interesting for the following reasons: E this is the "rst use of geotextiles in an earth dam; E it has been the subject of several investigations, observations and analyses on various aspects (Giroud et al., 1977; Delmas et al., 1992); E it presents the typical case of small and medium size earth dams (pro"le, material gradation curve and drainage system); E the geotextiles used in this structure have given satisfactory results for the last twenty years. The geotextile placed in the ground for a period of twenty one years has not been subject to any signi"cant change in behaviour since its installation (comparison of 1976 and 1992 tests). Therefore, a geotextile protected from UV rays has not lost its mechanical and hydraulic properties and there is no signi"cant evidence of ageing. It is not feasible to excavate a hole of any size in the body of the dam (where the geotextile is e!ectively subjected to #ow forces) because this could threaten the safety of the dam during its service life. Nevertheless, some remarks can be made about the
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downstream drain/"lter: the #ow rate measured downstream of the drain in 1992 is similar to the value measured in 1976. The out#owing water does not carry any soil particles. No seepage has been observed on the downstream facing of the dam and this implies that the entire downstream drainage system is working properly. In view of some imprints caused by the angularity of the drain gravel, it would seem advisable to prescribe a puncture strength criterion further to thickness criterion as proposed by Heerten (1992) for certain applications. Considering upstream protection "lter, for the geotextile protected by the rock"ll, no erosion under the geotextile was noted, thus proving the e$ciency of the geotextile as a "lter. Similar to Heerten's (1982) "ndings, a considerable volume of solid materials (up to 3 kg/m) was found on the geotextile, between the rip-rap blocks, which had not come from the soil making up the dam (Heerten, 1982) found up to 10 kg/m). The permeability of the geotextile is greater than that of the soil to be protected (permeability criterion), and the residual porosity of 40% to 80% is su$cient for the geotextile to maintain its "lter function. The microscopic study showed that the particles incorporated in the geotextile stay in the parts close to the surface of the "brous sheet, whereas the central parts of the sheet has shown very little entrapment.
Acknowledgements The authors thank very much J. Allemand (University of Savoie, Annecy, France) for his kind and e$cient collaboration to observe geotextile "lter of Valcros dam with his SEM.
References CFGG (French Geotextiles and Geomembranes Society), 1986. Recommandations geH neH rales pour la reH ception et la mise en oeuvre des GeH textiles. Avril 1986. Delmas P., Farkouh B., Faure Y.H., Nancey, A., 1994. Long term behaviour of a geotextile as a "lter in a 21 year-old dam: Valcros. Fifth Internat. Conf. on Geotextiles, Geomembranes and Related Products, Singapore, Vol. 3, pp. 1199}1202. Delmas, P., Nancey, A., Faure, Y.H., Farkouh, B., 1992. Long term behaviour of a nonwoven geotextile in a 21 year-old dam. First Internat. Conf. Geo"lters, Karlsruhe, Germany, pp. 331}338. Farkouh, B., 1994. Le "ltre geH osyntheH tique dans les ouvrages de drainage: essais de laboratoire et observations in-situ, Thesis University of Grenoble, France, 368p. Giroud, J.P., 1982. Filter criteria for Geotextiles. Second Internat. Conf. on Geotextiles, Las Vegas, USA, Vol. 1, pp. 103}108. Giroud, J.P., Gourc, J.P., Bally, P., Delmas, P., 1977. Comportement d'un textile non-tisseH dans un barrage en terre. Internat. Conf. on the use of fabrics in geotechnics, Paris 1977, Vol. II, pp. 213}218. Gourc, J.P., Faure, Y.H., Hussain, H., Sotton, M., 1982. Standard test of permittivity and application of Darcy's formula. Second Internat. Conf. on Geotextiles, Las Vegas, USA, Vol. I, pp. 139}144. Heerten, G., 1982. Dimensioning the "ltration properties of geotextiles considering long-term conditions. Second Internat. Conf. on Geotextiles, Las Vegas, USA, Vol. I, pp. 115}120. Heerten, G., 1992. A contribution to the improvement of dimensioning analogies for grain "lters and geotextile "lters. First Internat. Conf. Geo"lters, Karlsruhe, Germany, pp. 121}126.
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Kenney, T.C., Lau, D., 1985. Internal stability of granular "lters. Canadian Geotechnical Journal 22 (2), 215}225. La#eur, J., Mlynarek, J., Rollin, A.L., 1992. Filter criteria for well graded cohesionless soils. First Internat. Conf. Geo-"lters, Karlsruhe, Germany, pp. 97}106.
Analysis of geotextile "lter behaviour after 21 years in Valcros dam Y.H. Faure *, B. Farkouh , Ph. Delmas, A. Nancey LIRIGM, University of Grenoble, BP 53, 38041 Grenoble Cedex 9, France Bidim Geosynthetics, BP 80, 95873 Bezons Cedex, France Received 22 October 1998; received in revised form 13 February 1999; accepted 22 February 1999
Abstract In 1970, nonwoven geotextiles were used for the "rst time in an earth dam. The geotextile acted as a "lter for the toe drain and on the upstream slope below the rip-rap. In 1992, samples were taken from both locations and performance tests were conducted in the laboratory. This paper presents the main results of the hydraulic behaviour of the geotextile "lter in association with the soil of the dam. Also microscopic analyses are presented and, as the "lter is considered to be performing well, selected "lter criteria are checked. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Geotextile; Filter; Hydraulic properties; Mechanical properties; Durability; Filter criteria
1. Description of the structure The Valcros dam was built in 1970 and it is described in detail by Giroud et al. (1977). This homogeneous earth dam is made of silty sand originating from the surface weathering of the schistose rock forming the foundation and abutments (gradation curve of this material is given in Fig. 1). The embankment is 20 m high (Fig. 2), 140 m long at the crest and has upstream and downstream slopes of 3H/1V. This structure is the "rst of this size in which geotextiles have been used: } downstream, as a "lter for the toe drain, the drain gravel (8 to 13 mm range) is wrapped in a 300 g/m nonwoven geotextile; * Corresponding author. Tel.: 33-476-82-80-56; fax: 33-476-82-80-70. E-mail address: [email protected] (Y.H. Faure) 0266-1144/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 9 9 ) 0 0 0 1 0 - 2
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Fig. 1. Soil granulometry of Valcros dam.
Fig. 2. Cross section of Valcros dam.
} upstream, as a "lter under 250 mm rocks placed directly on a 400 g/m nonwoven polyester geotextile sheet placed in strips stitched together and "xed to the ground by metal pins. Valcros dam has already been the subject of several monitoring programmes. The "rst series of tests were performed in 1976 on specimens taken from the dam by Giroud et al. (1977). Results from the second programme (1992) have been published by Delmas et al. (1992, 1994). This present paper presents a synthesis of the main results of the tests performed on the geotextile samples to check if the selected geotextiles are well functionning and if they are well designed according to updated "lter criteria.
2. 1992 sample collection programme In February 1992, samples were taken (Fig. 3) from the dam: } Downstream, a 4 m deep pit was excavated (4.5 m long and 5 m wide, Fig. 4), in order to reach the main drain. Samples of geotextile were taken from three distinct
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Fig. 3. Distinct sections of geotextile sampling (1992 programme).
Fig. 4. 4 m deep pit downstream of the dam to take geotextile samples from the toe drain of the dam (1992).
parts: from the top and from the bottom of the drain and in the top part of the overlap (Fig. 3); } Upstream, a 4 m;3 m area was cleared of rip-rap rocks (Fig. 5) at water surface level, situated that day about 20 cm below maximum reservoir level. This part is often subjected to lapping waves. An approximately 0.5 m wide overlapping section of two geotextile strips was found in this cleared area with the overlap running parallel to the dam axis (Fig. 3). Geotextile samples were taken from the lower strip, at water level, from the lower part of the overlap, from the upper part of the overlap and from the upper strip, about 1 m above water level.
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Fig. 5. Area cleared from armour rocks at upstream of the dam to take geotextile "lter samples.
At both locations, soil samples were taken from the immediate vicinity of the textile (within a few millimetres) and about 0.10 m upstream of the geotextile. Cubic specimen boxes (0.20 m side length) were also used to obtain combined soil-textile samples with great care being taken to avoid disturbance.
3. Laboratory tests Systematic tests were carried out on the geotextile samples taken: mass per unit area, thickness, tensile strength, and hydraulic properties ("ltration opening size, soil entrapment level and permittivity of cleaned samples). All the results of these tests are presented in Tables 1}4, reported from Delmas et al. (1992). 3.1. Mechanical test: tensile test The same test was used as for the tests conducted in 1976: French Standard NF G 07-001. The test samples are 0.20 m long and 0.05 m wide. Before the test, the surface of the samples is cleaned of any soil and washed carefully. Then, they are dried before testing. The test results show no signi"cant di!erences between the tensile characteritics observed in 1976 and 1992 (Tables 1 and 2). The small di!erences are related to soil particles trapped inside the geotextile. After twenty years of use, the geotextiles are mechanically performing as well as originally.
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Table 1 Comparison of tensile tests results on new samples and samples taken in 1976 and 1992 from the downstream zone 1976 samples
1992 samples
Tensile tests NF G 07 001
New
Mean value
CV (%)
Mean value
CV (%)
Strength (long direc.) Strength (cross direc.) Strain (long direc.) Strain (cross direc.) Mass per unit area (g/m)
860 N 580 N 60% 64% 300
680 N 520 N 58% 51% Unknown
15 10 8 6 x
669 N 592 N 47% 50% 350
12 14 4 13
Notes: Samples of identical geotextile fabricated in 1976 and tested in 1976. CV is the coe$cent of variation. Table 2 Comparison of tensile tests results on new samples and samples taken in 1976 and 1992 from the upstream zone 1976 samples
1992 samples
Tensile tests NF G 07 001
New
Mean value
CV (%)
Mean value
CV (%)
Strength (long direc.) Strength (cross direc.) Strain (long direc.) Strain (cross direc.) Mass per unit area (g/m)
710 N 520 N 45% 41% 400
760 N 630 N 49% 36% Unknown
5 16 9 6
846 N 700 N 47% 44% 474
14 10 8 6
Notes: Samples of identical geotextile fabricated in 1976 and tested in 1976. CV is the coe$cent of variation. Table 3 Hydraulic test on samples taken in 1992, downstream zone Upper part
Mass per unit area (g/m) Entrapment ratio (%) Permittivity (s\) Filtration opening size (lm)
Lower part
Overlap upper side
No. 1
No. 2
276$40 4.35 2.7$0.6 95
346$51 1.9 2.1$0.2 115
328$20 0.8$0.3 2.2$0.2 95
329$15 1$0.2 2.2$0.2 235
3.2. Hydraulic tests: index tests Permittivity tests and "ltration opening size tests (Tables 3 and 4), were performed according to the French standards on cleaned specimens (washed and dried). Permittivity (NF G 38-016, 1989): the #ow is kept under constant head and the velocity is maintained 10 mm/s. The results show a permittivity greater than 1 s\
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Table 4 Hydraulic test on samples taken in 1992, upstream zone Lower part No. 1 SE
Overlap No. 2
EE
SE
EE
Upper part
lower side
upper side
SE
SE
EE
Mass/unit 468 518 531 603 538 514 485 area (g/m) Entrapment 21.6 39.1 14.4 54.8 7.4 11.2 13 level (%) Permittivity (s\) Washed samples 1.03 1.21 0.97 1.23 0.98 1.46 1.06 Unwashed samples 0.39 0.290 0.30 0.36 0.68 0.81 0.42 Filtration opening 72 73 72 68 72 size (lm)
EE
SE
EE
485
460
547
39.6
16.4
36.7
1.24 0.31 69
1.33 0.27 70
1.16 0.38 69
Notes: SE beneath stone; EE between stones.
upstream and greater than 2 s\ downstream. Upstream, permittivity is slightly lower for samples taken beneath stones than for samples taken beneath voids between stones. That could be due to the compressive stress at the points of contact. Filtration opening size (NF G 38-017, 1989): hydrodynamic sieving with 0.30 m diameter samples. The opening size corresponds to the d value of the soil passed through the geotextile sample. There is no particular observation upstream, but downstream a larger value is obtained for the specimen No. 2 taken from the upper part. As the geotextile is in contact with crushed gravel and compressed below 4 m of soil, imprints of drain gravels are observed and some small holes (5}7 per m) are visible. The hydraulic properties of the downstream and upstream geotextiles, have remained unchanged after more than twenty years use in the ground. The following section presents the results of tests carried out on the geotextile samples in association with the soil of the dam. 3.3. Permittivity of geotextile samples with entrapped particles The permittivity of the samples was measured with the permeameter used for the standard permittivity tests on uncompressed geotextiles (Gourc et al., 1982). To prevent particle migration under the drag force of the #ow, the sample was placed between two "lters (made up of a nonwoven heat-bonded textile sheet) reinforced by a grid and a perforated metal plate. The sample and the test circuit were brought to saturation very gradually, at very low #ow velocity, in order to avoid disturbing the soil}"bre structure. The measurement was also taken at very low #ow speed (5 mm/s). The system with the two "lters (textile#grid#perforated plate) was calibrated
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without any sample for reference purposes. For a given #ow rate, this measurement was used to determine the head loss created by a geotextile containing particles. The permittivity measurement was related to the entrapment level of the samples tested. The entrapment level (¹ ) is de"ned as the percentage (in volume) of geotextile voids < "lled with soil particles < . < ¹ " , < where M k and < "n"1! , <" o ;¹ o¹ i.e.
M o ;100 ¹ (%)" ¹ o !k o < "volume of the geotextile voids per unit volume ("n, porosity of the geotextile) < "volume of soil particles entrapped per unit volume of geotextile o "speci"c mass of the soil (2.65 10 g/m) o "speci"c mass of the "bers (1.37 10 g/m) ¹ "geotextile thickness (m) M "mass of trapped soil per unit area of geotextile (g/m) k "mass per unit area of the clean geotextile (g/m). The level of particles entrapment was calculated on the assumption that the soil inside the geotextile was uniformly distributed and that the mass per unit area of the sample was equal to that of the original geotextile. Before performing the test, the samples were dried and weighed to determine ¹ and a comparison of entrapment level after the test showed a 3% decrease on the average. This study was performed only on the geotextile samples taken from upstream where the soil entrapment level is higher. The maximum ¹ values remain less than 60% (Table 4), i.e., there are still 40% of voids compared to the initial volume of voids. Between stones, the entrapment level is higher than beneath stones (Table 4). This entrapment is mainly produced by external soil, not by soil of the dam as it can be observed for the geotextile from the overlap: the samples against the dam soil (lower side) contain less entrapped particles than the samples from the upper side. The permittivity decreases linearly with the logarithm of the entrapment level (Fig. 6). The minimum permittivity obtained with the highest entrapment level (55%) is 0.21 s\, which corresponds to a permeability of 6.3;10\ m/s. The geotextile, therefore, remains highly permeable compared to the soil permeability (10\ m/s).
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Fig. 6. Permittivity of upstream specimens (lower strip).
3.4. Filtration test The soil}geotextile samples were tested in a "ltrameter (Fig. 7) consisting of a base and cylindrical rings of 0.04 m height and 0.15 m inside diameter. These rings were interlocked to obtain the desired height. They were provided with ori"ces to connect piezometers. At the top, a hollow piston was used to compress the sample. The water was fed in through the base from a constant-level holding tank. After passing through the soil sample, the water was circulated through the piston and conveyed to a constant-level downstream tank. Given the nature of the soil and its low water content (w"12% downstream), the sample was humidi"ed using the capillary e!ect by placing the sample on a bed of sand immersed in water for several days in order to be able to handle it. This procedure was not necessary for the soil sample taken upstream. A 20 mm thick layer of "ne gravel was placed at the base of the cell and the upstream piezometer (No. 1, Fig. 7) was installed in this layer. The sample was placed in the cell (the geotextile at the top, with the #ow rising). To ensure tightness around the sample, the gaps between the sample and the inside wall of the cell were "lled with dry bentonite. A piezometer (No. 2) was installed in the sample, 25 mm from the geotextile. A 20 mm thick layer of "ne gravel was placed on top of the sample and another piezometer (No. 3) was installed there. The sample was lightly compressed by the piston and brought to saturation point very slowly under a small head of water with a rising #ow of water. This gave the bentonite time to swell and seal the gaps between sample and wall. During the tests the samples were subjected to an upward #ow at constant head, with an overall gradient of 6 to 8. The tests lasted 30 to 40 days. Downstream sample. Fig. 8a shows the time-dependent variation in hydraulic gradient (i ) in the area close to the geotextile, the variation in hydraulic gradient (i ) in the soil
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Fig. 7. Filtrameter for testing soil/geotextile system.
Fig. 8. (a) Time-dependent variation in hydrauilic gradient (downstream sample): i in the soil (1}2, cf. Fig. 7), i in the geotextile (2}3), i in the mean (1}3). (b) Time-dependent variation in hydraulic gradient
(upstream sample): i in the soil (1}2, cf. Fig. 7), i in the geotextile (2}3), i in the mean (1}3). K
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upstream, and the mean hydraulic gradient (i ). It seems that some washout of soil
occurred at the start of the test but this was of no consequence. The permeability near the "lter is slightly lower than that of the soil (k (k ). Upstream sample. Because of a leakage problem for discharge measurement, the "rst 5 days of the test were discounted. The #uctuations noted (Fig. 8b) would seem to indicate clogging of the "lter (i (i ) during the test but, in reality, they are linked to the di$culty inherent in this type of measurement (piezometers) in "ne soils of low permeability (not observed for the downstream sample). However, with a gradient ratio never noted greater than 3, it may be concluded that the "ltering area immediately in contact with the soil did not become clogged. 3.5. Soil grain size analyses The analysis of the soil (upstream area) at a depth of 100 mm was compared to that of the soil (over a 2}5 mm thickness) in contact with the geotextile between rock"ll and under rock"ll. It was found (Fig. 9) that there is no signi"cant di!erence between the soil in contact with the textile and deeper soil, except for the absence of large particles in the contact soil, which is not surprising in view of the small sampling depth. Other grain size analyses of soils trapped inside the geotextile "lters or taken from around the drains were presented by Delmas et al. (1992). The grain size analyses did not reveal any signi"cant rearrangement of the soil particles close to the "lter. On the other hand, a clear di!erence in grain size analyses was found between the sample from the upstream side and that from the downstream side. This was related to a di!erent sampling zone for the upstream facing during dam construction.
Fig. 9. Sieve analyses of the dam soil at di!erent points (upstream).
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4. Microscopic analyses 4.1. Optical microscope observations Thin 25 lm thick "lms of were made from the downstream and upstream geotextile samples. These "lms were examined in a transmission microscope. 4.1.1. Upstream area Figs. 10 and 11 represent a cross-section through a sample from the upper part of the overlap. An area with a relatively higher soil load, covering about 100 lm, can be seen compared to the rest of the sheet which remains very open and without particles entrapped (textile}textile contact at the base). The formation of bridges can be seen, linking the "bres and forming a `spongea structure. In sample of the lower strip (Figs. 12 and 13), deposits occurred inside the thickness of the geotextile. This deposit is more dense near the soil}geotextile interface. In the central part the voids percentage is very high. In the transition zone from the geotextile to the dam soil, there is less "nes particles than in the original soil. In spite of reversing #ows, bridges maintain equilibrium of the soil on the "bres.
Fig. 10. Upstream zone, upper part of the overlap.
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Fig. 11. Upstream zone, upper part of the overlap.
4.1.2. Downstream area All the observations made on the textile samples taken from the downstream side show that the textile sheet is highly undulated and deformed, that it is very clean and that very few soil particles or deposits are trapped in the sheet.
4.2. Scanning electron microscope observations A Valcros geotextile "lter sample observed with a scanning electron microscope (SEM) is not clogged, the "bers do not appear damaged and they are slightly surrounded by "ne particles (Fig. 14). Details show particles sticked on the "bers. Further researches are necessary to determine the kinetic of this increase and to know if it could cause clogging of pores.
5. Veri5cation of somme current 5ltration criteria The validity of current "ltration criteria was tested for this application where the geotextile "lters have given satisfaction for more than twenty years. Table 5 summarises the characteristics of the soil and geotextile required for application in the "ltration criteria. It is to be noted that the sieve analysis of the soil to be taken into account upstream is not the same as that for the soil downstream.
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Fig. 12. Upstream zone, lower part: geotextile between stones. Table 5 Characteristics of the soil and geotextile required in the "ltration criteria Downstream
Upstream
Soil d , d , and d d C "d /d k (sol)
20, 80, 500 lm 12 mm '100 10\ m/s
5, 20, 50 lm 1.2 mm 40 10\ m/s
Geotextile O k W
100 lm 3.7;10\ m/s 1.2 s\
75 lm 5.25;10\ m/s 2.4 s\
Remarks
'6, no uniform soils
French standard French standard
Notes: O is the "ltration opening size of the geotextile; k is the permeability of the geotextile; W is the permittivity of the geotextile.
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Fig. 13. Upstream zone, lower part beneath stones. Table 6 Static conditions Origin of criteria CFGG (1986) Retention Permeability Giroud (1982) Retention Permeability Heerten (1992) Retention Thickness Damage La#eur (1992) Retention Permeability
Criteria
Downstream "lter
Validity
O (d (C"1) W '10 ) k
O (12 mm W '0.01 s\
OK OK
O (13.5/Cu ) d
Cu "53 O (0.127 mm k 10\ m/s
OK
k '0.1 k O (d et O (10 d 30 O (T (50 O k '300 g/m
O (15 mm et O (5 mm 3(T (5 mm
O (d (gr.1, curve 1) k '20 k
O (0.5 mm k '2;10\ m/s
OK
OK NO OK OK OK
The soil internal stability was veri"ed by Farkouh (1994) according to Kenney and Lau (1985). The validity of the "ltration criteria are summarized in Tables 6 and 7. All selected "ltration criteria are well respected except, for the upstream
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Fig. 14. SEM views of Valcros geotextile from upstream zone. The scale is given by diameter of the "bers: 26 lm.
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Table 7 Dynamic conditions Origin of criteria CFGG (1986) Retention Permeability Giroud (1982) Retention Permeability Heerten (1992) Retention Thickness Damage La#eur (1992) Retention Permeability
Criteria
Upstream "lter
Validity
O (0.48 d (C"0.48) W '10 ) k
O (0.29 mm W '0.01 s\
OK OK
O (18/Cu ) d
OK
k '0.1 k
Cu "12 O (0.075 mm k 10\ m/s
O (d
O (0.05 mm
NO
30 O (T (50 O k '300 g/m
2.25(T (3.75 mm
OK OK
O (d k '20 k
O (0.05 mm k '2;10\ m/s
NO OK
OK
"ltration conditions, the retention criteria of Heerten (1992) and of La#eur (1992) and, for the downstream "ltration conditions, the thickness criterion of Heerten (1992). In this last case, we have seen that some small punctures occurred downstream con"rming that rounded gravel should be used for drainage material.
6. Conclusion The case of the Valcros dam is extremely interesting for the following reasons: E this is the "rst use of geotextiles in an earth dam; E it has been the subject of several investigations, observations and analyses on various aspects (Giroud et al., 1977; Delmas et al., 1992); E it presents the typical case of small and medium size earth dams (pro"le, material gradation curve and drainage system); E the geotextiles used in this structure have given satisfactory results for the last twenty years. The geotextile placed in the ground for a period of twenty one years has not been subject to any signi"cant change in behaviour since its installation (comparison of 1976 and 1992 tests). Therefore, a geotextile protected from UV rays has not lost its mechanical and hydraulic properties and there is no signi"cant evidence of ageing. It is not feasible to excavate a hole of any size in the body of the dam (where the geotextile is e!ectively subjected to #ow forces) because this could threaten the safety of the dam during its service life. Nevertheless, some remarks can be made about the
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downstream drain/"lter: the #ow rate measured downstream of the drain in 1992 is similar to the value measured in 1976. The out#owing water does not carry any soil particles. No seepage has been observed on the downstream facing of the dam and this implies that the entire downstream drainage system is working properly. In view of some imprints caused by the angularity of the drain gravel, it would seem advisable to prescribe a puncture strength criterion further to thickness criterion as proposed by Heerten (1992) for certain applications. Considering upstream protection "lter, for the geotextile protected by the rock"ll, no erosion under the geotextile was noted, thus proving the e$ciency of the geotextile as a "lter. Similar to Heerten's (1982) "ndings, a considerable volume of solid materials (up to 3 kg/m) was found on the geotextile, between the rip-rap blocks, which had not come from the soil making up the dam (Heerten, 1982) found up to 10 kg/m). The permeability of the geotextile is greater than that of the soil to be protected (permeability criterion), and the residual porosity of 40% to 80% is su$cient for the geotextile to maintain its "lter function. The microscopic study showed that the particles incorporated in the geotextile stay in the parts close to the surface of the "brous sheet, whereas the central parts of the sheet has shown very little entrapment.
Acknowledgements The authors thank very much J. Allemand (University of Savoie, Annecy, France) for his kind and e$cient collaboration to observe geotextile "lter of Valcros dam with his SEM.
References CFGG (French Geotextiles and Geomembranes Society), 1986. Recommandations geH neH rales pour la reH ception et la mise en oeuvre des GeH textiles. Avril 1986. Delmas P., Farkouh B., Faure Y.H., Nancey, A., 1994. Long term behaviour of a geotextile as a "lter in a 21 year-old dam: Valcros. Fifth Internat. Conf. on Geotextiles, Geomembranes and Related Products, Singapore, Vol. 3, pp. 1199}1202. Delmas, P., Nancey, A., Faure, Y.H., Farkouh, B., 1992. Long term behaviour of a nonwoven geotextile in a 21 year-old dam. First Internat. Conf. Geo"lters, Karlsruhe, Germany, pp. 331}338. Farkouh, B., 1994. Le "ltre geH osyntheH tique dans les ouvrages de drainage: essais de laboratoire et observations in-situ, Thesis University of Grenoble, France, 368p. Giroud, J.P., 1982. Filter criteria for Geotextiles. Second Internat. Conf. on Geotextiles, Las Vegas, USA, Vol. 1, pp. 103}108. Giroud, J.P., Gourc, J.P., Bally, P., Delmas, P., 1977. Comportement d'un textile non-tisseH dans un barrage en terre. Internat. Conf. on the use of fabrics in geotechnics, Paris 1977, Vol. II, pp. 213}218. Gourc, J.P., Faure, Y.H., Hussain, H., Sotton, M., 1982. Standard test of permittivity and application of Darcy's formula. Second Internat. Conf. on Geotextiles, Las Vegas, USA, Vol. I, pp. 139}144. Heerten, G., 1982. Dimensioning the "ltration properties of geotextiles considering long-term conditions. Second Internat. Conf. on Geotextiles, Las Vegas, USA, Vol. I, pp. 115}120. Heerten, G., 1992. A contribution to the improvement of dimensioning analogies for grain "lters and geotextile "lters. First Internat. Conf. Geo"lters, Karlsruhe, Germany, pp. 121}126.
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Kenney, T.C., Lau, D., 1985. Internal stability of granular "lters. Canadian Geotechnical Journal 22 (2), 215}225. La#eur, J., Mlynarek, J., Rollin, A.L., 1992. Filter criteria for well graded cohesionless soils. First Internat. Conf. Geo-"lters, Karlsruhe, Germany, pp. 97}106.