TITLE: HYDRAULIC COMPATIBILITY OF GEOTEXTILE DRAINS WITH FLY ASH IN PAVEMENT STRUCTURES

Transcription

1 Paper No TITLE: HYDRAULIC COMPATIBILITY OF GEOTEXTILE DRAINS WITH FLY ASH IN PAVEMENT STRUCTURES Author(s): M. Emin Kutay, and Ahmet H. Aydilek Department of Civil and Environmental Engineering 73 Glenn Martin Hall University of Maryland, College Park, MD Session Title: Sponsoring Committees: Recent Improvements in Performance of Pavement Drainage Systems A2KO6 on Subsurface Drainage and A2KO7 on Geosynthetics Paper accepted for presentation at the Transportation Research Board 82 nd Annual Meeting January 2-6, 2003 Washington, D.C. Abstract: The legislations have been promulgated in many states that remove barriers to large-scale beneficial re-use of waste materials. As a result, there is a renewed emphasis on incorporating suitable waste products into highway construction. Fly ash is one of these materials, and is increasingly being used as fill materials for highway embankments, and as grout mixes for road bases. In most cases, a geotextile is in contact with the fly ash or fly ash-treated soil, and drainage is one of the duties expected from the geotextile. Hydraulic compatibility of geotextile drains with fly ash was evaluated through laboratory gradient ratio tests. The results indicate that fly ash is compatible with a variety of woven and nonwoven geotextiles. Clogging of the geotextile or excessive piping of fly ash was not observed, even under relatively high hydraulic gradients. The gradient ratio test had some limitations when used with fly ash. Long-term tests should be performed for evaluating the performance of the fly ash-geotextile systems, and changes in hydraulic conductivity should be analyzed along with the measured gradient ratios.

2 Kutay and Aydilek Key words: fly ash, drainage, geotextile, geocomposite drain, gradient ratio test. INTRODUCTION Geotextiles are increasingly being used in transportation applications due their ease of construction and economy over traditional methods. Geotextiles are often used in embankment construction; their two most important roles being the reinforcement of the foundation and separation of the embankment fill from the foundation soil. In addition to these roles, geotextiles provide lateral drainage of percolating water and prevent the build-up of excess pore water pressure. Pavement subsurface and highway edge drainage systems are other application areas in which geocomposite (GC) drains are commonly employed. GC drains are composed of a geonet sandwiched between two geotextile layers, and have been cost effective alternatives over traditional drainage systems for the last two decades (Allen and Fleckenstein 99, Christopher et al. 2000). The current design of GC drains and geotextile drains is primarily dependent on their flow rate capacities. However, the hydraulic compatibility of a geotextile with the contact soil is an important issue and should be considered in design procedures. This compatibility is usually analyzed through laboratory soil filtration tests. The first main requirement for ensuring this hydraulic compatibility is that the drain should not be clogged throughout the life of the structure. The second requirement is that the soil piped through the geotextile should be minimal, so that the internal stability and modulus of the soil are not adversely affected. Allen and Fleckenstein (99) reported excessive clogging of GC drains in two different projects. The drain was completely clogged due to accumulation of soil fines at the surface and inside the geotextile (blinding and clogging phenomenon, respectively), resulting in excess pore water pressure build-up under the pavement. Similar problems, excessive clogging of geotextile component of GC drains with fine-grained soils, were also reported by Highlands et al. (99). These failures indicate that the hydraulic compatibility of contact soil with the geotextile component of a drain is an important issue, requiring consideration during pavement drainage system design. The problem of fine particle clogging becomes more cumbersome when industrial by-products are in contact with geotextiles in pavement drainage systems. The nature of these geomaterials is different than regular soils, often consisting of significant amounts of fines. The existing geotextile selection criteria may not be directly applicable to these materials and, in most cases, their filtration or drainage performance with geotextiles should be investigated by conducting laboratory tests. Fly ash is one of these industrial by-products, and has increasingly being used in transportation applications as fill materials or grout mixes for highway embankments, as well as grout mixes for road bases (Akram and Gabr 997, Edil et al. 2002). Beneficial reuse of fly ash in pavement bases has gained wide acceptance due to its abundance. For instance, 3.5 million tons of fly ash was used in pavement construction in the U. S. in 996 (American Coal Association 996). In spite of ongoing efforts to use fly ash in highway construction, limited information is available about its hydraulic compatibility with geotextiles (Gabr

3 Kutay and Aydilek 2 and Akram 996). Clogging of a geotextile drain by fly ash particles may cause significant reduction in permeability, thus reducing the flow capacity of the drain. Even though, the fly ash is mixed with other aggregates (e.g. sand), reduction of the permeability will be caused as a result of movement of fly ash particles through the drain. Therefore, the fly ash should be hydraulically compatible with the adjacent geotextile. In order to respond to this need, a series of laboratory gradient ratio tests were conducted to investigate the clogging behavior of various geotextiles with fly ash. The retention performance of these geotextiles was also investigated by analyzing the results obtained from the gradient ratio tests. MATERIALS AND METHODS Geotextiles One woven and three nonwoven geotextiles were employed in the testing program. The nonwoven geotextiles were selected from the ones most often used in filter applications and had a wide range of porosity, apparent opening size (AOS or O 95 ) and permittivity (ψ). The woven geotextile was a high-strength one (ultimate wide-width tensile strength=75 kn/m), and has commonly being used in reinforcement applications. In these applications filtration is usually the secondary function of the geotextile and therefore, it was included in the testing program. The physical and hydraulic properties of the geotextiles are given in Table. Fly Ash The fly ash used in this study was obtained from the Brandon Shores Facility of Baltimore Gas and Electric Company in Maryland ans was classified as Type F fly ash. The specific gravity of the material was determined as 2.2, in accordance with the ASTM D854. Particle size analysis indicated that 85% of the material passed through U.S. No. 200 sieve. Figure shows the particle size distribution curve of the fly ash. Physical and chemical characteristics of the fly ash are summarized in Table 2. Laboratory Tests Gradient ratio tests (ASTM D 50), were conducted to determine the clogging performance of fly ash-geotextile systems (Figure 2). The test apparatus consists of a rigid-wall permeameter, inflow and outflow devices, and a set of piezometers to monitor the water heads at different depths in the soil. Previous studies indicate that natural soils and recycled waste materials with hydraulic conductivities higher than 0-5 cm/s could be successfully be tested in rigid-wall permeameters with an insignificant amount of sidewall leakage (Ghosh and Subbarao 998, Aydilek and Edil 2002). Preliminary observations suggested that the fly ash has reasonable hydraulic conductivity values and using the gradient ratio test permeameters would be appropriate. Contrary to the 24-hour procedure prescribed in the ASTM D50, the tests were continued for more than 3 months to understand the long-term clogging performance of these combinations. Tests were performed at system hydraulic gradients of.5, 3, 6,

4 Kutay and Aydilek 3 and 8. Methods described in the ASTM D50 were followed for the specimen preparation. A fully automated system built at the University of Maryland deaired the tap water, and continuously supplied it to the test set-up. The dissolved oxygen content of water was regularly checked and maintained between 3.5 and 4 mg/l, which was less than a limit of 6 mg/l recommended for flow testing of geosynthetics (Aydilek and Kutay 2002). Preliminary analyses indicated that biological growth occurred due to the presence of microorganisms in the tap water. The biological growth decreased the hydraulic conductivities and led to erroneous measurements. To prevent this, deaired water was treated with slowly dissolving chlorine tablets once a week. GRADIENT RATIO TEST RESULTS For the analysis of gradient ratio test results, two different ratios were used: gradient ratio (GR) and permeability ratio (K R ). Gradient ratio is defined as the ratio of hydraulic gradient through the soil-geotextile system to hydraulic gradient through the soil alone (ASTM D50). For successful applications, a gradient ratio of less than 3 is recommended for compatible systems as part of the U.S. Army Corps of Engineers criterion (Haliburton and Wood, 982). Figure 3 presents the results of gradient ratio tests. The time required for stabilization of flow under each hydraulic gradient ranged from 300 to 600 hours. This was consistent with the findings of Gabr and Akram (996), and Aydilek and Edil (2002); indicating that a 24-hour procedure stated in the ASTM D50 is not sufficient, and long-term testing is required. Three distinct flow patterns were observed, as shown in Figure 3, similar to the behavior described by Gabr and Akram (996) for fly ash-geotextile systems. A piping pattern was observed in the first 200 hours (at i=.5). At this stage, the hydraulic conductivity increased from.5 x 0-6 m/s to 2.4 x 0-6 m/s, and GR decreased from.55 to 0.9. A blocking/blinding pattern was observed between 200 and 700 hours (at i=3). Hydraulic conductivity slightly decreased from 2.4 x 0-6 m/s to 2 x 0-6 m/s, and was accompanied by an increase in GR from 0.9 to.5. A mixed behavior was observed up to 500 hours (at i=6 and 8), after which steady state flow occurred. Rollin et al. (985) observed similar flow patterns during longterm filtration tests and classified them into three distinct groups (Figure 4). Each group was defined by the following criteria: () normal behavior where soil particles move through geotextile increasing the density of the soil just above the geotextile thus reducing permeability; (2) piping behavior, occurs after some normal behavior, where fine soil particles pipe through the geotextile resulting in an increase in permeability; (3) combined behavior where loss of soil particles is followed by a filter cake formation at the soil geotextile interface. Stabilized values of GR and hydraulic conductivity were plotted versus each applied hydraulic gradient in Figure 5. The figure shows that the GR is in a narrow range between 0.8 and.0 for all tests, independent of the magnitude of applied hydraulic gradient. On the other hand, the effect of hydraulic gradient on the flow regime was more clearly pronounced by a change in hydraulic conductivity values. Hydraulic conductivity generally decreased with increasing

5 Kutay and Aydilek 4 hydraulic gradient, consistent with the findings of Rollin et al. (985), Gabr and Akram (996), and Aydilek and Edil (2002). A GR value greater than one indicates that the flow over geotextile layer is impeded, and a value greater than three is usually considered as a sign of poor compatibility between soil and geotextile. On the other hand, a gradient ratio of less than one may imply the movement of soil particles through geotextile layer (Fannin et al. 994). Table 3 indicates that GR values ranged from 0.80 to.0, indicating that the hydraulic gradient in the fly ash-geotextile system was close to that of the fly ash. This was further verified by analyzing the water head distributions in the permeameter. A linear distribution of water heads indicates a GR value of one and implies that geotextile did not affect the flow regime in the system (Fannin et al. 994). This was evident from the nearly linear distributions observed within the fly ash-geotextile system (from bottom of geotextile to the location of the second and third manometers), as shown in Figure 6. The same figure also suggests that the gradient in the soil layer above ports 2 and 3 was slightly greater than the one in the soil. This is attributed to the formation of a blinding zone due to fine accumulation at the top of the soil layer, which possibly occurred during the soil placement before testing. Previous research has suggested that use of the gradient ratio alone may not be representative due to the analysis of a relatively small soil-geotextile contact zone in the gradient ratio apparatus, and use of alternative ratios have been recommended (Fischer 994, Aydilek and Edil 2002). This could be specifically true for fine-grained soils, such as fly ash, since piping of fly ash fines through a manometer port can potentially change the head registered in that particular manometer. In order to check the accuracy of the GR values, a different ratio, permeability ratio (K R ), was used for the analysis of gradient ratio tests. The permeability ratio was defined as the ratio of soil hydraulic conductivity (K soil ) to entire system hydraulic conductivity (K system ), and was designated as K R. The K system was determined using the applied hydraulic gradient on the soil-geotextile system (i.e.,.5, 3, 6, 8). For K soil calculations, i soil values were calculated using the readings registered by manometers located 25 mm and 75 mm from the top of the middle section of the permeameter. For both of the hydraulic conductivities (i.e., K soil and K system ) stabilized flow rates were used (determined by taking the average of the last five stabilized values for each test). A clogged geotextile results in a decrease in system permeability with a ratio of soil permeability to stabilized system permeability being greater than unity. Considering the heterogeneities in the test specimen, and similar to the U.S. Army Corps of Engineers criterion for GR, K R = 3 was set as the limit for acceptable clogging of fly ash-geotextile systems. Table 3 presents the measured hydraulic conductivities along with the calculated GR and K R ratios. GR values are highly comparable with the K R ratios. Both of the ratios are lower than the limit of 3, indicating that the geotextile did not have a significant effect on the flow regime of the overall system. Furthermore, piping was not observed during the testing, which would otherwise have adversely affected the water heads, and therefore the GR values. The amount of piped soil was about 0. g in all cases, corresponding to a piping

6 Kutay and Aydilek 5 rate of 2 g/m 2. This was significantly less than 2,500 g/m 2, the internal stability limit generally used for granular and geotextile filters (Lafleur et al. 989, Bhatia et al., 998). The piped amount of fly ash through geotextiles was found to be insignificant, agreeing with the findings of Gabr and Akram (996). CONCLUSIONS Geotextiles have been used to increase the drainage efficiency of pavement systems. Ideally, the geotextile drain should remain unclogged during the design life of the pavement. This becomes more important when unusual geomaterials, rather than natural soils, are in contact with the geotextile. A laboratory test program was undertaken to evaluate the hydraulic compatibility of fly ashgeotextile systems in a pavement structure. Long-term gradient ratio tests were conducted as part of this testing program, and the following conclusions are advanced: ) Measured GR values were close to.0, indicating that the fly ash did not clog the geotextiles. The observed trends for GR and system hydraulic conductivity throughout the test were comparable with the trends observed by previous researchers for natural soils. Amount of piped soil was insignificant, even under relatively high hydraulic gradients. This is in spite of its spherical shape, which would be expected to promote piping. The piping rate was about 2 g/m 2, significantly lower than a limit of 2,500 g/m 2 set for the internal stability of filters. Even though small amounts of fly ahs piped through the geotextiles, it is suggested that geotextile drains should be selected carefully, since the piping in the field could be different due to the existence of dynamic loads. 2) The gradient ratio test (ASTM D50) had certain limitations when used in the testing of fine-grained geomaterials such as fly ash. The 24-hour time period was usually not enough to achieve a steady hydraulic conductivity in testing fly ash, and the values obtained at 24 hours could be misleading. Therefore, the tests were run until the stabilized gradient ratios and hydraulic conductivities were obtained. 3) GR did not necessarily reflect the effects of hydraulic gradient increase on the clogging performance of fly ash-geotextile systems. However, this effect was observed more clearly by analyzing the change in hydraulic conductivity values. Therefore, permeability ratio (K R ) should be used along with GR to define clogging behavior of these systems. REFERENCES American Coal Ash Association, 996, Coal Combustion Product and Use, Alexandria, VA. Aydilek, A.H., and Edil, T.B., 2002, Filtration Performance of Woven Geotextiles with Wastewater Treatment Sludge, Geosynthetics International, Vol.9, No., pp

7 Kutay and Aydilek 6 Allen, D.L., and Fleckenstein, J., 99, Evaluation and Performance of Geocomposite Edge Drains in Kentucky, Transportation Research Record, No. 329, pp , Washington, D.C. Akram, M. H., Gabr, M. A., 997, Filtration of Fly Ash Using Non-Woven Geotextiles: Effect of Sample Preparation Technique and Testing Method, Geotechnical Testing Journal, ASTM, Vol. 20, No. 3, pp Aydilek, A.H., and Kutay, M.E., 2002, Automated Water Deairing System for Geotechnical Applications, Geotechnical Testing Journal, ASTM, submitted for publication. Bhatia, S.K., Moraille J., and Smith J.L., 998, Performance of Granular versus Geotextile Filters in Protecting Cohesionless Soils, Filtration and Drainage in Geotechnical and Geoenvironmental Engineering, ASCE, Geotechnical Special Publication 78, L.N. Reddi and M.V.S. Bonala, Eds., pp.-29. Christopher, B. R., Hayden, S. A., and Zhao, A., 2000, Roadway Base and Subgrade Geocomposite Drainage Layers, Testing and Performance of Geosynthetics in Subsurface Drainage, ASTM STP 390, L. D. Suits, J. B. Goddard, and J. S. Baldwin, Eds., ASTM, West Conshohocken, Pennsylvania, pp Edil, T. B., Benson, C. H., Bin-Shafique, M. S., Tanyu, B. F., Kim, W., and Senol, A., 2002, Field Evaluation of Construction Alternatives for Roadway over Soft Subgrade, Proceedings of the 8 st Transportation Research Board Annual Meeting, TRB, Washington, D.C., in print Fannin, R.J., Vaid, Y.P., and Shi, Y.C., 994, Filtration Behavior of Nonwoven Geotextiles, Canadian Geotechnical Journal, Vol.3, pp Fischer, G.R, 994, The Influence of Fabric Pore Structure on the Behavior of Geotextile Filters, Ph.D. Dissertation, University of Washington, Seattle, Washington, USA. Gabr, M. A., and Akram, M. H., 996, Clogging and Piping Criteria for Geotextile Filters for Fly Ash, Proceedings of the 3 rd International Symposium on Environmental Technology, San Diego, pp Ghosh, A., and Subbarao, C., 998, Hydraulic Conductivity and Leachate Characteristics of Stabilized Fly Ash, Journal of Environmental Engineering, ASCE, Vol. 24, No. 9, pp Highlands, K. L., Turgeon, R., and Hoffman, G.L., 99, Prefabricated Pavement Base Drain, Transportation Research Record, No. 329, pp , Washington, D.C. Haliburton, T.A., and Wood, P.D., 982, Evaluation of the U.S. Army Corps of Engineers Gradient Ratio Test for Geotextile Performance, Proceedings of the Second International Conference on Geotextiles, Vol., Las Vegas, Nevada, USA, pp Lafleur, J., Mlynarek, J., and Rollin, A.L., 989, Filtration of Broadly Graded Cohesionless Soils, Journal of Geotechnical Engineering, ASCE, Vol. 5, No. 2, pp Rollin, A. L., Broughton, R. S., and Bolduc, G., 985, Synthetic Envelopment Materials for Subsurface Drainage Tubes, paper presented at CPTA annual meeting, 985, Fort Lauderdale, FL.

8 Kutay and Aydilek 7 Wayne, M.H., and Koerner, R.M., 993, Correlation Between Long-Term Flow Testing and Current Geotextile Filtration Design Practice Proceedings of Geosynthetics 93, IFAI, Vol. Vol., Vancouver, British Columbia, Canada, TABLE Properties of geotextiles used in the study Mass/unit Thickness area (mm) (g/m 2 ) (b) Geotextile Structure, polymer type (a) Apparent opening size, AOS (mm) Porosity Permittivity (%) (b) (s - ) A W, MU, NR NA 0.07 PET B NW, NP, SF, PP C NW, NP, SF, PP D NW, NP, SF, PP Notes: (a) W: woven, NW: nonwoven, MU: multifilament, NP: Needle punched, STF: staple fiber, PET: Polyester, PP: polypropylene. (b) NA: Not applicable. NR: Not reported. All properties are the manufacturer s reported values with the exception of the porosity values, which were determined using the method described by Wayne and Koerner (993).

9 Kutay and Aydilek 8 TABLE 2 Properties of fly ash used in the study. Property Value Molding water content (%) -2 Solids content (%) 98 Specific gravity, G s 2.2 Organic content (%) None Atterberg limits (%) Nonplastic Fines content (grains < mm) (%) 83 D 5 (mm) D 50 (mm) 0.03 D 85 (mm) D 90 (mm) Coefficient of uniformity, C u 9.9 Calcium oxide, CaO (%) 0.9 Sulphurtrioxide, S 2 O 3 (%) None TABLE 3 Summary of the laboratory test results Geotextile Permittivity, ψ (s - ) Stabilized gradient ratio, GR Stabilized system hydraulic conductivity, K system (m/s) Stabilized soil hydraulic conductivity, K soil (m/s) Permeability ratio, K R A x x B x x C x x D x x

10 Kutay and Aydilek 9 00 Percent Finer (%) Particle diameter (mm) FIGURE Particle size distribution of fly ash

11 Kutay and Aydilek 0 Inflow constant head device Inlet Flow 2 4 Geotextile Soil 3 5 Outflow constant head device 6 Outlet Flow Manometer board Permeameter Constant head devices FIGURE 2 Gradient ratio test setup

12 Kutay and Aydilek.6 Gradient Ratio (GR) (a) Geotextile A Geotextile B Geotextile C Geotextile D Time (hours) Hydraulic Conductivity (m/s) (b) Geotextile A Geotextile B Geotextile C Geotextile D Time (hours) FIGURE 3 (a) Gradient ratio versus time, and (b) hydraulic conductivity versus time relationships for fly ash-geotextile systems

13 Kutay and Aydilek 2 Type - Type -2 Type -3 Permeability Permeability Permeability Filter Cake Formation Normal Behaviour Loss of Particles Loss of Particles Time Time Time FIGURE 4 Long-term filtration behavior of soil geotextile systems (After Rollin 985) 2

14 Kutay and Aydilek 3.6 Gradient ratio.4.2 (a) Geotextile A Geotextile B Geotextile C Geotextile D Applied hydraulic gradient Hydraulic conductivity (m/s) (b) Geotextile A Geotextile B Geotextile C Geotextile D Applied hydraulic gradient FIGURE 5 (a) Gradient ratio, and (b) hydraulic conductivity versus applied hydraulic gradient relationships for fly ash-geotextile systems.

15 Kutay and Aydilek i=.5 i=3 i=6 i=8 i=.5 i=3 i=6 i=8 Distance from bottom of geotextile (mm) &3 4&5 Location of manometer ports Distance from bottom of geotextile (mm) &3 4&5 Location of manometer ports Geotextile A Water head (mm) Geotextile B Water head (mm) i=.5 i=3 i=6 i=8 i=.5 i=3 i=6 i=8 Distance from bottom of geotextile (mm) &3 4&5 Location of manometer ports Distance from bottom of geotextile (mm) &3 4&5 Location of manometer ports Geotextile C Geotextile D Water head (mm) Water head (mm) FIGURE 6 Development of water heads inside gradient ratio test apparatus