Sea to Sky Geotechnique 2006

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1 DURABILITY EVALUATION OF VARIOUS GEOGRIDS BY INDEX AND PERFORMANCE TESTS Han-Yong Jeon, Division of Nano-Systems Engineering, INHA University, Incheon, Korea(South) Mun Sung Mok, SAGEOS, Saint-Hyacinthe, Quebec, Canada Jacek Mlynarek, CTT Group, Saint-Hyacinthe, Quebec, Canada ABSTRACT This dissertation is focusing on the test method that used for evaluating the long-term design strength of the various type of geogrid and suggestion of improved test method. Short-term tensile properties of the membrane drawn geogrid was very good compared to other types of geogrids (warp knitted and woven type). Estimated long-term creep deformation indicate that the 65% of T ult loading level is the optimum value that satisfying the creep criteria (curve becomes asymptotic to a constant strian line, of 10% or less) in the case of woven geogrid and 60% in warp knitted geogrid and 30~35 % in membrane drawn geogrid. From the durability test results, the membrane drawn geogrid had good resistance to the chemical, biological and UV circumstance compared with the textile geogrid is confirmed. The total reduction factor that considered creep, installation damage and durability (chemical & biological) of the warp knitted and woven geogrids are estimated as 2.00, 1.90 respectively. Creep data interpretation is performed by using performance limit strain and isochoronous curve. From this procedure we can obtain more accurate creep reduction factor at the aim design life. 1. INTRODUCTION A great number of permanent geosynthetic-reinforced soil structures were constructed due to their high costeffectiveness and stability and it should be admitted that geosynthetic-reinforced soil structures, including walls and abutments, are not as stiff as steel-reinforced concrete (RC) structures. Various rheological models were adapted in order to describe the creep of geosynthetics (Sawicki, 1998; Navarrete, 2001). For the study related with the installation damaged geosynthetics, more site performance tests were accomplished compared to index test (Jeon and Yuu, 2003; Thornton, 2001; Billing et al., 1990; Koerner and Soong, 2001; Baker, 2001, Nakamura and Mitachi, 2004). Construction induced damage testing on polymer geogrids has been conducted by manufacturers, independent researchers, and government agencies. Several studies have suggested that the level of damage induced by construction to a polymer geogrid is a function of the following primary factors (Watn, 2002); geogrid thickness, compacting effort and lift thickness, type and weight of construction equipment used for fill spreading, type and weight of compaction of backfill, angularity of backfill, polymer used in manufacturing the geogrid, geogrid manufacturing process. In this study, to estimate the RF values, short-term tensile test, long-term tensile test, installation damage test and durability test were performed and the test result compared to each samples. After comparison, it was found that each test methods to calculate the FS were not an efficient test method to consider its specific installation conditions and material properties because of its wide range of application. So, review about the index test for FS evaluation, then more site-specific and material specific test and data analysis methods are proposed especially to the creep, installation damage and chemical resistance test. 2. REPARATION OF GEOGRIDS Three types of geogrids were employed in this study. The textile type of geogrid is divided into woven geogrid(tw) and warp knitted geogrid(tk) again and made of polyester high tenacity yarn coated with PVC resin. The membrane drawn type geogrid(tm) is made of melted high density polyethylene with uniaxial conformation. So, for better comparison of these two types of geogrids, geogrids having same nominal strength (e.g., 8 ton/m-8tk, 8TW, 8TM, 10 ton/m-10tk, 10TW, 10TM) were selected as a sample. And, the tests were performed to only longitudinal directions because of the uniaxial drawn geogrid samples. 3. EVALUATION OF ENGINEERING PROPERTIES Rib tensile test method was used to determine the single rib tensile strength behavior of two types of geogrids. In determining the tensile strength of various geogrids, there are numerous difficulties in clamping and testing representative samples due to their thickness, rib spacing, stiffness and varying forms of construction. This method performs tensile tests on a single, representative rib unit, to determine its strength behavior. The results of these tests can be used to calculate the ultimate strength of the full geogrid structure in the direction of the test. In accordance with ASTM D4595, wide-width tensile test was done. Junction strength test is to be used to determine the strength of an individual geogrid junction. Creep test was intended for use in determining the unconfined tension creep behavior of geogrid specimens at constant temperature when subjected to a sustained tensile 885

2 loading in accordance with ASTM D Installation damage test was done under consideration the real installation field conditions. To consider weathering effect in this study, geogrid samples were placed at outdoor conditions. Total 40 days were taken for exposure time (in summer season). Samples were taken every 10 days, and then tensile strength test was performed to the original and exposed samples. Chemical resistance test were performed according to U. S. EPA 9090 standard, and the outline of the test is described in Table 3.7 and also the EPA 9090 is compared with the ASTM D5322. After sample exposure, rib tensile strength test was tested. For biological resistance tests of geogrid samples were performed. Samples were incubated in two types of conditioned box. First box was filled with soil composed with weathered granted soil and second box was filled with sewage sludge. At time periods of 30, 60, 90 and 120 days one sample from each container was removed, cleaned and cut for test specimens to be used in GRI test methods GG-1. The tensile strength before and after incubation were evaluated. Figure 1. Wide-width tensile strength-elongation curves of warp knitted geogrids (longitudinal direction). 4. RESULTS & DISCUSSION 4.1 Tensile properties Figure 1~3 show the results of tensile test of each geogrid samples. The textile geogrids have point of inflection, but the membrane drawn type geogrid have log style graph (we can't see any inflection point). These tendencies will effect to the long-term tensile properties. Tensile strength of each specimens are higher than its each product strength about 4~13%. Figure 2. Wide-width tensile strength-elongation curves of woven geogrids (longitudinal direction). In the case of warp knitted type geogrid specimens, the extra tensile strengths are above 13% to the product strengths, and about 4~12% higher in woven type geogrids. And the tensile strength of membrane drawn type geogrids are higher than the products strength about 8~13%. From these extra tensile strengths, we may say that the additional factor of safety has been connoted in the geogrid products itself. Also, the tensile strain at the ultimate strength are about 11.0~14.0%, 9.0~12.0% and 8~12% about the each geogrid samples (warp knitted, woven and membrane drawn type). Table 1 shows the results of junction strength tests for the textile geogrid samples. When we consider the junction efficiency of these geogrids, the efficiency of the woven type geogrid is slightly higher than the warp knitted type. From this result we can infer that the woven type geogrid has more united and load transferring structure than the warp knitted type geogrid. Figure 3. Wide-width tensile strength-elongation curves of membrane drawn geogrids (longitudinal direction). Table 1. Junction strength and junction efficiency of textile type geogrids. specimen 8T K 10T K 8T W 10T W Junction strength (kg) Junction efficiency (%)

3 4.2 CREEP TEST RESULTS Conventional ambient creep test results and accelerated test are shown to the each geogrid samples in Figure 4~8. From these test results we can estimate the strain values that the curve becomes asymptotic to a constant strain line, of 10 percent or less. In the case of textile geogrids, each woven and warp knitted geogrids have 60% of UTS. From the creep testing, it was observed that PET geogrids resist creep strain better than HDPE ones at similar temperatures and load levels, However, for both HDPE and PET specimens the increase in temperature and load level have a strong effect on the creep strain behavior, relatively larger for HDPE specimens. The increase in load level also increase the amount of creep strain in the specimens, but the influence is not as large as that due to the temperature. However, higher the temperature, the larger is the influence of the increase in load level. (a) 8T W (a) 8T K (b) 10T W Figure 5. Creep deformation curves of woven geogrids (b) 10T K Figure 4. Creep deformation curves of warp knitted geogrids. Figure 6. Long-term creep deformation curve of warp knitted geogrid (8T K). 887

4 Table 2 represents the tensile strength retention as degree of installation damage. From these results it was confirmed that there were some tensile strength decreases in each samples, in the case of textile geogrid, about 6~7% strength decreases were founded for each textile geogrids (woven and warp knitted type). And strength decreases about 1.4~2.2% were observed in membrane drawn geogrid samples. From these results, the installation damage resistance of the membrane drawn geogrids was stronger than the textile geogrids. Figure 7. Long-term creep deformation curve of woven geogrid (8T W). (a) 8T M The installation damage will be affected by the filling material, friction of geogrid and particles, compacting machine and compacting level. So to minimize the strength decrease after installation, it is considered that the filling material having particle size less than 20 mm and good grading condition should be used in a installation process. Table 2. Wide-width tensile properties of geogrids after installation damage test (longitudinal direction) Tensile strength (t/m) Tensile strain (%) Strength reduction (%) 8T K T K T W T W T M T M Durability test results OUTDOOR EXPOSURE RESISTANCE The tests results are reported in Table 3. There are some decrease of strength for the exposed sampled (< 2.6%), but these values can be contained within specimen variation and test errors. Actually, in most specifications, it is recommended that the maximum exposure day should not exceed 7 days in construction site, so these values can be ignored. Table 3. Rib strength after outdoor exposure test (8T K) (a) 8TK Exposure time (day) Parameters Rib strength (t/m) Strength reduction (%) (b) 10T M Figure 8. Creep deformation curves of membrane drawn geogrids. 4.3 Installation damage resistance (b) 8TW Exposure time (day) Parameters Rib strength (t/m) Strength reduction (%) (c) 8T M 888

5 Exposure time (day) Parameters Rib strength (t/m) Strength reduction (%) CHEMICAL RESISTANCE Figure 9~11 show the chemical resistance results of textile geogrids and membrane drawn geogrid under different deposition conditions (HCl; ph = 2, 5, NaOH; ph = 9, 13 and temperature; 23, 50). In the case of membrane drawn geogrid which was made by HDPE, there was merely small amount of decease (+0.7 % (increase) ~ -2.6 (decrease)) in both acidic and alkaline conditions. While in the cases of the textile geogrids, it resulted in very similar tendency with the membrane drawn geogrid in acidic conditions but the tensile strength decreased about max. 45% in severe alkaline condition, NaOH, ph=13, especially in knitted type geogrid. However, the strength decrease of woven type geogrid resulted in a reduction (%) as 17%, and this is smaller value compared with the results of knitted type geogrid. From these results, it can be concluded that the chemical resistance of HDPE geogrid is very strong to the both of severe acidic and alkaline conditions, but textile geogrids don't have strong resistance to the severe alkaline conditions. However, the actual site-specific condition (ph= 8.5~9.5) is considered, both type of geogrids can be used without problems and related durability safety factor for any soils having ph=9. weathered granted soil Sludge Exposure time (month) Material and parameters Rib strength (t/m) Strength reduction (%) Rib strength (t/m) Strength reduction (%) Table 5. Biological resistance of woven geogrid (8T W) weathered granted soil Sludge Exposure time (month) Material and parameters Rib strength (t/m) Strength reduction (%) Rib strength (t/m) Strength reduction (%) Table 6. Biological resistance of membrane drawn geogrid (8T M) weathered granted soil Sludge xposure time (month) Material and parameters Rib strength (t/m) Strength reduction (%) Rib strength (t/m) Strength reduction (%) BIOLOGICAL RESISTANCE A number of studies, both in the geosynthetic industry but especially in the plastic industry and in the water and gas piping industry, have shown that the synthetic polymers used for the manufacturing of the geogrid reinforcement are resistant to attack by micro-organisms (i.e. aerobic and anaerobic growth of bacteria, fungi and algae) and macroorganism such as rodents and termites. Moreover, for soil reinforced structures the biological activity is typically low due to the depth at which geogrids are typically buried and to the fact that fill soil is biologically inert since it is mainly composed of granular soil with no traces of organic soil or products coming from decomposition. Table 4~6 show the results of these biological resistance tests. There are some decrease of strength for the exposed sampled (<3.0%), but these values can be contained within specimen variation and test errors. From these results it can be concluded that all of these geogrid samples are not affected by any of biological affects. Table 4. Biological resistance of warp knitted geogrid (8T K) (a) HCl 889

6 (b) NaOH Figure 9. Rib strength change with exposure conditions (chemical resistance results of 8T K). (a) HCl (b) NaOH (a) HCl Figure 11. Rib strength change with exposure conditions (chemical resistance results of 8T M). 4.5 Evaluation of long-term design strength through partial factors-of-safety Usually, the tensile properties of geogrid are affected by the creep, installation damage, temperature, chemical, UV and biological environment. So, the long-term design strength of the geogrid reinforcement will be reduced by this reduction factors. In designing with geogrid reinforcement, considering of these reduction factor and applying to designing process is very important for more safe structure within the aimed design life time. The GRI test method GG-4 that used world widely is applying the reduction factors(table 7-12). (b) NaOH Figure 10. Rib strength change with exposure conditions (chemical resistance results of 8T W). Table 7. Reduction factors of warp knitted geogrids Reduction factor 8T K 10T K RF ID RF CR RF CD (ph 9) RF D RF BD Total RF D (ph 9) Total RF

7 Table 8. Long-term design strength of warp knitted geogrids specimen 8T K 10T K Nominal tensile strength (t/m) 8 10 Total RF Long-term design strength (t/m) Table 9. Reduction factors of woven geogrids Reduction factor 8T W 10T W RF ID RF CR RF CD (ph 9) RF D RF BD Total RF D (ph 9) Total RF Table 10. Long-term design strength of woven geogrids 8T W 10T W Nominal tensile strength (t/m) 8 10 Total RF Long-term design strength (t/m) Table 11. Reduction factors of membrane drawn geogrids Reduction factor 8T M 10T M RF ID RF CR RF CD (ph 9) RF D RF BD Total RF D (ph 9) Total RF Table 12. Long-term design strength of warp knitted geogrids 8T M 10T M Nominal tensile strength (t/m) 8 10 Total RF Long-term design strength (t/m) CONCLUSION In this study, the engineering properties related with the FS determination were tested, especially short term tensile test, long-term tensile test (creep deformation test), installation damage test, durability test were performed to the various geogrid samples. And the summary is following. 1. Short-term tensile properties of the membrane drawn geogrid was very good merely compared to other types of geogrids (warp knitted and woven type). But these properties could be changeable because of its low orientation properties. From additional short term tensile test, these poor elongation properties were found. 2. Estimated long-term creep deformation indicates that the 60% of T ult loading level is the optimum value that satisfying the creep criteria in the case of woven geogrid. In the case of warp knitted geogrid the vaule is 60% of ULT and 30~35 % in the membrane drawn geogrid. And this value indicates that the woven geogrid has strong resistance to the longterm deformation compared with the membrane drawn geogrid and warp knitted geogrid. It is due to the raw material of the geogrid and structure of products. The woven geogrid has stress dispersion structure, so its short-term and long-term tensile properties are better than the warp knitted geogrid that has same raw material and different manufacturing method. 3. In the case of durability test results, the membrane drawn geogrid had good resistance to the chemical, biological and UV circumstance compared with the textile geogrid. It is considered that the reason of these results is come from its raw material properties. Membrane drawn geogrid, used in this study, is made by HDPE polymer. So, the high resistance to the environmental is superior to the textile geogrids which are made by PET yarn. 4. From the test results, total reduction factor, RF total to influence the long-term design tensile strength of warp knitted geogrids represent 2.0 and elongation at break represent 11~14%. Total reduction factor, RF total of woven geogrids represent 1.90 and total reduction factor, RF total of membrane geogrids represent 2.94~3.46 this means that the real confined construction structure of woven geogrids should be more quite stable than that of warp knitted and memrbane drawn geogrid. References A. Sawicki(1998), "Creep Behaviour of Geosynthetics", Geotextiles and Geomembranes, Vol. 16, pp A. Watn(2002), "Geosynthetic damage-from laboratory to field", Proc.of 7th Geosynthetics, pp F. Navarrete(2001), "Creep of Reinforcement for Retaining Wall Backfills", Proc. of Geosynthetics Conference 2001, pp H. Y. Jeon and J. J. Yuu(2003), "Assessment of junction strength of geogrids by a multi-junction test method", Polymer Testing, Vol. 22, pp J. S. Thornton(2001), "Characterization of Short and Long Term Creep and Relaxation Properties of a Polypropylene ", Proc. of Geosynthetics Conference 2001, pp J. W. Billing, J. H. Greenwood and G. D. Small(1990), "Chemical and Mechanical Durability of Geotextiles" Proc. of 4th International Conference on Geotextiles, Geomembranes and Related Products, pp R. M. Koerner and T. Y. Soong(2001), "Creep Testing and Data Extrapolation of Reinforced GCLs", Geotextiles and Geomembranes, Vol. 19, pp T. L. Baker(2001), "Comparison of Results Using the Stepped Isothermal and Conventional Creep Tests on a Woven Polypropylene Geotextile", Proc. of Geosynthetics Conference 2001, pp T. Nakamura and T. Mitachi(2004), "In-soil Creep Deformation Behavior of and New Design Method of Reinforced Soil Structures", Proc. of the 3th Asian Regional Conference on Geosynthetics, Seoul, Korea, pp