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Designer’s Forum Recommended design strength for needlepunched geosynthetic liner products

By W. Allen Marr, Ph.D., P.E., and Barry Christopher, Ph.D., P.E.

A needlepunched geosynthetic clay liner tion above the liner system of 0.3 g will in- strength, the GCL loses strength with con- (GCL) is a composite material comprised crease this average shear stress to 32 kPa for tinued displacement. At large displacements of sandwiched between two geo- one or more instants in time, based on re- the internal strength of the GCL is the low- textiles and reinforced by needle punching sults from a pseudostatic stability analysis. est of all at 10 kPa and continuing to de- with synthetic fibers. The strength behav- Figure 2 shows some typical test results crease. The high internal strength is pro- ior of any GCL product is complex. Designs obtained on materials in the liner system vided by the needle punching fibers that act using a GCL must consider the internal from laboratory tests using a direct shear like reinforcement and hold the material to- strength of the composite product and the box. One test shows the internal strength of gether. After these fibers become stretched to interface strength between its outer surfaces the GCL where failure is forced to occur the point that they pull out or break, their and adjacent materials. within the bentonite and free swell of the contribution to internal strength of the GCL Figure 1 shows a cross section for a typi- GCL under low normal stress has been pre- decreases. With further displacement the cal with a composite liner system vented. The others show the interface strength contribution of the reinforcing fibers made up of the covered with a GCL, strength between the GCL and other ma- may become almost totally lost. The result a and a drainage geocompos- terials, including a textured geomembrane, is an internal at large dis- ite to form the primary liner system. A typ- a geocomposite and a clay . All tests used placement that is controlled by the shear ical failure surface determined by stability a normal stress of 69 kPa applied to hydrated strength of bentonite. Bentonite has a very analysis is shown. To illustrate concepts, con- materials and a shearing rate of 0.04 in./min. low shear strength, which is characterized by sider an “average” element “A” located mid- Hydration and consolidation of the mate- a angle of 8˚ at a normal stress of 70 way along the portion of the failure surface in rials were controlled to prevent squeeze out kPa and decreasing to 4˚ at 500 kPa (Olson the liner system. It experiences a normal of bentonite onto the interfaces. 1974). These low shear strength values rep- stress of 69 kPa and an average shear stress of The internal peak strength of the GCL of resent the lowest internal shear strength of 12.9 kPa. These are stresses created by the about 150 kPa is the highest of all the po- the GCL and thus the residual internal force of gravity acting on the waste mass. An tential failure surfaces included in Figure 2. strength. For the test shown in Figure 2, the that causes an average accelera- But after reaching a high peak internal internal strength of the GCL at 90 mm of displacement is 9˚, but is still decreasing. Figure 1. Cross section for a typical landfill. Composite liner system con- While it is not the true residual strength, the sists of subgrade covered with a GCL, a geomembrane and a drainage flat slope of the stress-displacement curve in- geocomposite to form the primary liner. dicates residual strength is almost reached. The close agreement in strength of the GCL at 90 mm of displacement and strength of bentonite obtained by the careful research Factor of Safety=2.1 work of Olson 30 years ago in a small triax- ial cell suggests that the large shear box is giving a realistic measurement of residual shear strength of the GCL (or we are lucky element A MSW enough to have compensating unknowns Liner System acting together in the shear box). The peak interface strength of the GCL with adjacent materials shown in Figure 2 is less than the peak internal strength of the GCL. The peak interface strength between the GCL and the textured geomembrane is less than half the peak internal strength of the GCL. The peak interface strength be- tween the GCL and the clay is about 1/3 the GFR • October/November 2003 www.gfrmagazine.info 20 m peak internal strength of the GCL. The peak interface strength between the GCL and the Designer’s Forum

geocomposite is about 1/5 the peak strength the temporary force from earthquake shak- 1966 and Herten et al. 1995). It seems very of the interface. If we sandwich these mate- ing is removed. As indicated by the results unlikely that creep will reduce the interface rials together to form a composite liner sys- in Figure 2, this slippage and that from ad- strength of a geosynthetic material against tem and subject them to a shear stress, slid- ditional earthquake cycles may cause a re- another geosynthetic material below the ing failure will occur when the applied shear duction in the interface strength of the value measured in a large shear box displaced stress exceeds the peak strength of the weak- GCL-geocomposite to as low as 23 kPa. to the residual value. It also seems very un- est material or interface. Once failure is ini- However, this reduced strength is still more likely that creep will reduce the interface tiated, displacement will continue along that than adequate to resist the static shear stress strength between a geosynthetic and a soil slip plane. For the materials and stress con- of 12.9 kPa that is maintained by gravity. below that measured in a large shear box dis- ditions used to obtain the data in Figure 2, This example illustrates that failure will placed to the residual value at a rate slow the weakest location is the interface between occur along the interface or in the material enough to avoid the creation of excess pore the GCL and the drainage geocomposite. with the lowest peak strength and not the water pressures along the interface during Substantial and rapid movements would de- one with the lowest residual strength. Use shear. This leaves open the question of the ef- velop along this interface once the shear of the lowest peak strength for design ap- fects of long-term creep and aging on the in- stress exceeded the peak shear strength of 30 plies to most GCL applications in caps and ternal strength of the GCL (and of geocom- kPa (equivalent to a friction angle of 23˚ in cover systems and in bottom systems of en- posites and for that matter). this case). Movement would continue until closed as supported by Koerner A significant decrease in strength of the something occurred to reduce the shear stress (2002). However, the example also indi- polymeric materials in GCLs due to aging is to less than about 23 kPa (the residual cates that if the interface or material with unlikely because of the oxygen level in satu- strength of this interface at larger displace- the lowest peak strength loses strength after rated bentonite is lower than the usual esti- ments which is equivalent to a friction angle straining through a peak, we should design mated amount in soil of 8%. The primary of 18˚ in this case). Since the GCL has an for gravity forces using the residual strength aging mechanism for polypropylene (PP) internal peak strength almost five times of that interface or material, if there is any fibers used in GCLs is oxidation. Manufac- higher than the peak strength of this inter- opportunity for that interface or material turers reduce the oxidation degradation po- face, it is inconceivable that a failure would to be stressed beyond its peak strength. This tential through the use of antioxidant addi- occur inside the GCL, even though it has a situation may occur where progressive fail- tives in the polymer. The service life (50% very low residual strength. ure is anticipated (e.g., in seismic events, strength loss) of one product with PP fibers The data for the cases shown in Figure 2 large settlements such as in waste materi- with a good antioxidation package in an oxy- indicate a design approach to use that will als resulting in downdrag, construction in- gen rich environment (circulating air at 21% avoid shearing the GCL to its residual duced deformations, migration of bentonite oxygen) was estimated to be on the order of strength––select an adjacent material or in- from the GCL to the interface, non-uni- 90 years in research performed by the Fed- terface that has a lower peak strength than form distribution of stresses such as in val- eral Highway Administration FHWA (Elias the internal strength of the GCL and does ley fills and sudden increases in pore pressure et al. 1999). In lower, stagnant oxygen envi- not experience a large loss of strength with as discussed by Thiel and von Maubeuge ronments, such as in granular soil, the life of continued displacement. We, in effect, de- 2002, and Gilbert 2001). the polymer is significantly extended. The sign the system to fail somewhere other than Design using the lowest peak strength as- FHWA found a service life of 240 years for through the GCL. For the materials used sumes that the peak strength of the inter- the previous polymer in 8% oxygen envi- in Figure 2 and a normal stress of 69 kPa, faces or materials do not change with time. ronment at 20˚ C. More recent studies by shear failure would occur at the interface The data in Figure 2 were obtained by shear- Thomas (2003) have confirmed longevity at between the GCL and the geocomposite ing in laboratory tests over a few hours. An low oxygen content. Based on aging tests per- GFR • October/November 2003 www.gfrmagazine.info when the shear stress reachs 30 kPa. obvious question is what happens to these formed on PP fibers taken from a GCL, they Returning to the example in Figure 1, materials over the much longer time that found a design life in buried applications with the average static shear stress in the liner they must perform in the field. It is 8% air to be more than 300 years. The ac- system is 12.9 kPa. All components of the known that polymeric materials in tension tual performance life may be even longer, be- liner system have sufficient short-term peak will eventually fail in creep at lower stresses cause in partially saturated GCL, only a few strength to withstand this shear stress. That than their short-term tensile strength. It is percent oxygen would be anticipated with the GCL has a residual strength less than also known that the strength of polymeric essentially no gas circulation (Hsuan and Ko- 12.9 kPa is not an issue because the GCL materials can decrease with aging. There is erner 2002). In a saturated GCL there would must first be stressed through its peak very little information on the behavior of essentially be no oxygen. The low oxygen strength. Failure will occur at other weaker these materials in a confined condition sub- level is why straw in adobe and wood beneath interfaces before that happens. The 0.3g jected to shear stresses over a long time. Such water has lasted for thousands of years. The earthquake increases the shear stress on the tests are expensive to perform and take condition of fibers within the GCL should average point to 32 kPa. This is sufficient months to years to run. Data from the few be of less concern than polymer degradation shear stress to exceed the shear strength of long-term tests that have been done in shear at other interfaces outside of the bentonite the GCL-geocomposite interface, and some boxes were for conditions that did not pro- where the oxygen content would be greater. slippage could occur at this interface until duce failure in the material (Trauger et al. If there is a question concerning aging of poly- mers on projects, aging test can be performed Interface/Internal Shear at 69 kPa Normal Load on the polymer following the FHWA testing 160 (1) protocol as outlined in Elias et al. (1999). 140 Temperature significantly effects aging. Where 120 100 elevated temperature is anticipated at the (3) liner, aging tests are recommended on all poly- 80 meric materials. 60 (4)

Shear Stress, kPa 40 As discussed above, the large internal (2) 20 peak strength of a GCL comes from the poly- 0 meric fibers used to bind the bentonite be- 0102030405060708090100 tween the external . Shearing of Displacement, mm the GCL places these fibers into tension. (1) GCL Internal (2) Geocomposite - nonwoven GCL (3) 60 mil textured HDPE geomembrane - nonwoven GCL (4) Nonwoven GCL - CLAY Much of the difference between peak and residual internal strength of a GCL must be Figure 2. Typical test results obtained on materials in the liner system created by these fibers working in tension. from laboratory tests using a direct shear box. Over time, creep may cause some of these fibers to break or pull out, which will reduce GCL internal peak strength. We could ob- proach. The displacement pattern devel- the internal peak strength of the GCL, as tain a lower bound estimate of the long-term oped within the GCL in a shear box test shown by Thies et al. (2002). Creep of the creep and aging reduced internal strength of does not match the displacement patterns fibers may be reduced by the composite soil- the GCL by adding 140/3.3 for the 100 year that develop in the field. Assuming the in- reinforcement interaction. As noted by Thiel performance period to the residual strength ternal strength measured at 2 in. in the shear and von Maubeuge (2002), the shear of 10 kPa, which equals 52 kPa at a normal box equals the strength available at 2 in. of strengths exhibited at high normal loads, stress of 69 kPa This value is well above the field displacement is incorrect. Test details, even under fully hydrated conditions, are peak strength on the GCL-geocomposite in- including how the GCL is gripped to cause much greater that the sum of the bentonite terface. It is highly unlikely that long-term failure by internal shear, affect the shape of shear strength and tensile strength. creep or aging would further reduce the the measured load-displacement curve. The reduction in long-term strength due to strength of the GCL to the point that failure Shear box tests by different laboratories on creep can best be addressed by performing would occur internal to this GCL. For the 300 the same material for the same test condi- long-term shear strength tests and develop- year case, a reduced internal strength of at tions can give substantially different results ing creep reduction factors to be applied to least 33 kPa would be anticipated, which also at displacements beyond the peak strength. short-term peak strength test results, as is exceeds the peak strength of the GCL-geo- This is an issue being addressed by ASTM currently done for soil reinforcement appli- composite interface. Even with conservative that appears to result from differences in cations. Creep is a time dependent issue that reduction factors to account for strength loss gripping systems used by different labora- does not actually reduce the strength of the due to creep and aging, the internal strength tories. Regardless of the outcome of this materials until incipient failure is ap- of this GCL is higher than most geosynthetic testing issue, it is not good practice to design proached. By applying a creep reduction fac- interfaces. Further protection is provided by re- any material for conditions where an addi- tor, a stress level is achieved that can pre- quiring a factor of safety in stability analyses tional increment of displacement causes a vent long-term failure due to creep and can that is greater than one. decrease in shear strength. Such a condi- provide full peak strength during transient If all designs include at least one inter- tion is inherently unstable. loads such as seismic events. An additional face with a peak strength less than this re- In summary, we recommend the follow- factor could be applied to account for aging. duced value for internal peak strength of ing approach to obtain long-term internal In the absence of long-term data for GCLs, the GCL, it will be unlikely that condi- design strength for GCLs: a reduction factor on the order of 3 could be tions will ever develop that would reduce Measure the short-term peak strength of applied to the portion of the GCL internal the internal strength of the GCL to its the GCL in a fully hydrated state at normal strength from reinforcement to account for residual value. Failure would most likely stresses representative of field conditions the contribution to peak strength from the occur at other locations first. This con- and a displacement rate of 0.04 in./min. in polymeric fibers (assuming the reduction fac- clusion has a conservative bias because it accordance with ASTM D 6243. tor for PP in tension in Koerner 1998). In the ignores the fact that the strength for other Apply reduction factors based on long- absence of aging data and considering the interfaces may also reduce with time due to term tests to this peak strength to obtain buried, low oxygen condition, an aging fac- creep and aging. the long-term internal design strength of tor of 1.1 to 2.0 as recommended by FHWA Some designers choose the internal the GCL. In the absence of project-specific (2001) would appear to be conservative re- strength of the GCL at some value of dis- test data, use a factor of three for creep to- duction for a 100 year to 300 year performance placement in the shear box, such as 2 in., as gether with a factor of 1.1 for 100 years of period, respectively. In Figure 2 the difference their design strength for the GCL. In Figure aging and 2.0 for 300 years of aging applied between peak and residual internal strength for 2, the internal shear resistance provided by to the difference between peak and resid- GFR • October/November 2003 www.gfrmagazine.info the GCL is 140 kPA, which is the assumed the GCL at 2 in. of shear box displacement ual strength. Add this to the residual contribution of the polymeric fibers to the is 30 kPa. We do not agree with this ap- strength to this reduced value to obtain the GFR • October/November 2003 • www.gfrmagazine.info Designer’s Forum Designer’s . ed. . Geosyn- Clay Geosyn- Second Sym- Proceedings of Clay Geosynthetic Designing with Geosyn- Designing with Proceedings of 15th GRI . Geosynthetic Institute, . Zanzinger, Koerner and Gar- . Zanzinger, . American Society of Civil Engi- . Zanzinger, Koerner and Garling . Zanzinger, . Prentice Hall, New Jersey. . Prentice Allen Marr, Ph.D., P.E., is a chief engi- Ph.D., P.E., Allen Marr,

esting Procedures to Obtain Peak and to Obtain Peak esting Procedures esting and Acceptance Criteria for Geosyn- Barry Christopher, Ph.D., P.E., is president Ph.D., P.E., Barry Christopher, W. thetic Barriers 73–86. Lisse. p. & Zeitlinger, ling (eds). Swets riers, i.e. reinforced GCLs.” riers, i.e. reinforced thetics for Geosyn- Use Peak Shear Strengths thetic Interface Design.” 16th GRI Conference: in Hot Topics III Folsom, Pa. T Residual Values.” in Geosynthethics II. Conference: Hot Topics Geosynthetic Institute, Folsom, Pa. p. 1–28. kaolinite, illite, and .” Journal of Division GT 11, 100, no. v. neers, Alexandria, Va. pp. 1215–1229. Industry Performance and Con- “Current struction Issues Related to GCLs.” Proceedings of 16th GRI Conference: in Geosynthetics III Hot Topics thetic Institute, Folsom, Pa. S. 2002. “Long-term shear testing of geosynthetic clay liners.” Barriers Lisse. pp 73–86. (eds.). Swets & Zeitlinger, Geotextile Used in a a Polypropylene Geosynthetic Clay Liner II—Oxidation of Air Rates in 8% Oxygen and the Effect Flow Rate on Oxidation.” posium on Geosynthetic Clay Liners Mackey, R.E. and von Maubeuge, K.P. ASTM International, Denver. behavior of a “Long-term shear strength needlepunched geosynthetic clay liner.” T ASTM Larry W. thetic Clay Liners. Ed. Well, International. STP 1308, 103–120. rauger, R.J., Swan, R.H. & Zehong, Y. 1996. R.J., Swan, R.H. & Zehong, Y. rauger, and owner of Christopher Consultants. Koerner, R.M. 1998. R.M. 1998. Koerner, Express Inc. neer with GeoTesting Koerner, R.M. 2002. “Recommendation to R.M. Koerner, “Interface and Internal 2001. Shear W.A. Marr, of Olson, R.E. 1974. “Shearing strengths 2002. Thiel, R. and von Maubeuge, K.P. Seeger, and W., C., Muller, Thies, M., Gerloff, of 2003. “Thermal Oxidation Thomas, R.W. T . . Geosyn- . FHWA-NHI-00-043. esting Protocols for Ox- T Mechanically Stabilized Earth Walls espect to reinforced geosynthetic clay bar- espect to reinforced Provide another material or interface Provide another material For earthquake loads with a pseudo static For conventional landfill design, we think thetic Institute, Folsom, Pa. p.29–30. 1995. “On the long-term Maubeuge, K.P. shear behavior of geosynthetic clay lin- ers (GCLs) in capping sealing systems.” p. 141–150. Nuremberg, bility and lifetime of polymer fibers with r U.S. Department of Transportation. Federal U.S. Department of Transportation. D.C. Highway Administration, Washington and Lu, S. 1999. idation and Hydrolysis of Geosynthetics In- National Technical FHWA-RD-97-144. formation Service, Springfield, Va. Containment.” for Waste Strength Proceedings of 15th GRI Conference: in Geosynthethics II Hot Topics DiMaggio, J.A. as technical consultant. 2001. and Reinforced Soil Slopes, Design and Con- struction Guidelines Heerten, G., Saathoff, F., Scheu, C., von F., Heerten, G., Saathoff, R.K. 2002. “Dura- and Koerner, Hsuan, Y.G. Elias, V., Salman, A., Juran, I., Pearce, E., Salman, A., Juran, I., Pearce, Elias, V., Residual Gilbert, R.B. 2001. “Peak Versus References B.R. and Berg, R.R. with Christopher, Elias, V., long-term internal design strength of the internal design strength long-term normally as- Temperature, GCL. [Note: at 20°sumed to be creep results C, will affect in and should be considered (Thies 2002), reduction factors.] selecting appropriate strength less than with a short-term peak design strength of the long-term internal from occurring the GCL to prevent failure the strength of this inside the GCL. Define as the design peak material or interface strength of this strength. Define the residual as the design residual material or interface factor of safety strength. Use a mimimum for design with the for global stability of 1.5 design peak strength and 1.1 for design with the design residual strength. factor of safety less than 1 using the design residual strength, perform a deformational analysis using the design residual strength. it unnecessarily conservative to design with the internal residual strength of a GCL that is sufficiently needlepunched to give it a high short-term peak strength relative to adjacent interfaces, provided the GCL is not permitted to free swell under normal stresses less than 10 kPa.