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Designer’s Forum Recommended design strength for needlepunched geosynthetic clay 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 bentonite 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 landfill with a composite liner system vented. The others show the interface strength contribution of the reinforcing fibers made up of the subgrade covered with a GCL, strength between the GCL and other ma- may become almost totally lost. The result a geomembrane and a drainage geocompos- terials, including a textured geomembrane, is an internal shear strength at large dis- ite to form the primary liner system. A typ- a geocomposite and a clay soil. 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 friction 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 earthquake 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 Foundation 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 geomembranes for that matter). this case). Movement would continue until closed landfills 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.

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