Part 1: Strength measurement
By Patrick J. Fox, Ph.D., P.E., and Chris Athanassopoulos, P.E.
The geosynthetic clay liner (GCL) has been with us for 30 years. Originally invented in the early 1980s as a flexible alternative to rigid waterproofing materials, GCLs are now used worldwide as hydraulic barriers in waste containment facilities, heap leach pads, ponds, canals, and other related engineered works. GCL shear strength is a primary design consideration for facilities involving slopes due to the low strength of hydrated bentonite. Thus, shear testing is a common and necessary part of engineering design and construction using GCLs.
GCLs are composite materials whose engineering behavior is governed by geosynthetic components, sodium bentonite, and their interaction. As a result, shear strength of GCLs and GCL interfaces can be affected by many factors, including normal stress, hydration/consolidation procedure, shear direction, shear displacement rate, drainage condition, specimen gripping surfaces, and total shear displacement. The shear strength behavior of GCLs is more complex than for any other geosynthetic material and proper care must be taken to achieve reliable results.
In response to an invitation from Geosynthetics editor Ron Bygness, we are pleased to contribute a three-part series on GCL shear strength. Our purpose is to share some of the knowledge gained during many years of shear testing with these unique materials. This first installment to the series addresses equipment and procedures for GCL shear strength measurement.
The objective of a GCL shear testing program is to obtain high quality measurements of the shear stress τ vs. shear displacement Δ relationship at different levels of applied normal stress σn. The defining features of the τ–Δ relationship are illustrated in Figure 1.
The peak shear strength τp is the highest measured value and occurs at displacement Δp. After the peak, a loss in shear strength, called “post-peak strength reduction,” typically occurs and yields a lower residual shear strength τr at much larger displacement Δr. No further strength reduction occurs beyond Δr. If displacement is insufficient to reach residual conditions, a “large displacement shear strength” is often defined. For example, τ75 is the shear strength at Δ = 75mm, which is a common displacement limit for GCL direct shear devices.
Examples of high quality τ–Δ relationships for internal shear of a double-nonwoven (NW/NW) needle-punched (NP) GCL are presented in Figure 2.
In this case, a final shear displacement of 200mm was possible because of the large size of the GCL specimens. The curves show good similarity and smooth transitions from the start of loading to peak shear strength and then to residual shear strength.
GCL internal and interface shear strengths show significant variability due to variability of component materials and manufacturing processes, differences in testing equipment and procedures, and changes in the design, manufacture, and application of GCLs over time. As a result, shear strength parameters must be obtained using project-specific materials tested under project-specific conditions. Performance shear tests are required at the design stage to provide design strength parameters and conformance shear tests are required during construction to ensure that the materials actually delivered to the project site are at least as strong as the original test materials. All materials should be sheared in the direction of applied shear stress in the field, typically the machine direction of the product.
The shearing device
Displacement-controlled direct shear is the preferred test method because it can be used for any type of GCL product, a large range of normal stress is possible, large specimens can be tested, post-peak response can be obtained, and shear strengths are measured in one direction with nominally uniform shear displacement. The primary disadvantage of the standard 300 × 300mm direct shear device is that the maximum shear displacement (typically 75–100mm) is not sufficient to measure τr for most GCLs and GCL interfaces. To address this limitation, large direct shear devices have been developed for specimens more than 1m long (Fox et al., 1997; Fox et al., 2006); however, τr may not be reached for some interfaces even after 200mm of shear displacement (Triplett and Fox, 2001).
The specimen gripping/clamping system is perhaps the most critical feature of any GCL shearing device. Ideally, to obtain accurate shear stress-displacement behavior a gripping/clamping system should enforce uniform shearing over the entire failure surface at all levels of applied normal stress. Progressive (non-uniform) failure will occur if the test specimen slips against the shearing blocks. This will yield conservative values of peak strength and unconservative values of large displacement strength (Fox and Kim, 2008). Figure 3 shows this effect under carefully controlled conditions for the interface between a textured geomembrane (GMX) and a NP GCL.
Gripping surfaces should provide high friction against a test specimen, good drainage, and no interference with the shearing process. Fox et al. (1997) found that modified metal connector plates, which are simply wood truss plates with short teeth, are low cost and work extremely well for GCLs. Likewise, the smooth side of a single-sided GMX specimen can be glued to a shearing block to prevent slippage. Clamping systems can be used to secure the ends of the geosynthetics but should not materially participate in the shearing process.
The type of gripping surface can significantly affect the quality of results. Figure 4 shows low quality shear stress-displacement relationships that were obtained using inadequate gripping surfaces.
These relationships display double peaks, unusually wide peaks, and poor similarity. One curve even shows no post-peak strength reduction for internal shear of an NP GCL (σn = 96 kPa). The examination of shear stress-displacement relationships provides an easy way to assess the quality of GCL shear tests. As such, these relationships should always be included with the results package for a GCL shear testing program.
GCL shear strength should be measured for hydration and normal stress conditions expected in the field. Full hydration is always expected unless the bentonite will be encapsulated between two GMs. Ideally, a GCL specimen should be hydrated to equilibrium and then slowly consolidated to the shearing normal stress; however, this procedure requires several weeks to complete. Our current recommendation for hydration/consolidation is to hydrate a GCL under low normal stress (often 20 kPa) for 48 hours and then increase the normal stress no more than three times per day using a load-increment-ratio no larger than 0.5 (for example: 20-30-45-68-101 kPa). The final load increment should remain on the specimen for a minimum of 48 hours prior to shearing. This procedure will still require about a week in the shearing device, which is a disadvantage.
As an alternative, Fox et al. (1998) developed an accelerated two-stage GCL hydration procedure that saves time. A GCL specimen is placed in a shallow pan and a specific amount of water is added to bring the specimen to the expected final water content after shearing is completed. These target water content values are obtained from previous shear tests on the same GCL product. The specimen is covered to minimize evaporation and placed under a low normal stress for 48 hours. The specimen then is moved to the shearing device and placed under the full shearing normal stress with free access to water for another 48 hours. Volume change measurements indicate that most GCL specimens reach equilibrium in the shearing device within 6 hours using this procedure (Fox et al., 2006; Fox and Ross, 2011). The drawback of the accelerated procedure is that the process of bentonite swelling and subsequent consolidation is not reproduced. Limited data suggest that the procedure does not affect GCL internal shear strength and has a slight effect on GMX/GCL interface shear strength (Fox et al., 1998; McCartney et al., 2004); however, more research is needed.
The shearing procedure
The issue of recommended shear displacement rates for GCL shear tests is still not fully resolved. ASTM D 6243 includes an equation for maximum displacement rate based on GCL consolidation data. However, this equation provides little guidance because it assumes mid-plane GCL shear, which may not be realistic, and typically gives a total shearing time of 1 month to 1 year, which is clearly prohibitive.
Available data indicate that dry GCLs show essentially no displacement rate effects and can be sheared at 1mm/min. For hydrated GCLs, current recommended displacement rates are 1mm/min for interface shear and 0.1 mm/min for internal shear tests (Fox and Stark, 2004). A slower rate is recommended for internal shear to allow more time for dissipation of shear-induced excess pore pressures. Note that some data suggest that an even slower displacement rate is necessary. As an example of the effect, Figure 5 shows τp and τr for internal shear of stitch-bonded (SB) and NP GCLs as obtained for σn = 72 kPa and displacement rate R ranging from 0.01-10mm/min (Fox et al., 1998). In that study, both values increased 3-5% for each log cycle of R.
Recent data obtained over a large range of R, including very rapid shear tests, have indicated that displacement rate effects are more complex than previously considered. Some of these results will be presented in the third installment to this article series on GCL dynamic shear strength.
The post-test procedure
GCL or GCL interface specimens should be inspected after shearing to determine the surface(s) on which failure occurred and the general nature of the failure. Any unusual distortion of the geosynthetics, such as necking, wrinkling, or tearing, should be recorded and may indicate problems with the gripping surfaces. Depending on the extent of localized distress, such a test may need to be repeated using improved gripping surfaces. Multiple final water content samples of the GCL and any soil materials should be taken after shearing to assess the level and uniformity of hydration that was achieved. All relevant features of failed specimens should be photographed.
Acceptance of GCLs during the last 30 years has been rapid because these products offer many advantages over compacted clay liners, not the least of which is lower cost for many applications. However, shear strength tests for GCLs and GCL interfaces are more difficult to properly perform than for any other geosynthetic material. One is reminded of the 1995 Geotechnical Fabrics Report article by Smith and Criley titled “Interface Shear Strength Is Not for the Uninitiated”—sentiments expressed therein regarding the challenges of GCL shear testing are still true today.
The state-of-the-art report by Fox and Stark (2004) provides a comprehensive source of information on shear strength of GCLs, including a checklist of what an engineer should require, provide, specify, and expect with regard to a GCL shear testing program. Costly pitfalls and mistakes can often be avoided with a clear appreciation of the following concepts:
- GCLs consist of both geosynthetic and sodium bentonite components, the interaction of which is more complex than either component acting alone.
- A GCL shearing device should be regularly calibrated to ensure accuracy of applied normal stress and shearing force.
- GCL shear strength should be measured for hydration and normal stress conditions expected in the field.
- Bentonite has low hydraulic conductivity and therefore consolidates and drains slowly.
- Bentonite has low shear strength and is susceptible to lateral squeezing under rapid changes in loading.
- Slower displacement rates are recommended for internal shear of hydrated GCLs than for interface shear of hydrated GCLs.
- Gripping surfaces of a GCL shearing device must have higher shear strength than all other potential failure surfaces within the GCL specimen.
- Examination of shear stress-displacement relationships provides an easy way to assess the quality of GCL shear test results.
- GCL shear strengths must be obtained using product-specific and project-specific shear tests.
- Conformance shear tests are needed during construction to ensure that materials delivered to the project site are at least as strong as the original test materials on which the design was based.
Patrick Fox, professor, University of California-San Diego, La Jolla, Calif.; firstname.lastname@example.org
Chris Athanassopoulos, technical manager, CETCO, Hoffman Estates, Ill.; email@example.com
Some of the data presented in this paper was obtained with funding provided by Grant No. CMMI-0800030 from the Geotechnical Engineering Program of the U.S. National Science Foundation and by CETCO of Hoffman Estates, Ill. This support is gratefully acknowledged.
ASTM D 6243. Standard Test Method for Determining the Internal and Interface Shear Resistance of Geosynthetic Clay Liner by the Direct Shear Method. ASTM International, West Conshohocken, Pa.
Fox, P.J., and Kim, R.H. (2008). “Effect of Progressive Failure on Measured Shear Strength of Geomembrane/GCL Interface,” Journal of Geotechnical and Geoenvironmental Engineering, 134(4), 459-469.
Fox, P.J., Nye, C.J., Morrison, T.C., Hunter, J.G., and Olsta, J.T. (2006). “Large Dynamic Direct Shear Machine for Geosynthetic Clay Liners,” Geotechnical Testing Journal, 29(5), 392-400.
Fox, P.J., and Ross, J.D. (2011). “Relationship between GCL Internal and GMX/GCL Interface Shear Strengths,” Journal of Geotechnical and Geoenvironmental Engineering, 137(8), 743-753.
Fox, P.J., Rowland, M.G., and Scheithe, J.R. (1998). “Internal Shear Strength of Three Geosynthetic Clay Liners,” Journal of Geotechnical and Geoenvironmental Engineering, 124(10), 933-944.
Fox, P.J., Rowland, M.G., Scheithe, J.R., Davis, K.L., Supple, M.R., and Crow, C.C. (1997). “Design and Evaluation of a Large Direct Shear Machine for Geosynthetic Clay Liners,” Geotechnical Testing Journal, 20(3), 279-288.
Fox, P.J., and Stark, T.D. (2004). “State-of-the-Art Report: GCL Shear Strength and its Measurement,” Geosynthetics International, 11(3), 141-175.
McCartney, J.S., Zornberg, J.G., and Swan, Jr., R.H. (2004). “Effect of Specimen Conditioning on Geosynthetic Clay Liner Shear Strength,” GeoAsia 2004, 635-643.
Smith, M.E., and Criley, K. (1995). “Interface Shear Strength Is Not for the Uninitiated,” Geotechnical Fabrics Report, 13(3), 28-31.
Triplett, E.J., and Fox, P.J. (2001). “Shear Strength of HDPE Geomembrane/Geosynthetic Clay Liner Interfaces,” Journal of Geotechnical and Geoenvironmental Engineering, 127(6), 543-552.