A series of monotonic (single-direction) and dynamic (bi-directional) direct shear tests were performed involving the interface of a textured geomembrane (GMX) and a needle-punched geosynthetic clay liner (GCL). A large dynamic direct shear machine was used to perform the test procedure. The GCL was hydrated and placed on a stationary bottom plate consisting of an aggressive gripping surface with steel teeth approximately 1.5 mm in height. The textured side of the GM was then placed against the top-half of the GCL. The smooth side of the GM was secured to a steel pullout plate with epoxy. The interface was tested in monotonic shear at varying displacement rates and in cyclic shear at multiple displacement amplitudes. All cyclic tests were followed by a post-cyclic static interface shear test.
The series of shear tests were performed to investigate a number of factors: (1) whether internal GCL failure could be induced at high normal stresses, (2) the effect that a range of normal stresses (13 to 1382 kPa) can have on interface behavior, and (3) the effects of displacement rate and displacement amplitude on static and dynamic shear response, respectively.
The results presented in this report indicate that monotonic displacement rates have no effect on shear strength at high normal stresses (348, 692, and 1382 kPa), and little effect at low normal stresses (13 kPa). Displacement rate may have an effect on failure mode as partial internal failures were present at slower displacement rates, and interface failures occurred at higher displacement rates. Partial internal failures only occurred at a normal stress of 1382 kPa. This normal stress appears to be close to the normal stress where complete internal GCL failure would occur for the geosynthetic material used in this study. Interface failures had smaller peak shear strengths and greater large-displacement shear strengths than internal GCL failures.
Cyclic shear tests indicate a displacement amplitude of approximately 12 mm as the critical displacement between pre-peak and post-peak behavior for the geosynthetic materials used in this study. Post-cyclic interface shear strength was greatly reduced following a cyclic test at a displacement amplitude of 15 mm. Nearly all of the interface peak shear strength is lost after a cyclic test with a displacement amplitude of 20 mm. At σn = 13 kPa, very little damage is caused on the interface during cyclic shear. All other normal stresses displayed a strong reduction in the secant shear stiffness with an increase in displacement amplitude. Continued cycling also furthered the degradation of interface stiffness. However, most of the interface shear strength was reduced within the first five cycles. Finally, frequency and waveform may have an effect on the amount of damage on the GMX/GCL interface, additional testing is needed.