وبلاگ تخصصی مهندسی ژئوتکنیک

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 σ_n = 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.

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Abstract

This thesis presents internal shear strength data for a needle-punched geosynthetic clay liner obtained with a large direct shear device at four normal stresses. Previous research in the literature studying geosynthetic clay liner shear strengths utilized normal stress levels of 100 kPa or less and no research has been performed with stress levels above 520 kPa (75 psi). These stresses are acceptable for landfill cover systems, but are not representative of the stresses observed by landfill liner systems. Additionally, current literature lacks detailed information regarding cyclic testing of internal shear strength of geosynthetic clay liners. The research discussed in this thesis intends to address these issues by providing a comprehensive testing program of internal shear strength under both monotonic and cyclic loading at four normal stress loads of 141 kPa (20 psi), 348 kPa (50 psi), 692 kPa (100 psi) and 1382 kPa (200 psi). The largest normal stress is similar in magnitude to the normal stress experienced for a geosynthetic clay liner in the bottom cover system of a landfill. Monotonic and cyclic shear tests illustrate the effect of shear displacement rate R and displacement amplitude Δ_a on material response. Monotonic peak shear strengths first increased and then decreased as R was increased from 0.1 to 28,000 mm/min. The highest peak strengths occurred at R = 100 to 1,000 mm/min. R = 0.1 mm/min peak strengths were found to be generally conservative at each normal stress. Displacements at peak strength generally decreased with increasing normal stress. Residual shear strengths increased with increasing R for R ≥ 1 mm/min, whereas the reverse trend was observed for R < 1 mm/min. Higher normal stresses resulted in both higher peak strengths and higher residual strengths. The displacement necessary to reach peak decreased with increasing normal stress. During cyclic shear, specimens exhibited the highest strengths during the first cycle of testing, and then decreased afterwards to reach a repeated hysteresis loop. Cyclic shear tests showed that peak strengths reached during cyclic testing initially increase with increasing displacement amplitude Δ_a, until reaching Δ_a = 15 mm when the strengths no longer are effected by increasing Da. This effect was observed at all normal stress levels and is believed to be due to the typical lengths of the needle-punched reinforcing fibers in the geosynthetic clay liner. After cyclic loading, specimens underwent monotonic shear at R = 0.1 mm/min. The peak shear strengths in this post-cyclic phase decreased with increasing cyclic displacement amplitude Δ_a for Δ_a < 10 mm and then decreased drastically for Δ_a > 10 mm. The residual shear strengths, however, were essentially constant at each normal stress and showed no trend with regard to previous cyclic testing.

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