Dynamic Shear Strength of a Needle-Punched GCL for Monotonic Loading

This paper presents an experimental investigation of the dynamic internal shear strength of a hydrated woven/nonwoven needle-punched geosynthetic clay liner (GCL) for monotonic (i.e., single direction) loading conditions. Displacement-controlled shear tests were conducted using a large direct shear machine for four normal stress levels ranging from 141 to 1,382 kPa and seven shear displacement rates $R$ ranging from 0.1 to $\mathrm{30,000}\mathrm{mm}/\min$. For each normal stress, peak shear strength first increased and then decreased with increasing displacement rate. Maximum values of peak strength occurred for $R=100–\mathrm{10,000}\mathrm{mm}/\min$ and were 16–23% higher than corresponding static values measured at $R=0.1\mathrm{mm}/\min$. For each normal stress, residual shear strength first decreased and then increased with increasing displacement rate, with minimum values occurring at $R=1\mathrm{mm}/\min$. On a relative basis, residual strengths show greater dependence on displacement rate than peak strengths. The standard displacement rate for static shear tests of hydrated GCLs ($0.1\mathrm{mm}/\min$) generally yielded conservative values of peak shear strength but unconservative values of residual shear strength, especially for higher normal stress levels. The GCL experienced large post-peak strength reduction for all test conditions.