Internal shear strength of three geosynthetic clay liners

A study of the internal shear strength of adhesive-bonded, stitch-bonded, and needle-punched geosynthetic clay liners (GCLs) is presented. Tests were performed using a large direct shear machine capable of measuring peak and residual (or near residual) shear strengths. For each product, failure occurred at the woven geotextile/bentonite interface and excess pore pressures remained zero on the failure plane during shear. The peak shear strength of the needle-punched GCL increased significantly with increasing normal stress because of the frictional connection of the reinforcing fibers. The peak shear strengths of the adhesive-bonded and stitch-bonded GCLs showed smaller corresponding increases. The residual shear-strength failure envelope was essentially independent of product type. A two-stage procedure for specimen hydration is described, which reduced the required in-machine hydration time to reach equilibrium conditions. For the reinforced products, small decreases in peak and residual shear strengths were observed with decreasing displacement rate. The findings of the study have implications for the design of facilities incorporating GCLs and for the manufacturing and shear-strength testing of GCL products.