Lower-bound shear strength for geosynthetic clay liners in base liners

The objective of this paper is to establish the minimum available shear strength for any type of geosynthetic clay liner (GCL) in a base liner. Direct shear tests are conducted on very thin (nominally I mm thick) specimens of reconstituted bentonite placed against a smooth geomembrane to measure the drained, residual strength of this interface. The use of thin specimens minimizes the time required to achieve drained conditions, and also minimizes the shear displacement required to mobilize the residual strength; testing times were several weeks and residual strengths were mobilized within 10 rum of shear displacement. The Mohr-Coulomb strength envelope developed from these test results corresponds to a secant friction angle of 4.7degrees over a range of effective normal stresses from 250 to 1500 kPa. Limited published information for the shear strength of bentonite and GCLs at these effective normal stresses supports the use of this envelope as a lower-bound strength for all types of GCL. This envelope establishes a conservative design check for slope stability and a benchmark for evaluating product-specific and project-specific test results with GCLs.