Extrapolating residual GMX/GDC interface strength from direct shear tests

The residual interface strength is mobilized along sideslopes of geosynthetic lined areas, such as landfill sideslope. Unfortunately, only a large displacement (LD) interface strength is measured in frequently used direct shear tests, which overpredicts the residual interface strength by about 40% because of limited continuous shear displacement and new geomembrane being introduced during the test. Commercial laboratories usually conduct direct shear tests in accordance with ASTM D5321 or D6243, which can only measure the LD interface strength. This paper suggests a method to estimate the residual interface strength from direct shear test results, which provides a better estimate of field interface strengths. The shear stress-displacement relationship from ring shear tests fits a power function, which can be used to extrapolate direct shear test results to a residual strength. This paper shows the power function can predict reasonable residual interface strengths from the shear stress-displacement relationship measured in direct shear tests. This method allows people to estimate the residual strength data when only direct shear test results are available and prevents overprediction of the Factor of Safety (FoS) during slope stability evaluations.