Interfacial shear strength of geosynthetics using the cylinder direct shear test

Various methods are available for interface friction testing of geosynthetics. The 0.3-m x 0.3-m direct shear device for testing of geosynthetics has been standardized through the development of ASTM Standard D-5321. The direct shear test provides both peak and residual shear strength. The primary limitation of the 0.3-m direct shear test is the limited displacement available. Many geosynthetics experience a loss of shear strength with increasing displacement, and the loss of interface shear strength in the field through creep, earthquake loading, construction activities, or other reasons could lead to failure. Knowing the interface properties and understanding the consequences of low shear strengths at large displacements is important. The displacement available for the 0.3-m direct shear is a few centimeters, while field failures often are measured in meters. A new interface shear strength test device that provides field-scale displacements in the lab was needed.; The Cylinder Direct Shear test device (patent pending) allows measurement of interface friction parameters and the residual shear strength at unlimited displacements. The new device has one geosynthetic material fastened to a rotating drum, with the second geosynthetic interface material rigidly attached to a stationary anchor bar. The normal stress is provided using a pressure chamber. Interface friction is determined between the two geosynthetics based on the torque necessary to turn the cylinder, and displacement is determined by measuring the cylinder rotation.; The Cylinder Direct Shear offers single direction unlimited displacement capabilities without the displacement limitations of the direct shear and without the rotational limitations of the ring shear test. Geosynthetic interface tests have been performed with up to 85 cm of displacement for single tests, and up to 3.3 m of total displacement for staged tests. The results of the Cylinder Direct Shear compare favorably with conventional 0.3-m direct shear testing. The new device significantly extends the range and capabilities of direct shear testing.