Direct Simple Shear Tests on Specimens with Thick Rings

Thick rings represent a relatively recent solution implemented in laboratory testing practice to prevent slippage during shear at the interface between the soil specimen and the top and bottom caps of the direct simple shear system. The setup of a specimen with thick rings involves two additional rings, approximately 9 mm in thickness, placed at the top and bottom ends of the regular ring stack. In order to laterally confine the soil–cap interface, the end of the specimen lies inside the thick ring where it makes contact with the porous stone of the top or bottom cap. The addition of thick rings essentially moves the shear test boundary away from the plane of contact between the soil specimen and the porous stone into the soil at the end of the thick ring. This paper describes the setup of soil specimens with thick rings in relation to the conventional setup with no thick rings and evaluates the impact of thick rings on stress–strain-strength measurements obtained from constant volume monotonic and cyclic direct simple shear tests performed on a high-plasticity marine clay. Because of the test boundary offset in direct simple shear tests on specimens with thick rings, there is a need to introduce an effective shear height in the calculation of shear strains based on ASTM D6528-17, Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Fine Grain Soils. This is different from the total specimen height at the start of the shear phase currently used in shear strain calculations.