Influence of Boundary Conditions on Undrained Simple Shear Tests

ABSTRACT This article presents the results of an experimental study conducted using kaolin clay to investigate the effects of different vertical boundary confinement methods and specimen geometry on the stress-strain-strength behavior of clay subjected to monotonic undrained simple shear loading. The confinement methods included using conventional reinforced membranes (RM), stacked rings (SR), and flexible membranes (FM). The FM required an applied cell pressure to maintain one-dimensional conditions during compression, and during shear, the cell pressure was controlled to either maintain constant vertical stress (FM(CVS)), or to provide constant confining pressure (FM(CCP)). The specimen geometry was varied with the diameter (D) varying from 60 to 80 mm and the diameter/height ratio (D/H) varying from 2 to 4. Measured values of the undrained shear strength (su) and simple shear friction angle (ϕss′), produced from the tests with FM(CCP), RM, and SR were comparable with each other. However, the tests with FM(CVS) produced higher values of the su and ϕss′ compared with the tests with FM(CCP), RM, and SR confinement systems. The gradually rising confining pressure required to maintain the constant vertical stress during the shearing stage of the FM(CVS) tests is associated with the higher values of the su and ϕss′ compared with the tests with other confinement systems. It is also demonstrated that the geometry has a minor influence on the undrained strength and is essentially independent of the D/H ratio of the specimen within the range of 2 ≤ D/H ≤ 4 consistent with other studies.