  Stress transmission in realistic granular media often occurs under external
load and in the presence of boundary slip. We investigate shear localization in
a split-bottom Couette cell with smooth walls subject to a confining pressure
experimentally and by means of numerical simulations. We demonstrate how the
characteristics of the shear zone, such as its center position and width,
evolve as the confining pressure and wall slip modify the local effective
friction coefficient of the material. For increasing applied pressure, the
shear zone evolves toward the center of the cylinder and grows wider and the
angular velocity reduces compared to the driving rate of the bottom disk.
Moreover, the presence of slip promotes the transition from open shear zones at
the top surface to closed shear zones inside the bulk. We also systematically
vary the ratio of the effective friction near the bottom plate and in the bulk
in simulations and observe the resulting impact on the surface flow profile.
Besides the boundary conditions and external load, material properties such as
grain size are also known to influence the effective friction coefficient.
However, our numerical results reveal that the center position and width of the
shear zone are insignificantly affected by the choice of the grain size as far
as it remains small compared to the radius of the rotating bottom disk.
