  We present a series of measurements examining the penetration force required
to push a flat plate vertically through a dense granular medium, focusing in
particular on the effects of the bottom boundary of the vessel containing the
medium. Our data demonstrate that the penetration force near the bottom is
strongly affected by the surface properties of the bottom boundary, even many
grain diameters above the bottom. Furthermore, the data indicate an intrinsic
length scale for the interaction of the penetrating plate with the vessel
bottom via the medium. This length scale, which corresponds to the extent of
local jamming induced by the penetrating plate, has a square root dependence
both upon the plate radius and the ambient granular stress near the bottom
boundary, but it is independent of penetration velocity and grain diameter.
