The next situation we describe is a pile on a flat, smooth
bottom, i.e. the particles in row M=0 are allowed to move.
Only the left- and rightmost particles are fixed horizontally
by the corresponding wall. In Fig. 6(a) we show
the results for two 
piles with 
and 
.
The vertical component V of the stress is not constant
and the horizontal component H is getting very large
close to the walls, since vertical stresses are transferred into
the horizontal direction and propagate directly outwards in row M=0.
In the case of we observe a relative minimum of the
vertical stress in the center, X=0.5.
Figure 6:
Components of the dimensionless stress tensor S(0) vs.
dimensionless horizontal coordinate X=x/l at row M=0, for a pile
with mobile particles at the smooth and flat bottom.
We indicate the vertical stress with 
, the horizontal stress
with 
, and the shear stress with 
.
(a) The slope of the pile is , and or .
(b) The results for 
and 
(solid line) or 
(symbols) are compared to the result for 
from
Fig. 2(b).
(c) Contact network for the left half of a pile
with and bumpy bottom.
(d) Contact network for the right half of a pile
with and smooth, flat bottom.
The dashed line in (c) and (d) gives the vertical stress V
for the corresponding piles.
In Fig. 6(b) we compare the result of Fig. 2(b),
i.e. to situations on smooth and flat bottom with
and . The dashed lines give the vertical stress
in row M=1 (c) and M=0 (d). We observe fluctuations
at the shoulders of the pile and again a dip in the
center of the pile, X=0.5. In order to find an explanation
for this behavior we plot the contact networks in Figs. 6(c) and (d)
for the large piles with bumpy, (c), and smooth, flat
bottom, (d). The dashed lines give the vertical stress
for the corresponding pile. In Fig. 6(c) we observe a contact
network similar to the result in Fig. 3(b) for c=0.
The center triangle is arranged on a diamond lattice and the
shoulders are arranged on a dense triangular lattice, i.e. the
horizontal contacts are closed. Only close to the surface we have a
few particles on a tilted diamond lattice. In Fig. 6(d)
the situation is more complicated. We observe three regions with
different structure. Firstly a diamond lattice in the center,
secondly a dense triangular lattice in outward direction and
thirdly, the diamond lattice tilted outwards at the ends
of the pile. In summary, we correlate the variations
of normal stress V to the change of structure in the
contact network.