Normal Stress (kPa)
Figure 7. Vehicle traction for snow over ice. (From Richmond et al.
Combined sinkage greater than
the snow. Thus, for shallow snow where the pres-
the snow depth
sure bulb is in contact with the ice, we assume
For some situations it is possible that the com-
that traction will be governed by the interfacial
bined sinkage z is greater than the snow depth. In
this case the motion resistance due to the sinkage
For undisturbed shallow snow deeper than 1
into the soil must be added to that from the snow
cm lying on top of ice, we use the following trac-
to determine the total resistance. Unfortunately
than the snow depth involves knowing the inter-
Tgrossi = 0.127 Ni .06 Ai
action of the snow and the substrate during de-
formation and shearing and is far beyond the
which was presented by Richmond et al. (1990)
scope of current understanding. Thus, we have
(Fig. 7). The value of 1 cm was chosen to reflect the
no provisions for calculating the additional resis-
point at which the snow/ice interface would have
tance or the effect on traction when z is greater
effectively no strength, and the traction generated
than h. The model calculates the resistance based
would be the same as that on a clear ice surface.
on the total sinkage considered to be in snow, and
Rterrain for this case is calculated using eq 3.
traction is based on the condition specified as if
this case did not occur.
and packed snow
Undisturbed snow over ice
The physical and mechanical properties of a
The algorithms above have all assumed that
snowpack change significantly when it is driven
shear displacement in the terrain as a result of the
over repeatedly. After a large volume of traffic,
tractive demand of the running gear occurs with-
the snow's characteristics cease to change dra-
in the snowpack. In this assumption we are
matically as the result of tire loads, and the snow
claiming that the internal shear strength of the
is then considered to be packed.
snow below a tractive element is less than or
Vehicle operators tend to follow in the tracks
equal to the shear strength of the substrate or the
of preceding vehicles. We assume that after four
interface between the substrate and the snow.
passes in the same track, the snow has reached its
This is most likely true for all situations except
critical density, and succeeding vehicles will be
when the snow overlies ice. This could occur on a
traveling on packed snow. Usually, packed and
frozen river or lake or on a road surface where
disturbed snow conditions can only be formed over
a firm substrate.
coming snow. In this case the interfacial shear
In some cases an area of snow is mechanically
strength between the snow and ice is almost
processed to produce a snow road that is capable
guaranteed to be less than the shear strength within
of supporting wheeled vehicle traffic (Abele 1990).