Undisturbed snow over ice

Undisturbed snow on a soft substrate

DEEP SNOW

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Figure 7. Vehicle traction for snow over ice. (From Richmond et al.

1990.)

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

properties.

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

tion equation:

calculating the resistance for a sinkage greater

than the snow depth involves knowing the inter-

Tgrossi = 0.127 Ni .06 Ai

(11)

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.

Disturbed, processed

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

precipitation started as freezing rain prior to be-

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).

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