For shallow snow it has been shown the pres-
tance), allowing vehicle manufacturers to design
sure bulb has virtually constant properties for a
for them. For travel in deformable terrain, such as
very wide range of vehicles and snow types
most snow, the dominate source of resistance is
(Blaisdell et al. 1990). Sinkage z is predicted by
deformation and displacement of the surface. By
virtue of the terrain surface not being able to sup-
z = h [1 (ρ0/ρf)]
(2)
port the running gear contact pressure, the ve-
hicle must sink to a level where adequate support
where h = snow depth
can be found. Thus, the vehicle is perpetually
ρ0 = undisturbed snow density
attempting to drive itself out of a rut.
ρf = the pressure bulb density.
The mechanical aspects of snow important to
mobility are the ability to support vertical loads
Pressure bulb density ρf for shallow snow is es-
and its resistance to horizontal shear displace-
sentially a constant (critical density) at 500 kg m3
ment. These two requirements are closely related,
(Young and Fukue 1977). A balance of forces re-
and hinge on the bearing capacity and shear
quires that
strength of natural and compacted snow. The ad-
hesion between a vehicle's running gear and the
L = Sbc Ah + Ss Av
(3)
snow is also a factor in some situations, but is
usually only a small contributor to traction. Effec-
where L = vertical load
tive running gear will shear the snow within the
Sbc = the natural snow's bearing capacity
snowpack, since the thrust available there is nearly
Ss = the shear strength of the natural
always greater than adhesion.
snow/pressure bulb interface
The bearing capacity of natural snow (density
Ah and Av = the horizontal and vertical areas of
less than 400 kg m3) is usually very low com-
the pressure bulb.
pared to the needs of a vehicle for support. This
This describes vertical equilibrium in the snow-
results in considerable snow deformation (nearly
pack. The height of the pressure bulb H can be
all in the form of compaction) leaving the vehicle
incorporated in the Av term; sinkage z and H are
founded some distance below the snow surface.
also related. However, the usefulness of eq 3 is
Compaction and vertical sinkage proceed until
limited by the unknowns Sbc and Ss. Further limi-
the pressure bulb (sharply defined zone of influ-
tations are the unknown properties of the pres-
ence under the running gear) reaches a height
sure bulb and the fact that Ss is a shear force
that can provide enough vertical shear area to
developed between two dissimilar snow masses.
make up the difference between the natural snow's
Once sinkage equilibrium is reached, the vehicle
bearing capacity and the load placed by the run-
running gear can engage to produce horizontal
ning gear.
shear to generate forward thrust. In shallow snow
Occasionally, during growth, the pressure bulb
the available horizontal shear strength was de-
encounters a firm snow layer or the base of the
rived empirically in Blaisdell et al. (1990). This was
snowpack. This increases greatly the effective
possible because of the "constant" pressure bulb
bearing capacity and thus reduces the vertical
properties found in the shallow snow condition.
shear area required. When a very firm base (ei-
Results of field mobility tests in deep snow are
ther soil, pavement, or ice) is contacted by the
very limited. The deep snow case is considerably
base of the pressure bulb, the snow depth is con-
more difficult since the pressure bulb has no firm
sidered shallow (note that this depth is depen-
base to assist in supporting the normal and shear
dent on the combination of vehicle and snow type).
forces. Additionally, during horizontal shearing
For some shallow snow conditions it is possible
it is common for some portion of the top of the
for vehicle loads to force the pressure bulb of
pressure bulb to be removed by shear displace-
compacted snow beyond the confines of the verti-
ment. This upsets vertical equilibrium (eq 3) and
cal projection of the edges of the running gear.
the vehicle suffers greater sinkage. This process is
This only occurs when vehicle load is signifi-
called slip sinkage and explains why tracked ve-
cantly greater than snow bearing capacity and
hicles operating at even a small degree of slip
the depth to a firm substrate is small. In all
always assume a "bow uptail down" attitude in
other circumstances the pressure bulb maintains
deep snow.
essentially vertical side walls that are aligned
While most mobility researchers agree that mo-
exactly with the lateral boundaries of the run-
tion resistance in snow is related principally to
ning gear.
8
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