ICE
When a wet, weak soil begins to freeze (Fig.
11a), the strong frozen layer on top will increase
Ice traction is a function of surface roughness,
the bearing capacity of the ground. The bearing
temperature, tire compound and vehicle speed
capacity--the ability of the soil to support a ve-
(Blaisdell and Borland 1992). While it may be pos-
hicle--can be expressed as a function of the frost
sible to easily incorporate ice temperature into the
depth t and the relative wetness of the frozen
NRMM/CAMMS database, it is unlikely that ei-
ground surface.
ther tread compound or surface roughness could
During spring and intermittent thaws, a thawed
be included with any degree of accuracy in the near
layer of soil develops over the frozen soil (Fig.
term. So, to treat the case of traction on ice, we have
11b). In the top layer the soil moisture is higher
reviewed published data (Fig. 9) (Shoop 1993b) and
than normal due to snowmelt, rain and the in-
taken a conservative value of 0.1. Therefore, we cal-
creased water drawn to the soil layer during the
culate traction on ice as
freezing process. This moisture is trapped in the
shallow thawed layer, creating a wet and weak
Tgrossi = 0.1 Ni A i .
(13)
layer of soil over the stronger frozen layer. The
reduction in vehicle mobility will be a function of
The traction equations for ice and snow are com-
the strength of the composite soil, which can be
pared in Figure 10. A model of ice traction utiliz-
expressed in terms of thaw depth S and the soil
ing traction aids exists (Blaisdell 1984), but it re-
moisture in the thawed layer. As the thaw pro-
quires as input geometric features of the devices,
gresses deeper (Fig. 11c), the frozen layer becomes
which are not available in the current Army mo-
too distant to add support to the vehicle or strength
bility database.
to the effective soil system, but it often continues
Because of the essentially undeformable na-
to restrict the soil drainage.
ture of ice under a vehicle's running gear, motion
Our current state of development of mobility
resistance Rterrain is assumed to be zero.
modeling for freezing ground is limited to go/
no-go predictions based on whether the ground
can support the vehicle. For thawing ground we
FREEZING OR THAWING
can make more quantitative predictions of the
GROUND CONDITIONS
effect on traction and motion resistance. Our
Freezing ground can often increase vehicle mo-
models assume a baseline traction and motion
bility, while thawing ground nearly always reduces
resistance for the soil of interest in its fully
mobility. An additional issue of importance is the
thawed state. The effects of freezing and thaw-
possibility of severe terrain damage when vehicles
ing are expressed as modifications (multipliers)
operate in areas with thawing conditions. Three
to the baseline values for traction and motion
critical conditions for vehicle mobility on freezing
resistance. The effects of other factors (e.g. veg-
and thawing soils are illustrated in Figure 11.
etation and slope) on mobility during freezing
a. Critical depth of frozen soil that will support a
vehicle.
b. Critical depth of a thawed wet layer where traction
is too low. If the tires can engage the strength of the
frozen layer, then the ground may be trafficable.
c. Frozen layer too far down to give support. The layer
still impedes drainage. Moisture content and soil proper-
ties are critical for trafficability.
U
Frozen Soil
Figure 11. Critical conditions for trafficability of
nfrozen Soil
freezing or thawing ground.
9
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