Table 1. Bearing strength of frozen peatland. (After Rummakainen 1984.)
Thickness of frozen peat layer (m)
Dry top
Wet top
Approx. equiv. load
peat layer
peat layer
Bearing capacity
(MN)
(lbf)
0.10
0.05
Will bear a horse
0.004450.008
10001800
0.150.20
0.10
Will bear 6-ton
0.05
12,000
horse-drawn sled traffic
0.200.35
0.150.25
Will bear empty
0.036
8000
4-ton truck
0.350.50
0.250.40
Will bear 10-ton truck traffic
0.09
20,000
dition the peat is less than saturated and has usu-
rather than that for the soil alone. However, since
ally been drained. (A frost depth of 0.5 m will pre-
we have no data to confirm this, no algorithm is
vent breakthrough failure of most vehicles.) Equa-
implemented to account for this effect.
tions 14 and 15 are shown schematically in Figure
12 in terms of the frost thickness necessary to sup-
Effect of thawing conditions on
port different vehicle classes (vehicle class is ap-
vehicle performance
proximately the vehicle weight in tons).
Thawing ground causes problems for vehicle
Mobility of heavy equipment may be limited
mobility when it is associated with thaw weaken-
by localized crushing failure of the surface mate-
ing of the soil. During thaw, otherwise freely drain-
rial, particularly for the dry conditions (Fig. 13)
ing soils can become saturated because drainage
(Shoop, in prep.). Therefore, the prediction equa-
is reduced by the underlying, nearly imperme-
tion for dry conditions, where surface crushing is
able frozen layer. Vehicle travel is then restricted
likely, is not applicable for vehicles with high
or impossible, and traffic can cause environmen-
ground pressure. We recommend that the dry equa-
tal damage (torn vegetation, mass soil flow and
tion be used for all tracked vehicles and wheeled
rutting, and subsequent erosion of sediments).
vehicles weighing less than 12 tons.
To predict traction and resistance in thawing
The predictive formulas are based on a best-fit
soils, we start from either measured or predicted
equation for the wheeled vehicles, and they over-
vehicle performance for the same soil at a tem-
estimate the frost thickness necessary for the sled
perate state. Adjustments are made to compen-
(Table 1). Thus, our load support predictions will
sate for the significant loss of shear strength in a
be conservative for vehicles with running gear
wet, thawing soil (resulting in a loss of traction)
that distributes load uniformly (e.g. tensioned
and the greater vehicle sinkage in the thaw-weak-
tracks, numerous road wheels, skis).
ened soil (causing an increase in motion resistance).
These equations can also be applied to frozen
Using established methods for predicting traction
ground other than peat. Once frozen, the strength
and motion resistance for unfrozen soil (in this case,
of ground depends primarily on its ice content,
models developed by WES), we apply multipliers
its density and its temperature. The time-depen-
to reduce traction and increase motion resistance
dent compressive strength of frozen peat is simi-
as the result of the thawing condition:
lar to that of frozen mineral soils (MacFarlane
Tthaw = f Tgross
(16)
1968) and falls within the range of frozen silt or
clay (Shoop, in prep.).
Rthaw = g Rterrain
(17)
Very few data exist for vehicle traction on frozen
ground. In general, frozen ground enhances mobil-
where Tthaw = gross traction in the thawing soil
ity and traction. Exceptions are if the ground has a
very high ice content or if the temperature is near
thawing soil
melting. In these cases traction may be reduced be-
f = traction reduction multiplier
cause of the slipperiness of the surface and may be
g = motion resistance multiplier
closer to the level of traction experienced on ice
11
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