The above analysis of soil strength assumes that the soil is not frozen. Cold regions soil
conditions present unique difficulties in characterizing the trafficability of soil. A cone
index for a soil with a surface frost of 12 cm is irrelevant because the high CI obtained is
indicative of the difficulty in penetrating the frozen soil with a handheld instrument. It is
not necessarily a measure of the bearing capacity of the soil for vehicles or repeated foot
traffic. Similarly, the trafficablility of a soil with a thawed layer is dependent as much on
thaw depth as it is on the CI of the thawed soil. A thaw layer so shallow that personnel or
vehicles sink through it to the underlying supportive frozen ground need not be
detrimental to mobility, despite the fact that the surface soil has a low CI. Another
complication is that because an average CI for the top 15 cm of soil is standard in
mobility applications, the bearing capacity of a thaw layer less than 15 cm deep will be
misrepresented by the standard CI.
Table 5.2.1 Default Soil Strength Coefficients.
CO, AS, RO, SN
Sullivan et al. (1997), p. 33 and 34
Despite these difficulties, information highly relevant to trafficability can be obtained
from FASST based on the predicted frost and thaw depths and the volume of meltwater
produced by the snow. The presence of 5 cm of frost at the surface will usually allow
unlimited cross-country operation if, when unfrozen, the soil is not extremely difficult to
traverse (Richmond 1991).Therefore, when FASST predicts a frost depth of at least 5 cm
in a soil that is predominantly silt or sand, the associated pseudo-CI is 300, the maximum
strength allowed. If the terrain is normally untrafficable, then a frost depth of 50 cm is
needed in order that most vehicles do not break through the frost layer (Richmond et al.
1995). For clays and peat with predicted frost depths of at least 50 cm, the pseudo-CI is