A finer resolution of bearing capacity when frozen soil overlies incompetent soil has been
formulated by Shoop (1995). The minimum frost depth to prevent breakthrough is
expressed in terms of the vehicle class (for example, 1 person = 0.1, pickup truck = 4,
5-ton truck = 12, and a D7H tractor = 25 [Shoop 1995 and http://155.217.58.58/cgi-
bin/atdl.dll/fm/3034.343/appb.htm]), which is the military load classification for bridge
and highway limits. It can be approximated using the gross weight of the vehicle (or
group of people) in tons. For wet soil beneath the frozen layer, the relation is
zwet = 0.10 vehicle class
(5.18)
and for dry soil the relationship is
zdry = 0.16 vehicle class
(5.19)
where z (m) is the frost depth.
Another contribution of FASST to trafficability is a quantification of the amount of water
available to infiltrate thawed soil from a melting snow cover. Once the wetness of the
thawed layer is known, FASST can assess the bearing capacity of the thawed layer as if it
were an infinitely thick layer (ignoring the underlying frozen soil). If the bearing capacity
of the soil that forms the thawed layer is high, then the presence of the thawed layer is
largely irrelevant. If the bearing capacity of the thawed layer is low or moderate, then its
thickness must be considered in determining its impact on trafficability.
Finally, FASST outputs a "slippery factor" that tells the mobility model whether the
surface is dry, wet, snow covered, or ice covered.
5.3 References
Guymon, G.L., R.L. Berg, and T.V. Hromadka (1993) Mathematical Model of Frost
Heave and Thaw Settlement in Pavements. U.S. Army Cold Regions Research and
Engineering Laboratory, CRREL Report 93-2.
Jordan, R. (2000) User's Guide for USACRREL One-Dimensional Coupled Soil and
Snow Model (SLTHERM).
Richmond, P.W. (1991) Notes for Cold Weather Military Operations, U.S. Army Cold
Regions Research and Engineering Laboratory, Special Report 90-30 (with excerpts from
U.S. Army Field Manual FM 90-13/FM 7-26, Ice Bridging, p. B-20).
Richmond, P.W., S.A. Shoop, and G.L. Blaisdell (1995) Cold Regions Mobility
Models. U.S. Army Cold Regions Research and Engineering Laboratory, CRREL Report
95-1.
Shoop, S.A. (1995) Vehicle Bearing Capacity of Frozen Ground Over a Soft Substrate,
Canada Geotechnical Journal, V. 32, p. 552556.
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