TM 5-852-4/AFM 88-19, Chap. 4
soils of the density and moisture content ranges there
penetration depths. Such local code values have been
represented. For paved areas kept free of snow, depth
selected to give generally suitable results for the types of
of frost penetration may also be estimated from TM 5-
construction, soil moisture, density, and surface cover
12
818-21 or TM 5-852-3 , entering the appropriate chart
conditions, severity of freezing conditions, and building
with air freezing index directly.
heating conditions which are common in the area.
(d) For given soil conditions, the
Unfortunately, specific information on how these factors
greatest depth of penetration will be for paved areas not
are represented in the code values is seldom available.
affected by any artificial heat, shaded from the sun, and
The code values may be inadequate or inapplicable
kept cleared of snow. For heated buildings, both slab on
under conditions which differ from those assumed in
grade and basemented, the heat flowing outward from
formulating the code, especially for unheated facilities,
the foundation tends to modify frost penetration next to
insulated foundations, or especially cold winters.
the foundation wall. However, a variety of possible
Building codes in the Middle and North Atlantic States
situations exists. A building with a basement offers a
and Canada frequently specify minimum footing depths
different condition than one with slab-on-grade
in the range of 3 to 5 feet. If frost penetrations of this
construction, and use of insulation or firming on
order of magnitude occur with fine silt and clay type soils,
basement or perimeter walls will change heat flow.
30 to 100 percent greater frost penetration may occur in
(e) Penetration depths for paved
well-drained gravels under the same conditions. With
areas will nearly always need to be determined by
good soil data and a knowledge of local conditions,
computation rather than from measurements. A deep
computed values for ordinary frost penetration,
snow cover may entirely prevent frost penetration;
unaffected by building heat, may be expected to be
however, the effect of snow cover should usually be
adequately reliable, even though the freezing index may
disregarded for design purposes, as snowfall may be
have to be estimated from weather data from nearby
very small or negligible in the years when temperatures
stations. In remote areas, reliance on computation of the
are coldest. Turf, muskeg, and other vegetative covers
design frost depth for the specific local conditions at the
also help substantially to reduce frost penetration. Some
proposed structure location may be the only practicable
additional guidance on effects of surface conditions is
or possible procedure, as opposed to reliance on
14
contained in TM 5-852-6/AFM 88-19, Chapter 6 .
measurements.
(f) In the more developed parts of
(2) Design depth of thaw penetration.
the cold regions, the building codes of most cities
Seasonal thaw penetration in permafrost areas typically
specify minimum footing depth, based on many years of
begins in May or June and reaches maximum depth in
local experience; these depths are invariably less than
the ground in the period July September, as illustrated in
the maximum observed frost penetrations. The code
figures 4-5 and 4-6. Under paved areas exposed to
values should not be assumed to represent actual frost
sunshine, particularly black bituminous pavements,
U. S. Army Corps of Engineers
Figure 4-5. Thaw progression under undisturbed surface, Camp TUTO (near Thule air Base), Greenland. Soils data:
average gradation SC, clayey sand; dry unit weight 120124 Ib/ft3; moisture content 8-12 percent, essentially no vegetative
cover on surface (by CRREL).
4-11
Previous Page
