TM 5-818-1 / AFM 88-3, Chap. 7
effective and economical means of maintaining a stable
values should not be assumed to represent actual frost
thermal regime under a heated structure, without
penetration depths. Such local code values have been
selected to give generally suitable results for the types of
degradation of permafrost, is by use of a ventilated
construction, soil moisture, density, and surface cover
foundation. Under this scheme, provision is made for the
circulation of cold water air between the insulated floor
conditions, severity of freezing conditions, and building
and the underlying ground. The same scheme can be
heating conditions that are common in the area.
used for the converse situation of a refrigerated facility
Unfortunately, the code values may be inadequate or
supported on unfrozen ground. The simplest way of
inapplicable under conditions that differ from those
providing foundation ventilation is by providing an open
assumed in formulating the code, especially for unheated
space under the entire building, with the structure
facilities, insulated foundations, or especially cold
supported on footings or piling.
For heavier floor
winters. Building codes in the Middle and North Atlantic
loadings, ventilation ducts below the insulated floor may
States and Canada frequently specify minimum footing
be used.
Experience has shown that ventilated
depths that range from 3 to 5 feet. If frost penetrations
foundations should be so elevated, sloped, oriented, and
of this order of magnitude occur with fine silt and clay-
configured as to minimize possibilities for accumulation
type soils, 30 to 100 percent greater frost penetration
of water, snow, ice, or soil in the ducts. Guidance in the
may occur in well-drained gravels under the same
thermal analysis of ventilated foundations, including the
conditions. With good soil data and a knowledge of local
estimation of depths of summer thaw in supporting
conditions, computed values for ordinary frost
materials and design to assure winter refreezing, is given
penetration, unaffected by building heat, may be
TM 5-852-4 and TM 5-852-6/AFM 88-19, Chapters 4 and
expected to be adequately reliable, even though the
6, respectively.
freezing index may have to be estimated from weather
(c) Natural or forced circulation
data from nearby stations. In remote areas, measured
thermal piles or refrigeration points may also be used for
frost depths may be entirely unavailable.
overall foundation cooling and control of permafrost
(2) Design depth of ordinary thaw
degradation.
penetration. Estimates of seasonal thaw penetration in
(4) Foundation insulation.
Thermal
permafrost areas should be established on the same
insulation may be used in foundation construction in both
statistical measurement bases as outlined in
seasonal frost and permafrost areas to control frost
subparagraph a(2)(b) above for seasonal frost
penetration, frost heave, and condensation, to conserve
penetration. The air thawing index can be converted to a
energy, to provide comfort, and to enhance the
surface thawing index by multiplying it by the appropriate
effectiveness of foundation ventilation. Unanticipated
thawing-conditions n-factor from TM 5-852-4/AFM 88-19,
loss of effectiveness by moisture absorption must be
Chapter 4. The thaw penetration can then be computed
avoided. Cellular glass should not be used where it will
using the detailed guidance given in TM 5-852-6/AFM
be subject to cyclic freezing and thawing in the presence
88-19, Chapter 6.
Approximate values of thaw
of moisture. Insulation thicknesses and placement may
penetration may also be estimated from a chart of the air
be determined by the guidance given in TM 5-852-4 and
thawing index versus the depth of thaw in TM 5-852-
TM 5-852-6/AFM 88-19, Chapters 4 and 6, respectively.
4/AFM 88-19, Chapter 4. Degradation of permafrost will
(5) Granular mats. In areas of significant
result if the average annual depth of thaw penetration
seasonal frost and permafrost, a mat of non-frost-
exceeds the average depth of frost penetration.
susceptible granular material may be used to moderate
(3) Thaw or freeze beneath structures.
and control seasonal freeze-and-thaw effects in the
(a) Any change from
natural
foundation, to provide drainage under floor slabs, to
conditions, which results in a warming of the ground
provide stable foundation support, and to provide a dry,
beneath a structure, can result in progressive lowering of
stable working platform for construction equipment and
the permafrost table over a period of years. Heat flow
personnel. Seasonal freezing-and-thawing effects may
from a structure into underlying ground containing
be totally or partially contained within the mat. When
permafrost can only be ignored as a factor in the long-
seasonal effects are only partially contained, the
term structural stability when the nature of the permafrost
magnitude of seasonal frost heave is reduced through
is such that no settlement or other adverse effects will
both the surcharge effect of the mat and the reduction of
result. The source of heat may be not only the building
frost penetration into underlying frost-susceptible soils.
heat but also the solar radiation, underground utilities,
TM 5-852-4/AFM 88-19, Chapter 4, provides guidance in
surface water, and groundwater flow. TM 5- 852-4 and
the design of mats.
TM 5-852-6/AFM 88-19, Chapters 4 and 6, respectively,
(6) Solar radiation thermal effects. The
provide guidance on procedures for estimating the depth
control of summer heat input from solar radiation is very
of thaw under a heated building with time.
important in foundation design in permafrost areas.
(b) The most widely employed,
Correc-
18-9
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