TM 5-852-4/AFM 88-19, Chap. 4
be designed as a permanent feature of the facilities. Mat
construction and subsequent facility operation and to
thickness criteria are discussed in paragraph 4-2. Many
pre-thaw and pre-consolidate the foundation within this
types of fibrous organic surface layers when of sufficient
zone. Thawing techniques are discussed in paragraph
thickness will support a few coverages of light
6-2. One major disadvantage of this scheme lies in the
construction equipment, but low-strength surface
difficulty of accurately anticipating the new thermal
materials may require end-dumping techniques even to
regime or thaw bulb position that will be stable,
enable placement of the working mat. However, such
particularly if permafrost is too thick to be thawed
mat or fill is not by itself a complete design solution when
completely through; continuing thaw of permafrost could
placed over frozen, highly compressible or high ice
result in settlement, but refreezing at the boundaries of
content deposits if there is any possibility of subsequent
pre-thawing would tend to produce heave. If only a
permafrost degradation.
In order to estimate the
relatively shallow layer of frozen fine-grained soil exists in
structural properties of the permafrost in its frozen state,
or on an otherwise satisfactory granular foundation, the
the temperatures at which it will be maintained must be
scheme may be more practical. The Corps of Engineers
estimated.
has constructed successfully performing facilities at both
(e) It
will
be
apparent
that
Anchorage and Fairbanks, Alaska, in which the major
maintenance of the existing thermal regime is much
portions of frost-susceptible soils have been prethawed,
easier to achieve in areas of continuous permafrost
consolidated and utilized in place with adequate heat
where permafrost temperatures are low than in the
permitted to escape to insure continuous thawed
discontinuous and borderline permafrost areas where
conditions. However, even under relatively favorable
there is less margin of safety and greater care is required
conditions, refreezing of the foundation when the building
in design analysis.
is vacated and heating discontinued for an extended
(2) Acceptance
of
thermal
regime
period can cause major facility damage under this
changes to be caused by the construction and facility.
scheme. Because possible changes in building usage
(a) This
design
approach
is
over long periods are relatively unpredictable and
applicable for both continuous and discontinuous
communication of requirements for continuous facility
permafrost zones.
heating to successor occupants cannot be relied upon,
(b) If small progressive thawing is
this approach should not be used except with specific
anticipated in the permafrost, settlement of structures
approval of HQDA (DAEN-ECE-G), WASH DC 20314.
(c) The same risk also occurs if a
may be avoided by supporting them on piles which are
foundation cooling system is installed to stabilize the
frozen into permafrost to a depth that is well below the
thawed regime of a foundation where degradation has
level of anticipated degradation during the planned life of
already been experienced. At a regional school at
the structures and that is also sufficiently deep to resist
Glenallen, Alaska, frost heave and structural difficulties,
any heaving forces during winter periods; it is approach
including differential movement of 2 inches, was
is usually only used for temporary structures such as
apparently caused by operation of a mechanical
construction camp buildings and the possibility of
refrigeration system for cooling under-floor air at
unacceptable environmental impact must be considered.
temperatures low enough to cause progressive
Piles or caissons may also be designed for end bearing
111
refreezing of underlying thawed soil.
on icefree bedrock or other firm, stable underlying
(d) Where the fine-grained settlement
formation. This method is particularly feasible when the
finegrained foundation soils containing ground ice form a
susceptible permafrost soils are limited to a relatively
relatively shallow cap. Designing for end-bearing is a
shallow upper layer, say up to about 20 ft thick, and
very good approach for bridge piers or similar structures
clean,
granular,
non-settlement-susceptible
soils
where foundation ventilation or similar systems are not
underlie, it may be feasible to remove the undesirable
practical. It must be kept in mind that once a residual
soils and replace them with compacted fill of clean,
thaw zone has developed as a result of the construction,
granular soils. Design and construction may then follow
the temperature of the underlying permafrost, and its
normal temperate zone techniques. The U.S. Army
structural capacities for members such as piles, will be
Engineer District, Alaska, has used this technique
seriously altered.
successfully at Fairbanks, Alaska.
(3) Modification of foundation conditions
(e) Occasionally it may be possible to
prior to construction.
alter surface conditions at a construction site up to
(a) This
design
approach
is
several years in advance so that adjustment of the
applicable almost solely in the discontinuous or
thermal balance may occur naturally over a long time.
(f) Where permafrost is to be pre-
borderline permafrost areas. It has only very limited
applicability for areas of continuous, low-temperature
thawed, the relative density of the soil in place, after
permafrost.
thaw, should be
(b) Under this alternative, one
procedure would be to compute the expected final extent
of thawed or unfrozen foundation materials produced by
4-5
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