TM 5-852-4/AFM 88-19, Chap. 4
nuencies, stress levels, and temperatures, applicable for
During construction in warm weather this granular layer
the specific site.
also provides some protection of underlying thaw
susceptible permafrost during the footing installation.
(5) For frost-susceptible soils in seasonal
4-7. Design of footings, rafts and piers.
frost areas footings should normally be supported below
a. General. Footings, rafts and piers are
the depth of seasonal frost penetration, whether the
considered together in this section because they share
structure is heated or not. While it is true that such
the common characteristic of developing load supporting
footing depth may appear unnecessary for many types of
capacity through the bearing of a horizontal surface of
heated structures in seasonal frost areas because of the
adequate dimensions on the supporting soil stratum.
protective effect of heat losses into the ground, it is
(1) Suitable foundations of these types
unrealistic to assume that a facility will continue to be
should:
heated if it is placed on standby status at some future
(a) be safe with respect to bearing
date.
Placement of a non-frost-susceptible fill of
capacity failure
sufficient depth on the site would avoid this requirement.
(b) keep displacement, settlement,
(6) If foundation soils are clean, granular
heaving, or creep deformations, and,
and nonfrost-susceptible, conventional temperate zone
when required, resonant frequency
foundation practice may be used in both seasonal frost
within acceptable limits.
and permafrost areas and footings should be placed at a
(c) be economical to construct and
minimum depth of 4 feet. This depth is recommended in
maintain.
order to minimize seasonal thermal and mechanical
(2) Foundation design must satisfy all
effects, such as seasonal expansion and contraction,
three of these criteria. However, in some cases one or
which are most intense in the upper layers of the ground
two of the criteria may have predominant importance.
and tend to impose stresses on the structure.
(3) A footing is an enlargement of a
(7) A variation of the continuous footing is
column or wall to distribute concentrated loads over
the sill or mud sill type foundation, usually used for small
sufficient area so that allowable pressure on the soil will
structures having light floor loads or for temporary
not be exceeded. A spread footing, individual column
buildings. If only small-dimension sill members are used,
footing, or isolated footing is usually an individual square
without ventilation, this option is limited to nonthaw-
or circular footing placed beneath a column or post. A
susceptible granular foundation materials or under less
combined footing is a footing carrying more than one
favorable foundation conditions to very temporary use
column or post. A footing that supports a wall is a
beneath small structures with the aid of a granular mat to
continuous or wall footing. A shallow footing is a footing
distribute differential movements and jacking and
whose width is equal to or greater than the vertical
shimming provisions to allow adjustment of differential
distance between the surface of the ground and the base
movements.
However, the latter usage is nor
of the footing. A sill or mud sill is a structural piece such
recommended, even for temporary construction facilities,
as a timber which rests directly on the soil and supports
unless heated.
If large dimension sills are used,
the structure.
marginal ventilated foundation designs are possible for
(4) With proper design care all these types
small structures, as by using through beams across the
can be used successfully for arctic and subarctic
minimum dimension of the structure.
structure foundations under the particular conditions to
(8) A raft or mat foundation is a combined
which they are suited, although they may not always be
footing which covers the entire area beneath the
as economical as alternatives such as piles. Individual
structure and supports the walls and structural columns.
column footings are preferred to continuous footings
It is usually employed where heavy floor loads are
because of the greater risk of structural foundation
required, such as in hangars, garages and warehouses.
damage for the continuous type of footing in frost areas.
It acts as a unit and minimizes differential movements
Examples of all these footing types are shown in figures
which could occur in the use of individual footings. As
4-13 through 4-17, 4-22 through 4-24 and 4-27. As
employed in permafrost areas, ordinary raft foundations
illustrated by these figures, footings on frost-susceptible
are reinforced concrete slabs on gravel mats, as
permafrost soils may be placed at the surface of
illustrated in figures 4-25 and 4-26, employing various
foundation gravel mats as in figures 4-13 through 4-15,
means of insulation and ventilation or refrigeration
and may be placed within the gravel mat as in figure 4-
between the floor and the underlying gravel mat (paras
24, or may be placed below the depth of maximum
4-2c and 4-2d). Care must be taken to insure that all
seasonal thaw which will exist after construction, as
parts of the foundation are protected by insulation and
illustrated in figures 4-16, 4-17 or 4-23. In the latter case
cooling provisions, including areas beneath the walls.
a granular layer of clean gravel or sand should be used
(9) In an intermediate type of foundation, a
immediately below the footing to provide a suitable
slab is used to support heavy floor loads, but the support
working and placement surface; it may also serve to
reduce bearing pressures on the underlying materials.
4-92
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