TM 5-852-4/AFM 88-19, Chap. 4
requirements, if any, may be computed as necessary. It
sufficient sand sizes to retain some moisture; this will
should be kept in mind that if heat escaping through the
help to control thaw and freeze penetration. If a very
foundation walls of an uninsulated basement is sufficient,
coarse gravelly mat material is to be placed over a fine-
it may prevent soil freezing at the outer face of the wall
grained subgrade, a sub-base of 6 in.
minimum
but if insulation is then added on these walls soil
thickness of dean, non-frost-susceptible sand should be
adfreeze may occur, with possible risk of frost heave in
placed directly on the subgrade in order to minimize the
frost-susceptible soils.
possibility of upward intrusion of fines into the mat during
(k) If a basement is completely below
thaw periods. Increased volumetric latent heat of fusion
grade and is not heated, the temperature in the
corresponding to the higher moisture-holding capacity of
basement normally will range between that in the rooms
this sand will also help reduce freeze and thaw
and that of the ground. The exact temperature which will
penetration into the subgrade.
naturally exist in an unheated basement or in crawl
(3) When the mat is needed only as a
spaces below floors is indeterminate and depends on
construction working platform, and control of freeze and
such things as the proportion of basement which is
thaw penetration into the subgrade is not a factor, the
below ground, the number and size of windows or wall
mat is made only thick enough to carry the loadings
vents, the amount of warm piping present, the extent of
which may be applied to it during actual facility
piping or floor insulation, and the heat given off by a
construction, during critical periods of reduced subgrade
basement heating plant. If the floor in the space above
strength. For this purpose, thicknesses should be
is at all cold, the using service or resident will try to
determined from the flexible pavement design curves
6
12
given in TM 5-818-2 or TM 5-852-3 . However, under
increase the floor temperature where it is not difficult to
summer soft ground conditions as much as 3 ft of
do so. It is necessary, therefore, for the design engineer
material may need to be placed by end dumping and
to evaluate the probable conditions carefully and to make
spread in one layer simply to support the hauling
a realistic basement or crawl space design temperature
equipment initially. If compressible materials underlie,
assumption in accordance with his best judgment.
e. Granular mats.
even more may be needed to meet design grades.
Under other conditions, as where the natural soils are
(1) In areas of both deep seasonal frost
free draining sands and gravels, little or no mat may be
and permafrost, a mat of non-frost-susceptible granular
needed except to provide, through elevation, a well-
material placed at the start of the field construction effort
drained surface during unfavorable periods of the year,
on the areas of planned construction serves to moderate
to provide a uniform work platform level, or to minimize
and control seasonal freeze and thaw effects in the
snow accumulation problems.
foundation soils, to provide stable foundation support and
(4) Figures 4-16, 4-17, 4-21 and 4-23
to provide a working platform on which construction
illustrate use of mats to provide thermal protection to the
equipment and personnel may move and operate with
foundation. Figures 4-13, 4-14, 4-15, 4-22 and 4-24
minimum difficulty regardless of seasonal conditions.
through 4-27 illustrate use of mats additionally to provide
The mat becomes the locus of part or all of the seasonal
stable bearing support for footings and rafts. Figure 4-28
freeze and thaw action, reducing or eliminating these
shows both thermal protection of a pile foundation
effects in the underlying in-place materials. Its thermal
supported in the natural ground and support of the
function is more nearly that of a heat sink than of an
hangar pavement on the gravel mat. When used for
insulator, dampening the effects of seasonal fluctuations
foundation support the mat should be designed with
relative to the subgrade.
The mat reduces the
sufficient thickness to achieve any needed heave
magnitude of any seasonal frost heave in underlying
reduction through its surcharge effect. For temporary
materials through its surcharge effect. It is usually
facilities or for light flexible structures, complete frost
convenient to place the mat at the start of the
heave control may not be necessary or economical. For
construction so as to serve both working platform and
facilities which require a foundation free from any frost
structure foundation purposes.
heave or thaw settlement effects the mat should be
(2) To insure a dry, stable working surface
made thick enough so that seasonal freeze and thaw are
during upward flow of melted water in the thaw period,
kept within the mat if the subgrade conditions are
the mat materials must be sufficiently pervious to bleed
unfavorable. Required thicknesses for thermal control
water away laterally without its emerging on the surface
8
(TM 5-820-2/AFM 88-5, Chap. 2 ). The mat is most
should be computed by procedures outlined in TM 5-
14
852-6/AFM 88-19, Chapter 6 . Approximate values
commonly composed of clean, well-graded bank run
may be estimated from figure 4-4. If thicknesses are
gravel of 2 and 3 inches maximum size, offering good
excessive, alternative foundation design approaches
compaction, trafficability and drainage characteristics.
must be investigated. It should be kept in mind that
Where such material is not available, alternatives such
during the thawing season building heat may add to the
as crushed rock or clean sand with a soil-cement surface
thawing in-
will have to be considered. Materials should contain
4-57
Previous Page
