TM 5-852-2/AFR 88-19, Vol. 2
to operating personnel because of its odor, but it
thawing) as a result of the proposed construction.
reduces the thermal disturbance of the hole wall,
Subsurface explorations for the design of runways,
increases wall stability, and markedly decreases the
taxiways, and roads should extend about 6 to 10
time required for installed temperature-sensing
feet below final subgrade elevation in cut areas, and
cables to come to equilibrium. (This method is no
to the same depths below the existing ground
longer
used
because
of
environmental
surface in fill areas. A few holes should be carried
considerations.)
deeper to determine the characteristics of the lower
(6) Samples obtained by drive sampling or cored
strata.
by rotary drilling methods generally give adequate
(9) When no cold air circulation can be provided
information on the nature and degree of ice
beneath the floor, the depth of test holes beneath
segregation and permit an estimate of the magnitude
large heated structures, such as powerhouses or
of subsidence that would result upon thawing. A
hangars, should be in excess of the theoretical depth
reliable, but slow and expensive, means of frozen
of thaw, with a minimum depth approximately equal
ground exploration is the use of test pits, employing
to the least width of the building. For building
compressed air tools or drilling (or both) for
foundations where cold-air circulation can be
excavation of frozen soils. Shape charges may be
provided, explorations should extend to a minimum
used to assist in excavation work. In frozen,
of 10 feet below the theoretical depth of thaw and,
gravelly soil, test pits may be necessary for
in all cases, be carried to approximately 10 feet
determining the existence of segregated ice, either
below the base of foundation support.
as buried ground ice or as pockets in frost-
(10) In sporadic or discontinuous permafrost
susceptible soils within the gravel deposit.
areas where a site is selected based upon the local
(7) Geophysical prospecting methods have been
absence of permafrost, great care should be taken to
used to delineate permafrost bodies; however,
assure that small permafrost bodies are not
procedures have not been fully developed to date.
overlooked.
b. Ground temperatures. Permafrost is defined
These methods can be used as a guide or in
conjunction with exploratory drilling. Seismic and
on a temperature basis; therefore, a knowledge of
resistivity methods have proved to be most useful
the ground thermal regime is usually needed for
because the frozen interstitial water in soils and
design studies. Various possible temperature mea-
rocks causes greater changes in seismic velocity and
suring methods are available, making use of probes
electrical resistivity than in other geophysically
in thermal equilibrium with the media whose
measured properties. Seismic refraction techniques
temperature is to be measured. Three types of
can be used to determine the extent and depth to the
equipment commonly used are resistance ther-
permafrost table. Theoretically, resistivity methods
mometers, thermocouples, and thermistors, each
can also be used to measure the thickness of
dependent on the accuracy desired. Thermocouples
permafrost bodies. The reliability of geophysical
encased in plastic tubes and installed in drill holes
prospecting methods depends to a great extent upon
have been used successfully for geothermal studies
experience of the personnel interpreting the results.
of permafrost and studies of the stability of struc-
(8) The amount and type of soil information
tures built on and in permafrost or ice. Subsurface
required depend upon the character of the structure
temperatures should be recorded in the summer and
or facility that is being planned at a particular
fall to observe the highest range of temperatures
location and the uniformity of soils and permafrost
reached in the ground. Figure 4-1 shows how
conditions. Where there are nonuniform conditions,
ground temperatures vary in a permafrost area
having a mean annual temperature of 26EF
such as spotty occurrences of permafrost or various
soil types, the explorations should be spaced as
(Fairbanks, Alaska). In addition, observations es-
closely as necessary to determine the extent of these
sential for design are the rate and depth of thaw
conditions. Explorations should be carried to
occurring under various terrain conditions, together
sufficient depth to obtain information for the entire
with observations of ground movements because of
zone that is estimated to be subject to future
freezing and thawing of the active zone.
4-2
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