TM 5-852-4/AFM 88-19, Chap. 4
maximum frost heave force which can be developed on
other viscous material or mixture which will be firm
the casing.
enough at normal summer temperatures to support
h. For permanent type datum points requiring
gravel or sand particles on the surface yet transmit
negligible frost heave forces to the datum rod in winter.
very high precision, the casing and viscous backfill
If oil-wax mixture is used, the oil and wax should be
should extend to a depth below which the annual
heated separately to about 200F, then mixed. The
temperature variation is 2 F or less, usually about 30
feet in northern areas. For extreme precision, a special
mixture then may be poured into the annular space while
study of requirements for attaining needed stability may
it is still warm enough for easy placement.
126
be required. Black has suggested that in areas such
as the northern part of Alaska the datum point may have
5-2. Temporary datum points.
to be supported at as deep as 65 feet (20 m) to obtain
a. Since these need to be stable only for
high stability.
limited periods of time, less elaborate precautions are
i.
The natural thermal regime and the method
required than for permanent installations. However,
of placing a datum point are important factors in datum
even though shorter times are involved, stability
point performance.
In permafrost areas, casing
requirements may be just as stringent. Thus, a stable
anchorage and datum rod stability depend on achieving
temporary benchmark must be solidly anchored in stable
rapid and thorough refreezing during installation and
ground and must be isolated from the effects of seasonal
maintaining this during the life of the datum point.
frost heave or thaw-settlement forces during its period of
Disturbance of the thermal regime may interfere with
use, in accordance with the same principles as for
this, particularly in marginal permafrost areas.
permanent benchmarks. However, it may be feasible to
j.
Dry
augering
or
refrigerated
fluid
use a smaller flange or no flange on the casing. For
mechanical drilling create the least disturbance of frozen
example, an unflanged casing embedded 2 ft below the
soil and permit much more rapid freezeback of the
annual frost zone and which may heave 8 in./yr can be
backfill material than steam thawing or drilling methods
expected to protect the benchmark from heave for at
using warm fluids. This is of utmost importance in the
least 3 years.
more southerly areas where permafrost is near the
b. For construction projects of limited duration
thawing point (32 F). Mechanical drilling may be the
it may suffice to install a temporary benchmark by hand
only practical method where bouldery soils or bedrock
augering or test pitting to below the depth of seasonal
are involved. On the other hand, test pit installation,
frost and then driving a relatively small diameter rod into
though very laborious, may sometimes be found the
the underlying material for sufficient depth to obtain
most practical approach in remote areas inaccessible for
stable embedment, placing an unflanged casing around
suitable drilling equipment. In all permafrost areas,
the rod, backfilling on the outside with soil and placing
freezeback of slurried-in datum points can usually be
the oil-wax or other mixture in the annular space. Small
expected to be complete in not over two weeks, often,
diameter pilot auger holes may be employed to facilitate
much less, provided dry or refrigerated-fluid drilling
driving. Hand-sledge driving is of limited effectiveness; a
methods are used, the annular space is not over 1 1/2 to
heavy weight (up to about 350 lb) operated with a tripod
3 inches and the slurry temperature when placed is not
and winch will extend effectiveness. Driving should be
more than a few degrees above 32 F, except that in
assumed impractical in other than in warm (above 25 F),
marginal permafrost areas this applies only during the
fine-grained frozen soils. Freezeback of datum points
period of February to early June, when temperatures in
driven into permafrost may be assumed almost
the upper permafrost are depressed. Refreezing times
instantaneous because of the relatively small amount of
for slurries can be estimated using the procedures
heat to be dissipated. Often these operations may be
outlined in paragraph 4-8d. Because refreezing does not
carried out easily after the end of the summer thaw
always occur at a uniform rate around the embedded rod
(though ground water may cause problems). Even if the
or pipe, some bending or small displacement may occur
casing is frost heaved several inches in winter the point
during this process. Therefore, the datum point should
may remain stable long enough to get the job completed.
not be used as a reference point until refreeze is
c. On the other hand, when extended post
complete.
construction performance feedback measurements on
k. The viscous substance placed in the
the structure are to be made, one or more permanent
annular space between the casing and the benchmark
type datum reference points should be installed.
rod may be a heavy grease, a special oil-wax mixture
such as 70 percent Mentor 29 oil with 30 percent Amber
Tervan wax or Socony Mobil Cerise AA by weight, or
5-3
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