TM 5-852-4/AFM 88-19, Chap. 4
mation on permafrost conditions.
(2) Explorations at the structure site
have reviewed the currently available
should extend to a depth at least equal to the least width
information on applicability of bore hole logging methods
of the foundation, unless icefree bedrock is encountered
in permafrost areas. Williams and VanEverdingen
at shallower depth. In addition, the explorations should
have also concluded that borehole geophysical logging
encompass any foundation materials subject to possible
methods can yield valid geophysical logs in frozen
thaw during the anticipated life of the structure, as
unconsolidated deposits and that interpretation is
illustrated in figure 3-1.
For structures with pile
possible in terms of bulk density, moisture content and
foundations, the explorations should establish the nature
of materials in which the piles will be supported.
b. Techniques of subsurface exploration.
Frozen soil may have compressive strength as great as
3-3. Detailed direct site exploration.
that of lean concrete (para 2-5). Frozen glacial till at very
Guidance for foundation investigation is given in TM 5-
low temperatures has been described as behaving
852-2/AFM 88-19, Chapter 2 and TM 5-818-1/AFM 88-
exactly like granite in excavation and tunnelling work.
3, Chapter 7 and is also discussed in Terzaghi and
These properties make subsurface exploration in frozen
Peck . Detailed direct investigations of site conditions
materials considerable more difficult than in unfrozen
are required at structure sites. Positive knowledge of
soils and sometimes have led to accomplishment of less
subsurface conditions is as important in foundation
subsurface exploration than needed, with disastrous
design as is knowledge of properties of construction
results. Persons responsible for subsurface exploration,
materials in design of above-surface structures.
therefore, should be prepared to bring extra money,
a. Extent of exploration. The number and
effort, talent, and equipment to bear on the problem.
extent of direct site explorations should be sufficient to
(1) Deep core drilling using refrigerated
determine in detail the occurrence and extent of
drilling fluid to prevent melting of ice in the cores gives
permafrost, ground ice, including ice wedges, moisture
the best results under the widest range of materials up to
contents and ground water, temperature conditions in the
and including frozen soils containing particles up to
ground, and the characteristics and properties of frozen
Cores obtained by
boulder size and frozen bedrock.
materials, soil and rock. It is desirable that the personnel
this procedure are nearly completely undisturbed and
who make the actual site investigations proceed in very
can be subjected to the widest range of laboratory tests.
close communication with the design engineers so that a
They permit ice formations to be inspected and
continuous process of feedback and adjustment of the
measured accurately after removal of the drilling-fluid-
investigation program can be maintained; as a minimum,
saturated out surface. Cores should be photographed
the field personnel must be aware of the features which
for record purposes when appropriate, and when low
are important in foundation design in general and of
temperature storage cannot be provided. In fine grained
criteria applicable for the particular facility.
soils above 25 F, drive sampling is feasible and is
(1) A thorough soil investigation should be
often considerably simpler, cheaper, and more rapid.
conducted for all new construction. Sites with granual
Samples obtained by this procedure are somewhat
soils free of ice masses are highly desirable for siting of
disturbed but they still permit accurate detection of
structures, and although sands or gravels of soil groups
ground ice and accurate moisture content determinations
GW, GP, SW, and SP are generally free of segregated
on specimens. Examination and sampling of natural and
ice, this is not necessarily true in all cases. Granular
man-made exposures in the general site area may be
soils often occur as a cap over finer grained soils
helpful but care is necessary to avoid being misled by
containing ground ice and superficial investigations
sloughing of the face or by rapid melting and
based only upon the nature of the surface materials may
disappearance of ice when air temperatures are above
lead to very serious problems, possibly many years after
completion of construction. Buried ice wedges, old
(2) Test pits are also widely useful,
stream channels, and peat deposits containing excess
especially in shallow granular deposits intended for
ice may be present. As figure 3-1 suggests, a single
borrow. For frozen soils, which do not contain very many
shallow exploration, or a few widely scattered ones may
cobbles and boulders, truck mounted power augers
fail to reveal the true subsurface conditions. Experience
using tungsten carbide cutting teeth provide excellent
also shows that bedrock often contains substantial
service where classification, gradation and rough ice
masses of ice which would produce substantial
content information will be sufficient; this procedure is
settlement on thaw; bed rock thus cannot automatically
very useful for expansion of subsurface information
be assumed to provide a sound foundation. Bedrock
where critical details have already been established by
should be explored by core boring methods to obtain
more widely spaced undisturbed core drilling techniques.
undisturbed frozen cores whenever this possibility would
In both seasonal frost and permafrost areas a saturated
be a factor in the foundation design.