TM 5-818-1 / AFM 88-3, Chap. 7
strata, permafrost and excess ice including ice wedges,
Classification System (table 2-3), including the frozen soil
moisture contents and groundwater, temperature
classification system, as presented in TM 5-852-2/ AFM
conditions in the ground, and the characteristics and
88-19, Chapter 2.
properties of frozen materials and unfrozen soil and rock.
(5) In seasonal frost areas, the most
(1) The need for investigation of bedrock
essential site date beyond those needed for nonfrost
requires special emphasis because of the possibilities of
foundation design are the design freezing index and the
frost heave or ice inclusions as described in paragraph
soil frost-susceptibility characteristics. In permafrost
18-2c(3). Bedrock in permafrost areas should be drilled
areas, as described in TM 5-852-4/AFM 88-19, Chapter
to obtain undisturbed frozen cores whenever ice
4, the date requirements are considerably more complex;
inclusions could affect the foundation design or
determination of the susceptibility of the foundation
performance.
materials to settlement on thaw and of the subsurface
(2) In areas of discontinuous permafrost,
temperatures and thermal regime will usually be the
sites require especially careful exploration and many
most critical special requirements. Ground temperatures
problems can be avoided by proper site selection. As an
are measured most commonly with copper-constantan
example, the moving of a site 50 to 100 feet from its
thermocouples or with thermistors. (6) Special
site
planned position may place a structure entirely on or
investigations, such as installation and testing of test
entirely off permafrost, in either case simplifying
piles, or thaw-settlement tests may be required.
foundation design. A location partly on and partly off
Assessment of the excavation characteristics of frozen
permafrost might involve an exceptionally difficult or
materials may also be a key factor in planning and
costly design.
design.
(3) Because frozen soils may have
18-4. Foundation design.
a. Selection of foundation type. Only sufficient
compressive strengths as great as that of a lean
concrete and because ice in the ground may be melted
discussion of the relationships between foundation
by conventional drilling methods, special techniques are
conditions and design decisions is given below to
frequently required for subsurface exploration in frozen
indicate the general nature of the problems and
materials. Core drilling using refrigerated drilling fluid or
solutions. Greater detail is given in TM 5-852-4/AFM 88-
air to prevent melting of ice in the cores provides
19, Chapter 4.
(1) Foundations in seasonal frost areas.
specimens that are nearly completely undisturbed and
(a) When foundation materials within
can be subjected to the widest range of laboratory tests.
By this procedure, soils containing particles up to boulder
the maximum depth of seasonal frost penetration consist
size and bedrock can be sampled, and ice formations
of clean sands and gravels or other non-frost-susceptible
can be inspected and measured. Drive sampling is
materials that do not develop frost heave or thrust, or
feasible in frozen fine-grained soils above about 250F
thaw weakening, design in seasonal frost areas may be
and is often considerably simpler, cheaper, and faster.
the same as for nonfrost regions, using conventional
Samples obtained by this procedure are somewhat
foundations, as indicated in figure 18-4. Effect of the
disturbed, but they still permit ice and moisture content
frost penetration on related engineering aspects, such as
determinations. Test pits are very useful in many
surface and subsurface drainage systems or
situations. For frozen soils that do not contain very many
underground utilities, may need special consideration.
cobbles and boulders, truck-mounted power augers
Thorough investigation should be made to confirm the
using tungsten carbide cutting teeth will provide excellent
nonfrost susceptibility of subgrade soils prior to design
service where classification, gradation, and rough ice-
for this condition.
content information will be sufficient. In both seasonal
(b) When foundation materials within
frost and permafrost areas, a saturated soil condition is
the annual frost zone are frost-susceptible, seasonal
common in the upper layers of soil during the thaw
frost heave and settlement of these materials may occur.
season, so long as there is frozen, impervious soil still
In order for ice segregation and frost heave to develop,
underlying. Explorations attempted during the thaw
freezing temperatures must penetrate into the ground,
season are handicapped and normally require cased
soil must be frost-susceptible, and adequate moisture
boring through the thawed layer. In permafrost areas, it
must be available. The magnitude of seasonal heaving
is frequently desirable to carry out explorations during
is dependent on such factors as rate and duration of
the colder part of the year, when the annual frost zone is
frost penetration, soil type and effective pore size,
frozen, than during the summer.
surcharge, and degree of moisture availability. Frost
(4) In
subsurface
explorations
that
heave in a freezing season may reach a foot or more in
encounter frozen soil, it is important that the boundaries
silts and some clays if there is an unlimited supply of
of frozen and thawed zones and the amount and mode
moisture available. The frost heave may lift or tilt
of ice occurrence be recorded. Materials encountered
should be identified in accordance with the Unified Soil
18-6
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