UFC 3-220-01N
15 AUGUST 2005
11-3.2
Remote Sensing and Geophysical Investigations. These techniques
are particularly valuable in selection of the specific site location, when a choice is
possible. They can give clues to subsurface frozen ground conditions because of
effects of ground freezing upon such factors as vegetation, land wastage, and soil and
rock electrical and acoustical properties.
11-3.3
Direct Site Investigations. The number and extent of direct site
explorations should be sufficient to reveal in detail the occurrence and extent of frozen
strata, permafrost and excess ice including ice wedges, moisture contents and
groundwater, temperature conditions in the ground, and the characteristics and
properties of frozen materials and unfrozen soil and rock.
11-3.3.1
Bedrock. The need for investigation of bedrock requires special
emphasis because of the possibilities of frost heave or ice inclusions. Bedrock in
permafrost areas should be drilled to obtain undisturbed frozen cores whenever ice
inclusions could affect the foundation design or performance.
11-3.3.2
Discontinuous Permafrost. In areas of discontinuous permafrost, sites
require especially careful exploration and many problems can be avoided by proper site
selection. As an example, the moving of a site 15 to 30 m (50 to 100 ft) from its planned
position may place a structure entirely on or entirely off permafrost, in either case
simplifying foundation design. A location that is partly on, and partly off, permafrost
might involve an exceptionally difficult or costly design.
11.3.3.3
Frozen Soils. Because frozen soils may have compressive strengths as
great as that of a lean concrete and because ice in the ground may be melted by
conventional drilling methods, special techniques are frequently required for subsurface
exploration in frozen materials. Core drilling using refrigerated drilling fluid or air to
prevent melting of ice in the cores provides specimens that are nearly completely
undisturbed and can be subjected to the widest range of laboratory tests. By this
procedure, soils containing particles up to boulder size and bedrock can be sampled,
and ice formations can be inspected and measured. Drive sampling is feasible in frozen
fine-grained soils above about -4 C (25F) and is often considerably simpler, cheaper,
and faster. Samples obtained by this procedure are somewhat disturbed, but they still
permit ice and moisture content determinations. Test pits are very useful in many
situations. For frozen soils that do not contain very many cobbles and boulders,
truck-mounted power augers using tungsten carbide cutting teeth will provide excellent
service where classification, gradation, and rough ice-content information will, be
sufficient. In both seasonal frost and permafrost areas, a saturated soil condition is
common in the upper layers of soil during the thaw season, so long as there is frozen,
impervious soil still underlying. Explorations attempted during the thaw season are
handicapped and normally require eased boring through the thawed layer. In
permafrost areas, it is frequently desirable to carry out explorations during the colder
part of the year, when the annual frost zone is frozen, than during the summer.
In subsurface explorations that encounter frozen soil, it is important that
the boundaries of frozen and thawed zones and the amount and mode of ice
11-8
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