Site Investigations - Areas of Significant Frost Penetration.

a. General.   In addition to the needed site investigations and data described in the manuals for nonfrost conditions, design of foundations in areas of significant frost penetration requires special studies and
data because of factors introduced by the special frost- related site conditions.  Detailed site investigation
procedures applicable for arctic and subarctic areas are described in TM 5-852-2/AFM 88-19, and TM 5-852-4/AFM 88-19, and may be adapted or reduced in scope, as appropriate, in areas of less severe winter freezing.  Methods of terrain evaluation in arctic and subarctic regions are given in TM 5-852-8.

b. 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 accoustical properties.

c. 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.

(1) The need for investigation of bedrock requires special emphasis because of the possibilities of frost heave or ice inclusions as described in paragraph 18-2c(3).  Bedrock in permafrost areas should be drilled
to obtain undisturbed frozen cores whenever ice inclusions could affect the foundation design or performance.

(2) 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 50 to 100 feet from its planned position may place a structure entirely on or entirely off permafrost, in either case simplifying foundation design. A location partly on and partly off permafrost might involve an exceptionally difficult or costly design.

(3) 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 250F 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 cased 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.

(4) 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 occurrence be recorded.  Materials encountered should be identified in accordance with the Unified Soil Classification System (table 2-3), including the frozen soil classification system, as presented in TM 5-852-2/ AFM 88-19.

(5) In seasonal frost areas, the most essential site date beyond those needed for nonfrost foundation design are the design freezing index and the soil frost-susceptibility characteristics.  In permafrost
areas, as described in TM 5-852-4/AFM 88-19, the date requirements are considerably more complex; determination of the susceptibility of the foundation materials to settlement on thaw and of the subsurface temperatures and thermal regime will usually be the most critical special requirements.  Ground temperatures are measured most commonly with copper-constantan thermocouples or with thermistors. 

(6)Special  site investigations, such as installation and testing of test piles, or thaw-settlement tests may be required.

Assessment of the excavation characteristics of frozen materials may also be a key factor in planning and
design.

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