UFC 3-220-01N
15 AUGUST 2005
occurrence be recorded. Materials encountered should be identified in accordance with
the Unified Soil Classification System (ASTM D 2487), including the frozen soil
classification system, as presented in UFC 3-130-01.
11-3.3.4
Seasonal Frost Areas. In seasonal frost areas, the most essential site
date beyond those needed for nonfrost foundation design is the design freezing index
and the soil frost-susceptibility characteristics. In permafrost areas, as described in
UFC 3-130-01, 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.
11-3.3.5
Special Site Investigations. 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.
11-4
FOUNDATION DESIGN
11-4.1
Selection of Foundation Type. Only sufficient discussion of the
relationships between foundation conditions and design decisions is given below to
indicate the general nature of the problems and solutions. Greater detail is given in
UFC 3-220-10N.
11-4.1.1
Foundations in Seasonal Frost Areas. When foundation materials
within the maximum depth of seasonal frost penetration consist of clean sands and
gravels or other non-frost-susceptible materials that do not develop frost heave, thrust,
or thaw weakening, design in seasonal frost areas may be the same as for non-frost
regions, using conventional foundations, as indicated in Figure 11-4. Effect of the frost
penetration on related engineering aspects, such as surface and subsurface drainage
systems or underground utilities, may need special consideration. Thorough
investigations should be made to confirm the non-frost susceptibility of subgrade soils
prior to design for this condition.
When foundation materials within the annual frost zone are
frost-susceptible, seasonal frost heave and settlement of these materials may occur. In
order for ice segregation and frost heave to develop, freezing temperatures must
penetrate into the ground, soil must be frost-susceptible, and adequate moisture must
be available. The magnitude of seasonal heaving is dependent on such factors as rate
and duration of frost penetration, soil type and effective pore size, surcharge, and
degree of moisture availability. Frost heave in a freezing season may reach a foot or
more in silts and some clays if there is an unlimited supply of moisture available. The
frost heave may lift or tilt foundations and structures, commonly differentially, with a
variety of possible consequences. When thaw occurs, the ice within the frost-heaved
soil is changed to water and escapes to the ground surface or into surrounding soil,
allowing overlying materials and structures to settle. If the water is released by thaw
more rapidly than it can be drained away or redistributed, substantial loss in soil
11-9
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