Environmental Engineering Reference
In-Depth Information
Pingos:
These are dome-shaped hills resulting from the uplifting pressure of water freez-
ing to form large ice lenses in the ground. Pingos can rise more than 150 ft above the sur-
rounding terrain and can measure more than several hundred feet in circumference.
Engineering Characteristics
Active zone:
The zone of seasonal freeze-thaw cycles is the most significant factor.
Construction of structures, roadways, and fills causes the depth of the active zone to
increase, often resulting in a saturated weak material providing poor support for struc-
tures. The result is differential settlement in the summer months when the ground thaws,
and differential heave in the winter when the ground freezes. Heated structures placed
near the ground surface are particularly troublesome. Structures are commonly supported
on pile foundations steam-jetted into the permafrost to a depth equal to twice the thick-
ness of the active zone. The piles must be protected from uplift caused by active zone
freezing, in the same manner as they are protected from uplift from swelling clays (see
Section 10.6.4).
Insulation between the ground surface and the underside of heated build-
ings is provided by an airspace or a gravel blanket.
Strength characteristics of ice:
Terzaghi (1952) noted that the unconfined compressive
strength of ice depended on ice temperature, structure, and loading rate, and ranged from
21 to 76 tsf. Ice, however, has the capacity to creep under constant load. At a load less than
about 2 tsf and a temperature of
C, creep was found to be imperceptible, but under
greater loads, the creep increased rapidly as the load increased. The tendency for creep to
occur under relatively low deviator stress is responsible for the movement of glaciers.
Solifluction
is the downslope movement resulting from the freezing and thawing of silty
soils. The phenomenon is most common between the southern boundary of seasonal frost
(the 5
5
°
C mean annual temperature isotherm) and the southern boundary of the permafrost
region. At the foot of slopes subject to solifluction, the soil strata may be intricately folded
to a depth of more than 10 ft.
°
Seasonal Frost
General
In areas of seasonal frost, the depth of frost penetration influences the design of pavements
and foundations, which is usually based on the maximum depth of frost penetration, as
given for the United States in
Figure 7.110.
Whether frost actually develops to the maximum depths depends on factors other than
climate, mainly soil type and the depth of the static water table. Free-draining soils above
the water table will develop very little frost. The frost susceptibility of soils increases with
increasing fineness, which influences both internal drainage and capillary as given in
Table 3.10.
Soil susceptibility to frost is discussed in
Section 5.3.5.
Pavements
As the depth of frost penetration increases, pavement thickness is increased accordingly.
Full protection against frost heaving is considered to be achieved with a pavement thick-
ness equal to the maximum depth of frost penetration; a thickness of onehalf the depth of
frost penetration is generally considered as the minimum required protection, as long as
some risk of pavement deflection can be tolerated, such as in parking lots, for example.
Foundations
Exterior foundations and other foundations in soils subject to freezing are normally placed
below the depth of maximum frost penetration. During construction, interior foundations
