Environmental Engineering Reference
In-Depth Information
Occurrence
Its distribution, although erratic where it does occur, appears to have general boundaries
beyond which it does not extend. In particular, waterways and major channels (washes)
are free of caliche, as are areas where soil is aggrading at a relatively rapid rate, such as
some portions of alluvial slopes extending out from the base of a mountain.
Significance
It is often necessary to resort to blasting for excavation, and caliche can be highly ques-
tionable material for foundation support, since it varies from hard to soft with erratic
thickness, and is usually underlain by weaker materials.
Silcrete
A siliceous duricrust, silcrete covers up to 200,000 square miles of semidesert in Australia
in addition to areas of Africa. In Australia, the formation begins at a depth of about 2 ft
and extends to depths of 30 ft or more. The parent rock is commonly a granitic gneiss.
7.7.3
Permafrost and Seasonal Frost
General
Frozen ground can be divided into two general classes:
•
Permafrost
, ground that remains permanently frozen
•
Seasonal frost
, ground that thaws periodically
Permafrost
Distribution
Black (1964) has graded permafrost in the Northern Hemisphere into continuous, discon-
tinuous, and sporadic (distributed in patches), corresponding accordingly with mean
annual ground surface temperatures and permafrost depths as shown in
Figure 7.109.
The
southern boundary corresponds roughly with the 0
°
C mean annual temperature isotherm
(about latitude 55
N). Along the southern boundary, it is common to find relict permafrost
at a depth of 30 to 50 ft below the surface and about 30 ft thick.
°
Occurrence
Where the mean annual temperature drops below 0
C, the depth of ground freezing in
winter will exceed the depth of ground thawing in summer and a layer of permanently
frozen ground will grow downward from the base of the seasonal frost (the active zone).
The position of the top of the permafrost layer is the depth where the maximum annual
temperature is 0
°
C, whereas the position of the bottom of the permafrost is determined by
the mean surface temperature acting over long time periods. Heat flow from the Earth's
interior normally results in a temperature increase of the order of l
°
C for every 100 to 200
ft of depth. It could be anticipated, therefore, that the depth of the bottom of the per-
mafrost layer would be about 100 to 200 ft for each
°
°
C, below mean ground surface tem-
perature of 0
C. This has, in fact, been found to be a good rule of thumb in places remote
from water bodies (Lachenbruch, 1968). On the Arctic slope of Alaska, where the surface
temperatures range from
°
C, permafrost has been found to extend to depths of
600 to 1200 ft. The ground beneath large rivers and deep water bodies such as large lakes
6 to
9
°
