These categories of classification use the coefficient of
count y survey maps prepared after 1970, if available,
linear extensibility (COLE), which is a measure of the
provide more detail on surface soils than do the other
change in linear dimension from the dry to a moist
maps discussed in b above. The other maps used in con-
state, and it is related to the cube root of the volume
junction with the SCS maps provide a better basis for
change. Premises guiding the categorization of the
election of the construction site.
Krohn and Slosson map include: degree of expansion
(1) Recognition of the problem area at the construc-
as a function of the amount of expandable clay; cover
tion site provides an aid for the planning of field ex-
of nonexpansive glacial deposits; and low-rated areas
ploration that will lead to the determination of the
with nonexpansive and small quantities of expansive
areal extent of the swelling soil formations and sam-
soils. Environmental factors, such as climatic effects,
ples for the positive identification and evaluation of
vegetation, drainage, and effects of man, were not con-
potential swell of the foundation soils and probable
sidered.
soil movements beneath the structure.
(3) Soil Conservation Service county soil surveys.
(2) Problem areas that rate highly or moderately
Survey maps by SCS provide the most detailed surfi-
expansive on any of the hazard maps should be ex-
cial soil maps available, but not all of the United
plored to investigate the extent and nature of the
States is mapped. Soil surveys completed during the
swelling soils. Structures in even low-rated areas of po-
1970's contain engineering test data, estimates of soil
tential swell may also be susceptible to damages from
engineering properties, and interpretations of proper-
heaving soil depending on the ability of the structure
ties for each of the major soil series within the given
to tolerate differential foundation movement. These
county. The maps usually treat only the upper 30 to 60
low-rated areas can exhibit significant differential soil
inches of soil and, therefore, may not fully define the
heave if construction leads to sufficiently large
foundation soil problem.
changes in soil moisture and uneven distribution of
(4) U.S. and State Geological Survey maps. The
loads. Also, low-rated areas on hazard maps may in-
U.S. Geological Survey is currently preparing hazard
clude some highly swelling soil that had been neglect-
maps that will include expansive soils.
ed.
(3) Figure 2-1 indicates that most problems with
c. Application of hazard maps. Hazard maps provide
swelling soils can be expected in the northern central,
basic information indicative of the probable degree of
central, and southern states of the continental United
expansiveness and/or frequency of occurrence of swell-
States. The Aliamanu crater region of Fort Shafter,
ing soils. These data lead to initial estimates for the lo-
Hawaii, is another example of a problem area.
cation and relative magnitude of the swelling problem
to be expected from the foundation soils. The SCS
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