TM 5-818-7
CHAPTER 2
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RECOGNITION OF PROBLEM AREAS
2-1. Site selection
sites is the experience gained from previous construc-
tion and observation of successful or unsuccessful past
The choice of the construction site is often limited. It
performance. Local builders should be consulted to ob-
is important to recognize the existence of swelling soils
tain their experience in areas near the site. Existing
on potential sites and to understand the problems that
structures should be observed to provide hints of prob-
can occur with these soils as early as possible. A sur-
lem soil areas.
face examination of the potential site as discussed in
(b) The soil moisture may tend to be much closer
paragraph 3-2 should be conducted and available soil
to an equilibrium profile than that of an undeveloped
data studied during the site selection.
site. Differential movement may not be a problem be-
a. Avoidance of potential problems. If practical,
cause previous irrigation, leaking underground water
the foundation should be located on uniform soils sub-
lines, and previous foundations on the site may have
ject to the least swelling or volume change. Discon-
stabilized the soil moisture toward an equilibrium pro-
tinuities or significant lateral variations in the soil
file. Significant differential movement, however, is
strata should be avoided. Swampy areas, backfilled
still possible if new construction leads to changes in
ponds, and areas near trees and other heavy vegetation
soil moisture. For example, trees or shrubs planted too
should be avoided, Special attention should be given to
close to the structure or trees removed from the site,
adequate compaction of filled areas, types of fill, and
change in the previous irrigation pattern following
leveling of sloped sites (para 7-1).
construction, lack of adequate drainage from the struc-
(1) Undeveloped sites. Undeveloped sites general-
ture, and improper maintenance of drainage provi-
ly have little or no subsurface soil information avail-
sions may lead to localized changes in soil moisture
able and require subsurface exploration (para 3-3).
and differential heave. Edge movement of slab-on-
(a) Substantial differential heave may occur be-
grade foundations from seasonal changes in climate
neath structures constructed on previously undevel-
may continue to be a problem and should be minimized
oped sites where trees and other heavy vegetation had
as discussed in chapter 7.
been removed prior to construction, Soil moisture will
(3) Sidehill or sloped sites. Structures construct-
tend to increase since loss of heavy vegetation reduces
ed on sites in which the topography relief is greater
the transpiration of moisture. Construction of the
than 5 degrees (9 percent gradient) may sustain dam-
foundation over the soil will tend to further increase
age from downhill creep of expansive clay surface soil.
soil moisture because of reduced evaporation of mois-
Sidehill sites and sites requiring split-level construc-
ture from the ground surface.
tion can, therefore, be expected to complicate the de-
(b) Swampy or ponded areas may contain great-
sign. See chapter 7 for details on minimization of foun-
er quantities of plastic fine particles with a greater
dation soil movement.
tendency to swell than other areas on the site.
b. Soil surveys, Among the best methods available
(c) Future irrigation of landscaped areas and
for qualitatively recognizing the extent of the swelling
leakage from future sewer and other water utility lines
soil problem for the selected site is a careful examina-
following development of the site may substantially
tion of all available documented evidence on soil condi-
increase soil moisture and cause a water table to rise or
tions near the vicinity of the site. Local geological rec-
to develop if one had not previously existed. Filled
ords and publications and federal, state, and institu-
areas may also settle if not properly compacted.
tional surveys provide good sources of information on
(2) Developed sites. Subsurface exploration
subsurface soil features. Hazard maps described in
should be conducted if sufficient soil data from earlier
paragraph 2-2 document surveys available for esti-
borings are not available for the site selection and/or
mating the extent of swelling soil problem areas.
problems had occurred with previous structures. Some
subsurface exploration is always necessary for site se-
2-2. Hazard maps
lection of any structure of economic significance, par-
Hazard maps provide a useful first-order approxi-
ticularly multistory buildings and structures with spe-
mation of and guide to the distribution and relative ex-
cial requirements of limited differential distortion.
(a) An advantage of construction on developed
pansiveness of problem soils. These maps should be
2-1
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