TM 5-818-7
CHAPTER 3
FIELD EXPLORATION
ferential heave of earlier or nearby structures. Differ-
3-1. Scope
ential heave depends on conditions such as variation of
The field study is used to determine the presence, ex-
soils beneath the structure, load distribution on the
tent, and nature of expansive soil and groundwater
foundation, foundation depth, and changes in ground-
conditions. The two major phases of field exploration
water since construction of the earlier structures.
are surface examination and subsurface exploration.
(2) Soil gilgai. The surface soil at the site should
The surface examination is conducted first since the
also be examined for gilgai. Soil gilgai are surface
results help to determine the extent of the subsurface
mounds that form at locations where the subsurface
exploration. In situ tests may also be helpful, particu-
soil has a greater percentage of plastic fines and is
larly if a deep foundation, such as drilled shafts, is to
thus more expansive than the surface soil. Gilgai begin
be used.
to form at locations where vertical cracks penetrate
into the subsurface soil. Surface water enters and
3-2. Surface examination
swelling takes place around the cracks leaving frac-
a. Site history. A study of the site history may re-
tured zones where plastic flow occurs. These mounds
veal considerable qualitative data on the probable fu-
usually have a higher pH than the adjacent low areas
ture behavior of the foundation soils. Maps of the pro-
or depressions and may indicate subsurface soil that
posed construction site should be examined to obtain
had extruded up the fractures.
information on wooded areas, ponds and depressions,
(3) Site access and mobility. Indicators of site ac-
water-courses, and existence of earlier buildings. Sur-
cess and mobility (table 3-1) may also influence behav-
face features, such as wooded areas, bushes, and other
ior of the completed structure. For example, nearby
deep-rooted vegetation in expansive soil areas, indi-
water and sewer lines may alter the natural moisture
cate potential heave from accumulation of moisture
environment. Flat land with poor surface drainage, as
following elimination of these sources of evapotran-
indicated by ponded water, may aggravate differential
spiration. The growth of mesquite trees, such as found
heave of the completed structure if drainage is not cor-
in Texas, and other small trees may indicate subsur-
rected during construction. Construction on land with
face soil with a high affinity for moisture, a character-
slopes greater than 5 degrees may lead to structural
istic of expansive soil. Ponds and depressions are often
damage from creep of expansive clay surface soils.
filled with clayey, expansive sediments accumulated
Trees located within a distance of the proposed struc-
from runoff. The existence of earlier structures on or
ture of 1 to 1.5 times the height of mature trees may
near the construction site has probably modified the
lead to shrinkage beneath the structure, particularly
soil moisture profile and will influence the potential
during droughts.
for future heave beneath new structures.
c. Local design and construction experience. Local
b. Field reconnaissance. A thorough visual examina-
experience is very helpful in indicating possible design
tion of the site by the geotechnical engineer is neces-
and construction problems and soil and groundwater
sary (table 3-1). More extensive subsurface explora-
conditions at the site. Past successful methods of de-
tion is indicated if a potential for swelling soil is evi-
sign and construction and recent innovations should
dent from damages observed in nearby structures. The
be examined to evaluate their usefulness for the pro-
extent of desiccation cracks, plasticity, slickensides,
posed structure.
and textures of the surface soil can provide a relative
indication of the potential for damaging swell.
3-3. Subsurface exploration
Subsurface exploration provides representative sam-
of cracking in nearby structures should be especially
noted. The condition of on-site stucco facing, joints of
ples for visual classification and laboratory tests. Clas-
brick and stone structures, and interior plaster walls
sification tests are used to determine the lateral and
vertical distribution and types of foundation soils. Soil
can be a fair indication of the possible degree of swell-
swell, consolidation, and strength tests are needed to
ing that has occurred. The differential heave that may
evaluate the load/displacement behavior and bearing
occur in the foundation soil beneath the proposed
capacity of the foundation in swelling soil. The struc-
structure. however, is not necessarily equal to the dif-
3-1
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