TM 5-818-1 / AFM 88-3, Chap. 7
walls of river or stream valleys may reveal stratigraphy
purpose of preliminary subsurface explorations is to
and offer opportunities to obtain general samples for
obtain approximate soil profiles and representative
basic tests, such as Atterberg limits and grain-size
samples from principal strata or to determine bedrock or
analysis for classification.
stratigraphic profiles by indirect methods. Auger or
(2) Evidence of in situ soil performance. A
splitspoon borings are commonly used for obtaining
representatives samples. Geophysical methods together
study of landslide scars contributes greatly to the design
with one to several borings are often used in preliminary
of excavation slopes; it may indicate need for bracing or
exploration of sites for large projects, as they are rapid
suggest slope maintenance problems because of
and relatively cheap.
Procedures for geophysical
groundwater seepage. Evidence of general or localized
exploration are described in standard textbooks on
subsidence suggests compressible subsoils, subsurface
geotechnical engineering. Borings are necessary to
cavities, or ongoing sink-hole formations as in areas of
establish and verify correlations with geophysical data.
limestone formations or abandoned mine cave-ins. Fault
Preliminary reconnaissance explorations furnish data for
scarps or continuous cracks suggest bedrock
planning detailed and special exploration of sites for
movements or mass soil movements.
(3) Existing
structures.
large and important projects. The preliminary exploration
Careful
may be sufficient for some construction purposes, such
observation of damage to existing structures, such as
as excavation or borrow materials. It may be adequate
cracks in buildings (or poor roof alignment), misaligned
also for foundation design of small warehouses,
power lines, pavement conditions, corrosion on pipelines,
residential buildings, and retaining walls located in
or exposed metal and/or wood at water lines, may
localities where soil properties have been reasonably
suggest foundation problems to be encountered or
well established as summarized in empirical rules of the
avoided.
(4) Groundwater.
local building code.
The extent
of
e. Detailed subsurface explorations.
For
construction dewatering may be anticipated from factors
important construction, complex subsurface conditions,
such as the general water level in streams, spring lines,
and cases where preliminary subsurface explorations
marshy ground, and variations in vegetal growth. The
provide insufficient data for design, more detailed
effects of lowering the water table during dewatering on
investigations are necessary. The purpose is to obtain
surrounding
structures,
as
well
as
potential
detailed geologic profiles, undisturbed samples and
environmental effects, should be appraised in a
cores for laboratory testing,
or larger and fairly
preliminary manner. Drainage problems likely to be
continuous representative samples of possible
encountered as a result of topography, confined working
construction materials. Test pits and trenches can be
space, or increased runoff onto adjacent property should
used to depths of 15 to 25 feet by using front-end
be noted.
loaders or backhoes at a cost that may compare
(5) Availability of construction materials.
favorably with other methods, such as auger borings.
The availability of local construction material and water is
Test pits allow visual inspection of foundation soils; also,
a major economic factor in foundation type and design.
high-quality undisturbed block samples may be obtained.
Possible borrow areas,
quarries and commercial
Continuous (2 1/2 - to 5-foot intervals) sampling by
material sources, and availability of water should be
means of opendrive, piston, or core-boring samplers is
noted.
used for deeper explorations. Penetration, sounding or
(6) Site access. Access to the site for
in situ tests, such as vane shear, or pressuremeter tests
drilling and construction equipment should be appraised,
may be conducted depending on sampling difficulty or
including the effects of climate during the construction
desired information.
season.
(7) Field
investigation
records.
4-2.
Soil boring program.
a. Location and spacing. Borings spaced in a
Considering the value and possible complexity of a field
rigid pattern often do not disclose unfavorable
investigation, a well-kept set of notes is a necessity. A
subsurface conditions; therefore, boring locations should
camera should be used to supplement notes and to
be selected to define geological units and subsurface
enable a better recall and/or information transfer to
nonconformities. Borings may have to be spaced at 40
design personnel.
c. Local experience. Special attention should
feet or less when erratic subsurface conditions are
encountered, in order to delineate lenses, boulders,
be given to the knowledge of inhabitants of the area.
bedrock irregularities, etc. When localized building
Farmers are generally well informed about seasonal
foundation areas are explored, initial borings should be
changes in soil conditions, groundwater, and stream
located near building corners, but locations should allow
flood frequencies. Owners of adjacent properties may
some final shifting on the site. The number of borings
be able to locate filled areas where old ponds, lakes, or
should never be less than three and preferably five-one
wells have been filled, or where foundation of
at each corner and one at the center, unless subsurface
demolished structures are buried.
d. Preliminary subsurface exploration. The
conditions are known to be uniform and the
4-2
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