c. Criteria. No definite criteria for geophysical methods can be given
because they are highly specialized and require experienced operators and
interpreters for each application.
2. LIMITATIONS. Geophysical surveys are able to outline boundaries between
strata, but can only indicate approximate soil properties.
a. Sources of Errors. Differences in degree of saturation, presence of
mineral salts in groundwater, or similarities of strata that effect
transmission of seismic waves may lead to vague or distorted conclusions.
b. Check Borings. Geophysical surveys should be supplemented by
borings and sampling to determine soil properties and confirm the
stratification revealed by the survey.
Section 5.
SOIL BORINGS AND TEST PITS
1. SOIL BORINGS. Soil borings are probably the most common method of
subsurface exploration in the field.
a. Boring Methods. See Table 5 for applicability of the several
methods of making soil borings. For details of boring techniques and
equipment, see Reference 4, Subsurface Exploration and Sampling for Civil
Engineering Purposes, by Hvorslev.
b. Boring Layout. General guidance for preliminary and final boring
layout is presented in Table 6 according to the type of structure or problem
being investigated. Boring layout should also be governed by the geology of
the site.
(1) Geological Sections. Arrange borings so that geological
sections may be determined at the most useful orientations for final siting
and design. Borings in slide areas should establish the full geological
section necessary for stability analyses.
(2) Critical Strata. Where detailed settlement, stability, or
seepage analyses are required, include a minimum of two borings to obtain
undisturbed samples of critical strata. Provide sufficient preliminary
sample borings to determine the most representative location for undisturbed
sample borings.
c. Boring Depths. The depth to which borings should be made depends on
the sizes and types of proposed structures (see Table 7). It is also
controlled to a great degree by the characteristics and sequence of the
subsurface materials encountered.
(1) Unsuitable Foundation Strata. Extend all borings through
unsuitable foundation strata, such as unconsolidated fill; peat; highly
organic materials; soft, fine-grained soils; and loose, coarse-grained soils
to reach hard or compact materials of suitable bearing capacity.
7.1-65
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