CHAP. 4.1 FOUNDATION INVESTIGATIONS
Foundation investigations are made to determine the character and condition
of the natural materials upon which an engineering structure will be founded.
Such investigations are accomplished by geologic surface mapping, subsurface
exploration which may include ground-water and geophysical studies, and
field and laboratory testing of representative samples.
.2 The Chief Engineer is responsible for making the final decision as to the
suitability of all structure foundations. Found tion investigations are per-
formed by Regional or operating office personnel in accordance with policies,
standards, and procedures established by the Commissioner. The Regional
Director is responsible for furnishing field data and samples to the Chief
Engineer. Since the information and data obtained by the Regional and operat-
ing offices are used by the Chief Engineer in determining the suitability of
structure foundations, these operations are under the technical supervision
of the Chief Engineer. The Chief Engineer's technical supervision and
general coordination of foundation investigations are accomplished through
periodic and special visits to Regional and operating offices by engineers
and geologists of his staff; through correspondence with field offices; through
the preparation of specialist manuals such as the Earth Manual and the
Geology Manual; and through the formulation of training programs for field
engineers.
.3 Experience has shown that geological investigations fall logically into six
stages defined as follows: reconnaissance, site selection, preliminary
design, investigations for specifications design, construction, and operation
and maintenance. These are discussed in the following paragraphs.
Reconnaissance is the inventory of possible sites to determine the kind of geological and engineering problems that different sites may be expected to impose. Investigations should cover the broad aspects of the local structures, the stratigraphic and foundation conditions, and the relations of these to the general geology of the region. The engineering geologist must study overburden, bedrock, ground water, and other natural conditions of prime importance in the proposed construction. In most cases, the major geologic problems of the site can be solved by visual study of surface features, obviating the necessity of making subsurface explorations.
Detailed mapping is not required in the reconnaissance stage. However,
any maps prepared at this stage should show rock outcrops, areas and
estimated depths of overburden, and location and attitudes of important
geologic structural features, such as regional joint systems, folds and
faults, as well as prominent features of sedimentary stratification, lava
bedding, and foliation in metamorphic rocks. Appropriate geologic
sections, geologic interpretation of subsurface (including ground water)
conditions, and the tentative stratigraphic sections should accompany the
maps. Surface geology may be plotted on aerial photographs if topographic
maps are not available.
B. Laboratory test samples are not ordinarily taken during this stage. Sampling is usually limited to small-size inspection and classification samples picked up from the stream or other convenient cuts.
If site-selection studies are carried on immediately following or in
conjunction with the reconnaissance investigation, one geologic report
covering both stages should be prepared (see Subparagraph 4.1.5C). If the
investigations terminate with the reconnaissance stage, a report should
be prepared at the conclusion of this stage. The geological report should
set forth all that is known of the geologic and engineering aspects of the
area studied, including comments on probable availability and types of
natural construction materials. Sound judgment is essential, since
decisions made at this stage are necessarily based more on an appraisal