Civil Engineering Reference
In-Depth Information
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Geologic mapping:
The first step is to supplement the results of the field reconnais-
sance (see Sec. 5.2) with geologic mapping, which can be used to further identify such
features as existing landslides and surficial deposits of unstable soil.
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Subsurface exploration:
The results of the screening investigation and geologic map-
ping are used to plan the subsurface exploration, which could consist of the excavation
of borings, test pits, or trenches. During the subsurface exploration, soil samples are often
retrieved from the excavations. Field testing could also be performed in the excavations.
Subsurface exploration is discussed in Sec. 5.4.
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Laboratory testing:
The purpose of the laboratory testing is to determine the engineer-
ing properties of the soil to be used in the seismic hazard analyses. Laboratory testing is
discussed in Sec. 5.5.
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Engineering and geologic analyses:
An important parameter for the engineering and
geologic analysis of seismic hazards is the peak ground acceleration. This is discussed in
Sec. 5.6.
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Report preparation:
The results of the screening investigation and quantitative evalu-
ation are often presented in report form that describes the seismic hazards and presents
the geologic and geotechnical recommendations. Section 5.7 presents guidelines on the
report content for seismic hazards.
5.4 SUBSURFACE EXPLORATION
There are many different aspects of subsurface exploration. The most important part of the
subsurface exploration typically consists of the excavation of borings, test pits, and
trenches. Soil samples are usually retrieved from these excavations and then tested in the
laboratory to determine their engineering properties. In addition, field tests, such as the
standard penetration test (SPT) or cone penetration test (CPT) could also be performed.
These aspects of the subsurface exploration are individually discussed in the following sec-
tions. In addition, App. A (Glossary 1) presents a list of field testing terms and definitions.
5.4.1
Borings, Test Pits, and Trenches
Objectives of the Excavations.
The main objectives of the borings, test pits, and trenches
are to determine the nature and extent of the seismic hazards. In this regard, the Division of
Mines and Geology (1997) states:
The subsurface exploration should extend to depths sufficient to expose geologic and sub-
surface water conditions that could affect slope stability or liquefaction potential. A sufficient
quantity of subsurface information is needed to permit the engineering geologist and
or civil
engineer to extrapolate with confidence the subsurface conditions that might affect the project,
so that the seismic hazard can be properly evaluated, and an appropriate mitigation measure can
be designed by the civil engineer. The preparation of engineering geologic maps and geologic
cross sections is often an important step into developing an understanding of the significance
and extent of potential seismic hazards. These maps and
or cross sections should extend far
enough beyond the site to identify off-site hazards and features that might affect the site.
Excavation Layout.
The required number and spacing of borings, test pits, and trenches
for a particular project must be based on judgment and experience. Obviously the more test
excavations that are performed, the more knowledge will be obtained about the subsurface
