SUMMARY OF GEOTECHNICAL EARTHQUAKE ENGINEERING - Geotechnical Earthquake Engineering - page 484

geologic factors dominated the project behavior. They conclude that geologic misjudg-
ments can be disastrous for the project. The assistance of an engineering geologist is essen-
tial for the following:
● Location of faults, evaluation of active and inactive faults, and historic record of
earthquakes.
● Evaluation of the peak ground acceleration and earthquake magnitude.
● Investigation of geologic hazards such as fault rupture, regional subsidence, and tsunami.
● Site-specific investigations of landslides, formational materials, and groundwater.
● Evaluation of rock properties, such as hardness, mechanical properties, fracture condition,
depth determinations, and rock classification.
● Evaluation of soil properties, such as type of soil deposit (i.e., alluvium, colluvium, etc.),
geologic age, uniformity, etc.
● Preparation of subsurface logs, subsoil profiles, and geologic cross sections.
As discussed in Secs. 5.2 and 5.3, the site investigation is divided into two parts, the
screening investigation and the quantitative evaluation.
Screening Investigation. The purpose of the screening investigation is to screen out
those sites that do not have geologic hazards or to assess the severity of the geologic
hazards at the site (Sec. 5.2). If the site is likely to have geologic hazards, then the screening
investigation is used to define those hazards before proceeding with the quantitative
evaluation. The screening investigation should be performed for both the existing site
conditions and the final constructed condition. In addition, off-site potential haz-
ards should be evaluated. The screening investigation consists of a review of available
documents, such as preliminary design information, history of prior development, earth-
quake records, aerial photographs, geologic maps, building codes, and other regulatory
specifications. Once the site research is completed, the screening investigation should
include a field reconnaissance to observe the site conditions and document any changes
that may not be reflected in the available reports. The results of the screening investigation
should either clearly demonstrate the lack of geologic hazards or indicate the possibility of
geologic hazards, in which case a quantitative evaluation is required.
Quantitative Evaluation. The purpose of the quantitative evaluation is to obtain sufficient
information on the nature and severity of the geologic hazards so that mitigation measures
can be developed. The quantitative evaluation consists of geologic mapping, subsurface
exploration, laboratory testing, engineering and geologic analyses, and report preparation.
Engineering and geologic analyses, including the determination of the peak ground
acceleration, are summarized in Secs. 15.3 and 15.4.
A preliminary subsurface exploration is often performed in order to get a rough idea
of the soil, rock, and groundwater conditions at the site. Once the preliminary subsurface
data is analyzed, additional detailed subsurface explorations are performed in order to
better define the subsoil profile, further explore geologic hazards, and obtain additional
information on the critical subsurface conditions that will have the greatest impact on the
design and construction of the project.
For sites having sand and gravel deposits, the standard penetration test (SPT) and cone
penetration test (CPT) should be performed in order to evaluate the liquefaction potential
and/or volumetric compression of these deposits. For sites having sloping ground and thin
layers or seams of potentially liquefiable soil, the CPT is preferred because it will be better
able to discover the thin layers or seams as compared to the SPT. For sites having layers of
cohesive soil, such as silts and clays, soil samples should be obtained in order to determine