Earthquakes - Geotechnical Engineering Investigation - page 982

In addition to the limitations of the analytical procedures, there are uncertainties in
measuring soil properties and ground-motion characteristics. Thus the problem is not only
extremely complicated, but it is not well-defined. The result is necessary conservatism in
the design of important structures.

11.5 Investigation: Important Structures in High-Hazard Areas

11.5.1 Introduction

Purpose
This chapter tends to follow procedures and guidelines employed in studies for nuclear
power plants in the United States, but is intended as a conservative guide for important
structures located in any high-hazard seismic region. References are NRC (1979, 1997,
2003) and USAEC (1995, 1999).
To establish the design earthquake a “probabilistic seismic hazard analysis” (PSHA) is
performed. The methodology quantifies the hazard at a site from all earthquakes of all
possible magnitudes, at all significant distances from the site of interest, as a probability
by taking into account their frequency of occurrence (Thenhaus and Campbell, 2003). The
elements of the PSHA are given in Figure 11.50. One of the elements, “The Logic Tree,” is
illustrated in Figure 11.51. It provides a systematic approach to evaluating a region with a
number of seismic sources and an earthquake history.

Objectives and Scope
Investigation objective is basically safe and economical construction, which requires:

Identification and treatment of geologic hazards (avoid, reduce, or eliminate)

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Estimation of the design earthquake

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Establishment of foundation design criteria

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Evaluation of structural response to dynamic forces

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Study scopes range from simple to complex, depending upon several major factors, including:

Importance of structure and the degree of risk (see Section 11.4.5)

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Regional seismicity (the degree of hazard) and adequacy of available data

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Physiographic conditions (mountains, coastline, plains, etc.)

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Regional and local geology (hazards, rock types and structure, soil types, and
characteristics, and groundwater conditions)

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An investigative team to accomplish the objectives includes geologists, seismologists, geo-
physicists, geotechnical engineers, and structural engineers.

11.5.2 Preliminary Phase

Purpose
During the preliminary phase, existing data on regional and local seismicity and the nat-
ural environment are collected and reviewed either to provide a database for a site selec-
tion or a feasibility review of a previously selected site.