SITE INVESTIGATION FOR GEOTECHNICAL EARTHQUAKE ENGINEERING - Geotechnical Earthquake Engineering - page 141

LOAD
CELL

VERTICAL DEFLECTION
TRANSDUCER

BACKPRESSURE REGULATOR

CONFINING
PRESSURE

LOW FRICTION
BRUSHING

VACUUM REGULATOR

CELL

VOLUME CHANGE
INDICATOR

PISTON

DATA
ACQUISITION

LD DEF PWP
STRIP CHART

X-Y
RECORDER

SPECIMEN

FLUSHING

PORE PRESSURE
TRANSDUCER

FIGURE 5.16 Schematic diagram of the cyclic triaxial test equipment. ( Reproduced from ASTM D 5311.
Reproduced with permission from the American Society for Testing and Materials. )

5.6 PEAK GROUND ACCELERATION

5.6.1 Introduction

As indicated in Fig. 2.14, the ground motion caused by earthquakes is generally characterized
in terms of ground surface displacement, velocity, and acceleration. Geotechnical engineers
traditionally use acceleration, rather than velocity or displacement, because acceleration is
directly related to the dynamic forces that earthquakes induce on the soil mass. For geotech-
nical analyses, the measure of the cyclic ground motion is represented by the maximum hor-
izontal acceleration at the ground surface a max . The maximum horizontal acceleration at
ground surface is also known as the peak horizontal ground acceleration. For most earth-
quakes, the horizontal acceleration is greater than the vertical acceleration, and thus the peak
horizontal ground acceleration also turns out to be the peak ground acceleration (PGA).
For earthquake engineering analyses, the peak ground acceleration a max is one of the
most difficult parameters to determine. It represents an acceleration that will be induced
sometime in the future by an earthquake. Since it is not possible to predict earthquakes, the
value of the peak ground acceleration must be based on prior earthquakes and fault studies.
Often attenuation relationships are used in the determination of the peak ground accel-
eration. An attenuation relationship is defined as a mathematical relationship that is used
to estimate the peak ground acceleration at a specified distance from the earthquake.
Numerous attenuation relationships have been developed. Many attenuation equations
relate the peak ground acceleration to (1) the earthquake magnitude and (2) the distance
between the site and the seismic source (the causative fault). The increasingly larger pool
of seismic data recorded in the world, and particularly in the western United States, has
allowed researchers to develop reliable empirical attenuation equations that are used to
model the ground motions generated during an earthquake (Federal Emergency
Management Agency 1994).