Environmental Engineering Reference
In-Depth Information
the slopes of a volcano. In its most destructive mode the mass is very large and moves at
high velocities. The greatest danger exists where the volcano has a deep snow cover that
can be melted by volcanic activity. Such was the case with Mt. St. Helens; at Mt. Pinatubo,
a lahar was initiated by heavy rains. In 1985, a small eruption at a Columbian volcano,
Nevado del Ruiz, caused a lahar that took more than 20,000 lives in Armero, located in a
valley about 75 km from the volcano's summit.
In the United States there is concern over a potential lahar impacting on the urban area
near Portland Oregon, around the base of Mt. Rainier. Evidence of lahars, about 500 years
old, have been identified in the valleys near the base of the volcano. This has prompted
the USGS to set up a network of stations to monitor and send out alarms when ground
vibration thresholds are exceeded so that evacuation can be initiated (Internet: http;//vol-
canoes.usgs.gov/RainierPilot.html).
Volcanic Hazard Manuals
have been prepared by the USGS and UNESCO
.
11.4
Earthquake-Resistant Design: An Overview
11.4.1
Introduction
Ground Motion
Dynamic Forces
The large amounts of energy released during earthquakes travel through the Earth as var-
ious types of seismic waves with varying oscillation frequencies and amplitudes or dis-
placements. The oscillating particles in the wave possess velocity and exert a force due to
the acceleration of gravity (see
Section 11.2.3)
.
Earthquake ground motions for structural
analysis are usually characterized by peak ground acceleration (or a fraction thereof),
response spectra, and acceleration time histories.
In rock and other nearly elastic geologic materials, these dynamic forces result in transient
deformations, which are recovered under the low strains of the seismic waves. Interest in the
properties of these materials lies primarily in their ability to transmit the seismic waves.
In weaker deposits, such as alluvium, colluvium, and aeolian soils, however, the base-
rock excitation transmitted to the soils is usually amplified. In addition, some materials
respond to the cyclic shear forces by densifying, liquefying, or reducing in shear strength
(unstable soils).
Significance
The effect of the dynamic forces, therefore, can be divided into two broad categories:
1.
The effect on structures subjected to the forces transmitted through the ground,
which result in ground shaking
2.
The effect on the geologic material itself, primarily in the form of response to
cyclic shear forces
Field Measurements
Ground displacement (amplitude), used in Richter's relationship to compute magnitude,
is measured by seismographs.
The force imposed on structures, in terms of the acceleration due to gravity, which has
both horizontal and vertical components, is measured by accelerographs.
