Environmental Engineering Reference
In-Depth Information
Some of the applications include:
Monitoring natural slopes, open-pit mines, and other cut slopes; excavations for
tunnels, caverns and underground storage facilities and pressure chambers; and
the stability of dams and embankments
Locating leakage paths in dams, reservoirs, pressure pipelines, and caverns
Monitoring fault zones
Inspecting steel and concrete structures under test loading
The Phenomenon
General
A material subject to stress emits elastic waves as it deforms, which are termed acoustic
emissions.
Rock Masses
Testing has shown that both the amplitude and number of emissions increase continu-
ously as macroscopic cracks initiate and propagate first in a stable manner, then in an
unstable manner. Near rupture, friction along crack surfaces, as well as crack propagation
and coalescence, contributes to the acoustical emission activity. Mineralogical and
lithological differences, moisture content, and stress conditions affect the emissions
(Scholz, 1968).
Soils
Frictional contact in well-graded soils produces the greatest amount of “noise” during
stress, and activity increases with the confining pressure. Clays exhibit a different form of
response than sands (Figure 4.22) , and the emission amplitude for sands can be 400 times
that of clay (Koerner et al., 1977).
Detection
General
Sensors detect microseismic activity at a specific location in an earth mass by monitoring
displacements, velocities, or accelerations generated by the associated stress waves. At
times, the sounds are audible, but usually they are subaudible because of either low mag-
nitude or high frequency, or both. In application, if after all extraneous environmental
noise is filtered and there are no emissions, the mass can usually be considered as stable.
If, however, emissions are observed, a nonequilibrium condition exists, which may even-
tually lead to failure.
Instrumentation
Accelerometer or transducer sensors are used to detect acoustic emissions by converting
mechanical energy associated with the microseisms into an electrical signal proportional
to the amplitude of sound or vibration that is detected. The detected signal is then passed
through a preamplifier, amplifier, and filter, and finally to a display on a cathode-ray oscil-
loscope or into a recorder as shown in Figure 4.23.
The system components are selected for a specific study to provide suitable frequency
response, signal-to-noise ratio, amplification, and data-recording capacity. The design,
construction, and calibration of equipment are discussed in Hardy and Leighton
(1977).
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