Environmental Engineering Reference
In-Depth Information
“acoustic counts” were high, then tapered off. During one operation rainfall caused an
increase in acoustic counts. During the fifth cut, the emission rate followed the trend for
the previous cuts, but 30 min after the cut was made, the rate increased rapidly and
reached its maximum when a large portion of the fill slid downslope.
Acoustic count
: For the equipment used, acoustic count was defined as being registered
each time the amplitude of the electric signal exceeded the threshold level of 0.025 V as
recorded on the electronic counter. From several studies (Koerner et al., 1978) it was con-
cluded that:
1.
Soil masses with no emissions are stable.
2.
Granular soil masses generating moderate levels of emissions (from 10 to 100
counts/min) are considered as marginally stable.
3.
High-emission levels (100 to 500 counts/min) indicate that the soil mass is
deforming substantially and that immediate remedial measures are required.
4.
At above 500 counts/min the mass is in the failure state.
Active Slide (Novosad, 1978)
The inclinometer and the acoustic emissions device were compared during the study of an
active landslide. The emissions identified the shear zone after only three measurements,
even though movements were very slow, of the order of 5 to 10 mm/year. Novosad con-
cluded that the emissions method had the advantages of a simple, common borehole cas-
ing, smaller casing diameter (40 cm I.D. is sufficient), and a deeper reach. The
disadvantage was the qualitative character of the measurements.
Open-Pit-Mine Slopes (Kennedy, 1972)
Rock noises emanating from an unstable zone in an open-pit-mine slope are identifiable
by their characteristic envelope shape, a frequency between 6 and 9 Hz, and a character-
istic irregular amplitude. Experience is required to distinguish extraneous noise from
drills, trucks, shovels, locomotives, etc. operating in the pit. Earthquakes are easily identi-
fiable because they have a very definite waveform envelope.
In application, records of the number of rock noises for a given period (counts) and of
the frequency, duration, and equivalent earth motion of each seismic disturbance are
maintained. As the instability increases, so does the number of rock noises released dur-
ing a given period. Plotting the cumulative number of rock noises against time may give
a useful prediction chart similar in shape to those obtained from plotting displacement vs.
time.
Advantages of the method include its extreme sensitivity and continuous recording
capabilities. Early warnings of impending failure allow the removal of workers and equip-
ment from the threatened area.
4.4
In Situ
Pressures and Stresses
4.4.1
General
Significance
Deformations occur as the result of response to changes in stress conditions
in situ
, which
can be measured under certain conditions. The data obtained by instrumentation are used
