Environmental Engineering Reference
In-Depth Information
4.
Shotcrete placed to cover the shallow rock slope between the upper and lower
roadways
5.
Low anchored concrete walls to be installed along the toe of the upslope and
downslope side of the roadways
6.
The roadway grade is not to be raised as this would add load to the unstable
mass
9.5.5
Instrumentation and Monitoring
Purpose
Instrumentation is required to monitor changing conditions that may lead to total failure
where slope movement is occurring and safety factors against sliding are low, or where a
major work would become endangered by a slope failure.
Slope-stability analysis is often far from precise, regardless of the adequacy of the data
available, and sometimes the provision for an absolutely safe slope is prohibitively costly.
In this case, the engineer may wish to have contingency plans available such as the instal-
lation of internal drainage systems or the removal of material from upslope, etc., if the
slope shows signs of becoming unstable.
In unstable or moving slopes, instrumentation is installed to locate the failure surface
and determine pore-water pressures for analysis, and to measure surface and subsurface
movements, velocities, and accelerations which provide indications of impending failure.
In cut slopes, instrumentation monitors movements and changing stress conditions to pro-
vide early warning and permit invoking remedial measures when low safety factors are
accepted in design.
Instrumentation Methods Summarized
Instrumentation is discussed in detail in Chapter 4, and for slopes is illustrated in Figure
4.41 and Figure 4.42.
Surface movements are monitored by survey nets, tiltmeters (on benches), convergence
meters, surface extensometers, and terrestrial photography. Accuracy ranges from 0.5 to
1.0 mm for extensometers, to 30 mm for the geodimeter, and to 300 mm for the theodolite
(Blackwell et al., 1975). GPS systems are showing promise for continuously monitoring
and recording slope movements.
Subsurface deformations are monitored with inclinometers, deflectometers, shear-strip
indicators, steel wire and weights in boreholes, and the acoustical emissions device.
Accuracy for extensometers and inclinometers usually ranges from 0.5 to 1.0 mm, but the
accuracy depends considerably on the deformation pattern and in many instances cannot
be considered better than 5 to 10 mm.
Pore-water pressures are monitored with piezometers.
All instruments should be monitored periodically and the data plotted as it is obtained to
show changing conditions. Movement accelerations are most significant.
GPS Installations
Mission Peak Landslide, Fremont, California
On the Internet during 2003, the USGS reported on a GPS installation to monitor the
Mission Peak Landslide in Fremont, California. Installed in January 2000, the system
included a field station with a GPS antenna, receiver, controller card, and radio modem
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