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Borehole extensometers are used to measure movements of
soil and rock along the the axis of a borehole. A wide range of
extensometers is available, each designed for a particular application. In
general, typical applications include:
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Monitoring settlement or heave in excavations,
foundations, dams, and embankments.
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Monitoring subsidence above mines, tunnels, and other
underground openings.
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Monitoring convergence in tunnel walls and other
underground openings.
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Monitoring movements in rock slides, walls, and
abutments.
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Monitoring consolidation of soil under embankments and
surcharges.
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Monitoring compression of piles and soil under piles.
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Borros Anchor Settlement Point
The settlement point is used to monitor
settlement of soil under an embankment. It consists of an anchor and and
two concentric riser pipes that are extended up through the embankment.
Measurements are made with a graduated tape and optical survey.
Advantages:
Simple to install and inexpensive.
Limitations:
Provides only measure of total
settlement; requires a man on site; extensions to pipe must be recorded
carefully; top of pipe must be surveyed; anchor works best in soft
clays, vertical installation only.
Increx Mobile Extensometer
The Increx mobile extensometer is used in
rock or stiff soils for high-resolution measurements of deformation along
the axis of the borehole. It consists of a number of brass rings that are
positioned at one-meter intervals along inclinometer casing, and a probe
and readout that are used to measure the distance between rings.
Advantages:
Provides high-resolution measurements
at one meter intervals, can be operated in any orientation, supplements
inclinometer measurements.
Limitations:
Requires a skilled man on site; cannot
be monitored remotely.
Slope /
CEP Magnet Extensometer
The magnet extensometer consists of a series of magnets that are installed
with an access pipe. The magnets are anchored at specified depths.
Measurements are taken by lowering a probe through the access pipe to detect
the depth of the magnets.
Advantages:
Can monitor large settlements; works
with inclinometer casing and can supplement inclinometer data,
relatively easy to operate, indicates incremental settlements.
Limitations:
Requires a skilled man on site; not
easily automated, difficult to install more than 15 or 20 magnets,
vertical installation only.
Settlement Hook
The Settlement hook is used to monitor settlement in telescoping
inclinometer casing. Measurements are taken by lowering the hook device
through the casing. The hook is catches on the telescoping joints and a
depth reading is obtained from a steel tape.
Advantages:
Works with inclinometer casing, nothing
extra to install. The USBR-type settlement hook is easy to use and
delivers reliable readings.
Limitations:
Works only with telescoping casing. The
inexpensive settlement hook requires a skilled operator.
Sondex Settlement System
The Sondex system consists of a series of rings attached to a flexible
corrugated pipe. Measurements are lowering a probe through an inner access
pipe to detect the position of the rings.
Advantages:
Can monitor large settlements; works
with inclinometer casing and can supplement inclinometer data, indicates
incremental settlements, no limitation on number of measured rings.
Limitations:
Requires a skilled man on site; cannot
be automated, vertical installation only.
Rod Extensometer
The rod extensometer consists of anchors set at specified depths, rods
inside protective tubing, and a reference head. Measurements are taken at
the reference head by micrometer or by an electric sensor.
Advantages:
Can be automated, can be read remotely,
works in any orientation.
Limitations:
Limited measurement range (50 to 100
mm).
Tape Extensometer
This instrument is not a borehole extensometer. It is a structural
instrument used to monitor convergence of the walls of underground openings,
such as tunnels.
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