Extensometers
Applications
 Extensometers are used to measure movements of soil and rock along a single axis. Applications for extensometers include:
- Monitoring settlement in excavations, foundations, and embankments.
- Monitoring subsidence above mines and tunnels.
- Monitoring movements in rock slides, walls, and abutments.
- Monitoring consolidation of soil under embankments and surcharges.
- Monitoring compression of piles and soil under piles.
- Monitoring spread in embankments.
- Monitoring convergence in underground openings, such as tunnels.
Borehole Extensometers
- 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,
can measure multiple points.
- Limitations: Limited measurement range (50 to 100 mm).
- 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: Cannot be automated,
practical limit of 15 or 20 magnets, vertical
installation only.
- 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: Cannot be automated, vertical installation
only.
- The Borros Anchor Settlement Point 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.
- 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: Cannot be automated, works only with telescoping casing,
requires a careful operator.
Special-Purpose Borehole Extensometers
This borehole extensometer is a special purpose, sub-surface extensometer with a range of 25 inches or more and an 4-20mA output. It consists of an anchor set in stable ground, rods inside protective pipe, and a waterproof reference head. Special order only.
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. The Increx system must be ordered from Holland and requires a long lead time.
Other Types of Extensometers
The soil strainmeter is used to monitor spreading of embankment material. It is typically installed in trenches.
The tape extensometer is used to monitor convergence in underground openings, such as tunnels.
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