Magnet Extensometer
Applications
The
magnet extensometer is used to monitor settlement and heave in excavations, foundations,
dams, and embankments. It can also be installed behind retaining structures, such
as sheet piles and slurry walls, and above underground openings, such as tunnels
and shafts. Data from the extensometer indicate the depths at which settlement
has occurred as well as the total amount of settlement.
Operation
The system consists of a probe, a graduated cable, a
tape reel with built-in light and buzzer, and a number of magnets positioned
along the length of an access pipe. The magnets are coupled to the surrounding
soil and move up or down as heave or settlement occurs. Readings are obtained
by drawing the probe through the access pipe to find the depth of the magnets.
When the probe enters a magnetic field, a reed switch closes, activating
the light and buzzer. The operator then refers to the graduations on the
cable and notes the depth
of the magnet.
When the access pipe is anchored in stable ground, the depth of each magnet
is referenced to a datum magnet that is fixed to the bottom of the access pipe.
If the bottom of the access pipe is not in stable ground, the depths of the magnets
must be referenced to the top of the pipe, which is optically surveyed before
readings are taken.
 Installed Components
Access Pipe: Inclinometer casing or one-inch
access pipe can be used. After pipe and magnets are installed, the borehole is
backfilled with grout.
Datum Magnet: The datum magnet is fixed directly
to the bottom section of access pipe to serve as a reference. A datum magnet is
used when the bottom of the pipe is anchored in stable ground.
Spider Magnet: The spider magnet, named for
its spring-steel legs, is used in boreholes. The legs are compressed for installation
and are released when the magnet is positioned at the specified depth. The spider
magnet is typically attached to the access pipe prior to installation, but can
also be pushed into place after the pipe is installed.
Plate Magnet: The plate magnet is used in
fill. It is positioned at the specified elevation and then covered with fill material
that is compacted to the same specifications as the surrounding fill.
Telescoping Sections: Telescoping sections
are installed when settlement or heave is expected to exceed 3%.
Features
Easy to handle: Instead of a measuring tape, the magnet extensometer uses
a graduated cable that is easy to handle and winds up neatly. The cable is
3.5 mm (0.125") in diameter and has twin steel conductors inside for good
dimentional stability. The polyurethane cable jacket is flexible and has
very high resistance to abrasion. Graduations on the cable jacket are marked
by a laser and are as durable as the cable jacket itself.
Accurate: Readings are typically repeatable to +/- 3 mm ( +/- 0.1 inch).
Greater precision can be achieved by building a reference stand that incorporates
a vernier dial gauge.
For more information
Please download the datasheet
and the manual for
the Magnet Extensometer. These documents can be viewed and printed with
Adobe Acrobat Reader.
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