Magnetic Extensometer
For Vertical Settlement Profiles
Data from the magnetic extensometer can reveal the depths at which
settlement has occurred as well as the total amount of settlement.
The magnetic extensometer consists of a probe, a steel survey tape, a
tape reel with built-in light and buzzer, and a number of magnets po-
sitioned 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 lowering the probe to the bottom of the
access pipe and drawing it up to find the depth of each magnet.
When the probe enters a magnetic field, a reed switch closes,
activating the light and buzzer. Two buzzes are emitted, the first in
the general vicinity of the magnet, and the second at the exact loca-
tion of the magnet. The operator then refers to graduations on the
survey tape and notes the depth of the magnet.
When the pipe is anchored in stable ground, the depth of each mag-
net is referenced to a “datum” magnet that is fixed to the bottom of
the access pipe. Settlement and heave are determined by comparing
the current depth of each magnet to its initial depth.
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.
Advantages
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Provides multiple measurements, indicating not only the total
settlement, but also the “incremental” settlement at the depth of
each magnet.
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Optical survey not required if bottom is fixed in stable ground.
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Inexpensive.
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Easy to read.
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Can be combined with inclinometer in certain soils.
Limitations
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Interferes with site operations. Not easily automated.
Borehole Extensometers
Flush Coupled
Access Pipe
Plate Magnet
Probe
Earth Fill
Telescoping
Section
Datum Magnet
Stable Ground
MAG E XT- 0. cd r
Grout
Spider Magnet
Original Ground Level
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Rembco Geotechnical Contractors specializes in
slope stabilization,
soil nail wall,
foundation repair
and support,
grouting,
shoring,
micropile
construction,
rock drilling, and we remediate
sinkholes. We use a
problem-solving approach in the geotechnical portions of buildings, bridges, water plants, sewage plants, tunnels,
installing caisson bottom, and roads. We are experienced in these applications:
micro piles,
sinkhole repair,
permeation and
pressure
grouting,
soil nails,
cement grouting,
chemical grouting,
micropiles,
compaction grouting, and
rock anchors.
Rembco techniques apply when adding new structures: especially
rock anchor,
microfine cement,
soil nailing, and
micropile
installations for foundation support. Our specialties are
soil nails,
rock anchor,
chemical and
compaction grouting,
mini piles,
sinkhole repair,
micropiles, and
soil nailing.
