Magnet Extensometer

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.

Magnet Extensometer

Operation

The magnet extensometer 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 PipeInclinometer 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%.

Schematic diagram of Magnet Extensometer in use

Magnet Extensometer 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.

Magnet Extensometer Specifications

READOUT (PROBE AND REEL)
Readout with 30m cable 51817203
Readout with 50m cable 51817205
Readout with 100m cable 51817210
Readout with 100' cable 51817310
Readout with 150' cable 51817315
Readout with 300' cable 51817330
The readout consists of a reed-switch probe, a graduated cable, and a reel that contains a power switch, indicator light, and batteries.
Probe Stainless steel, 16 x 203mm (5/8 x 8")
Cable 3.5mm (0.125") diameter, twin steel conductors for good dimensional stability, polyurethane jacket for high resistance to abrasion, indelible graduations marked by a laser marking machine. Metric cable have 2 mm graduations. English cables have 0.01' graduations
Reel 230 or 280mm (9 or 11") in diameter, depending on cable length. Mounted on a steeltube stand with a durable finish
Batteries The readout operates on three 1.5 volt alkaline AA cells. Battery life depends on usage
Precision Readings are typically repeatable to ±3 to 5 mm or ±0.1 to 0.2 inches
MAGNETS FOR 1-INCH PIPE
Datum Magnet 51817303 Datum magnet has 34 mm ID (1.34"). Body is PVC plastic
Spider Magnet 51817503 Spider magnet has legs that are compressed for downhole installation and released when magnet is in place. Compression wires and release pin are included. Release cord is not included. OD of magnet is 86 mm (3.4") with legs compressed, so borehole should have a diameter of 100 to 200 mm (4 to 8"). ID of magnet ID is 34 mm (1.34"). Spring force of legs is 2.7 kg (6 lb). Legs are spring steel, body is PVC plastic
Plate Magnet 51817703 Plate magnet measures 300 x 300 mm (12 x 12"). ID is 34mm (1.34"). Body is PVC plastic
Pipe Magnet 91821450 Pipe magnet is fixed directly to telescoping sections of 1-inch pipe (50711048), eliminating spider magnets and simplifying installation. Up to two magnets can be attached to each telescoping section
1-INCH PIPE
Pipe Section, 3.05m (10') 50711408 Pipe section is 3.05 m (10') long Schedule 80 PVC pipe. Threaded ends (4 TPI) provide flush joints
Telescoping Section 50711458 Telescoping section is 3.05m (10') long when expanded and 2m (7') long when collapsed. It has threaded ends for coupling to PVC pipe above. OD is 43 mm (1.7")
Pop Rivet, AD-42H 51003303 Pop rivets hold telescoping sections open during installation. Use 2 per telescoping section
End Cap 50711428 End cap is glued on with PVC cement
MAGNETS FOR 70MM CASING
Datum Magnet 51817346 Datum magnet has an ID of 71mm (2.8")
Spider Magnet 51817546 Spider magnet has legs that are compressed for downhole installation and released when a pin is pulled. Compression wires and release pin are included. Release cord is not included. Magnet OD is 122 mm (4.8") with legs compressed, so borehole diameter should be 125 to 305 mm (5 to 12”). Magnet ID is 71 mm (2.8"). Legs have spring force of 2.7 kg (6 lb). Legs are spring steel, body is PVC plastic.
Plate Magnet 51817746 Plate magnet measures 300 x 300 mm (12 x 12"). ID is 71 mm (2.8")
MAGNETS FOR 85MM CASING
Datum Magnet 51817366 Datum magnet has an ID of 85 mm (3.36") and is made of PVC plastic
Spider Magnet 51817566 Spider magnet has legs that are compressed for downhole installation and released when a pin is pulled. Compression wires and release pin are included. Release cord is not included. Magnet OD is 122 mm (4.8") with legs compressed, so borehole diameter should be 125 to 305 mm (5 to 12"). Magnet ID is 85 mm (3.36"). Legs have spring force of 2.7 kg (6 lb). Body is PVC plastic. Legs are spring stee
Plate Magnet 51817766 Plate magnet measures 300 x 300 mm (12x12") and has an ID of 85 mm (3.36"). Body is PVC plastic