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.
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.
Source:http://www.slopeindicator.com/instruments/ext-magnet.php
Link to report of this instrument: ETM