The magnet extensometer is designed to measure settlement and heave in excavations, foundations, dams, and embankments. It is also useful to monitor sheet piles, slurry walls and other retaining structures. It allows to measure the total settlement and to determine the depth at which the settlement occurs.
Basically the magnet extensometer in a borehole where magnetic rings called “spider” are placed at different depth. The rings move together with the ground layers and the relative position between them can be measure with a magnetic fields probe.
The probe is very similar to a water level meter with the difference that it signals the presence of a magnetic ring instead of the water.