Extensometers are used for measuring vertical displacements in ground.
The most common forms of extensometers are probe (magnet) extensometers and rod extensometers.
Probe extensometers
A hollow flush PVC tube is installed in a borehole. Spider magnets consisting of donuts with equally spaced magnets embedded inside and sprung legs are threaded over the PVC tube and pushed to their required depths using an insertion tool.
When a magnet reaches the required depth the insertion tool is withdrawn and the legs are forced into the ground.
The position of each spider is measured using a reed switch, which is lowered inside the access pipe and gives an audible signal when it passes the spider.
Rod extensometers
Rod extensometers consist of hydraulically operated anchors, connecting rods and a measuring head.
Each anchor is positioned where a measurement is required and joined to a connecting rod, which runs to a measuring head located at the ground surface.
Measurements can be recorded manually using a dial gauge or precise level, or automatically using an electric displacement transducer.