Extensometers represent a wide family of instruments, mainly designed to monitor relative displacement between two or more point.
The most common applications are:
- Deep excavations;
- Pile testing;
- Retaining walls.
Multipoint borehole extensometers (also called rockmeters) are often used where displacement measurement between one fixed point and two or more point on the same axis is requested. It consists of an instrument head, up to 7 individual measuring rods of different lengths, each one attached to a bottom anchors. In the head electrical or vibrating wire displacement transducers, that allow remote monitoring, are installed. Alternatively also manual readings using a dial gauge can be taken. Measuring rods are available in different material to suit the project requirements (fiber glass, stainless steel or invar-steel).
The MEXID extensometer is a miniaturized version of the multipoint extensometer. Installation is permitted within a 50mm diameter borehole. The MEXID extensometer is supplied completely mounted with pre-installed electrical or vibrating wire displacement transducers to enable very fast and simple installation. Just unpack the MEXID, unreel it, insert the instrument into the borehole and grout the borehole.
Tunnel convergence measurements are carried out with a tape extensometer. Usual convergence scheme include 5 reference point distributed within a tunnel cross section. The tape extensometer permits to interconnect all points with the measuring device and to measure the relative distance and displacement between all measuring points. If reference points permit to fix also optical targets or prisms, convergence measurements will have also topographic redundancy.
Embankment extensometers are installed in large earth structures like earth fill dams where monitoring of horizontal deformations within the dam body is requested. It consists of a number of anchor plates interconnected by telescopic sensed elements equipped with a displacement transducer to form a continuous chain for strain and deformation monitoring.