Borehole rod extensometers
The extensometers with fiberglass rods are used to control the displacement between one or more points located along the shaft of a hole and the reference head installed at the extremity of the hole.
The tool is formed by an anchor cemented in the bottom hole, placed in direct contact with a pole in fiberglass enclosed in a nylon cover, so that it is free to slide forward and transmit the movement between the anchor and the surface head. In case of multiple strain gauge, the anchoring points are distributed at different depths along the hole.
High reliability for measurement is granted by the use of a fiberglass rod (a material that has a very low coefficient of thermal expansion) and the minimization of the friction between the rod and the ground thanks to the protective nylon sheath.
The measurements are usually made manually with a depth gauge or automatically by installing a displacement transducer connected to a data acquisition system.
They are used for subsidence problems, foundation failures, convergence measurements during the digging, landslides, wall deformations, bulkheads and pilings, control on deep movements of clusters and rocky walls. Furtherly ther are used everytime high precision surveying is needed, even at great distances.