An extensometer is an instrument used to measure the movement of cracks on unstable rock slopes. This is done by measuring the distance between bolts installed on either side of the crack, over a long time period.

Picture: Extensometer for measuring the width of a fracture (Foto: T. Oppikofer, NGU)

An extensometer is a compact, portable and easy tool to measure distances. It is used by assembling fixed permanent bolts on the rock surface and measuring the distance between them. Repeated measurements over time will capture any displacements of the bolts with respect to each other. A typical use of an extensometer for NGU is to measure the width across the back fracture or internal cracks in an unstable rock slope.

NGU using a digital extensometer from Soil Instruments, which includes a 20 m long stainless steel measuring tape with precise evenly distributed holes. The measuring tape is wound on a spool that also includes a tensioning device with an optical indicator for the tension in the tape. Distance measurement is read digitally. NGU’s extensometer has a measurement accuracy of 0.01 mm and a repeatability of 0.1 mm. Repeat tests show a standard deviation between 0.04 and 0.19 mm.