Borehole extensometer is a precision instrument designed to help civil engineers and geologists in measurement of deformation of rock mass and adjacent or surrounding soil. Together with anchor bolt load cells, it is an essential piece of equipment for investigation and monitoring of foundations, slopes & embankments and for studying behaviour of rock around underground cavities, tunnels and mines. The borehole extensometer is an important instrument especially in investigation of underground cavities and landslides.
The borehole extensometer measures the extension (displacement) which takes place with time in sub-surface bored holes (rock mass or ground).It essentially consists of an anchor grouted in a borehole and connected to a reference plate with connecting rods (fiberglass or stainless steel). The connecting rod is enclosed inside heavy duty protective tubing to allow for free movement. The change in displacement between the anchor and reference plate is accurately monitored with passage of time. The initial reading is taken as the base. Subsequent readings are then compared with the initial reading to determine the magnitude of change in displacement across the opening.
The borehole extensometers (BHE) are broadly classified as single point BHE (SPBX) and multi-point BHE (MPBX).
Single point BHE are of following types:
Mechanical SPBX – read manually with a micrometer depth gage. The measurement is economical, reliable and is essentially preferred at locations where access to the mouth of the borehole is easily available.
Model EDS-64U/D- Model EDS-64Usuitable for upward borehole; Model EDS-64D suitable for downward borehole
Electrical SPBX – The electrical output can be accurately measured by any suitable readout unit. The readings can also be automatically collected at desired frequency and stored by a suitable datalogger, with wireless options to transfer data to remote location/server.
Model EDS-71V – with vibrating wire displacement sensor; Model EDS-71V-U suitable for upward borehole; Model EDS-71V-D suitable for downward borehole
Model EDS-71P -with potentiometric displacement sensor; Model EDS-71P-U suitable for upward borehole; Model EDS-71P-D suitable for downward borehole