A borehole extensometer is a precision instrument designed to help civil engineers and geologists in the 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 the behaviour of rock around underground cavities, tunnels and mines. The borehole extensometer is an important instrument especially in the investigation of underground cavities and landslides.
The borehole extensometer measures the extension (displacement) which takes place with time in subsurface bored holes (rock mass or ground). It essentially consists of anchors grouted in a borehole at different depths and connected to a reference plate with connecting rods (fibreglass or stainless steel). The connecting rods are enclosed inside heavy-duty protective tubing to allow for free movement. The change in displacement between the anchors and the reference plate is accurately monitored with the 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).
Multipoint BHE (2-6 points) are of the following types:
Mechanical MPBX –It is used to read manually with a micrometre depth gauge. The measurement is economical, reliable and is essentially preferred at locations where access to the mouth of the borehole is easily available.
Model EDS-63U suitable for upward borehole; Model EDS-63D suitable for downward borehole.
Model EDS-63U/D is a mechanical multipoint borehole extensometer that is economical, reliable and preferred to use at locations where access to the mouth of the borehole is easily available. The reading is taken manually with a micrometre depth gauge.
Electrical MPBX – The electrical output can be accurately measured by any suitable readout unit. The readings can also be automatically collected at the desired frequency and stored by a suitable data logger, with wireless options to transfer data to a remote location/server.
Borehole extensometer measures extension that takes place with the time in a bored hole or several bored holes in a rock mass.
Model EDS-70V – with vibrating wire displacement sensors; Model EDS-70V-U suitable for upward borehole; Model EDS-70V-D suitable for the downward borehole.
Model EDS-70P -with potentiometric displacement sensors; Model EDS-70P-U suitable for upward borehole; Model EDS-70P-D suitable for the downward borehole.