Borehole Extensometer – EXCEXX-100
Borehole extensometers are instruments which record the rock mass displacements at certain points (anchor). They are installed in a borehole and are fixed to the ground with different methods, either by grouting or mechanically by spider anchors. Stability and movement behavior of structures in rock can be determined by these instruments. A typical extensometer consists of a reference head, one or more anchors and rods by which the displacement of each anchor is transferred to the reference head. These rods are usually made out of fiberglass which is light, strong and elastic. A PE pipe covers all the rods and de-bonds them from the grouting agent. When the soil or rock deforms, the distance between anchors change and these are recorded relative to the reference head. By using extensometers, one can accurately determine distribution of rock mass displacement along the length of extensometer and calculate the magnitude and rate of displacements. Reading displacements can be either manually or remotely by attaching an electrical head with linear sensors.
Some of the applications of this instrument are:
- Measurement of deformation around tunnels.
- Monitoring rock displacement in open pit mines.
- Stability assessment in layered formations.
- Subsidence of surface buildings adjacent to open excavations.
- Settlement of ground under plate loading and Jacking tests.
- All kinds of mine stability studies.
Operation and Installation
Groutable extensometers are placed in a borehole either in an upward or downward direction. The hole is washed and dried prior to installation. Grout and vent tubes are attached to the instrument then the whole package is inserted to the hole. The head is connected and sealed to the excavation surface by cement mortar. Once the mortar is set, grouting commences from the grout hose until fresh grout exits from the vent hose. After grout has set, the readings begin and this is repeated in the followings days.
- Extensometer length
- Number of anchor points and their locations
- Type of ground (to select anchor type)
- Manual reading or electrical head
- Borehole diameter