Multipoint Borehole Extensometer- Introduction, Application, and System Requirements

Multipoint Borehole Extensometer is a geotechnical instrument designed to assist civil engineers and geologists in the estimation of deformation of rock mass and adjacent or surrounding soil. Clubbed with anchor bolt, load cell, and tape extensometer, it constitutes an integral piece of equipment used to investigate and monitor the foundations, slopes, and embankments in order to study the behaviour of the rock around underground cavities, tunnels and mines.

The borehole extensometer Model EDS-63U/D is commonly used for the upward or downward sloping hole with the help of fibreglass connecting rods or stainless steel extension rods. Making the use of EDS-63U/D system, up to three extensometers can be installed in a borehole of φ 76 mm with a diameter at the mouth of borehole increased to 90 mm up to a depth of 200 mm. Up to six extensometers can be installed in a borehole of φ 102 mm with a diameter at the mouth of borehole increased to 125 mm to a depth of 225 mm.

Multipoint Borehole Extensometer-Application

Let’s see where we find Multipoint Borehole Extensometer useful.

• To learn the behaviour of the roof or wall of mine, tunnel or underground cavity during an excavation.
• To examine the effectiveness of the roof/wall support system of a mine, underground cavity or tunnel.
• It used to be a step ahead of the accidents by predicting the potential roof or wall fall before it happens. This is detected as the fall is invariably preceded by measurable sags as the strata open up, and the movement usually occurs at an increasing rate as fall conditions are approached. If the roof or wall fall goes undetected, it may result in serious accidents or may require costly patch-up and repair operations.
• To observe and measure the movements in slopes and foundations which occur due to the excavation of underground cavities or due to construction of heavy structures like concrete, rock fill, masonry or earth dams over the foundation.

Measurement method and anchors- Multipoint Borehole Extensometer

The measurement method used for the multipoint borehole extensometer is a micrometre depth gage. As for the anchors, Encardio-rite presents two types of anchors. Let’s have a look at them:

The first type is a groutable anchor with a length of φ 20 mm x 500 mm, especially used for hard rocks. These anchors are connected to a rod of appropriate length, pushed in a borehole and fixed in position by cement grout. The connecting rod is shielded from the cement grout as it is enclosed in nylon tubing. These anchors can be installed in vertical boreholes or upward inclined holes. However, in the case of holes inclined upward, special precautions are needed to retain and prevent the grout from flowing out of the borehole.

The other type of anchors, known as packer anchors, are used for soft rocks and soil. Similar to the groutable anchor, they are lowered into the borehole along with a connecting rod and fixed in position by pumping cement grout into the packer for taking a firm grip with the surroundings.

Multipoint Borehole Extensometer- The explanation

The multipoint borehole extensometer finds its application in measuring displacement taking place in the borehole overtime. It helps to precisely determine the change in distance between various anchors with respect to reference plate (6.9) and monitor their relative displacement with the passage of time.

It is typically assumed that the deepest anchor is in the stable ground and hence any change in anchor spacing is described as sag of roof bed, settlement of the foundation, movement of …[Continue Reading]