Single Point Borehole Extensometer — Introduction & Operating Principle
Encardio-rite single point borehole extensometer is a geotechnical instrument devised to assist civil engineers and geologist in the measurement of deformation of rock mass and adjacent or surrounding soil. Coupled with anchor bolt load cell and tape extensometer, it makes for an essential piece of equipment for the complete monitoring of foundations, slopes & embankments and for studying the behaviour of rock around underground cavities, tunnels and mines.
Encardio-rite Model EDS-64UD borehole extensometer is used for upward or downward sloping hole working with stainless steel or fibreglass connecting rods. It is capable to be installed in a borehole of φ 50 mm with a diameter at the mouth of borehole increased to 62 mm up to a depth of 200 mm.
Let’s have a look at the application of borehole extensometer.
Borehole Extensometer- Application
- To understand the behaviour of roof, wall of mine, underground cavity, or tunnel during excavation.
- To analyse the effectiveness of the roof/wall support system of a mine, underground cavity or tunnel.
- To determine and monitor the movements in slopes and foundations which occurs due to excavation of underground cavities or heavy construction of structures like concrete, rock-fill, masonry or earth dams over the foundation.
Single Point Borehole Extensometer- Introduction
As discussed, borehole extensometer determines the displacement taking place in a borehole with reference to time.
Once installed, the single point borehole extensometer successfully measures the distance between the anchor (1) and the reference plate (5.8) and monitors it with time.
Assuming that the anchor is in the stable ground, any change in the spacing of anchor is understood as a movement of sidewall or slope, sag of roof bed, settlement of foundation etc.
Single Point Borehole Extensometer- Operating Principle
Encardio-rite Model EDS-64U/D is a mechanical borehole extensometer in which a single anchor is positioned in a borehole. The relative displacement is then measured over time with respect to a reference plate.
The model is especially efficient to measure the displacement when it comes to borehole with easy access to the mouth. It becomes economical and reliable to use the single point extensometer rather than any other.
A micrometre depth gage is used to capture the displacement reading by measuring the depth of the reference button at the near end of connecting rod from a reference plate.
A borehole extensometer is comprised of three major elements:
- Reference head assembly
- Stainless steel/Fiberglass assembly
- Anchor
Single Point Borehole Extensometer — Reference Head Assembly
The image below explains the parts and the reference head assembly. It is installed in a borehole of φ 50 mm. The diameter of the mouth of the borehole is then expanded to 2 mm up to a depth of 200 mm. The flange consists of four 15 mm diameter mounting holes at a PCD of 105 mm.
Single Point Borehole Extensometer -Stainless steel/Fibreglass rod assembly
The rod assembly consists of a stainless steel/fibreglass rod of specified length shielded inside an outer PVC tubing or continuous nylon tube. The rod is equipped with two ends connected, with one end having a male thread which goes into the anchor. The other end fitted with female thread is reserved for the reference button.
One end of the PVC/nylon tube is tightly sealed to the anchor to prevent any leaking of the grout. The other end if of the PVC/nylon tube is firmly seated in the reference head assembly.
Make sure that the reference head assembly is firmly sealed especially while installing in upwards to horizontally slanting holes to prevent the grout from leaking and entering the reference head assembly.
Even while grouting, the stainless steel/fibreglass rod and reference button are free to move inside the outer …[Continue Reading]
