Measuring and monitoring the status of excavations and structures within mines, as well as the behaviour of the rock and soil deposits in and around a mine, are a vital part of overall control and management. The key to effective measuring and monitoring is data—accurate, reliable data. In order to gather this data, mine engineers make use of a variety of geotechnical monitoring instruments – of which one of the key types is the extensometer.
How Extensometers are Used
Extensometers are commonly used in underground mines to measure:
- Compression (piles, pillars, and the soil under them, etc.)
- Consolidation (soil under tunnel, room and stope floors, embankments, etc.)
- Movement (walls, roofs, abutments, etc.)
- Settlement (embankments, excavations, and foundations)
- Spread (e.g. in embankments)
- Subsidence (e.g. above tunnels, rooms, stopes and other underground structures and workings)
And to monitor:
- Underground excavations (including walls and backs of mining drifts)
- Surface crown-pillars
- Walls (in open-pit mines)
- Backfill (in underground mines)
- Convergence (in tunnels, rooms, stopes, and other underground openings)
How Extensometers Work
Extensometers are fixed into the rock or soil mass they are being used to measure. One of the most common ways to do this is by fitting them into a narrow borehole and grouting the entire instrument solidly in place (hence the term “borehole” extensometer).
The device then provides data on movement in the rock or soil by measuring the distance between one or more anchor points and the reference head (extensometers with several anchor points are commonly referred to as “multi-point extensometers”, and if they are of the type that is fitted in a borehole, as “multi-point borehole extensometers”). Any change in this distance over a period indicates movement/shifting.
In older, manual-based extensometers, measurements were made using a caliper and written down. However, it is becoming increasingly common for extensometers to be fitted with a digital readout head that can be connected to devices that progressively transfer the data to a central point, where it can be utilised by mine engineers.
In the world of multi-point borehole extensometers, the SMART MPBX from Mine Design Technologies (MDT) really stands out. This device features 6 anchor points and an electronic readout head. The SMART MPBX is light, flexible, durable, easy to install and so small that it can be installed in a borehole of only 50mm (2 inch) diameter.