What is a Contractometer?

A contractometer, as used in mining, is basically an extensometer that works in reverse. Contractometers are designed to measure convergence or compression (settling or loading), important for ensuring that a mine tunnel or shaft is safe to operate in.

Because they measure convergence, contractometers can provide data that warns mine engineering staff that a particular area of the mine is under increasing stress, often due to the nearby movement of rock, soil or water.

Some contractometers (e.g. the MDT SMART Contractometer) can be fitted inside a regular 50mm (2”) borehole, or cast in backfill. This makes them easy to install, and they can be easily protected by shotcreting over the leadwires to prevent damage.

How Does It Work?

A contractometer has a collapsible structure that contains one or more sensors and is secured inside a borehole, often with grout. If the area around the contractometer is subjected to increased stress, usually due to external factors, convergence will occur, and the structure of the contractometer will partially collapse. This movement will be detected and measured by a sensor, and data will be recorded.  Contractometers can also be cast in backfill, or embedded inside a shotcrete pillar to measure compression.  By using MDT’s shear washer option, the surface area of the standard anchor is increased and allows the instrument to bond better in a pillar or backfill scenario.

This interesting case study explains how shotcreted pillars were built inside the Golden Giant Mine (Hemlo, Ontario) to provide additional support when it was found that shotcreted ground support and cable bolts were not sufficient. Contractometers were utilized inside the pillar to measure deformation.

Data from contractometers can be gathered manually, one at a time by a staff member using MDT’s SMART Reader, or automatically using a SMART Log or SMART Log3. A better data gathering method, though, is increasingly being used. This method consists of connecting the individual contractometers three at a time to wireless nodes (e.g. the MDT-RTU), which then form a wireless mesh network. The wireless network connects at one or more points to the mine’s existing LAN network via Ethernet gateways.

Mine engineering staff can access the data by logging in to the network, or even access it remotely via web interface if required. Custom software, like the MDT MineMonitor or MineHop packages, is used to interpret and display the data.