Breakthrough in Measuring Stress

Measuring stresses in hard rock has always been a big problem for rock mechanics engineers. Dr Gerhard Herget should know. The research scientist at the Canada Centre for Mineral and Energy Technology (CANMET), in Ottawa, has been measuring stresses in mine pillars for about 20 years now. All the instruments he has used as a rock mechanics professor in Kentucky and as a scientist at CANMET’s mining research labs have one major drawback, however. The glue used to cement the instruments in a borehole tends to creep with time. Because of this, these instruments do not offer the precision over extended periods of time that mine engineers need to accurately predict changes in rock stresses as mining progresses. Also, the instruments tend to get knocked around when a blast is set off in an adjacent stope and have a tendency to detach. There was always a 10-20-MPa “grey area” when we used these instruments, Herget says. By combining several ideas developed at CANMET, Herget and his lab technologist Frank Kopeller have come up with an instrument which is mechanically wedged in the borehole yet which can be retrieved after testing is complete. Herget says this instrument is “a breakthrough in high- precision and high-resolution monitoring.” By measuring the frequency of a continuously vibrating piano wire (5-9 cm long and mounted in a stiff, steel ring), the instrument can detect movement as minute as 50 micrometres or five one-hundred-thousandths of a millimetre.

Field tests at the Niobec mine in Quebec between July, 1986 and March, 1988 have proven him right. Having made accurate measurements of stresses in mine pillars there, Herget can say with confidence that mine engineers could make pillars in the niobium mine a bit smaller.

The instrument has since been installed at the Creighton mine in Sudbury, and the Montauban mine in Quebec. The unit, which is relatively inexpensive and rugged, is being marketed by Roctest of Montreal.

Herget is now working on an instrument which can measure strain in the axial direction of the borehole and one for measuring strain in softrock mines.