Wall to wall stability

Monitoring rock slopes is essential to the success of most op mines. Slopes may become unstable during blasting, during spring thaw or because of geological factors such as an undetected layer of kaolinitized sandstone, for example, which can play havoc with the distribution of stresses in a pit wall.

Wherever possible, pit slopes are steepened to increase ore recovery and reduce the handling of overburden. But by doing so, mining companies increase the risk of harming workers in the pit, mining equipment and, consequently, ore production. Various wall stability monitors have been tried in the past with varying success. Micro-seismic monitoring, for example, was developed by the U.S. Bureau of Mines and test-marketed briefly in Canada in 1986 by RETOM Geo-Research & Engineering of Calgary, Alta. “The system still exists,” says Thomas Vladut of RETOM, “but there hasn’t been sufficient support for it in the industry.” Macro-seismic, blast monitoring type methods have fared much better. To monitor groundwater levels and hydrostatic pressures, pieziometers are often installed in boreholes around the pit.

The simplest and least expensive way to monitor rock slopes, however, is the geodetic survey method, or electronic distance measuring (EDM). If a visible target can be installed on a pit wall, then wall stability can be monitored by this method. And EDM lends itself well to computerized data analysis, storage and graphic output. Quebec Cartier Mining (now owned by Dofasco) has installed a survey system operating 24 hours a day in two separate pits at the company’s big Mt. Wright iron ore operation in northeastern Quebec. The system cost about C$700,000 and may be unique in mining. Prof. Garston Blackwell of Queen’s University in Kingston, Ont. was hired by Quebec Cartier as a project consultant. He developed the programs for the computer that the system employs. “It’s really an insurance policy,” says Richard Bowan, a mining analyst for Quebec Cartier.

The company lost between $5 million and $6 million worth of iron ore when it closed a small pit because of unstable walls, he says. However, without the system, the company probably would have continued to mine that deposit, without knowing the risk involved to men and equipment.

The system consists of two total station survey instruments manufactured by Geodimeter of Sweden: a radio telemetry system and a personal computer capable of color graphics for data analysis. Each instrument is equipped with two motors, Blackwell explains. One rotates the theodolite on a horizontal plane and the other on a perpendicular plane. The computer activates the motors, which point the instrument in the general direction (azimuth and dip) of an individual retroreflector on the pit wall. Once the instrument is positioned, the computer directs it to execute routines to pinpoint the target’s exact location. The instrument scans the target horizontally and then moves up and down over the target. A distance-finder on the instrument transmits infra-red energy to the target during the routine. Infra-red energy reflected by the prism-like target back to the theodolite is measured as the instrument scans across the target. The computer draws two graphs showing the amount of infra-red energy reflected at each point along the scanned distasta. The computer then instructs the survey instrument to point to the exact spot where the maximum amount of reflected energy is indicated by the two graphs. The azimuth and distance to this point are then measured and recorded. Measurements automatically compensate for weather conditions such as atmospheric pressure and temperature.

If the instrument can not find a given target, the control computer instructs the survey instrument to execute a search. This consists of scanning a diamond-shaped area, the dimensions of which systematically increase until a reflected signal is detected. To determine if the target has moved, the azimuth and distance are compared with the previously recorded azimuth and distance. If the difference is greater than a predetermined limit, an alarm sounds and an engineer inspects the field target for excessive ground movement. The software allows operators to accelerate the data to determine the direction and consequences of detected movement, Bowan says. The movement of large blocks of ground are plotted and overlain with various geological information. It takes up to six minutes for the instrument to obtain a target reading.

Quebec Cartier has a total of 67 retroreflectors, monitored by two instruments in two pits. Instructions from the computer and data from the instruments are transmitted by radio telemetry to and from the mine office, where the personal computers are based. Four targets have also been positioned well outside the pit perimeter. These serve as control points o determine whether the instrument station itself is moving. The maximum height of walls at Mt. Wright is 150 metres.

The open pit portion of the Doyon gold mine in northwestern Quebec is about 25 metres deeper than Quebec Cartier’s Mt. Wright pits. MiMing of the main pit was completed last year and the company is now designing a smaller open pit. Monitoring of the south wall of the pit played an important role in the design of Doyon, says Dr. Trevor Carter, an associate with Golder Associates of Mississauga, Ont., who designed the open pit .

The weak, shistose nature of the hangingwall demanded close monitoring from the outset. Pieziometers were installed in boreholes around the pit perimeter and later, as mining progressed, in boreholes on pit benches. Together with this monitoring, LAC Minerals installed an EDM system using Wild survey instruments, manufactured in Switzerland. The initial design called for a pit slope of 45deg, but several toppling failures of individual benches eventually forced the company to reduce that angle to about 40deg.

To prevent further toppling failures, Golder came up with an elaborate system of artificial support consisting of 6-metre Swellex bolts and steel straps.

This spring, as the frost leaves the ground, engineers will be taking a close look at the stability of their open pit walls. Perhaps previous operating experiences will help them design remedial measures to ensure the safety of the men and equipment employed in open pit operations.