Mining iron ore under difficult pit wall conditions at Ferteco Fabrica Mine, Brazil.
CIM Montreal 2003
Gerrit Vos, Carlos Nieble,
Ferteco Mineracao, belonging to the CVRD group, is one of the several iron ore mines in the south east of Brazil. Some of these mines have pit walls of over 350-m height and operations under difficult geotechnical conditions are steadily increasing.
In order to mine the 15 million tonnes of high grade hematite ore remaining in the Fabrica pit, Ferteco had to deal with the following restrictions: warrant power to its concentrator and pellet plant, relocate the load-out rail road loop, stabilize a poor phillyte slope, relocate a creek on a bench, lower the water table and mine at maximum angles at a slope with weathered phillytes and poor quality itabirites directly under the rail road loop.
Considering these aspects, 3 stages of operations were designed, guarantying a safe operation, but with the lowest possible security factors at the last mining stage, before the pit would be filled in with tailings.
Stage one above the rail road loop and below the high tension power transmission line mast, where the poor phillyte slope was secured by a flexible retaining wall, using old rails and anchors up to 27 meters (80ft) with working loads of up till 30 tonnes. Stage two below the train road loop, was related to mine with different slope angles, deep wells, high wall blasting perpendicular to the highwall with relief holes and re-routing the creek in steel half culvert. Stage three considered the filling in with tailings and recuperation of the deep well pumps.
The following studies and analysis were made: geotechnical and geological studies of all rock formations including rock laboratory tests, a hydrological study to define the lowering of the ground water level, slope stability analysis for every bench and all possible combination of benches, definition of blasting parameters close to the pit slope, a monitoring plan, including topographical movements, vibrations due to blasting, piezometers and pressure cells as well as a plan to fill up the pit with tailings in combination with the rising of the ground water level due to the interruption of pumping.
A close co-operation between the geotechnical consultant, pit operators, the blasting contractor and the geological department resulted in an operation without any incidents,
lowering the pit with an additional 3 benches below the initial planning and consequently mining an additional 5 million tonnes.
Slope stability, de-watering, geotechnical, Open pit mining, Blasting, retaining wall, Tailings