Occupational safety and health

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The Cuiabá Expansion project in Brazil will involve the extraction of deeper blocks of ore (about 1,562 m below surface). Among the major infrastructural developments planned as part of the project, a major upgrade to the ventilation system is currently under way to cater for the projected increase in production from deeper levels (and hence the increased load on the air supply) and a consequent increase in diesel-powered mining equipment.

A study conducted by independent mine ventilation consultants in conjunction with AngloGold Ashanti Technical Services and the technical team at Cuiabá, indicated the future demand in air quality and quantity for ventilation and cooling purposes as a result of the expansion of mining areas. The study determined that the two new ventilation shafts would be required to be raise-bored between Level 11 (770 m below surface) and the surface, with one shaft to serve as a main intake downcast airway, and the second as a main return airway for upcast air, both of 5.1 m in diameter as recommended diameter.

Raiseboring, which in this case consisted of the upward drilling of a tunnel through the Nova Lima Group rock formation, from one position underground to surface, has been challenging. A number of technical features have had to be considered in the process and an extensive risk assessment exercise was undertaken.

Previously, raisebored smaller-diameter holes showed a tendency to deviate significantly from a projected path with the deviations caused by, among others factors, dense and highly foliated schist formations. The possibility of large deviations occurring while drilling the pilot-hole that guides the raiseboring cut was an important concern. The impact of deviation could certainly compromise integrity during reaming operations and eventually shaft stability.

The risk of large deviations occurring during the required 770 m-deep pilot-holes (up-cast and down-cast holes) was mitigated by using active deviation-correction technology in the Rotary Vertical Drilling System (RVDS). This instrument allows for corrections to be carried out while drilling takes place by systematically computing changes in the original trajectory. This instrument is pre-programmed to enforce that the drilling trajectory follows a given path and is activated when deviations from vertical reach a set value. The control permits drilling accuracy up to a 0.05%. The RVDS deviation control instrument was used in the drilling of about 50% of the full length of the first raisebored upcast hole. Its use resulted in a deviation of only 13 m from a planned holing position, quite within the acceptable range. To further minimise deviations in the second ventilation hole, a RVDS instrument will be used when raiseboring the entire length of this hole, i.e. over 770 m.

The independent ventilation study recommended that the upcast ventilation hole should have an optimum diameter of 5.10 m over its collar on surface. A comprehensive geotechnical risk assessment was carried out to assess the risks associated with the raiseboring of the two ventilation holes.

The geotechnical surveys carried out revealed poor ground conditions near surface, where various layers of weathered and weakly competent rocks prevail. Risk of instability arising from poor ground conditions was thus identified. From a geotechnical view point, therefore, it would be ideal not to affect the integrity of a raisebored excavation with further enlargement operations, in particular with the use of explosives.

On the basis of the geotechnical stability analysis, it was recommended that a diameter of 4.8 m for the up cast shaft would be preferable.

Though the reduction from 5.1 to 4.8 m in shaft diameter is optimal from the point of view of excavation stability, the adopted size should not compromise the previously designed ventilation capacity. Indeed, a suitable ventilation system model was run, which indicated that a marginal loss in air volume would result from the reduction in shaft diameter. However, it was found that the magnitude of this loss could be compensated by routing air through currently existent airways. As a result, it was decided to retain the 4.8 m size as the final diameter for the ventilation shaft.

For further information on the safety and health aspects of underground ventilation, (see Multimillion dollar refrigeration project for Obuasi).