National Steel Raw Material Exploration Agency (1994) TABLE 1 PARAMETERS OF THE MAIN ORE LAYER OF AJABANOKO IRON ORE DEPOSIT TABLE 2 ORIENTATION OF JOINT SET (FIRST AND SECOND CORNER) FIGURE 3 POTENTIAL WEDGE FAILURE OF SLOPE FIGURE 5 POTENTIAL WEDGE FAILURE AT THE SLOPE FACE DIPPING WEST When the designed pit dips to the West of the deposit and uses a slope angle with orientation 63/270, there are potential wedge failures of intersection of planes 3 and 4, and intersection of planes 4 and 6 as shown in Figure 5. In order to prevent plane failure and wedge failure at this section of the mine, a slope angle with orientation 50/270 was used as shown in Figure 6. When the designed pit dips to the north, there is no potential failure; the slope angle can be as steep as 60° or more. However, slope angle of 61/359 was used as shown in Figure 7. Setting the slope angle at 68/179 would lead to potential wedge failure of the intersection of mean joints 3, 6 and 1 when the designed pit dips to the south as shown in Figure 8. To eliminate this, a slope angle of 46/178 was used as shown in Figure 9. In addition, because the deposit trend towards north-east and south-west direction, it is important that the slope direction follow this direction i.e. parallel so that the slope does not cut into the iron ore but parallel to the ore. This would allow more iron ore deposit to be mined and little waste to be excavated. FIGURE 4 DESIGNED SLOPE ANGLE AT 440 FIGURE 6 DESIGNED SLOPE FACE OF DISCONTINUITIES DIPPING WEST FIGURE 7 DESIGNED SLOPE FACE OF DISCONTINUITIES DIPPING NORTH FIGURE 8 POTENTIAL WEDGE FAILURE OF SLOPE FACE DIPPING SOUTH FIGURE 9 DESIGNED SLOPE ANGLE OF DISCONTINUITIES DIPPING SOUTH FIGURE 11 FACTOR OF SAFETY FOR SECTION WHERE GRANITE GNEISS IS DOMINANT Factor of safety is defined as the ratio of the total force to resist sliding to the total force tending to induce sliding. Other sections of the deposit where granite and gneiss are predominant have their FOS as 3.05 and 2.89 respectively as shown in Figures 11 and 12. It is evident from the table below that as the friction angle increases the FOS decreases as shown in Table 3. These values further show that mineral deposit containing various rock types must have high frictional values to maintain stable slopes. values further show that mineral deposit containing The slope geometry in an open pit mine is influenced by the factor of safety, blasting pattern and other ore characteristics such as friction angle and the unit weight of the overburden. The ore characteristics themselves are governed by the discontinuities such as folds and faults. The factors, which mainly influence the stability of a typical opencast slope, are the shear strength parameters of slope forming material, the presence and characteristics of structural discontinuities in the slope mass and the ground water conditions (Singh and Monjezi, 2000; Singh et al., 2008) The factor of safety (FOS) is a tool used to define the stability of rock slopes. FOS of slopes where iron ore is predominant using the limit equilibrium analysis of the SLOPE/W program is 3.84 which is greater than 1, this is the condition of limit equilibrium in which the resisting and disturbing forces are equal and the factor of safety, F = 1 as shown in Figure 10. FIGURE 10 FACTOR OF SAFETY FOR SECTION WHERE IRON ORE IS DOMINANT Conclusions Slope angles of 44°, 50°, 61°, and 46° were considered safe for the Eastern, Western, Northern and Southern section of the open pit. The result of the stability analysis carried out for the designed pit shows that Factor of Safety (FOS) varies from a lower limit of 2.89 to an upper limit of 3.84. This is higher than 1, considered as the bench mark for stable slope and this indicates stable conditions for the designed slopes. To eliminate slope failures, mine operators should not use slope angle less than 44” to ensure safe open pit while the slope angle at the Northern part of the pit can reach an upper limit of 61°. Efforts should also be made to ensure the maintenance of the slope angle as recommended above and the establishment of an appropriate drainage system for the open pit. 445462, 1936. FIGURE 12 FACTOR OF SAFETY WHERE BIOTITE GNEISS IS DOMINANT TABLE 3 FACTOR OF SAFETY FOR AJABANOKO IRON ORE DEPOSIT
