Open Pit Slope Design of Ajabanoko Iron Ore Deposit, Kogi State, Nigeria

Journal of Mining World Express, Volume 3 2014 www.mwe‐journal.org doi: 10.14355/mwe.2014.0302.01

Open Pit Slope Design of Ajabanoko Iron Ore Deposit, Kogi State, Nigeria Adebimpe, R.A1*, Akande, J.M2 and Arum, C3 Department of Mineral Resources Engineering. The Federal Polytechnic, Ado‐Ekiti, Nigeria Department of Mining Engineering. The Federal University of Technology, Akure, Nigeria Department of Civil Engineering. The Federal University of Technology, Akure, Nigeria rasheed4u1@yahoo.com; 2akandejn@yahoo.com; 3arumcnwchrist@yahoo.co.uk

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Received 8 March 2013; Revised 2 January 2014; Accepted 9 January 2014; Published 16 April 2014 © 2014 Science and Engineering Publishing Company

Abstract An appropriate open pit slope design is the one that considers both the safety and cost aspects to minimize waste excavation. Open pit slope design of Ajabanoko iron ore deposit, Nigeria was carried out using DIPS and SLOPE/W mine softwares. Compass clinometer and Geographic Positioning System (GPS) were used to determine the dip, dip direction and location respectively of joints on the iron ore deposit and this serves as an input data in the slope design. Fifty‐two dip/dip direction values were plotted using DIPS mine software. The factor of safety of the designed slope was determined using limit equilibrium method included in the slope SLOPE/W program. In order to calculate the factor of safety for the designed slopes, inputs parameters, namely unit weight of the rock mass, internal friction angle and cohesion were used in the SLOPE/W program. The obtained result indicates that slope angles with orientation of 44/071, 50/270, 61/359, 46/178 were considered safe for the eastern, western, northern and southern section of the deposit respectively. The slope design carried out shows a factor of safety that varies from a lower limit of 2.89 to an upper limit of 3.84, and this indicates safe slopes in all sections of the deposit.

Keywords Open Pit; Slope Design; DIPS; SLOPE/W; Factor of Safety; Slope Angle.

Introduction Over the years slope design has become the domain of specialist geotechnical practitioners (Tebrugge et al., 2008). Slope design is an integral part of open pit planning and requires an adequate knowledge of rock mass characteristics and the type of discontinuities that are dominant on the deposit. Over the years the slope design process in large open pit mine, has been hampered by critical gaps in our knowledge and

understanding of the relationships between the strength and deformability of rock masses and the likely mechanism of failure (Read and Ogden, 2006).

While this has benefited the technology of the slope design processes, it has also alienated the responsibility of the risk versus reward relationship from the mine design engineer (Tebrugge et al., 2008).

Increasingly more ore characteristic data are required for the design of safe and cost effective slope in the mines. This is because these data are used as input in most of the commercially available slope design software. Variability in rock mass conditions can be as a result of major geological structures, large fault zones, and areas of closely spaced jointing, geological structures carrying water, weak rock, intense alteration and excessive rock bridges (Dempers et al., 2011).

Basically limit equilibrium method makes use of the Mohr‐Coulomb equation to determine the critical slip surface. The equation is defined by the friction angle and the cohesion. However, the idea of discretizing a potential sliding mass into vertical slices was introduced early in the 20th century (Krahn, 2003).

There are numbers of approaches to assess the behaviour of rock slope using different modelling methods like limit equilibrium, analytical and kinematic tools, physical and numerical models as well as intelligent models (Verma et al., 2011). The numerical methods allow the analysis of slope stability problems involving complexities related to geometry, material anisotropy and non linear behaviour (Li et al,.

2009; Kainthola et al., 2011). Numerical methods such as the Finite Element Method (FEM) have now been successfully applied to slope stability analysis over the years (Kainthola et al., 2012). Because of certain

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advantages of finite element methods, like no assumptions needs to be made in advance about the location of failure surface, it has been widely used for slope stability analysis over traditional equilibrium methods (Verma et al., 2013).

developed for stability analysis of slopes includes (i) Ordinary Method (OM) (Fellenius, 1936) (ii) Bishop’s Simplified Method (BSM) (Bishop, 1955) (iii) Janbu’s Methods (Janbu, 1954) (iv) Lowe‐Karafiath’s Method (Lowe and Karafiath, 1960) (v) Corps of Engineers Method (US Army Corps of Engineers, 1967) (vi) Sarma Method (Sarma, 1973) (vii) Morgenstern‐ Price Method (Morgenstern and Price, 1965) and (viii) Spencer’s Method (Spencer, 1967). Limit equilibrium methods use representative geometry, material and/or joint shear strength, material unit weights, groundwater and external loading/support conditions to determine slope safety factors based on a set of simplifying mechanical assumptions (Lorig et al., 2010).

Limit equilibrium methods consist in cutting the slope into fine slices so that their base can be comparable with a straight line then to write the equation (Baba et al., 2012).

Limit equilibrium formulations based on the method of slices are also being increasingly applied to the stability analysis of structures such as tie‐back walls, nail or fabric reinforced slopes, and even the sliding stability of structures subjected to high horizontal loading arising, for example, from ice flows (Krahn, 2003). A reliable geotechnical model is the cornerstone of all slope design (Read and Ogden, 2006). From a reviewer’s perspective slope designs must not only be technically sound, but also address the broader context of the mining operation as a whole, taking into account such factors as safety aspects, the available equipment to implement the designs, and the acceptable risk levels for the company (Stacey, 2006).

Factor of safety should consider both the technical and the cost aspect of the slope design.

Discrete element method (DEM) treats the problem domain as an assemblage of distinct, interactive bodies or blocks subjected to external loads and are expected to undergo significant motion with time (Eberhardt, 2003). Numerical methods such as the Finite Element Method (FEM) have now been successfully applied to slope stability analysis over the years (Kainthola et al., 2012). Continuum approaches used in slope stability include the finite‐difference and finite‐element methods, in both these methods the problem domain is divided (discretized) into a set of sub‐domains or elements (Eberhardt, 2003). Due to its power and flexibility, the finite element method (FEM) is increasingly being applied to slope stability analysis (Hammah et al., 2004). The FEM offers a number of advantages over traditional method‐of‐slices analysis (Griffith and Lane, 1999) which include: (i) elimination of priori assumptions on the shape and location of failure surface (ii) elimination of assumptions regarding the inclinations and location of interslice forces (iii) capability to model progressive failure (iv) calculation of deformations at slope stress levels, and (v) robustness ‐ ability to perform successfully under a wide range of conditions. In addition, Trivedi et al (2012) used FLAC/Slope to perform factor‐of‐safety calculation for slope stability analysis in limestone mines.

Major input variables for calculating FOS are unit weight, internal friction angle and cohesion (Han and Kim, 2003). The friction angle and the cohesion are usually determined using the shear box assembly. The FOS determines how safe a slope is. The factor of safety can be defined in three ways: limit equilibrium, force equilibrium and moment equilibrium (Abramson, 2002). The optimum slope design of a pit requires the determination of the most economic pit limit that normally results in steep slope angle as in this way the excavation of waste is minimized (Steffen et al., 2008). In general, as the slope becomes steeper, the stripping ratio (waste to ore ratio) is reduced and the mining economics improves (Steffen et al., 2008).

Surface deformations and slope stability are very important issues to the open pit mining industry (Jarosz and Wanke, 2003). Large amount of surface deformations can lead to slope instability. Slope instability can be a potential source of danger for people and equipment, also it disrupt mine scheduling and increase the cost of mining production (Lilly et al., 2000; Kido et al., 2000; Bromhead, 1992). To deal with these slope stability issues, various approaches have been adopted and developed over the years however, the approaches have been more of computational rather than manual (Trivedi et al., 2012).

Location and Geology of Ajabanoko Iron Ore Deposit The study area for this project is Ajabanoko, located at Okene, Kogi State, Nigeria. Ajabanoko Iron Ore deposit is on longitude 60 14’ 0”E and latitude 70 33’ 0”N and lies 4.5 km Northwest of Itakpe hill.

Slope Stability Analysis Methods Limit equilibrium method (LEM) is the most widely used method for slope stability analysis. The LEM

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The Ajabanoko deposit area falls within the Nigerian Precambrian basement complex, a suite of crystalline rocks exposed in over nearly half of the country extending west into Dahomeyan of Benin Republic and east into Cameroon (Amigun and Ako, 2009). The Ajabanoko area consists of a set of three closely related hills of basement rocks in which some large bands of iron ore occur. The dominant lithologic units of Ajabanoko deposit area are gneiss of migmatite, biotite and granite, ferruginous quartzites, granites and pegmatite (Amigun and Ako, 2009). The ferruginous quartzite is the source of the iron ore mineralization in the area (Olade, 1978).

clinometers and measuring tape. In addition, Geographic Positioning System (GPS) was also used to determine the location of the area of the deposit mapped. A 1 m scaline was established on the deposit, joint set within the scaline were captured and the joint spacing measured. DIPS, slope design software were used to design the slope of the designed pit. Fifty‐two dip/dip direction values were plotted on the stereonet using DIPS software at a magnetic declination of 10 35’ West.

The cohesion value, C and friction angle, Ф was obtained from direct shear test conducted on the rock samples using shear box assembly.

The nature of Ajabanoko iron ore deposit and the associated rocks indicate that they are residual concentrates derived from iron rich sediment, a volcanogenic sedimentary material (National Steel Development Authority, 1976). This suggests that all the rocks in the area including the high grade metamorphic ones such as the gneisses and the low grade metamorphic ones such as the quartzites may have been derived from sedimentary materials which in turn were probably derived from an ancient volcanic source (National Steel Raw Materials Exploration Agency, 1994). Four principal ore layers have been identified as the different ore zones (Nnagha, 1997). Four thick bands ranging from 1 m to 5 m in thickness and measuring 1.22 km along strike have been identified in the deposit, and are classified as orebody I, orebody II, orebody III and orebody IV as shown in Table 1 (National Steel Raw Materials Exploration Agency, 1994). Petrological studies of the ore have revealed four major types of ore composition similar to Itakpe Hill, Itakpe Nigeia: (i) magnetite quartzites (ii) magnetite‐hematite quartzites (iii) hematite‐magnetite quartzite (iv) hematite‐quartzite.

The sum total of iron ore reserves in the entire deposit is 62.104 million tons.

SLOPE/W software was used to determine the minimum factor of safety of the designed slope.

Morgenstern‐Price method using the half‐sine was used to make piezometric line as the pore water pressure condition. The Morgenstern‐Price method can use interslice force functions such as (i) constant (ii) half‐sine (iii) clipped sine (iv) trapezoidal and (v) data point specified. Entry and exit method was used to determine the critical slip surface. The input data used for the slope stability analysis in SLOPE/W were friction angle, cohesion and unit weight which were entered in layers (bedwise).

The minimum factor of safety (FOS) determined is for force equilibrium. The reason why we use Morgenstern‐Price method (Morgenstern and Price, 1965) is that it allows selection for interslice force function and computes FOS for both force and moment equilibrium. SLOPE/W operates based on three commands. Define which is used to define both the region and profile of the ore; Solve application is used to solve the problem display computed result. In order to calculate the factor of safety for the designed slopes, inputs parameters such as unit weight of the rock mass, internal friction angle and cohesion were used in the SLOPE/W program.

TABLE 1 PARAMETERS OF THE MAIN ORE LAYER OF AJABANOKO IRON ORE DEPOSIT

Results and Discussion

Ore layer

Length along strike(m)

Average thickness(m)

Average Fetot

Orebody I Orebody II Orebody III Orebody IV

1100 925 750 ‐

14.7 10 3.6 4.3

40.4 30.3 37.28 34.04

Slope design software (DIPS) was used for the design of the slope angle at the four major sections of the mine. Figure 1 shows the pole plot of the joint data.

Figure 2 shows the pole contour of the discontinuities.

The mean orientation of joint sets 1, 2, 3, 4, 5 and 6 are as tabulated in Table 2 below. The friction angle used for the slope design is 360 which is the friction angle obtained for the hematite‐magnetite iron ore using shear box assembly for the direct shear test. Using a slope angle of 60/090 for slope, which dips to the East of the deposit, could lead to potential wedge failure of

National Steel Raw Material Exploration Agency (1994)

Methodology Geologic data of joints such as dip, dip direction and joint spacing values were measured using compass

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intersection of planes 1 and 2, and planes 2 and 5. It can also lead to planar failure of plane 2 as shown in Figure 3. In order to prevent any of these failures, it is good to use a slope angle with orientation 44/071 as shown in Figure 4.

TABLE 2 ORIENTATION OF JOINT SET (FIRST AND SECOND CORNER)

Mean joint set

Orientation of joint set (first corner)

Orientation of joint set (second corner)

1 2 3 4 5 6

53/112 41/50 62/206 45/257 54/167 07/218

78/153 64/096 81/241 68/293 70/189 30/282

FIGURE 4 DESIGNED SLOPE ANGLE AT 440

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 1 POLE PLOT OF JOINT DATA

FIGURE 2 POLE CONTOUR OF JOINT DISCONTINUITIES

FIGURE 5 POTENTIAL WEDGE FAILURE AT THE SLOPE FACE DIPPING WEST

FIGURE 3 POTENTIAL WEDGE FAILURE OF SLOPE

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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 6 DESIGNED SLOPE FACE OF DISCONTINUITIES DIPPING WEST

FIGURE 7 DESIGNED SLOPE FACE OF DISCONTINUITIES DIPPING NORTH

FIGURE 10 FACTOR OF SAFETY FOR SECTION WHERE IRON ORE 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.

FIGURE 8 POTENTIAL WEDGE FAILURE OF SLOPE FACE DIPPING SOUTH

FIGURE 9 DESIGNED SLOPE ANGLE OF DISCONTINUITIES

FIGURE 11 FACTOR OF SAFETY FOR SECTION WHERE GRANITE GNEISS IS DOMINANT

DIPPING SOUTH

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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.

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Adebimpe, Rasheed Adeshina was born in Ibadan of Oyo State, Nigeria. The author obtained B.Eng Mining Engineering of The Federal University of Technology, Akure, 1990, M.Eng, mine environment of The Federal University of Technology, Akure, 2004. He is currently a Chief Lecturer in the Department of Mineral Resources Engineering, The Federal Polytechnic, Ado‐Ekiti, Nigeria.

Engr. Adebimpe is a member of Nigerian Society of Engineers and member Nigerian Society of Mining Engineers. The area of research interest is in mine environment, mine planning and design.

Professor of Mining Engineering at The Federal University of Technology, Akure, Nigeria. Dr Akande belongs to many professional bodies such as Nigerian Society of Engineers and Nigerian Mining and Geosciences Society. His research interest is in mine design and mine mechanization.

Arum, Chinwuba was born in Enugu of Enugu State, Nigeria. The author is of the Civil Engineering Department of the Federal University of Technology, Akure and a Visiting Associate Professor and Founding Dean of the Faculty of Engineering, Elizade University, Ilara‐Mokin, Nigeria. He obtained a PhD degree from Moscow Civil Engineering Institute, Moscow with specialization in Structural Engineering. Dr Chinwuba is a member of Nigerian Society of Engineers. He has published widely in Learned Engineering Journals and Conference Proceedings.

Akande, Muili Jide was born in Nguru, Nigeria. The author obtained M.Sc and PhD Degrees in Mining Engineering from Donetsk National Technical University, Ukraine between 1979‐1984 and 1990‐1993 respectively. He is an Associate

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