Open access peer-reviewed chapter

# Open Pit Mining

Written By

Awwad H. Altiti, Rami O. Alrawashdeh and Hani M. Alnawafleh

Submitted: October 8th, 2019 Reviewed: March 20th, 2020 Published: February 17th, 2021

DOI: 10.5772/intechopen.92208

From the Edited Volume

## Mining Techniques

Edited by Abhay Soni

Chapter metrics overview

View Full Metrics

## Abstract

Open pit mining method is one of the surface mining methods that has a traditional cone-shaped excavation and is usually employed to exploit a near-surface, nonselective and low-grade zones deposits. It often results in high productivity and requires large capital investments, low operating costs, and good safety conditions. The main topics that will be discussed in this chapter will include an introduction into the general features of open pit mining, ore body characteristics and configurations, stripping ratios and stripping overburden methods, mine elements and parameters, open pit operation cycle, pit slope angle, stability of mine slopes, types of highwall failures, mine closure and reclamation, and different variants of surface mining methods including opencast mining, mountainous mining, and artisan mining.

### Keywords

• open pit mine
• slope stability
• mine reclamation
• stripping ratio
• production cycle

## 1. Introduction

Open pit mining is defined as the method of extracting any near surface ore deposit using one or more horizontal benches to extract the ore while dumping overburden and tailings (waste) at a specified disposal site outside the final pit boundary. Open pit mining is used for the extraction of both metallic and nonmetallic ores. Open pit mining is considered different from quarrying in the sense that it selectively extracts ore rather than an aggregate or a dimensional stone product.

Open pit mining is applied to disseminated ore bodies or steeply dipping veins or seams where the mining advance is toward increasing depths. Backfilling usually occurs until the pit is completed; even then, the high cost of filling these pits with all of the waste removed at the end of the mine life would seriously risk the project’s economics. Few large open pits in the world could support such a costly obstacle. Open pit method is usually nonselective, and it includes all high and low-grade zones; whereas mining rate is nearly over 20,000 tons mined per day and often necessitates a large capital investment but generally results in high productivity, low operating cost, and good safety conditions [1]. The main purpose of this chapter is to discuss the general features of open pit mining, ore body characteristics and configurations, stripping ratios and stripping overburden methods, mine elements and parameters, open pit operation cycle, pit slope angle, stability of mine slopes, types of highwall failures, mine closure, and reclamation. The chapter will also discuss different variants of surface mining methods including opencast mining, mountainous mining, and artisan mining.

### 1.1 Features, technical and economic indicators of open pit development

Compared to underground mining methods, the open pit mining method requires removing significant amount of overburden from the pit and moving it outside the mine. The cost of extraction of the ore from open pit constitutes the bulk of the total cost of mining operations, because the access to the ore body is so fast and requires less time compared to underground mining, i.e., extracting the ore below overburden can only begin with some lag time from the start of removing overburden. Also, open pit has virtually an unlimited ability to create and use high-performance large-sized mining and transportation equipment that can provide the highest technical and economic parameters. Open pit mining has higher productivity (3–5 times of underground methods), lower production costs, more safe and hygienic working conditions, more complete recovery of a mineral, and lower per unit production cost.

Open pit mining is characterized not only by its high share of total minerals production, but it is also considered as one of the surface mining methods that contributes to the construction of powerful performance quarries (100–150 million tons of rock a year reaching to a depth of 500 m). Capital cost of such huge open pits/quarries is very high, and the total cost for excavation of rock in the long term reaches hundreds of millions of dollars or more. Therefore, decisions on the construction of new or existing quarries should be economically justified. Table 1 shows the advantages and disadvantages of open pit mining method [2, 3].

High productivity, i.e., highly mechanized and labor conserving (around 100–400 tons per employee-shift including both ore and waste)Limited by depth ∼500 m; technological limit imposed by equipment; and deposit beyond pit limits must be mined underground or left in place
High production rate (essentially unlimited, although small surface mines also possible)Limited by stripping ratio
Lowest cost along with open cast miningHigh capital investment associated with large equipment
Early production, development can be programmed to permit early start-upSurface damaged may require reclamation; a bond has to be added to the production cost
Low labor requirement; can be unskilled except key operators (e.g., drill, shovel)Requires large deposit to realize lowest cost, unless very high grade
Relatively flexible; can vary output if demand changesWeather can be detrimental; it can impede operations.
Suitable for large equipment; permit high productivitySlope stability must be maintained; proper design and maintenance of benches plus good drainage are essential
Fairly low rock-breakage cost (drilling and blasting); superior to underground mining where bench faces are less easily maintainedRequires provision of large waste disposal/dump area
Simple development and access; minimal openings required although advanced stripping may be considerable
Little if any bank support required; proper design and maintenance of benches can provide stability
Good recovery; good health and safety; no underground hazards

### 1.2 Ore body characteristics and configurations

Open pit mining is widely used with metallic ore bodies (aluminum, bauxite, copper, iron), and nearly all nonmetallic (coal, uranium, phosphate, etc.). It is a traditional cone-shaped excavation (although it can be of any shape, depending on the size and shape of the ore body) that is used when the ore body is typically pipe-shaped, vein-type, and steeply dipping stratified or irregular [4]. The major open pit and ore body configurations are classified into the following:

• Flat lying seam or bed, flat terrain (e.g., platinum reefs, coal), as shown in Figure 1.

• Massive deposit, flat terrain (e.g., iron-ore or sulfide deposits), as shown in Figure 2.

• Dipping seam or bed, flat terrain (e.g., anthracite), as shown in Figure 3.

• Massive deposit, high relief (e.g., copper sulfide), as shown in Figure 4.

• Thick-bedded deposits, little overburden (e.g., iron ore, coal) as shown in Figure 5.

### 1.3 Stripping ratio

The parameter known as the stripping ratiorepresents the amount of uneconomic material that must be removed to uncover one unit of ore, i.e., the ratio of the number of tons of waste material removed to the number of tons of ore removed. Also, the ratio of the total volume of waste to the total volume of ore is defined as the overall stripping ratio.A lower stripping ratio means that less waste has to be removed to expose the ore for mining which generally results in a lower operating cost [5]. The major types of stripping ratios are overall, instantaneous, and break-even.

In order to specify the maximum allowable stripping ratio (SRmax) of a surface mine, break even ratio can help to establishes the pit limits. SR max defined as the ratio of overburden to ore at the ultimate boundary of the pit, where the profit margin is zero. It can be calculated as:

SRmax=Value oforeProduction cost/Stripping costE1

or,

SRmax=Stripping allowance$/ton/Stripping costm3/tonE2 The maximum allowable stripping ratio enables us to locate the ultimate pit boundary or limit based on prevailing economic, physical, and geometric conditions in the pit. A copper pit designed in this manner with varying ore grades and critical SRmax ∼ 2.5 m3/ton (3.0 yd3/ton) is shown in Figure 6. Ore occurring in the ore body beyond this maximum stripping ratio will have to be left or mined underground. Example 1: A seam of coal has a density of 1.36 t/m3 and is 2.5 m thick. It is covered by 27 m of shale which has a density of 1.7 t/m3. Calculate the stripping ratios? Solution: • Volumetric stripping ratio= 27/2.5 =10.8 m3 of overburden per m3 ore • Weight stripping ratio= (27 × 1.7)/(2.5 × 1.36) = 13.5 tons of overburden per ton ore • Stripping ratio= 27/(2.5 × 1.36) = 7.9 m3 of overburden per ton ore Example 2: The head assay of a copper ore is 0.8% Cu. The expected overall copper recovery from the ore is 88%. Calculate the maximum stripping ratio if the total cost of production (excluding overburden removal) is$5.90 per ton of ore and overburden removal costs are $0.3 per ton of waste. Assume copper values of$1.00, $1.25, and$1.50 per kg of refined metal at the smelter.

Solution:

SRMax= Value of ore − Production/Stripping cost per ton overburden

Recoverable copper per ton ore= 0.8% × 88% × 1000 = 7.04 kg

At $1.00/kg SRmax= (7.04 − 5.90)/0.3 = 3.8 tons waste per ton ore At$1.25/kg SRmax= (7.04 × 1.25) − 5.90/0.3 = 9.7 tons waste per ton ore

At $1.50/kg SRmax= (7.04 × 1.50) − 5.90/0.3 = 15.53 tons waste per ton ore To check that maximum stripping ratio has been reached; for$1.50, it is possible to strip 15.53 tons waste for each ton ore.

Profit= value of ore − production costs − stripping cost

## 3. Pit slope angle and stability

Slope angle is required during the early feasibility study. The degree of confidence on calculating slope angle depends upon the condition applicable. The major pit slope angle conditions can be divided into:

1. mining a shallow high-grade ore body in favorable geological and climatic conditions. Slope angles are unimportant economically and flat slopes can be used. No consideration of slope stability is required;

2. mining a variable grade ore body in reasonable geological and climatic conditions. Slope angles are important but not critical in determining economics of mining. Approximate analysis of slope stability is normally adequate; and

3. mining a low-grade ore body in unfavorable geological and climatic conditions. Slope angles are critical in terms of both economics of mining and safely of operation. Detailed geological and groundwater studies followed by comprehensive stability analysis are usually required.

During the pre-production period, the operating slopes should, however, be as steep as possible. The working slope can then be flattened until they reach the outer surface intercepts. The horizontal flow of stress through a vertical section both with and without the presence of the final pit is shown in Figure 17.

With the excavation of the pit, the preexisting horizontal stresses are forced to flow beneath the pit bottom. The vertical stresses are also reduced due to the removal of the rock. The rock lying between the pit outline is largely distressed. As a result of stress removal, cracks and joints can open. Cohesive and friction forces restraining the rock in place are reduced. Groundwater can more easily flow reducing the effective normal force on potential failure planes. With increasing pit depth, the extent of the stressed zone increase and the failure becomes more severe.

### 3.1 Types of highwall failures

There are several mechanisms by which highwall instability can occur. While we cannot expect to prevent all highwall failures, a better understanding of these mechanisms will enable us to identify potential problems before they become actual problems and to limit exposure to dangerous conditions. The most common types of failure include plane failure, wedge failure, toppling failure, and circular failure. Except for the circular failure, these usually occur along preexisting discontinuities. Example of each are the following:

#### 3.1.1 Planar failure

This slide in Figure 18 illustrates a typical plane failure of a highwall. Notice that the rockslide occurs along this discontinuity which daylights on the highwall and dips toward the pit. If this sliding plane does not daylight, or dips away from the pit, the slope is stable. Even if the joint daylights, in order for the slide to occur, the weight of this sliding block must exceed the frictional resistance along the discontinuity. Figure 19 shows an example of a slope, which is plagued by large planar failures, and leads to a slide off rocks along natural, parallel, and bedding planes.

#### 3.1.2 Wedge failure

A wedge failure occurs when two discontinuities meet and their intersecting line daylights on the slope face and dips toward the pit. If these conditions do not occur, you cannot have a wedge failure. The weight of the block also has to exceed the frictional resistance along the failure surface to have failure, Figure 20.

As shown in Figure 21, the failure can follow trends since joints tend to occur in repeating patterns. Note here the failure on the top bench, and on the next bench, should probably expect another at the next level down.

#### 3.1.3 Toppling failure

Toppling failures look like Figure 22. A toppling failure can occur when the discontinuities dip very close to vertical but away from the pit. The discontinuities can be natural or they can be caused by the mining process.

If the mine progresses from left to right, there will be continuous problems, because of the way these cracks are oriented. On the other hand, if the mine goes from right to left, mine operators do not have to worry about toppling-type failure; so, decisions made during mine planning can have a profound effect on the stability of the highwalls. Figure 23 shows a picture of a toppling failure that resulted in a fatality to a blast hole drill operator.

#### 3.1.4 Circular failure

In slopes excavated in soil or highly jointed and weathered rock mass where there are no geological structures to control the failure, the most unstable failure surface is approximately a circular arc. This circular failure surface (Figures 24 and 25) results from a process of localization of deformations. It is an arch type of landslides; however, the specific shape of this failure surface and the associated failure mechanism cannot be generalized [11].

## 4. Mine closure and reclamation

In general, mining has a significant negative impact on environment. Due to its nature, it leads to severe degradation of the landscape. Many factors such as drainage, air, soil and water quality, noise levels, ground vibrations, human health, and habitation are mostly affected by mining activities. When the extraction of mine reserve is over, the distorted landscape has to be reclaimed in order to reduce the damaging effects of open pit mining and bring back the landscape and its surroundings, see Figure 26.

Land use plan at the end of mining has to be set in order to determine what the mine site will look like and how the lands will be used after the mine is closed and fully reclaimed. The mine must operate and close such that the land and water in and around the mine site are less disturbed and environmentally safe and sound like original. It is the responsibility of the mining company to pay for reclamation and closure costs. To ensure that funds are available for closure, the mining company will normally be required to post a financial security (a reclamation bond) before mining starts.

“Progressive reclamation” is usually part of the overall closure plan. Progressive reclamation means that once a part of the mine site is no longer needed, it will be reclaimed rather than waiting for all aspects of operation to cease. For example, waste rock piles will be reclaimed as soon as they have reached their permitted size.

The general rehabilitation goals require rehabilitation of areas disturbed by mining to result in sites that are safe to humans and wildlife, nonpolluting, stable, and able to sustain an agreed postmining land use. The process of reclamation normally involves the following steps [12, 13, 14]:

1. Recontouring: the ground is re-sloped and contoured to a profile that will be stable and that provides proper drainage, facilitates the growth of vegetation, and provides various habitats for wildlife.

2. Capping with a growth medium: waste rock piles and other areas of the mine site will need to be covered with a soil material that is suitable for the growth of plants.

3. Seeding and fertilizing: this usually takes place over many years. Fast growing grasses may be planted in order to stabilize the soil followed by shrubs and trees depending on the end use plan.

4. Monitoring: plants in areas that are to be used for grazing will be tested to ensure they contain acceptable levels of metals and other possible contaminants.

## 5. Variants of surface mining methods: strip mining, mountainous mining, and artisan mining

Variants of open pit mining are limited to a number of other surface mining methods, which include strip mining, high wall mining, and quarrying. Strip (open cast) mining is used extensively for the surface mining of important commodities such as coal and phosphate ores. Casting is the process of excavation and dumping into a final location. This type of mining involves removing the overburden and extracting the valuable mineral deposit. Strip mining is applicable to shallow, flat-lying deposits [15]. It is a method that is generally applied on a large scale with low mining costs and high productivity and that has minimum land degradation [16, 17]. In Jordan, strip is used for the extraction of oil shale and phosphate ores. These mines are located at the central and southern parts of the country (e.g., Figure 27).

Strip mining differs from open pit in that the overburden is not transported to waste dumps but cast directly into adjacent mind-out panels, i.e., reclamation is contemporaneous with extraction. These mines often occupy a large area of land for ore excavation and overburden disposal. Strips are large rectangular parallel pits that extend to more than a mile in length [18]. After the removal of vegetation and topsoil, the mining begins with an initial rectangular box cut. The dragline is used for overburden removal. As the overburden is removed from one portion of a mineral deposit, it is used to fill in the trench left by the previous removal [19]. The backfilled area is then replanted during the reclamation process.

Figure 28 shows typical dragline operation. Stripping process continues along parallel strips. Where the deposit becomes thinner, or dipping more below the surface, or in the case of dramatic increase in the stripping ratio, the mining operation must be ceased [19]. Shovel-truck system is currently adapted for extracting phosphate ore in several phosphate mines in Jordan (Figure 29); especially in Al-Shidiyah, Al-Abiad, and Al-Hasa mines. Since shovel truck removal of overburden generally costs at least three times as much as dragline stripping, the dragline is currently implemented for removing overburden from phosphate ore in those mines (Figure 30). On the other hand, shovel truck removal of overburden is currently used in Attarat oil shale mine (Figure 31).

In the mountainous and hilly terrains, contour mining is applied. It is also known as mountaintop mining. The mining of flat deposits in these areas follows the contour around the hill and into the hillside up to the economic limits. The extraction becomes difficult with inclination and depth increase. The top of a mountain is removed to recover the ore contained in the mountain that resulted in huge quantity of excess spoil that is placed in valleys that affected the streams flowing within these valleys [21].

Artisanal mining is a small scale mining method, which includes enterprises or individuals that employ workers in developing countries who are poor and have few other options for supporting their families and who usually use manually intensive methods for mining (e.g., panning in case of gold). Artisanal miners use elementary techniques for mineral extraction and often operate under hazardous, labor-intensive, highly disorganized, and illegal conditions [20, 22].

## References

1. 1. Wetherelt A, Peter K, Wielen V. Introduction to open pit mining. In: SME Mining Engineering Handbook. 2nd ed. Colorado: Society for Mining, Metallurgy, and Exploration Inc; 2011. p. 2161
2. 2. Tomakov P, Naumov I. Technology Mechanization and Organization of Surface Mining. 2nd ed. NEDRA: Moscow; 1986. p. 312
3. 3. Bullivant D. Current surface mining techniques. Journal for the Transportation of Materials in Bulk: Bulk Solids Handling. 1987;7:827-833
4. 4. Hustrulid W, Kuchta M, Martin R. Open Pit Mine Planning and Design—Volume 1: Fundamental. 3rd ed. London: Taylor and Francis; 2013. p. 995
5. 5. Introductory HH. Mining Engineering. 2nd ed. New York: Wiley-Interscience Publication; 2002. p. 564
6. 6. Oggeri C, Fenoglio T, Godio A, Vinai R. Overburden management in open pits: Options and limits in large limestone quarries. International Journal of Mining Science and Technology. 2019;29(2):217-228
7. 7. Harraz H. Mining Methods: Surface Mining Planning and Design of Open Pit Mining [Internet]. 2016. Available from:https://www.slideshare.net/hzharraz/surface-mining-planning-and-design-of-open-pit-mining[Accessed: 15 November 2019]
8. 8. Ascarza W. Mine Tales: Huge Haul Trucks Changed the Face of the Mining Industry. 2014. Available from:https://tucson.com[Accessed: 15 November 2019]
9. 9. Dessureault S. Equipment Operations Technology, University of Arizona Mining and Geological Engineering Course Notes–2005; 2015. p. 208
10. 10. McShane J. Metal Products Advances in Material Hauling for Mining Operations. 2020. Available from:https://www.mcshanemetalproducts.com/blog-post/advances-material-hauling-mining-operations/[Accessed: 10 November 2019]
11. 11. Fleurisson J. Slope design and implementation in open pit mines: Geological and geomechanical approach. Procedia Engineering. 2011;46:27-38
12. 12. Stevens R. Mineral Exploration and Mining Essentials. British Colombia: Pakawau GeoManagement Inc./British Columbia Institute of Technology; 2014. p. 317
13. 13. Kuter N. Reclamation of Degraded Landscapes Due to Opencast Mining. Cankiri, Turkey: Department of Landscape Architecture, Faculty of Forestry, Cankiri Karatekin University; 2013. DOI: 10.5722/55796
14. 14. Liu H. Strategic Planning for Dragline Excavation Sequencing [PhD thesis]. Australia: The University of Queensland; 2018. Available from:https://espace.library.uq.edu.au/[Accessed: 18 April 2020]
15. 15. Haldar S. Mineral Exploration: Principles and Applications. 2nd ed. Oxford: Elsevier; 2018. p. 378. ISBN: 9780128140222
16. 16. Brown I. Strip mining. Ch. 10.11. In: Darling P, editor. SME Mining Engineering Handbook. Vol. 1. Colorado: SME; 2011. pp. 1031-1046
17. 17. Harraz H. Mining Methods: Part I- Surface Mining. 2010. Available from:https://www.researchgate.net/publication/301824314_Mining_Methods_Part_I-Surface_mining. [Accessed: 15 December 2019]
18. 18. Tatiya RR. Surface and Underground ExcavationsMethods, Techniques and Equipment. London: A.A. Balkema Publishers, a member of Taylor & Francis Group plc.; 2005. ISBN: 90 5809 627 0
19. 19. Liu H. Strategic Planning for Dragline Excavation Sequencing [PhD thesis]. Australia: The University of Queensland; 2018. [Unpublished]
20. 20. Erdem B, Duran Z, Çelebi N. A model for direct dragline casting in a dipping coal-seam. The Journal of the South African Institute of Mining and Metallurgy. 2004:9-16
21. 21. Copeland C. Mountaintop Mining: Background on Current Controversies. Congressional Research Service.7-5700; 2015. Available from:www.crs.gov. RS21421
22. 22. Hinton J, Veiga M, Beinhoff C. Women and artisanal mining: Gender roles and the road ahead. Chapter 11. In: Hilson G, editor. The Socio-Economic Impacts of Artisanal and Small-Scale Mining in Developing Countries. The Netherlands: August Aime Balkema, Swets Publishers; 2003

Written By

Awwad H. Altiti, Rami O. Alrawashdeh and Hani M. Alnawafleh

Submitted: October 8th, 2019 Reviewed: March 20th, 2020 Published: February 17th, 2021