1)Students will describe how the ground's water pressure effects the stability of the layout of the open pit mine.

2)Students will explain how economics of the mine are determined by design, available equipment, and mining ratios to production ratios.

Metaliferous deposits that occur near the surface are mined by open pit methods. Quarrying and strip mining methods are used for other deposits such as sand, gravel, iron ore and coal. The advantages of open-pit mining over underground mining are lower costs, greater safety, and mechanically easier operations. These open pit mines range in sizes from "dog holes" to one half cubic mile. Many low-grade deposits are mined this way because of the lower costs and higher productivity.
Soil and barren rock must be removed to expose the ore bodies. This process is known as pre-production stripping. This process of stripping this surface away may take a short time or as long as years. A series of benches that are arranged in spiral or connecting ramps are developed. The ramps or benches are usually 25ft to 100ft wide, with heights being 25 to 70 feet. Equipment and stability of the rock determine the height and width of the each bench. Bench height and width are also related to slope stability. The slope can vary between 20% to 70%, with the limit being determined by slope stability and economics.
The amount of barren rock that is to be mined is the major consideration in open-pit mining planning.

Open Pit Mining
The basic concept of an open pit mine is simple. Open pit mines, however, require a lot of planning to make sure that as much ore as possible can be extracted. Note: the problems and cost of removing overburden often dictate the limits of the pit.

Mining is done at large pits by track-mounted electric shovels. Diesel-powered front end loaders are generally used at smaller operations. Truck, railroad, or conveyors usually do haulage.

Open Pit Mine- an excavation or cut made at the surface of the ground for the purpose of extracting ore and which is open to the surface for the duration of the mine's life. To expose or mine the ore, it is generally necessary to move large quantities of waste rock. The purpose of the mine is to make a profit so careful planning and engineering must take place from the very beginning. The planning of an open pit mine is an exercise in economics, constrained by geologic, and mining engineering aspects.

Increased population, created greater demand, which in turn, initiated increased productivity. Production increased with improved engineering and technology and for the most part, greater productivity by workers. The shift was from underground mining to surface mining even though the grade and quality was declining.

It is generally conceded that surface mining is more advantageous than underground mining in terms of recovery, grade control, economy, and flexibility of operation, safety, and the working environment. There are many deposits, that are too small or irregular, and or deeply buried to be extracted economically by surface mining methods. When the minerals extend deep in the ground, the removal of the waste rock becomes too cumbersome and expensive and the mine must be converted to underground operations or abandoned.

Open pit design is conducted in several stages.

Devising a scheme or set of alternatives Evaluation Selection of optimum scheme

The most economic final pit design is often out of the hands of the designer. The design depends of factors such as:

Geometric outline of the ore body, The topography Maximum allowable slope angles

In the end, the economics depend on the engineers' choice of plans, equipment and the mining ratio to production rates.

Exploration Input for Open Pit Planning
The data from the core samples is crucial at this point. Besides the information collected thus far being used to determine the shape and size of the ore body, these core samples help determine the slope stability. The water pressure plays and an important role in the stability of the slope also.
The detrimental effects of the presence of ground water on surface mining are:

Water pressure reduces the stability of the slope and tends to induce sliding of materials in the slope Increased weight of the unit rock thus higher rates for transportation. Freezing water can block drainage paths Erosion can cause instability and silting up of drainage systems Increased operating costs from discharge of water for equipment and blasting costs

Bench Plan Preparations
Technology plays an important role in planning and the design of a modern mine operation. A model representation is created called a block model or ore body model. The models enable mining planners to effectively select the most promising means of extracting the ore body both physically and economically. With the assistance of modern computer facilities, sophisticated and complex bench plans can be prepared.

Stripping Ratio Considerations
The term stripping ratio is almost universally used and represents the amount of uneconomic materials that must be removed to uncover one unit of ore. The ratio of total volume of waste to ore volume is defined as the overall stripping ratio.
R=volume of waste removed to depth d = ABD
____________________________________________
Volume of ore recovered to depth d = BCED

While a volume relationship, calculated in cubic yards/cubic yards (cubic meters/cubic meter). It is more commonly expressed as tons/tons. Note that in mining certain mineral commodities, however, stripping ratio is expressed in units of cubic yards/ton.

Cutoff stripping ratio is the one that the costs of mining the ore and waste are matched by the revenue for that block of ore. Factors used to determine costs should include the added costs of mining as the mine deepens and the interest charges on the prestripping of waste.

In the most complete analysis, the entire ore body is mined on paper. The production from each time period is determined, the costs and revenues listed, and a cash flow generated. The profits are projected. The result is to be the value of the mine or production. Mining is continued until it no longer increases the value, and so a pit limit is determined. The ratio of the total volume of waste to total volume of ore is then the overall stripping ratio,

Mining Plan Options
Declining Stripping Ratio Method- this requires that each bench of ore be mined in sequence, and all the waste on the particular bench is removed to the pit limit.

Advantages	Disadvantages
1. Operating space available 2. Accessibility of the ore on subsequent bench 3. All equipment working on the same level 4. No contamination form waste blasting above the ore 5. Equipment requirements are minimum towards the end of the mine's life	1. Operating costs are a maximum during initial years when profits are required to handle interest and repayment of capital.

Increasing Stripping Ratio Method- stripping is performed as needed to uncover the ore. The working slopes of the waste faces are essentially maintained parallel to the overall pit slope angle.

Advantages	Disadvantages
1. Maximum profit in the initial years 2. Greatly reduces investment risks in waste removal for ore to be mined at future date 3. Method is popular where the mining economics or cutoff ratio is likely to change	1. Impracticality of operating a large number of stacked, narrow benches simultaneously to meet production needs.

Constant Stripping Ratio- This method attempts to remove the waste at a rate approximated by the overall stripping ratio. The working slope of the waste face starts very shallow, but increases as mining depth increases until the working slope equals the overall pit slope.

Advantages	Disadvantages
1. Equipment fleet size and labor requirements throughout the project are relatively constant. 2. Good profit initially to increase cash flow. 3. Distinct mining and stripping areas can be operated simultaneously, allowing for flexibility in planning.	1. Disadvantage and advantages is the compromise that removes the extreme conditions of other two stripping methods.

Mine Plan Frequency- Mine plans vary in frequency from short to long range. The plans can be daily, weekly, monthly, yearly or a life-of-the mine plan. The shorter the plan the more concise it will need to be. The longer plans will have to establish financial forecasts for replacement of equipment, variations in operating costs due to haul distances, and dewatering requirements. Changes in the ore type may dictate plan changes also.

Feasibility Studies- Once the mineral deposit has been discovered and evaluated thoroughly enough to be considered an ore body, the next problem is how to mine the ore that will maximize the the net present value (NPV). The plan must figure in the best way to get the most from the mine early in the operation to cover high interest and to improve cash flow. The best ore grade may be removed first to increase finances or the early plan may include beginning to operate in a low waste-to-ore ratio area of mine site.

(Littleton, Colorado:  Society for Mining, Metallurgy, and Exploration, Inc., 1992),  pp. 1274-1278.

Vocabulary:
Open Pit Mine- an excavation or cut made at the surface of the ground for the purpose of extracting ore which is open to the surface for the duration of the mine's life.
Bench-a ledge that forms a single level of operation above which mineral or waste materials are mined back to a bench face.
Bench height-vertical distance between the highest point (crest) of a bench and the lowest (toe) of the bench.
Bench slope- the angle, measured in degrees between the horizontal and an imaginary line joining the crest and the toe of the bench.
Pit limits- the vertical and lateral extent to which the open pit mine may be economically conducted.
Berm- a horizontal shelf within the pit wall slope that is left for stability. The berm slope angle and width are determined geotechnical configuration.
Haul road- a road is maintained in a uniform spiral arrangement from top to bottom of the pit. A zigzag or switch back haul road system is dependent upon the shape and size of the ore body and equipment and the stability of the pit.
Angle of repose- is the maximum slope at which a heap of loose material will stand without sliding.
Suboutcrop depth- is the depth of the waste that has to be removed before ore is exposed.
Surface Mining- Metaliferous deposits that occur near the surface are mined by open pit methods. Quarrying and strip mining methods are used for other desposits such as sand, gravel, iron ore and coal. The advantages of open-pit mining over underground mining are lower costs, greater safety, and mechanically easier operations. These open pit mines range in sizes from "dog holes" to one half cubic mile. Many low-grade deposits are mined this way because of the the lower costs and higher productivity.
Soil and barren rock must be removed to expose the ore bodies. This process is known as pre-production stripping. This process of stripping this surface away may take a short time or as long as years. A series of benches that are arranged in spiral or connecting ramps are developed. The ramps or benches are usually 25ft to 100ft wide, with heights being 25 to 70 feet. Equipment and stability of the rock determine the height and width of the each bench. Bench height and width are also related to slope stability. The slope can vary between 20% to 70% with the limit being determined slope stability and economics.
The amount of barren rock that is to be mined is the major consideration in open-pit mining planning.

References:
**This article was adapted from G.A. Fourie and Gerald C Hohm, JR.  SME Mining Engineering Handbook, 2nd Edition, Volume 2.  (Littleton, Colorado:  Society for Mining, Metallurgy, and Exploration, Inc., 1992),  pp. 1274-1278.