An open-pit mine is an excavation or cut made at the surface of the ground for the purpose of extracting ore and which remains open to the surface for the life of the mine. The selection of physical design parameters and the planning of the ore and waste extraction program are complex engineering decisions of enormous economic significance. Several benches may be in use simultaneously in different parts of and at different heights in the open-pit mine.
The bench height is the vertical distance between the highest point of the bench or the top of the bench and the toe of the bench. Pit limits are the vertical and lateral extent to which open pit mining can be economically conducted. The berm interval, berm slope angle and berm width are controlled by the geotechnical configuration of the slope.
The total slope angle of the pit is the angle at which the wall of the open pit stands, as measured. During the open pit, the access road must be maintained into the pit. A spiral system is an arrangement in which the haul road is arranged spirally along the perimeter walls of the pit, so that the slope of the road is more or less uniform from the top to the bottom of the pit.
The choice of spiral or zigzag depends on the shape and size of the ore body, truck economy and pit slope stability.
In many cases, it may be desirable to drill inclined rather than vertical holes for the purposes of
Slaking and classification tests on weathered or suspected weak materials that could influence dump and pit stability
Shear strength tests on soft or weak layers, particularly in the floor, and on overburden materials showing strong
Compressive strength, point load strength, and drillability tests to provide information for estimation of overburden and
Water pressure reduces the stability of pit slopes by reducing the shear strength of potential fault surfaces. Water pressure in tension cracks or similar nearly vertical cracks reduces stability by increasing the forces that tend to induce sliding. High moisture content results in an increased unit weight of the rock and thus gives rise to increased transport costs.
Changes in the moisture content of some rocks, especially shales, can cause accelerated weathering with a decrease in durability. Freezing of groundwater during winter can cause scour in water-filled cracks due to temperature-dependent ice volume changes. Freezing of surface water on slopes can block drainage paths, resulting in an increase in water pressure on the slope with a decrease in stability.
Surface soil erosion and fissure filling occur due to the velocity of groundwater flow. This erosion can cause a decrease in stability and also siltation of drainage systems. Discharge of groundwater into an open pit causes increased operating costs because the water must be pumped, as well as the difficulty of operating heavy equipment on very wet ground.
Liquefaction of exposed soils or waste dumps can prevail where the water pressure in the material builds up to the point where uplift forces exceed the mass of the soil. This can happen if the drainage channels are blocked or if the soil structure suddenly changes in volume, as can happen under. Deduction of the overall groundwater flow pattern from consideration of rock mass permeability and groundwater resources.
Deduction of the overall groundwater flow pattern from consideration of the permeability of the rock mass and sources of groundwater
Direct measurement of water levels in boreholes or wells or of water pressure by means of piezometers installed in boreholes
Bench Plan Preparation and Presentation
Stripping Ratio Considerations
- A good profit can be generated initially to increase cash flow
- The labor and equipment fleet can be increased to maximum capacity over a period of time
- The labor and equipment requirements decrease gradually toward the end of the mine’s life
- Distinct mining and stripping areas can be operated simultaneously, allowing for flexibility in planning
First, the pit slope is determined for geotechnical and other considerations and then the stripping ratio is calculated. The cutoff strip ratio is the one at which the costs of mining ore and waste match the revenue from that block of ore. It is necessary to regularly update the long-term plan of the mining project.
This method (Fig.) requires that each bank of ore be mined in sequence, and all the waste on the relevant bank be removed to the pit boundary. The advantages of this method are the working space available, the accessibility of the ore on the subsequent bench, all equipment working at the same level, no contamination from waste that shoots above the ore, and equipment requirements are a minimum towards the end of the my life The primary disadvantage of this method is that the overall operating cost is a maximum during the initial years of operation when it is maximum.
The working slopes of the waste faces are kept essentially parallel to the general slope angle of the pit. This method is very popular where the mining economics or outage ratio is likely to change at very short notice. The disadvantage of this method is the impracticality of operating a large number of narrow benches assembled simultaneously to meet production needs.
This method (Fig.) attempts to remove debris at a rate approximating the overall stripping ratio. The working slope of the tailings surfaces starts out very shallow, but increases with the depth of the mine until the working slope equals the overall pit slope. This method, from the point of view of advantage and disadvantage, is a compromise that removes the extreme conditions of the two previous stripping methods.
In practice, the best stripping sequence for a large ore body is one where the rate is slow during the initial stages and towards the end of the project life. The labor and equipment fleet can be expanded to maximum capacity over time. The shorter the time span covered by the plan, the greater the degree of accuracy and confidence that actual performance will meet the forecast.
For example, a daily mine plan details the production schedule on a 24-hour shift basis, while long-term plans can be used to make financial forecasts related to the purchase of new equipment, changes in the workforce, variations in operating costs due to increased hauling distance, and requirements for water drainage. Changes in ore type may dictate long-term blending criteria and potential changes in ore sales revenue.
OPEN PIT FEASIBILITY STUDY
