Stripping of Mineral Deposits

Stripping of Mineral Deposits


the execution of major mining operations to provide access from the surface to the entire deposit or part of it and to ensure the possibility of conducting preliminary mining operations that are necessary for extraction work at the mine faces.

The chief goals in stripping a mineral deposit are to establish transportation connections between the working faces (the place from which the mineral is being extracted) and the receiving point on the surface, to ensure safe conditions for the movement of personnel, and to supply pure air to the working sections (in the shafts).

Underground operations. Major stripping workings are divided into main and auxiliary categories. The main workings include those that have a direct outlet onto the surface (vertical and inclined mine shafts and tunnels); auxiliary workings include crosscuts, blind shafts, dip headings, and inclines. Preliminary workings are usually drifts driven through the mineral deposit. The methods of stripping a deposit are extremely diverse, and they vary according to the type of main opening workings, their location relative to the strata or ore beds, the presence of auxiliary stripping workings, and the number of underground transportation levels. The method of stripping a mineral deposit depends on the topography of the area; the value of the mineral; the form, dimensions, and depth of its bedding; the thickness and pitch of the bed or ore body; and the number of ore beds and the distances between them.

The effects of the geological and mining-engineering factors mentioned above are all taken into consideration in selecting a method for stripping a deposit. Among the most important factors are the minimum initial capital expenditures and the time required for the construction of the shaft, the concentration of production under conditions of maximum increase of extraction from the working face, the concentration of mining operations to a limited number of simultaneously worked beds, and the reduction of the length of supported mines through intensification of extraction operations and periodic renewal of the mining system in the shafts, either by preparing new levels or by reconstructing them. Stripping a mineral deposit by means of vertical shafts is an all-purpose method. At least two shafts are sunk (two safety exits from the mine to the surface); one serves to supply fresh air to the mine, and the other serves to carry off exhaust air to the surface.

In uncovering a coal deposit extensive use is made of a scheme according to which two shafts are sunk to the full planned depth; the facilities for the haulage horizon—the area around the shafts, the crosscut, and the main drifts—are erected around them. A complex of sloping main mining facilities (the dip heading and passages) are driven upward through the mineral, and the hauling and ventilation drifts are driven from them. Long working faces, which are equipped with mechanized units for extracting the mineral, are located between the drifts. The workings of the airway horizon are installed at the upper boundaries of the deposit. Coal from the working face is transported through the drift, the dip heading, and the crosscut and is conveyed through one of the shafts to the surface.

Ore deposits are usually characterized by inconsistent thickness and pitch of the ore body in the ore beds, by the presence of a large number of dislocations, and by small length but great depth. Therefore, ore deposits are stripped by means of vertical shafts and crosscuts, which are sunk successively as the shafts become deeper. The shafts for hoisting the mineral to the surface are situated approximately in the central section of the deposit, and the ventilation shafts are located on the flanks.

Cave-ins of rock, as well as subsidence of the overlying strata, occur during the extraction of a mineral. Therefore, in stripping steep and inclined mineral deposits, mine shafts are sunk in rock of the bottom wall, outside the displacement zone, to avoid shaft deformations. This also eliminates losses of valuable ore in the protective pillars, which are necessary to preserve the shafts. Inclined shafts are usually used in stripping isolated strata or ore beds that lie at comparatively shallow depths. These shafts are sunk at an angle of up to 18°; in opening coal seams they are located along the mineral seam, whereas in the case of ore beds they are sunk in barren rock of the bottom wall. Initially such inclined shafts strip reserves from the upper horizon; as these reserves are exhausted, the shafts are deepened to the next level, and so on.

Mineral deposits are stripped by means of tunnels where the topography of the area is strongly dissected and when the utilization of vertical or inclined shafts would be technologically impossible or economically unfeasible. Depending on the location of the deposit with respect to the slope of the hill, tunnels are driven either in the same direction as the mineral deposit or through barren rock. Combination of the main opening workings—for example, vertical and inclined shafts (the combination stripping method)—is also possible. In this case the inclined shaft is utilized for conveyor transportation of the mineral to the surface, whereas the vertical shaft is used for auxiliary purposes.

Open-pit operations. In open-pit operations, the stripping of a mineral deposit involves the conducting of inclined (main) open-pit operations with a terraced, trapezoidal (pit) or triangular (half-pit) cross section from the ground surface or the part of a quarry that is being worked to the newly created operational levels. The horizontal excavation with a trapezoidal or triangular cross section—the sectional pit or half-pit, which is excavated to create an initial face for mining operations—is a direct continuation of the main pit.

The determining elements of a pit are its terminal depth, the longitudinal incline of its base, the width of its foundation, and the length and slope of its sides. The depth of the principal pits is equal to the height of one or several of their benches. The transportation facilities are located on the base of the pit, and the width of the base is determined by the size of the vehicles—for example, dump-cars and dump trucks. In most cases the longitudinal incline of the sloping main pits designed for rail or motor-vehicle transportation does not exceed 4 and 8 percent, respectively. The steep pits designed for conveyors have an incline ranging up to 18°, and those designed for skips are as steep as 45°. If the direction in which the rock is moved from the pit (load flow) varies, each bench may be opened by a separate main pit. Grouped pits are utilized to split up the load flows of stripped rock and of the mineral. Open stationary pits are used to open quarries used for wording horizontal and gently sloping deposits. Opening mineral deposits by means of sliding ramps permits a decrease in the volume of mining operations during the period when the pit is being constructed. Such sliding ramps are customarily used to open two to four lower operational benches in working sharply sloping deposits. Internal main pits are used to open mineral deposits in working mineral beds with slopes ranging from 27° to 30°. The route of a system of main pits—the spatial position and direction of the longitudinal axis of the pits—may be simple if the pits are located on one side of the quarry and the traffic direction of the means of transportation does not change. A complex route consists of two or more sections moving in different directions, interconnected by dead ends (in the case of rail transportation) or loops of small radius (usually in the case of motor-vehicle transportation). A spiral route passes around all sides of the quarry, encircling it one or more times. Frequently the two or three topmost benches of a quarry are opened by external pits, and the lower benches are stripped by internal main pits. Sometimes quarries are opened by underground excavations, using inclined and vertical shafts with galleries or tunnels.

The selection of a rational method of stripping a mineral deposit is made during the period of planning of the mining enterprise, and it is a complex engineering problem with respect to the specifics of mining production: the instability of production conditions (the variability of natural factors), the dispersion and constant shifting of work sites, and the need for continual replacement of exhausted working faces.

In addition to the methods of classical mathematical analysis, the method of overall optimization of design decisions is also used in planning; in this method, several variant methods of stripping the mineral deposits are developed, followed by the compilation of an economic-mathematical model of the mine or quarry. In the process of making the final decision the best version is found using electronic computers.


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