Factors determining the application of underground metal mining methods are: shape, size, regularity and dip of the orebody; distribution of ore, strength and physical character of the ore and wall rocks or overlying material; relation of deposit to surface and to other orebodies or to existing shafts on the same property; availability, character and cost of timber and material for filling. These factors are interdependent and of varying importance. The method chosen must be safe and should give maximum profit and extraction.
A logical classification of mining methods based on the factors outlined is impossible, because of their complex relations. Type of stope is used as a basis for classification; the stopes themselves are grouped according to modes of supporting walls and men, as follows: open stopes, shrinkage stopes, filled stopes, timbered stopes, caving methods, and combined methods. In present day mining practice one finds such a broad variety of modifications of mining methods that they may be only genetically referred to their original basic group.
An open stope is a stope in which no timber or filling is used to support walls or men; a finished stope is an open cavity, the walls are supported by pillars of ore, or some simple forms of timbering. The term room-and-pillar covers many different methods of cutting up a deposit by excavating rectangular rooms and leaving pillars at regular intervals; the pillars may be left for permanent support or recovered by robbing operations. Suitable deposits for exploitation by room-and-pillar are flat or slightly dipping beds of uniform tenor and character, and of large area. Cheap, abundant, strong mineral, and a strong roof and floor are necessary if permanent pillars are left; where ore in pillars is recovered by robbing, a very strong roof may hang over large areas and cause trouble by dropping suddenly.
Sublevel stoping was developed in Michigan iron mines, the USA. It is an open stope method. It is a relatively low cost method, in terms of cost per ton of ore mined. It does not permit very effective sorting and is therefore applied where values are fairly uniform. Ore should be strong enough to stand well after trimminig. Walls must be firm or they will cave prematurely and dilute the ore. This method may not be advantageous in very hard ore due to the high cost of driving sublevels; it is best adapted to steep dips, but has been successful on flat dips too.
Shrinkage stopes are overhand stopes where part of the broken ore remains in the stope as a means of support and affords a working platform until the stope is completed. In mining large orebodies it can be applied where the orebody itself is self-sustaining across its width and where the walls are sufficiently firm to stand without support over a considerable length along the strike of the vein. The range of dip permissible is from 50 to 90 deg.
The method is not confined to wide veins alone but may be applied to narrow steeply inclined veins. With veins of great length, the stopes are usually separated by pillars. In mines with stopes 60 ft. wide along the length of the vein and extending from hanging to footwall, pillars 42 ft. wide separate the stope. The vertical height of a stope depends upon the dip of the vein. With flat veins of moderate thickness and 50- to 55-deg. dip a level interval of 100 ft. is used. For a vein of 60-deg. dip a level interval of 150 to 175 ft. is permissible. It is evident that an increased interval between levels prolongs the time required to complete the stope and until the stope has been completed only the surplus ore can be drawn off. This ordinarily amounts to 1/3 of the broken ore. For every ton of ore produced from a given stope 3 tons must be broken and 2 tons left in the stope. For orebodies of moderate length and height it is possible to plan for the winning of the orebody in one stope from a single level.
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Filled stopes, as the term is used here, is the stope in which (a) support for walls and men and, at times, for the back of ore is furnished by waste rock, tailing or sand, which are called filling; (b) filling is an integral part of stoping; generally the orebody is excavated in small sections, filled wholly or in part before the adjacent ground is attacked; (c) the use of timber, if any, is for temporary support of back and is not systematic as in square set stopes.
Most filled stoping is done in overhand flat back stepped face or rill stopes. The surface of fill is kept roughly parallel to the stope back. As the stope progresses, chutes (usually of timber) are carried up through the fill, giving access to the stope and delivering the broken ore to the level below. The height of the section mined before filling depends on the character of the ore and walls; details vary with the size and the shape of the stope and the source of filling; modes of arranging haulage ways and of robbing level pillars vary with the strength of walls and ore, and the width of the orebody.
In overhand filled stopes, the operations of breaking or cutting slice from the back of the stope and then filling the excavated area, have led to the wide use of the term cut-and-fill. A flat-back filled stope is called a horisontal cut-and-fill; a filled rill stope, an inclined cut-and-fill stope. The term filled stope is also applied to a stope mined by some other method and then filled with waste to prevent caving or subsidence.
Caving methods are those in which the ore is first undercut and then broken down by its own weight or by the weight of the overlying rock, or by a combination of both. But as a result of custom, operations involving caving of the material overlying an orebody, as a systematic and essential part of the work, are also classed as caving methods, though practically all the ore is broken by drilling and blasting. Three distinct methods, sublevel caving, block-caving, and top-slicing, result from this classification, each having many modifications.
Top-Slicing. The field of use is in wide veins, masses, or thick beds of weak ore, where clean mining and high extraction are desired. Each floor is mined in small sections, the roof of each being allowed or forced to cave before an adjacent one is attacked. The work on each floor retreats from the limits of ore toward the points of an entry; all the ore is broken by blasting.
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Sub-Level Caving, a logical development of top-slicing, is largely used in the Lake Superior Iron Ranges, but rarely elsewhere in the USA. Suitable orebodies are wide deposits of moderately soft or moderately firm ore, overlain by ground which will cave readily but coarsely, to form a capping which will arch and support itself temporarily over small opening. The latter conditions is neither necessary nor desirable in top-slicing.
This method resembles top-slicing in that the ore is mined in horizontal slices in descending order, so that the overburden, or capping, will break up and subside as the ore beneath is removed. Fundamental difference is that the height of slices in sublevel caving is usually 15-25 ft. as against 10-12 in top-slicing. Timbered slice-drifts are driven as in top-slicing, but a back of ore 7-15 ft. thick is left between the top of the sets and the bottom of the mat, this back of ore being removed by mining and caving, starting at the far end of the slice drift and retreating toward the entrance.
Block-Caving. Large sections or blocks of the orebody, sometimes to a height of 400 ft. or more, are successively undercut and allowed to slough and cave above the undercut portion. Drawing off the caved ore causes further caving, often aided and controlled as to its lateral extent by weakening the boundaries of the block by narrow shrinkage stopes or superimposed cut-off drifts. The ore is caved and crushed by its own weight and the weight of the overlying capping into pieces of suitable size for handling. Caving usually extends eventually to the surface, the overburden settling as the support of the underlying ore is removed. Drawing continues until the appearance of overburden material at drawpoints indicates exhaustion of the ore. This method is a natural development of sub-level caving through gradual increase of the height of ore caved in operation.
Suitable orebodies are wide veins, thick beds, or massive deposits of homogeneous ore, overlain by ground which will cave readily. The ore must be such that it can be supported while blocks are developed and undercut, and will break up when caved.
To increase the rate of caving and for mining hard rocks which do not break easily, a modification of the block caving method has been introduced, namely the forced block caving which differs from the former method in using deep blast hole drilling to assist in breaking the ground.
(7320)
NOTES:
stoping ;
open stope ;
robbing , ;
sublevel stope ;
trimming ;
shrinkage stope ;
to draw off ;
filled stope ;
chute , ;
cut-and-fill ;
caving method ;
sublevel caving ;
block caving () ;
top-slicing ;
capping , ;
cut-off drifts , ;
forced block caving () .
Text 11. MINING GEODESY
Mining geodesy is a branch of mining dealing with the dimensional-geometric measuring of mines and open-pits and the ways of solving mining-geometrical problems.
Mining geodesy has for its object obtaining data as to the form of mineral deposits, determining the size and position of mine workings as well as laying out mining. Vertical and horizontal surveys are known to be run with the help of theodolite and levelling traverse, measuring angles, elevations and distances up to the sighted object. For this purpose the following instruments are employed: transit, levels, rods, depth measurers, protractors, tacheometer-telemeters, self-recording devices, photo-theodolites and others.
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Levels. Precise levels belong in their essentials either to the Y or dumpy type. In both the telescope is mounted on a vertical axis about which it can swing horizontally and is levelled by levelling screws. The distinguishing feature of the Y-level over the dumply level is the possibility of its telescope being reversed in the Y-supports.
The Zeiss level is known to be the most widely used dumpy level. As compared with other dumpy levels used for ordinary levelling the most distinctive features of the Zeiss level are the methods of mounting the telescope and level tube, the design of the telescope and the arrangement for showing the observer if the bubble is central while he is sighting. The level tube is fixed on one side of the telescope. The metal tube carrying it is cut away both on top and bottom. The bubble is illuminated by means of the reflector. The telescope with the level tube is capable of rotation through 180 about its own axis, so that the level can lie either on the left- or right-hand side. The level tube is not graduated but the observer can tell when the bubble is central by means of a combination of prisms contrained within the casing.
In addition to the means for reversal of the telescope about its own axis, the telescope fittings are such that the ends can be reversed, the eyepiece being inserted in the objective cap. Observations can therefore be taken in four positions: 1 and 2 eyepiece direct, bubble tube on left (right) of telescope; 3 and 4 eyepiece reversed, bubble tube on left (right) of telescope. The prism is reversed for position 3 and 4.
Levelling rods. Levelling rods are of two types: target and self levelling; the former is read by a rodman, the latter by a levelman. The rods are usually made of strips. The graduations (meters and decimeters) are painted on the wood. The figures indicating the heights are inverted for ease in reading with an inverting telescope. Sometimes for more accurate work the centimeters are painted on a strip of invar which is fastened securely to a metal foot piece which lies against the face of the rod held by flat-headed screws.
Verticality of the rod is determined by means of a circular spirit level or two levels attached at the back. A similar scale of feet (or meters) is often painted on the back of the rod for checking the readings.
(2530)
NOTES:
levelling traverse ;
transit - ;
levelling rod ;
protractor , ;
Y-level ;
Y-supports ( );
dumpy level - ( );
level tube ;
casing , , , ;
self levelling rod , ;
invar , - .
