Some 80 per cent of the containers used to transport coal from the face to the surface in British mines had, until recently, a capacity of less than 15 cwt, the bulk of these carrying between 10 and 15 cwt. The size of tub adopted has in the past depended on the natural conditions, particularly the thickness and inclination of the seams and the method of working employed. With the advent of increased mechanization particularly where trunk conveying is adopted and the tub need not leave the main roads, the size of tub has greatly increased to between 2 and 6 tons, and in one case to 10 tons; and such larger cars will continue to replace tubs of 25 cwt or less capacity when more installations of locomotive haulage, now in development stage, come into operation.
Where main roads are driven through the measures in the so-called horizon system of mining, mine cars are necessary to exploit the system to the best advantage.
The inherent advantage of mine cars over tubs are that the capacity of the whole transport system is increased and becomes more economic, since the useful load is increased relatively to the gross load, the power required is reduced since the friction is lessened, and the number of cars in circulation and in consequence the men required to manipulate them - is also reduced.
(1100)
NOTES:
tub - , ( 0,7 3); [TB1]
car - ( 1 );
bulk - , ;
gross load - .
Text 8. CONVEYERS
A system of conveyers is commonly divided into two sections: (a) transport on the face; (b) transport by gate or room and trunk conveyers.
There are three main types of face conveyers: 1) jigging or shaker conveyers; 2) belt conveyers; 3) scraper conveyers.
Each of these suits best certain conditions of inclination, output or custom in the coal-field.
The shaker conveyer is essentially a downhill conveyer and although it may function satisfactorily in level conditions it is rarely suitable for even a slightly rising gradient.
This conveyer consists of a line of troughing in 10 or 11 ft sections connected to each other by bolts and nuts, ropes, or by wedges and other quickly-connected fasteners. The pans (troughing) are mounted on rollers and cradles shaped to give an abrupt stop at the end of the forward stroke of the conveyer, or on steel balls confined in frames which give only a horizontal motion to the troughing.
For a high concentrated output whether the coal is delivered uphill or downhill, the belt conveyer, consisting of a driven drum, a double length of belting and a tension drum, is generally preferred for face work. The width of the belting employed varies from 20 in. to 26 in. and exceptionally 30 in., and the speed is from 100 to 200 ft per minute. The output from a 20-in. wide belt conveyer is about 50 tons per hour when running at 10 ft per min. and 85 tons at 175 ft; a 26-in. belt at the same speeds will give 65 and 110 tons per hour respectively. The height varies from 6 ft to 13 in. according to the thickness of the seam. The rollers supporting the top belt are at intervals of 3 ft, those-supporting the return belt every 6 ft.
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A feature of recent years has been the return to popularity of the scraper chain conveyer. Its simplicity, robustness and reliability in its modern form has led to its increasing use in both longwall, and bord and pillar systems of mechanized mining.
In the latter the room type of easily extendible scraper conveyer is used to receive the output from loaders where shuttle cars cannot be employed, and this can deal with an output of 1.2 tons a minute with a chain speed of 126 ft per minute.
For longwall work either the single chain or the double chain type is used with capacities from 32 to 100 tons per hour On the level, using 12 in. and 20 in. troughing and motors of 8 to 35 hp.
On the Ruhr an armoured scraper conveyer of this type has been developed for use with the coal plough. It is driven by motors of 50 hp at each end of the face with a 50 hp compressed-air engine which can be clutched in if an overload occurs. Resonance conveyers are also used with the plough and other power loaders.
(2200)
NOTES:
shaker (or jigging) conveyer - ( ) ;
belt conveyer - ;
troughing - 1) , 2) ;
longwall - ;
bord and pillar systems - ;
room type - ;
armoured scraper conveyer - ;
plough - .
Text 9. ROCK PRESSURE
The development excavations and the excavations in the orebody may become dangerous if no support be used to resist rock pressure. The greater the depth below surface, the greater will be the pressure due to the weight of rock. Rock pressure may result also from movement along fault planes and along lines of weakness in the rock foundation. Rock pressure is the cause of rock-bursts, falls of hanging walls, and the premature caving of workings; the main proportion of underground accidents may be attributed to these causes.
The roof of an excavation in rock is not subjected to a pressure equal to the weight of the whole of the overlying mass; were it so, it would be impossible to mine to greater depths than a few hundred feet below surface. Rock pressure is vertical; but the lines of pressure from the rock far above an excavation will tend to combine into resultants which follow the line of an arch; the weight of rock above the arch is supported by the solid ground adjoining the excavation. Otherwise the roof of an excavation at say 4,000 ft. below surface, would be subjected to a pressure of 330 t. per square foot in a rock such as quartzite and 360 t. per sq. ft. in a rock such as hornblende schist.
According to its physical and mechanical properties a rock may be: homogeneous, as granite; stratified, as quartzite; laminated, as slate; schistose; broken or weakly-consolidated; unconsolidated, as sand.
Causes of heavy ground. There are three principal causes of heavy ground: ground flow, sometimes referred to as earth pressure, is present to a more or less degree in all underground openings and is particularly troublesome when the openings are made through soft materials such as dikes, slates, fault gouges or ore. Ground subsidence, another cause of heavy ground, may be due to the proximity of mining operations or to weakness of the bedding or cleavage planes in certain types of formations because of faulting or lack of adhesion, permitting the material to slip on itself and become unstable when an opening is made in it. The third, ground swelling or expansion is due to the exposure of an excavation to air or water or both.
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Type of Support and Factors Affecting its Choice. The above mentioned principal causes of heavy ground are present in most deep mines, but each must be contended with in a somewhat different way. The problem is not how to support, but rather how to cope with these types of heavy ground so that the supporting members whether of rigid steel, yielding steel, concrete, or conventional timber will give the maximum life before replacement is necessary.
There are many factors which enter into the choice of the type of support such as: location, the distance taken either horizontally or vertically from the center of disturbance; the orientation of the opening, whether it runs perpendicular or parallel to planes of weakness; the use to which it is to be put, that is, if it is going to be subject to blasting and the impact of scrapers, etc.; the time you expect to hold it open; the shape and size of the opening; and possibly others.
(2590)
NOTES:
premature caving - , ;
hornblende - , ;
ground flow - ;
gouge - , ;
ground subsidence - ;
adhesion - ;
ground swelling - , , .
