Compilation of exam questions we got on mining technology for civil engineering students.
Exam Question # 1. What are the types of d
etonators used in mining?
Ans. High explosives are initiated by detonators or detonating fuses. A detonator is a small copper or aluminium tube containing essentially a small auxiliary charge of special explosive. A chemical reaction initiated by a flame or electric current in the special explosive can build up very rapidly into an explosion of sufficient intensity to project a detonation wave throughout a high explosion enclosing the detonator.
Detonators are of the following types:
1. Plain Detonators:
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These are fired by safety fuses, the spark or “spit” from the fuse causing the detonator to explode; these are sometimes called “ordinary” detonators.
2. Ordinary Electric Detonators:
These are fired by passage of electric current through the detonator.
They are further subdivided as:
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(a) Low tension detonators, and
(b) High tension detonators (not generally used in mining).
Ordinary electric detonators are of instantaneous type, i.e., without any delay element. They are of copper or aluminium tubes.
3. Delay Detonators:
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These are essentially low tension electric detonators with a delay element, the object in their use being to phase the firing of shots, so that time and effort are saved in charging and firing several successive rounds of shots.
These are subdivided as:
(a) Half second or long delay detonators, and
(b) Milli-second delay detonators (also known as short delay detonators).
ADVERTISEMENTS:
A plain detonator, i.e. non-electric consists simply of an aluminium tube 6 mm dia., 37 to 50 mm long, filled 1/3 with A.S.A. composition and penta-erythritol tetra-nitrate (P.E.T.N.)., The A.S.A. composition consists of a mixture of lead sized. (A), lead styphnate, (S) and a little aluminium powder (A). The A.S.A. priming composition initiates the base charge of P.E.T.N. which is a much more powerful explosive.
No. 6 detonator is suitable for normal requirements of mining work. No. 8 detonator is more powerful than No. 6 but is not generally used.
In an ordinary electric detonator, i.e. non-delay type or instantaneous type, the priming charge and base charge are the same as for plain detonator, but they are fired, not by ignition of a safety fuse, but by passing electric current through a fusehead. The current ignites a flashing composition in the fusehead, which in turn, initiates the priming charge.
The resistance of low tension detonators with a 45 m long shot firing cable is about 7 ohms. Current required for ignition of the fuse head is 0.5 amps so that a single detonator can be blasted with minimum voltage of 3.5 V. The circuit continuity of a L.T. detonator can be tested by a galvanometer and simultaneous shot firing of a number of detonators with series connection is possible.
ADVERTISEMENTS:
In appearance and composition, this is like the L.T. detonator. The L.T. detonator described above is of instantaneous type; the moment voltage is applied to it, the detonator explodes and along with it, the enclosing explosive. A delay detonator has a delay element introduced between the fusehead and the priming charge.
The delay element consists of a copper or brass sleeve filled with a special composition which burns at a specified rate and the delay is obtained by varying the length of the sleeve containing the special composition.
Delay detonators of some manufactures have antistatic sleeve over the fusehead as a protection against static electricity hazard.
The delay detonators and non-delay detonators are distinguished by the colours of lead wires. The delay period is marked on a tag attached to the wires. Moreover the delay number is stamped on the bottom of tube.
In underground coal mines aluminium detonators are not permitted but only copper detonators should be used.
Permission from the D.G.M.S. is required before using delay detonators in underground coal mines. Delay detonators and non-delay detonators should not be kept in the same box.
Exam Question # 2.
Briefly explain shaft sinking at Sudamdih and Monidih collieries.
Ans. Sudamdih was the first mine in India where two shafts were sunk in the early sixties with the help of Polish engineers in a manner that was completely new for the mining industry in the country.
The distinctive features adopted for the first time in shaft sinking at Sudamdih, and later at Monidih, were:
1. Sinking was carried out with the help of one head gear and 2 winders, diametrically opposite to each other on either side of the shaft.
2. A double deck platform was used for simultaneous sinking and concrete lining.
3. Delay detonators were used.
4. Shot firing was carried out with the current from the electric power line at 550 V.
5. Grab loaders, suspended from the double deck platform, were used for loading of muck at shaft bottom.
6. High rates of sinking with completion of shaft lining were achieved. The average progress per month was 25 metres and the maximum was 50 metres.
7. Entire shaft depth was lined with concrete.
At Sudamdih No. 1 shaft is 450 m deep, finished dia. 7.2 m and the adjacent No. 2 shaft is 420 m deep, 6.5 finished dia. All the coal production comes from 400 meters horizon (400 m below M.S.L.) by means of skips in No. 1 pit. During sinking at shaft No. 1 a temporary head gear and two winders were installed but the headgear and winders were replaced later by permanent ones.
A general time-study of different operations:
I. Drilling 3 to 4 hours.
II. Charging and blasting 1/2 to 2 hours.
III. Clearance of smoke and cleaning of D.D. platform 1/2 to 1 hour.
IV. Lowering of pipes, equipment and grab 1/2 to 1 hour.
V. Loading of much 18 to 21 hours.
VI. Lowering the steel shuttering 45 min to 1 hour.
VII. Placing and plumbing the shuttering 1 to 1/2 hours.
VIII. Extension of concrete pipe line (flexible portion 1/2 to 1 hour).
IX. Pouring concrete 3 to 4 hours.
Exam Question # 3. Give an example of mine in India.
Ans. Some of the coal mines planned for large production (million tonnes per year) are- Kusmunda – 6.0, Mukunda – 15.1, Gevra – 14.0, Nighai-12, Rajmahal-10.5, Jayant/ Dhudhichuva/Khadia-10.0 each.
Dipka Expansion/Anant-8.0:
For mechanised quarries, employing heavy earth moving machinery the DGMS makes bye-laws coverings bench sizes, roads, etc. These have to be studied before planning a mechanised quarry.
Mechanised opencast mining is preferred when there is a thick mineral bed of mild inclination, practically continuous and not in pockets, at a low depth and the reserves are plentiful. For coal, a seam of less than 6 m thickness and with less than one million tonnes of quarriable reserves will not justify the heavy capital expenditure, large amount of interest on it and the depreciation charges.
The overburden may be removed by a combination of dozers and scrapers if the rocks are soft. If they are hard, blast holes are drilled by wagon drills or well hole drills and blasted with explosives. The blasted rock is loaded into dumpers by dipper shovels or tractor shovels. Draglines are also used where the overburden is alluvium sand or soft rock, but if it consists of hard rock it is loosened by sprase blasting for loading.
Exam Question # 4. What is the difference between crawler chain and pneumatic tyred equipment?
Ans. The machines employed in a mechanised quarry are mounted on pneumatic tyred wheels if they have to be towed from one place to another e.g. compressors, wagon drills, etc. Self-propelling units having their own engines (line tractors, dozers, shovels, cranes, etc.) may however be mounted on pneumatic tyres or crawler chains. Table 1 shows the approximate rolling resistance of various road surfaces to pneumatic tyres and crawlers.
A resistance of 9 kgf per 1,000 kgf is roughly equivalent to that offered by an upgradient of 1% i.e., a road rising 1 m vertical through 100 m horizontal.
It will be seen that the advantage of pneumatic tyres receders rapidly as the ground conditions worsen and for rough roads a crawler unit is at an advantage. Machines using crawler chains are not allowed to cross public roads which are damaged by the cleats of grouser plates and the latter should be replaced by plain plates preferably with rubber soles, for such crossing.
Speed of crawler chains is slower than that of tyres. Where public roads have to be crossed often, or where the flitting is frequent, it is an advantage to use pneumatic tyred equipment. In the field crawler mounted equipment scores over the tyred one for climbing steep gradients and has better traction on rough roads and better gripping capacity on bench floor when the machine has to dig hard, e.g., when extracting toe.
Crawlers can negotiate sharp turns, a big advantage in a quarry of limited size. The quantity of mineral and rock that remains frozen in the roadways of a quarry where tyre mounted dozers and tractor shovels are used is much larger compared to that when similar equipment on crawlers is deployed.
Exam Question # 5. What is the difference between diesel-driven and electrical equipment.
Ans. The equipment used in quarries may be diesel operated or electrically operated. Machines which have to move frequently from place to place are operated by diesel engines e.g., bulldozers, scrapers, graders, dumpers, tractors dozers.
But the equipment which has to work from one site over long periods in a shift may be electrically operated or diesel operated e.g., dipper shovels, draglines, compressors, well hole drills, bucket wheel excavators. Permanent or semi-permanent machines are always powered electrically e.g., large centrally located compressors, pumps, etc.
An electric shovel avoids fuelling problem, is comparatively quiet, simple and maintenance cost is cheaper than for the diesel one. In electric shovel motors can be placed so as to eliminate complex gear trains and chain drives, and controls are easy to operate.
Initial cost of electric shovel is however higher than for a diesel unit. Ward-Leonard system is the standard method of drive and control on large H.P. shovels and draglines. Dipper shovels of smaller size (upto 3.5 m3) are usually diesel driven, but electric drive is preferred for larger machines.
The electric shovel, due to limited length of the trailing cable, operates only within a restricted area and the trailing cable being heavy needs men to handle it during movement from one place to another. Some shovels are diesel-electric.
In this type of drive a diesel engine mounted on the shovel itself drives a generator that supplies electricity to motors which to the heavy work of the machine and the various controls on the shovel. Such drive is found in medium large shovels.
The voltage of small machines is usually medium, 400 or 500, though H.T. voltage (3300-6600) is often essential for large machines like dipper shovels which have to move very little during a shift. The bucket wheel excavators at Neyveli are supplied power at 11 kV through T.R.S. cables and transformers in the machine step down the voltage to 3.3 kV for some motors and to 400 V for some other motors.
Overhead power transmission lines bring the power to a convenient point near the quarry and from there the pliable armoured cables which are sufficiently flexible carry it to the machines. In mechanised quarries it is difficult to construct the power lines entirely outside the blasting zone as trailing cable lengths are restricted by the Electricity Rules and such overhead lines and the insulators sometimes break due to flying stones of blasting.
Exam Question # 6. What are the a
dvantages of delay detonators?
Ans. 1. Reduced consumption of explosive as blasting is more efficient due to availability of a free face for each row or round of shots e.g. blasting due to No. 1 delay detonator gives a free face for the blasting effect of shots fired by No. 2 detonator.
2. Increased fragmentation and ease of loading the rock or coal. Broken rocks from successive shots collide in air, thereby increasing the fragmentation.
3. Considerable time is saved in that the whole round of shots is fired in a fraction of a second. This is the chief advantage. If individual shots or even groups of 5 or 6 shots simultaneously by L.T. detonators are fired, the time required for inspection and clearance of fumes and gases between successive firings is considerable. In steep seams, the exertion involved in frequent trips for such examinations and connections is saved.
4. The millisecond delay (short delay) detonators have been observed to produce less ground vibrations than the half second delay detonators and are therefore used in mechanised quarries where blasting of large diameter holes containing heavy charges is likely to produce excessive ground vibrations and damage to nearby surface buildings and other important engine foundations, etc.
Exam Question # 7. Name the materials used in mining support.
Ans. The following materials are commonly used for mine supports entirely as such or in combination:
1. Timber, usually sal (and in some areas, teak) is used for props, bars, chocks or cogs, and laggings,
2. Iron and steel in the form of bars, props, arches, corrugated sheets and roof bolts.
3. Brick or building stone in masonry walls, or archings.
4. Reinforced concrete or precast concrete blocks as roadway lining.
5. Roadway ripping, dirt bands and shales as packwalls.
6. Sand, earth, boiler ash, washery rejects, mill tailings, slage from blast furnace for smelting iron and crushed stone as packing of goaf and filling or voids.
Research into strong, lightweight materials for underground supports has shown that the most promising one is glass fibre-reinforced plastics. These have not been introduced in our mines as yet. Tests in Russia have shown the CBAM framed supports or development roads weigh one seventh or one eighth as much as precast reinforced concrete and one third as much as timber frames. CBAM is a Soviet glass fibre product.
Precast concrete assemblies as support have been seldom used in our mines. They have the serious disadvantage of great weight and difficulty in handling.
The type of support to be built up depends on the importance of the place to be supported, the period for the support, its cost and availability.
Exam Question # 8. What are the types of roof bolt? Explain the principles, advantages and disadvantages of it.
Ans. Types of Roof Bolt:
Slot and Wedge Bolt:
It consists of ms rod 25 mm dia., nearly 1.3 m long, split at one end for nearly 160 mm, and threaded at the other end for about 125 mm. Into the slot is fitted a wedge 150 mm long.
To install the bolt in position a vertical hole, nearly 125 mm less than the length of the bolt, is drilled and the bolt, having the wedge fitted at the split end, is inserted into the hole and hammered in position. The split end having the wedge somewhat widens in the hole and grips the rocks. A bearing plate of mild steel, 150 mm x 150mm is placed on the bolt and a nut is tightened.
The better the grip, the sounder is the installation.
This type of bolt is popular as it can be easily made in the colliery workshop, is cheap in the first cost and can be installed in holes drilled by the usual coal drill. It cannot be recovered.
Expansion Shell Type Bolt:
One of the expansion shell bolts works as follows:
A forged head or stud type bolt complete with expansion shell and plug of Nanda Manufacturing Company. Complementary taper of plug and shell provides full contact with the wall of hole.
Its installation is as follows:
1. Drill hole of recommended diameter at right angles to bearing surface, to a depth larger than the bolt to be used.
2. Place washer on bolt thread. Plug on bolt four to eight turns.
3. Insert in hole until washer and bolt heads are against mouth of hole.
4. Apply torque as per recommendation. This expands the serrated shett against the wall of hole.
Perfo Bolts (N.M.C.):
A ribbed/torsteel rod is threaded at one end and chamfered at the other end for easy insertion. Sleeve of recommended diameter made of flexible galvanized wire mesh (size 5 mm x 1 mm dia.) is used to insert the sand cement mortar into the drill hole.
Two semi-cylinders are manufactured from 0.63 mm thick perforated steel sheet; alternatively wire mesh cylinder is manufactured from 5 mm mesh x 1 mm dia. They are usually 3 to 4 mm less than bore hole dia. It is advisable to use perforated semi cylinder tubes for lengths longer than 1500 mm.
For installation of a perfo bolt, drill hole longer than bolt to be used. Take wire sleeve and fill it with mortar (cement, sand and water 1:1:0.6 by vol.).
Principles of Action of Roof Bolt:
There are three theories put forward to explain the action of roof bolt:
1. Beam theory.
2. Keystone theory.
3. Suspension theory.
The beam theory is widely accepted. According to this theory, the roof bolt assists in joining together the rock beds through which the bolt passes. The joined beds form a sort of thick beam which supports the weight of the strata.
For example, if a single wooden plank, say 6 mm thick and 6 m long is supported at its two ends, it will bend considerably in the middle under its own weight, and may even break, but if several of such planks are clamped or bolted together, the composite beam so formed will be much stronger than the single plank and will support not only its own weight, but also some heavy weight over it.
For deficient roof support bolts in sufficient number should be fixed immediately after the roof is exposed, i.e. immediately after loading of coal in an advancing heading is over. Roof bolts are not very successful in watery holes as the grip of bolts is reduced.
During roof bolting instead of a plate, a girder or a bar may be used to support the roof. The girder or bar provides supports to a crack or slip in the roof.
Advantages of Roof Bolting:
1. It is simple to apply, easy to mechanise and moderately cheap in cost; manpower required for fixing supports is less than with conventional supports. In the U.S.A. the complete roof bolting equipments are mounted on a single self-propelled trolley.
2. It gives greater headroom and clearance in the roadway. This facilitates easy manoeuvring of the machines, e.g., shuttle cars, coal cutters, mechanical loaders etc. and offers least resistance to the air flow;
3. The hazards due to accidental dislodgement of conventional supports caused by derailed tubs, blasting, etc. are reduced if systematically carried out. It results in greater safety and less accidents due to roof falls. The supports are fireproof;
4. Handling and transport of heavy support materials involved in conventional supports are eliminated;
5. Storage space required is small;
6. The protruding ends of bolts can be used to suspend water or air pipes and cables;
7. Stability of the support does not depend upon the condition of the floor and this is a considerable advantage where floor is soft;
8. Where excavations are wide, e.g., open stopes in metal mines, underground engine rooms, pit bottom excavations, etc., roof bolting is useful.
9. Bolts can be used to secure sides in headings and sinking pits.
The disadvantages are that:
1. It cannot be applied in all cases;
2. It gives no warning of impending failure;
3. Some types of bolts are not recoverable e.g., slot and wedge type.
One Company in U.S.A. Climax Molybdenum Company has successfully used in the late sixties fibre glass rock bolts in varied ground support applications. Two-metre bolts cemented in place within resin have been installed in brows at draw points and are also used in supporting under-cut sub-drifts. Since then fibre-glass rock bolts have gained wide popularity in metal mines.
Exam Question # 9. What is the application of floor bolting?
Ans. In many roadway supported by conventional methods, having of the floor is of such magnitude that the roadway is completely disrupted. The erection of conventional supports may promote such floor having in some cases because the roof weight is transmitted to the floor through the props or arches and these then pierce the floor, breaking it up and setting it free to move.
Floor bolting, like roof bolting, has as its object the formation of a strong compound beam of clamped floor beds which will be strong enough to resist the lifting forces acting upon it. In some cases, it is found sufficient to concentrate the holes near the middle of the roadway where maximum movement is liable to occur. But the best pattern of floor bolting to prevent heaving of floor can be found by experiments.
Exam Question # 10. What are the advantages of rigid steel props over timber?
Ans. 1. A row of steel props offers uniform resistance if properly set.
2. A steel prop has a greater ultimate strength than timber.
3. Recovery of steel props and frequency of its re-use is greater. If buckled, it can be straightened and re-used but a timber prop once broken cannot be re-used except as lagging.
4. A steel prop does not deteriorate to the extent timber does; timber decays in a number of ways, particularly by dry rot.
5. They are incombustible.
Exam Question # 11. What are the disadvantages of steel joist props?
Ans. 1. Large props are difficult to tighten against the roof and may, therefore be knocked out.
2. A large manpower is required for handling them.
3. Wooden props give warning of heavy weighting, which steel props do not.
4. A simple joist prop cannot be used in seams of varying thickness. Cutting of a long prop and turning its flanges cannot be done at the site of erection.
Exam Question # 12. Compare between friction and hydraulic props.
Ans. 1. A hydraulic prop is by far superior to any friction prop in so far as acceptance of a sufficiently high and uniform load in the underground application is concerned. The friction prop has irregular behaviour under load and it slides more than a hydraulic prop under a similar load.
2. A high setting load is obtained in a hydraulic prop.
3. Friction prop is lighter in weight and has a superior extensibility.
4. Friction props serve a long life of sand stowing faces.
5. Friction as well as hydraulic props can be easily withdrawn.
Wooden lids are not used with friction props and hydraulic props. Where such props have accommodated steel roof bars, the prop heads have prongs which prevent the prop from being joilted or forced out of position by an accidental heavy blow or roof pressure.
Exam Question # 13. Explain the mechanism resin capsule bolting.
Ans. Roof bolts of the wedge-and-split type an expansion shell type are not suitable in soft rocks due to their poor anchorage in the latter. The wire mesh grouted bolts or roof stitching, though suitable for soft rocks, require about 24 hours for the cement mortar to set.
The use of styrene resins instead of cement grout reduces the interval between the moment the bolt is installed and the moment it is ready to carry a load. After a lapse of 15 minutes the bolt can support a 5-te load, and 20 minutes thereafter, a 10 te load. Styrene resins have been used in USSR and some foreign countries.
A system of roof bolting which can be adopted in soft rocks and which does not require a long time for the cement mortar to set to make the bolt effective in anchorage, has been developed by the CMRS and is known as resin capsule bolting.
The resin capsule is a polythene cartridge, 28 to 35 mm dia. and 330 mm long. The components of a resin capsule are resin, a hardener and filler. The resin is a synthetic polyester resin which has the consistency of a jelly at ambient temperatures. The filler is coarse sand or small sized dolomite chips.
The hardener is kept in a separately sealed glass tube which is placed at the centre of the polythene cartridge of resin and filler. The composition of the resin and the hardener are trade secrets. If the glass tube is broken and the hardener allowed to come in contact with the resin and the two are mixed well, the mixture hardens within 30-40 seconds and becomes solid.
In actual application vertical or inclined holes are drilled in the soft rock of roof with the usual coal drill fitted with a slightly larger dia. drill rod for 40 mm dia. hole. The roof bolt to be used is 22 mm dia. m.s. rod, with a length of 230 mm at one end in the form of a scroll (like an auger).
The other end of the bolt is threaded to receive a nut. A thin circular washer of 40 mm dia. is provided at 250 mm below the bolt head to prevent falling of resin while mixing inside the hole. The roof bolt is then attached to the coal drill through an adaptor.
The resin capsule and the bolt are inserted in the hole, the capsule leading. The bolt is pushed as far as it can go and then the drill, switched on. As the bolt starts revolving it is pushed inside the capsule. The rotation of the scrolled end tears open the polythene wrapper and breaks the thin-walled glass tube of the hardener.
The bolt is rotated only for 15 seconds. The hardener and the resin, coming in intimate contact with each other, result in a solid compound within 30-40 seconds with the scrolled end firmly embedded in it. The solid compound that is formed grips the side of the hole.
The solidifying process inside the hole is indicated by the gradually diminishing speed of the drill. The roof bolt is thus anchored in the hole. A bearing plate and a nut are then fixed to the threaded end outside the hole. The anchorage strength is 8-9 te even with boundage for a length of only 300 mm. Wooden dowels 36 mm dia. made of hard wood, capable of taking more than 10 te tensile load, could also be used as bolt material.
Capsule:
It contains separated pre-measured quantity of filled polyestered and resin catalyst. Outer skin of capsule is made of PP, nylon or special paper. Filler is crushed rock- silica, barytes, soap-stone, etc. usually of 250 mesh. Catalyst tube is usually made of glass tube with cork on both sides to prevent mixing of catalyst with resin unless desired.
For a capsule the gel time is the time between the start of the running operation and the onset of the solidification process. This time varies from 2 to 5 minutes, depending upon the type of bolt used. The curve time follows immediately after the gel time and is the time for resin to harden completely and become load bearing. The bolt should not be loaded for the first half an hour.
Under normal conditions the capsule can be stored for 3 months from the date of manufacture.
Cured resin has properties as follows:
Compressive strength – 125 MN/m2 (12.5 kg/mm2)
Tensile strength – 50 MN/m2 (5 kg/mm2)
The capsule has a length of 310 mm but the diameters are suitable for two hole sizes, 40-43 mm and 31-33 mm. The sp. gr. is 1.9.
For using the resin bolt the procedure is as follows:
1. Air blow or water flush the hole to remove the dust.
2. Check that the bolt is free to rotate in the hole and that the hole is of correct depth, i.e., 75 mm less than bolt length.
3. Insert the capsule into hole, push gently home with bolt.
4. Drive bolt home, spinning it all the time with drill. Do not try to rush this stage.
5. Spin bolt for further 30 seconds once it is home. Continue longer if an impact wrench is being used and bolt is rotating very slowly.
6. Removes drill and can nut disturbing bolt as little as possible until 5 minutes after “gel” has occurred. Gel time can be detected when the bolt becomes too stiff to rotate slightly between fingers.
Fit plate washer and nut over bolt and tighten up.
The system was tried at Sudamdih colliery for friable coal roof and proved quite effective. It can also be used for side bolting in coal or other corcks if the sides have a tendency to spill off and has been observed to be suitable in holes having water percolation.
Exam Question # 14. Explain the design of Salzgitter/MAMC hydraulic prop.
Ans. This is a hydraulic prop of open circuit type, normally used with sliding roof bars. The roof bar itself can be pushed or retrached, when in position, by hydraulic pressure. The Salzgitter/MAMC hydraulic prop can be extended, according to prop type, by mounting of extension pieces.
Setting of Salzgitter/MAMC hydraulic props is effected by hydraulic pressure, which, produced by a central high-pressure pump, is delivered to the setting gun via a lead in the supporting system. The setting gun is put on the filling valve of the prop, mechanically fastened by one-hand locking device and operated by one hand.
The pressure medium flows under the piston, pushes out the inner ram tube and sets the prop safely with the setting load pre-set by the hydraulic pressure between cap and bottom. The hydraulic oil used is a 5% oil emulsion, and 95% water; pH value 5-8 pH.
The prop can take the roof load until the nominal load adjusted in the operating valve has been reached. As soon as the nominal load has been reached, the operating valve opens and some pressurised fluid spurts out until a pressure spring closes the operating valve again. This operation occurs whenever the nominal load has been reached. Thus the prop is always protected from over-load of rock pressure.
Large stroke admit large convergence as well. Due to the extension feasibilities according to prop type, the prop lengths can be easily adapted to seam heights underground.
When the prop is to be withdrawn at the goaf edge, the releasing valve can be actuated by means of a releasing key which is extended by means of a rope or chain from the miner’s safe position effecting the flow-off of the pressure fluid out of the cylinder space.
An installed powerful return spring rapidly retracts the inner ram tube. After this the prop is ready again for another setting process. Releasing keys are available in accordance with service conditions (service heights) in lengths of 150, 280 and 800 mm.
Exam Question # 15. Explain the design of CMRS designed hydraulic prop.
Ans. Most of the hydraulic pros for use in mines do not incorporate any device:
(i) To indicate load coming on the prop at any particular moment, and
(ii) To change the capacity of the prop without dismantling it.
CMRS has developed a new type of hydraulic prop in which these two devices have been incorporated.
Other notable features of this prop are:
(i) If any part of the prop gets damaged underground, it can be easily repaired without taking it to the surface, and
(ii) The load indicating system also acts as a safety release capsule and can be operated when the release valve gets damaged during working conditions.
Prop density is the term used to denote the number of props required to supports one square metre of the roof area. This is the practice adopted in Germany.
In U.K. prop density is frequently referred to as the area of roof supported per prop. The prop density is calculated under least favourable conditions.
Bars:
A steel bar or beam is used when there is a prop at each end. A corrugated bar is stronger and offers more resistance to bending. On the prop-free-front system of roof support, cantilever bars are used, thereby reducing the number of props and the risk associated with their installation.
They are supported by a prop in the middle or at one third length from one end behind the conveyor (on the goaf side), the other end remaining unsupported and therefore liable to bend slightly with roof weight. A cantilever bar has a length which is a multiple of the face advance per cut by coal cutting machine or shearer.
Sliding bar is a long rigid light section steel girder which rests in special heads fitted to the vertical props and slides between the vertical slides posts of the heads. The bar is raised and tightened against the roof or lowered or released from roof pressure prior to sliding into the next position by means of a sliding wedge.
Bar slide method is used in the prop-free front. At the start of cut, the bar resting on the heads of three props, is raised and tightened against the roof by means of the wedges. As coal is loaded away the bar is dropped from the roof by releasing the wedge and pushed forward.
The bar is re-tightened in the new position and if necessary, temporary prop is erected under the free end of the bar next to the face. Conveyor is then advanced and a prop is fixed in the new position just behind the conveyor. During the period the bar is released and advanced, the roof load is carried by props as if there was no bar. In case of an irregular or weak roof sliding of the bar presents difficulties.
Though the hydraulic prop is a powered supports, the term “powered support” is confined to cover such supports as the hydraulic chock, shield or canopy.
Exam Question # 16. What are the factors to be governed during blasting practices followed in mines?
Ans. If the blasting is not off-the-solid, the position, direction, depth and number of holes at a face are governed by the following factors in an underground coal mine:
1. Position and Depth of Cut:
A hole should be at least 150 mm shorter than the depth of the cut, made by coal cutting machine.
2. Type of Roof:
If immediate roof is shale the hole should terminate 0.3 m below the roof; if the roof is sand stone, the hole should terminate 150 mm below the roof.
3. Type of Bands and their Position:
The hole should be drilled near the band and if it is of hard rock, it should be drilled towards it, slightly below or above so that the inclined hole terminates near the band where the explosive force is maximum. The holes should be drilled below the band if it is near the roof, specially when the roof is weak or friable.
4. Cleavage in the Coal:
Shot holes should cross the cleavage planes as near to right angles as possible.
5. Strength of explosives and hardness of coal: Harder coal requires more density of holes or explosives. If output of lump coal from the mine is important it is advisable to use a low density explosive and this influences the number and position of holes.
6. Dimensions of the Gallery:
A gallery of large cross section gives better yield of coal per kg of explosive. Gallery with more height, say between 1.5 m and 2.4 m, requires two rows of holes but only one row suffices for low height.
Exam Question # 17. Describe the layout of a mechanised quarry with suitable diagram.
Ans. The layout of a quarry depends primarily on:
1. Shape, size and dip of the deposit.
2. Proposed depth upto which mining activity is planned to extend.
3. Thickness of the overburden,
4. Surface topography,
5. Desired production,
6. Transport system for mineral and OB,
7. Arrangement for disposal of debris, and
8. Type of mechanisation and finance available for it.
The layout of a quarry should depict the following:
1. Position of OB benches and mineral benches.
2. Access to the benches and exit roads for the dumpers.
3. Position of back filling area.
4. Location of machinery which operates from one site over a long period such a sovel, dragline, bucket wheel excavator, in relation to the benches.
Fig. 5.19 shows the layout of a mechanised quarry using dipper shovels, dumpers and well hole drills, the common equipment in most of our mines. The property is divided into areas along the strike, each area being nearly 100 to 300 m long. In the initial stages a box cut (trench) RK is made with the help of scraper, dozers or small capacity (2 m3) shovels to suit the depth of the trench.
Such trench is essential for a shovel which stands on the floor of the quarry and operates on benches. Some amount of blasting may be necessary in making a trench in hard rocks. In the advanced stage of the quarry two trenches RK and R1K1 are in use, one for the OB and the other for the mineral which goes to the crushing plant. The OB bench and the mineral bench are usually parallel to each other and advance towards the quarriable limit of the mine.
In the figure, h1 and h2 are OB bench and mineral bench respectively. If the OB and mineral are of considerable thickness, there may be two or more benches for OB as well as for the mineral. In such cases ramps have to be provided between adjacent benches.
Provision of two trenches, one for OB and the other for mineral, avoid congestion of dumpers on the haul roads. This layout can be modified to have a crusher and outgoing belt conveyor on the lower end of mineral trench.
In-pit crusher with feeder breaker is the trend in open cast mines these days. Mineral is then transported by conveyor belts to coal handling plant or mineral processing plant. Some of the mechanised opencast mines in South Eastern Coalfields Ltd., are now having in-pit crushers.
Such layout of two trenches is not suitable if the mineral is at much depth as formation of trenches would involve removal of large volume of earth and rock. Layout of one trench for both mineral and OB may then be adopted.
For deeper quarries switchback -system of track layout is advantageous. Fig. 5.20 b shows the tracks as they would appear in a vertical projection (exaggerated) for a locomotive- minecar combination of transport system. Numbers 0,1,2,3, mark the levels of the floors.
Thus to get from level 0 to working floor 3 a train must follow the route marked by the arrows. Switchback entails a big loss of time for shunting operations. Each switchback should be sufficiently long to hold a train and railway switches, or else to allow enough room for minecars to turn round.
Fig. 5.21 shows a modified layout depicting the position of various coal and OB benches in a quarry (shovel-dumper combination) at an advanced stage if OB is backfilled in the decoaled area. The seam is considered to be 18 m thick dipping at 1 in 5.
The benches formed are sometimes designated as Bench No. 1, Bench No. 2, etc. or by the reduced level of the floor of the bench e.g. 500 metre bench, 510 metre bench, 520 metre bench, etc.
Pumps are installed in the quarry which has to be kept nearly dry for operation of shovel and dumpers. The usual arrangement is to have two or three pumps as a semi-permanent installation fed by small portable pumps.
The area may be divided into strips of 45 to 60 m width along dip rise. A width of 45 to 60 m permits easy movement of dumpers, the minimum width required being 18 metres. Each strip provides a bench, which is equipped with a shovel and a well hole drill.
Daily progress of shovel in the strips L, N, is along dip rise direction, but the advance of the bench is along the strike when considered over a quarter or half year. The quarry excavation of course advances towards the dip to the quarriable limit.
The shovels work on stone benches and coal benches. The overburden may be taken to dumping yard away from the quarry, or dumped in the decoaled area. Suitably graded roads, not steeper than 1 in 10, with ramps between the strips. Shortage of shovels and ancillary equipment may require the management to work only a few benches at a time and not as many as shown in the figure.
As the haul road runs along the periphery of the quarry, excavation should commence from the boundary of the quarry and proceed downward forming one bench at a time. Such layout is an advantage in hill-top deposit which may be worked in horizontal slices.