In this article we will discuss about the depillaring:- 1. Introduction to Depillaring 2. Preparatory Arrangement before Depillaring 3. Depillaring By Caving (Method) 4. Depillaring of Contiguous Seams 5. Behaviour of Roof and Its Control during Depillaring 6. Precautions against Fire During and After Depillaring 7. Precautions against Inundation during Depillaring and 8. Pillar Extraction Method.
Contents:
- Introduction to Depillaring
- Preparatory Arrangement before Depillaring
- Depillaring By Caving
- Depillaring of Contiguous Seams
- Behaviour of Roof and its Control during Depillaring
- Precautions against Fire During and After Depillaring
- Precautions against Inundation during Depillaring
1. Introduction to Depillaring:
After pillars have been formed on the bord and pillar system, consideration has to be given to the extraction of coal from the pillars; the operation is known as pillar extraction. It is also referred to as depillaring, pillar-cutting or broken working.
ADVERTISEMENTS:
In a method of depillaring, known as the caving method, the coal of the pillars is extracted and the roof is allowed to break and collapse into the voids or the decoaled area, known as goaf. As the roof strata about the coal seam break, the ground surface develops cracks and subsides, the extent of damage depending upon depth, thickness of the seam extracted, the nature of strata, thickness of the subsoil and effect of drag by faults.
Depillaring with stowing is a method of pillar extraction in which the goaf is completely packed with incombustible material and in generally practised where it is necessary to keep the surface and strata above the scam intact after extraction of coal.
The following circumstances would require adoption of depillaring with stowing:
1. Presence of water-bearing strata above the coal seam being extracted, e.g. Kamptee series in Wardha Valley coalfield of Maharashtra. Enormous quantities of water beyond the economic pumping capacity may enter the mine through cracks in the strata.
ADVERTISEMENTS:
2. Railways, rivers, roads, etc. situated on the surface, which cannot be diverted.
3. Presence of fire in a seam above the seam to be extracted.
4. Existence of one or more seams of marketable quality extractable in the near future.
5. Restrictions imposed by local or Government authorities for the protection of the surface.
ADVERTISEMENTS:
6. Extraction of the full thickness of a seam thicker than 6 m, as thicker seams cannot be extracted fully by caving method.
7. Extraction of seams very prone to spontaneous heating, of very gassy nature or liable to pumps.
8. Surface buildings which cannot be evacuated.
9. Tanks, reservoirs, etc., which cannot be emptied.
ADVERTISEMENTS:
Where it is not practicable or economic to adopt complete stowing of goaf but all the same it is essential that the surface and strata above the seam remain intact, a method known as “splitting” the pillars is practised as a final operation. Maximum possible coal is extracted form the pillars by driving galleries through them so that the portions of the pillars left in situ support the overlying strata.
Caving is the general method adopted in majority of our mines at it does not involve the high cost of stowing arrangements. For depillaring with stowing availability of stowing material at a reasonable cost is a pre-requisite. River sand is the abundant stowing material commonly used and its hydraulic conveying is the general practice.
2. Preparatory Arrangement before Depillaring
:
The depillaring operations are so planned and conducted that:
ADVERTISEMENTS:
(a) There is the least danger to the safety of the mine and the men employed underground or on the surface,
(b) Maximum possible extraction is achieved,
(c) Extraction of other seams or sections of seams is not rendered difficult,
(d) The extraction is economic, and
(e) Dangers from crush or collapse of pillars, fire, influx of noxious gases, or inundation are avoided. Effect of fire or water-logged areas in the adjacent mines has also to be considered.
Before undertaking depillaring operations, permission has to be obtained from the D.G.M.S. in all cases, from the Railways if the operations are likely to affect a railway, and from the district authorities if District Board roads or other buildings are affected.
Diversion of railway, stream, road, power transmission lines, telephone lines, aerial rope way, etc. has also to be considered. Rehabilitation should receive attention. Surface land likely to subside has to be purchased. All these arrangements take 1-2 years.
We shall consider here the arrangements preparatory to depillaring by caving method:
(1) If there is a seam above, developed and filled with water, or “goafed” and water logged, it has to be dewatered. In case the top seam is developed, and abandoned temporarily, pumps can be installed in it for dewatering. Top seam may be dewatered through a shaft if it is on the dip side and is accessible on the surface.
The shaft may be equipped with a submersible borehole pump. In most of the cases of depillaring of top seam the shaft pillar might have been extracted and the access road to it on the surface subsided. Under such circumstances pumps cannot be installed in the top seam and its dewatering is by boreholes drilled from bottom seam workings.
Such borehole drilling calls for accurate surveying. The boreholes should be made by a drilling machine like the VOLSAFE drilling machine which can control flow of water during drilling. Dewatering of the top seam from the bottom flow of water furring drilling. Dewatering of the top seam from the bottom seam may be done in stages if the depillaring in bottom seam is to proceed from rise to dip.
(2) Plans:
The plans are brought up-to-date showing all the new constructions, position of boreholes, staple pits, etc. The position of fault planes and dykes in the leasehold and also in the area of an adjacent colliery should be shown. The goaf area of all these seam in the same mine and the goaf areas of the adjacent mines also should be indicated. History of mining operations in the adjacent mines should be ascertained from role experienced Managers, over men or surveyors.
Sufficient allowance should be made for inaccuracies in the plans which may be intentional or unintentional. Plans and history of the adjacent abandoned mines will be available from the office of the D.G.M.S.
The thickness of barrier in adjacent mines that have been depillared should be treated with caution as the barrier might have been “robbed” during retreating though the plan shows adequate thickness.
Plans of the area to be depillared are prepared on a large scale showing the panels, the pillars to be extracted, the sequence in which pillars have to be split and extracted, and the pillars to be left intact. Pillars underground should be numbered with red lead and the pillar to be left intact should be marked all-round the perimeter with a band of lime wash 300 mm wide at a height of 1.5 m from the floor.
Tracings of such large plans should be given to the under managers, over men and also to the mining sirdars.
(3) The adjacent mines likely to be affected by depillaring e.t.; mines on the dip side that will have to pump extra water, have to be informed.
(4) Adequate numbers of supervisory staff, workers and specially the mines who have experience of depillaring, have to be appointed; strength of timber men may have to be increased,
(5) There should be adequate stock of the following materials-
(i) Timber – The demand is heavy during depillaring. In a thick seam the size of props normally not required during development will be in demand during depillaring. Construction of cogs requires logging sleepers in a large number.
(ii) Firefighting equipment and fire sealing equipment e.g., stone dust, bricks, lime, cement, C.G.I, sheets, flame proof brattice cloth, etc.
(iii) Protective equipment like helmets, hoes, etc.
(6) Production during depillaring is likely to be high and the magazine capacity, number of tubs in circuit, haulage capacity and strength of trammers may have to be increased.
(7) Capacity of pumps should be increased to deal with extra water coming underground through surface cracks.
(8) Out-bye pillars have to be stabilised by sand stowing or substantial timber supports, preferably chocks.
(9) Isolation stoppings must be constructed to form artificial panels of convenient size within which any heating or fire caused during depillaring may have to be confined and sealed. Sized of artificial panels should be such as to complete the extraction within the incubation period.
(10) Systematic timbering rules are to be framed and explained to the supervisory staff and timber men. A copy of the rules, suitably illustrated, has to be posted at conspicuous places in the mine. Systematic timbering is compulsory in depillaring areas.
(11) In a mine every working place, where practicable, should be provided with at least two ways affording means of escape to the surface. The fact should always be borne in mind for depillaring districts where the operations are usually hazardous. It is, however, not always possible two escape routes e.g., during splitting.
3. Depillaring By Caving
(Method):
We shall consider the operations in a seam of:
(A) Thickness upto 3.0 m
(B) Thickness 3 to 6 m
The pillars formed during development are split into small pillars called “stooks” which are then extracted one by one. A gallery driven in the pillar for this purpose is called a “split”. At shallow depth, below 100 m depth, pillars during development are small and they can be extracted without splitting.
General, a pillar greater in size than 18 m x 18 Tm is split up into two stooks. In a large pillar of 30 m x 30 m size between centres, one split 3.5 m wide in the middle of the pillar along the strike and another split of similar width in the middle of the pillar along the dip, are driven so that the pillar is divided into 4 stooks. The splitting may start from 4 middle points of the pillar simultaneously.
The Coal Mines Regulations permits splitting of a maximum of four pillars when pillar extraction is about to begin. The width of split gallery should not exceed the width of development galleries at that depth (Reg. 100). Once the pillar extraction is commenced, the splitting of pillars may be done upto 2 pillars (maximum) ahead of the pillar under extraction.
During development, height of a gallery does to exceed 3 m. If the seam is thick, the development galleries and the split galleries are heightened so that all the coal upto the roof is removed using portable wooden stools to stand on. If the roof consists of shale, 0.6 m thickness of coal is left intact in the roof as shale has a tendency to part from roof rocks when exposed to atmosphere for some days. Props are erected generally 1.2 m apart and cogs 3 m apart for roof support.
In circles indicate the pillars which will be attached at a time. The plain numbers (uncircled) indicate the stooks which will be extracted simultaneously to maintain a diagonal line of fracture.
Due to weight of roof on the pillars their coal is loosened and can be obtained easily by miner’s picks without blasting.
A pillar 30 m size from centre to centre is generally 26 m corner to corner. Splits of 3.5 m width give stooks of nearly 11 x 11 m. The miners attack the stook from the side remote from the goaf or barrier so that the coal in the stook always stands between the miner and the goaf and protects him from the falls of roof in the goaf.
The arrows indicate the direction of attack by miners. The smaller pillar M indicates the fender or Chowkidar, that is, the solid coal of stook or pillar left intact during extraction of the stook or pillar. Roman numerals indicate the sequence of extraction of pillars and stooks.
(a) Roof is good and reliable.
(b) “Half moon method”. Roof reliable.
(c) Roof is bad.
(d) Roof is bad and unreliable.
(e) Least area of roof is exposed.
The loaders or miners form a team of 3 to 4. They drill the holes the shot-firer blasts them and the miners dress down the coal and load into tubs or mine cars. One team requires a span of about 4 to 4.5 m on the side of the stook.
At one time 3 to 4 such stooks are under extraction and the other miners are employed on splitting and gallery heightening jobs.
During extraction of coal a rib about 1 metre thick is left against the goaf. When all the slices are removed, and the area is to be abandoned, the rib coal is “robbed” to the maximum extent possible. When no more coal is extractable from the stooks, without endangering safety, props and roof supports are withdrawn and the area is then abandoned. Chocks are erected at goaf edges and the area is fenced off.
4. Depillaring of Contiguous Seams
:
The method of extraction of contiguous seams depends primarily on the thickness of the parting. In the development galleries are driven in the seams such that galleries and pillars are vertically coincident. In some mines, a seam thicker than 7.5 m is developed in two sections, leaving a parting of 3 m and extraction takes place as if there were two contiguous seams.
If the stone in the parting between the two continuous seams, less than 1 m thick only one seam is developed and the two seams are treated as one seam during pillar extraction. The stone parting is blasted out and thrown in the goaf.
If the stone parting is between 1 and 3 metres so that it is not economic to blast out the stone, the seams or sections are depillared simultaneously, that is, the depillaring is conducted in the two seams or sections such that the line of extraction of the lower seam is vertically (or nearly so) below that of the upper seam (Fig. 12.6).
If one of the seams or sections is not developed, entry is made into the virgin seam through a drift starting at a suitable point in developed seam. The undeveloped seam is partially developed for one or two pillar-lengths at a time and pillar extraction is carried out simultaneously, keeping the supports almost vertically coincident.
The withdrawal of supports is done simultaneously. If the top section supports are withdrawn before those in the lower, the roof fall in the top section punctures the parting and affects the bottom section’s supports; if bottom seam supports are withdrawn first, the parting may collapse and supports in the top section may be lost in the collapse. Coal of both the sections is loaded at one point wherever possible.
When the stone parting is thicker than 3 m the two seams are developed with galleries and pillars vertically coincident either throughout the mine or a panel, and extraction proceeds in the two seams at the same time almost simultaneously with line of goaf in the top seam not more than one or two pillars ahead (i.e., out bye) of the bottom seam line of goaf.
To co-ordinate the work in the seams, it is better to have the depillaring districts of the two seams under the supervision of one overman. A good system of communication is essential.
This takes the form of:
(i) Telephones
(ii) Signaling bells connected by wires laid through a borehole drilled in the parting
(iii) A 18 mm diam. pipe fitted in a borehole in the parting; the pipe is recoverable.
With the parting between the seams thicker than 9 m it is not necessary to insist on simultaneous pillar extraction. The usual arrangement is that the seams are extracted in a descending order.
The following points may be borne in mind:
1. The extraction in the lower seam should be conducted under a settled goaf of the top seam so that the parting is not subject to the impact of kinetic forces let loose when the strata movements in the goaf takes place. A period of 3 to 5 years is considered sufficient to allow the goaf to settle.
2. Steps should be taken to guard against inundation from water-logged top seam goaf.
3. The main fall is much quicker if top seam is goafed and extraction must be planned with this in view.
4. Heavy timbering is essential because of the dead weight of top seam goaf.
Working over the Goaf of a Lower Seam:
Under Regulation 104, no work in a higher seam or section shall be done over an area in the lower seam or section which may collapse. The working in the upper seam is, therefore possible only when it is virgin and the lower seam goaf has settled down.
It takes 3 to 5 years for a goaf to settle down. An attempt can be made to work such virgin seam if the parting between the seams is 30 m or more. With a parting less than this, the top seam may be assumed to be unworkable.
If the top seam is developed before depillaring in the bottom seam, roof of top seam develops numerous cracks with depillaring in bottom seam and it is too dangerous to work in top seam.
A virgin seam over goaf of a bottom seam was worked at Bhatdih Colliery (Jharia field) and at another mine in Raniganj field working Koithi/Poniati seams. The following observations are based on that experience.
In one area where a higher seam (Koithi), was to be worked over the goaf of a lower seam (Poniati), it was found that the top seam was disturbed in patches, and in some places, titled. Parting between the two seams is 40 m.
Where the lower seam was extracted in panels, upper seam portions over the panel barriers remained intact while there was noticed an abrupt dislocation over the periphery of the excavated portion and the virgin seam became somewhat trough shaped in the area corresponding to the middle of the excavated panel of lower seam.
These conditions made it difficult to work the higher seam with coal-cutting machines and to plan for normal direct or endless rope haulages. Workings were, therefore, done by pick mining and main and tail haulages were installed to cope with undulating gradients.
In the zone of sudden dislocation and fractured ground, roof conditions were bad and close timbering, mostly cogs, with occasional use of fore poling, had to be resorted to. Care had to be taken to dewater waterlogged areas of the lower seam when approaching a level in the higher seam which was likely to be at par or lower than the water logged workings of the lower seam.
Precautions have to be taken against gases from the lower seam finding an access to the higher seam through cracks. Ventilation by forcing fan was found to be advantageous for this reason. Doing to the risks of cracks and hardened coal, permitted explosives of adequate strength had to be used. Now-a-days solid blasting is the general practice since coal cutting machines cannot be used and precutting at a face is out of question.
Coal bumps generally occur in deep mines. These are sudden violent bursts of coal from pillars, usually accompanied by an air blast and shooting of coal pieces at terrific speed. The amount of coal ejected in a bump may be a few hundred tonnes.
Gas may also be released. Coal bumps are the result of a sudden release of elastic strain energy stored in the pillars. They are in this respect similar to rock bursts. Coal bumps are found only in coal seams with specific geological and mining conditions. There are two types of coal bumps; shock and pressure bumps.
Shock bumps occur where a strong massive stratum lying immediately above the coal ruptures as a beam or flat arch and sets up a shock wave that is transmitted downward to the coal pillars. If the underlying coal pillar is already highly stressed, the transmitted shock wave may cause the pillar to bump.
Pressure bumps occur when weak pillar is stressed beyond its strength and fails suddenly and violently. It will be obvious that bumps occur when stresses larger than coal strength are induced, whatever be the cause.
Bumps occur usually under the following conditions:
(1) A strong and brittle coal that does not crush easily,
(2) Depth of over 150 m
(3) A strong overlying rock stratum such as sandstone that overhangs into the goaf,
(4) A strong floor rock that does not heave readily, and
(5) Improper mining methods that create localized extensive stress concentration.
The last item depends upon the planners of the mine working whereas the first four items are the creations of Nature.
5. Behaviour of Roof and Its Control during Depillaring
:
When the galleries are widened or heightened, or splits formed, the equilibrium of pressure that had stabilised during development stage in the strata overlying the seam and immediately below it is affected and the re-adjustment of pressure causes roof movement.
When the stook or pillar is extracted, such roof movement is more pronounced due to the removal of the support so far offered by the solid coal of stook or pillar. Sudden transfer of pressure to any pillar or stook may result in crushing and the aim during depillaring is to avoid sudden accumulation of pressure over standing pillars or stooks and the supports erected out-bye the goaf edge. Once the roof movement sets in, the kinetic energy is difficult to control by supports.
During depillaring, no splitting or reduction of pillars or heightening of galleries should be done beyond a length of two pillars ahead of the pillar under extraction. Galleries which are heightened or widened should be supported by rows of props, 1.2 m apart and by chocks 3 m apart. Chocks are necessary at junctions of the galleries, or splits and at goaf edges. Systematic timbering rules specify the manner of supports and the spacing of supports.
Supports should be erected without delay in the void formed due to pillar extraction, leaving enough room for movements of workers, swing of tools, movement of tubs, etc.
Even when supported by timber the area exposed should be kept to a minimum. With a good sandstone roof, the area explored may not be more than 40 m2 at one stook extraction. With 3 or 4 stooks under attack at one time, the total area should not exceed 900 m2. Each case, however, should be decided on the experience gained about the nature of the roof and its behaviour.
The roof strata are relieved of the accumulate pressure energy when the pressure breaks up the roof. Once such energy is dissipated in the breakage of roof the pillars are not under stress and working conditions are comparatively safe. It is therefore, necessary to see that the roof in the goaf area is induced to fracture at regular intervals.
The Regulations require that the depillaring operations should be conducted in such a way as to leave as small as area of uncollapsed roof as possible, and there possible, suitable means shall be adopted to bring down roof in the goaf at regular intervals.
Such suitable means consist of:
(1) Withdrawing all the roof supports beyond the goaf edge so that the goaf is unsupported and the roof breaks due to pressure of superincumbent strata.
(2) Drilling a few holes in the roof near the goaf edge and blasting them, or
(3) Changing the line of goaf or line of face.
The collapse of roof in the goaf which is deliberately brought about by withdrawal of roof support is called normal collapse as distinct from a premature collapse. Such normal collapse is an everyday incident in the depillaring area and steps can be taken to prevent danger to workers or property when the collapse or roof fall is anticipated. Premature collapse, as the adjective implies, occurs at a place where collapse is not expected and is therefore dangerous.
A strong thick stratum of massive sandstone or a sill in the roof, immediately above the seam, will delay the fall of roof and when a sufficiently large number of pillars are extracted the main roof will ultimately break with the build-up of pressure. The weight of the strata is transferred, before the fall takes place, over the pillars on the outbye side. If the pillars are not strong enough, they may be crushed due to such sudden heavy pressure.
This phenomenon is known as overriding of pillars and stooks and results ultimately into premature collapse in the area. This shows the importance of stabilising weak pillars outbye of the area under extraction in a depillaring district and such stabilisation is an essential step before commencing depillaring.
Ensuring Regular Roof Fracture:
The supports are withdrawn when all, or nearly all, the coal in the stook is extracted, or when the roof shows signs of heavy roof pressure.
The indications available of roof weight are:
1. The prop, when tapped with a stick, produces a dull and drummy sound.
2. The lid over the top of the prop bends.
3. The prop buckles in the middle or at weaker points in its length.
4. If the floor is soft, the prop may penetrate it.
5. There may be rumbling sound in the goaf.
6. If a small crack existed in the roof and a wedge is hammered in it when supports an erected, the wedge becomes loose and falls as the crack widens when the roof pressure increases and roof tends to come down. This method has been used to get indication of roof pressure at Giridih Colliery and at other collieries.
7. In some cases water percolation from the roof takes place as the cracks widen. This is the first indication of roof movement. With more roof movement the crack may widen further with increased flow of water or the flow may cease altogether as the entrance of water into the crack may be closed by movement of rocks. This had been observed at Giridih Colliery.
8. Intermittent fall of loose rock piece from the roof in goaf increases, and small coal pieces shoot off the sides.
9. Booming sound of roof movement increases.
10. Heaving of floor is noticed.
The time of withdrawal of props has to be decided by experience. If the withdrawal is delayed, the increasing roof pressure breaks them and the weight has to be borne by the outbye stooks which may get crushed and be eventually lost.
All the props and chocks have to be withdrawn only by mechanical prop withdrawer or similar contrivances. Hammer should not be used to dislodged a prop from its position. The furthest row of props in the goaf should be withdrawn first and the withdrawal should proceed towards the safer place.
As far as possible, no supports should be left in the goaf in erected position as they retard the regular roof break. If a prop cannot be withdrawn it may be destroyed. The chocks should be erected at the goaf edges before prop withdrawal and the openings out bye the goaf edges should be strengthened by erection of props and bars. Such goaf edges should be fenced off after all the timber is withdrawn. It is not always possible to withdraw all the props and cogs, particularly those at the goaf edge.
After timber has been withdrawn the unsupported roof tends to fracture. The lower strata of the immediate roof separate and fill in the goaf. The fall which takes place soon after withdrawal of supports is called local fall. Where the stratas are well stratified sandstone or shales, the local fall takes place within 24 to 48 hours of the withdrawal of timber.
Such local fall does not extend to the surface and rocks within a height of only a few metres above the seam is affected. The intensity of the booming sound in the goaf due to roof movement gradually increases after withdrawal of supports and ceases after the local fall.
Air Blast:
When a roof fall takes place, the air in the goaf area is displaced as the latter is filled by the broken roof rocks. The displacement is so quick that the air is pushed out at the high speed resulting in air blast if the air has limited outlets to escape. The intensity of the air blast depends upon the volume of the air that is displaced.
If the immediate roof is massive sandstone or has a sill, the local fall does not occur for a long period even after withdrawal of supports. The volume of air that is displaced in such case when a roof fall takes place is large. Similarly the volume of air displaced in a thick seam is also large.
The air blast in these cases is strong and dislodges roof supports and ventilation doors, damages isolation stoppings and injures workers in the path of the air blast. The dislodged roof supports may initiate some additional roof collapse.
Precautions against Air Blast:
1. Steps should be taken to see that an extensive area of uncollapsed goaf does not exist at a time. Where possible, steps should be taken to bring down the roof at regular intervals.
2. In any depillaring district, all the workers should be withdrawn to a safe place when the usual indications of imminent roof fall are observed. Where immediate roof is well stratified sandstone and shale, the work has often to be suspended for 2 to 3 hours or more before the roof fall takes place.
If the roof consists of massive sandstone, the sound of roof-movement in the goaf lasts for 8 to 16 hours and coal-getting operations in the area have practically to be suspended for such a long period. The mining sardar or overman should warn the workers by a whistle to retreat to a safe place which is not directly in the path of air blast.
3. Apart from the entries for ventilation and haulage, additional roadways should be kept open in a thick seam.
4. After every blast there is usually a thick cloud of coal dust in suspension in the air. The electric switches should be put off before such anticipated air blast and put on only after coal dust has settled down.
5. Construction of a few isolation stoppings with an easily breakable zone (by air blast) should receive attention.
After the roof fall has taken place all the supports and the roof should be examined before workers are allowed to the working place. Special attention should be given to the goaf edges which should be well fortified by supports after the roof fall.
It has been observed in some collieries where the immediate roof is a massive sandstone that some districts may have areas of uncollapsed roof as extensive as 60 m x 50 m. One or two props erected furring pillars extraction are left erect in the area when all other roof supports are withdrawn.
Such props serve to indicate roof weight and the mining sardar observes the condition of the props from the goaf edge by a powerful torch. The roof fall in such extensive goaf area is a serious matter.
The local does not extend to the surface as the broken shale and sandstone occupy a large volume after breaking and fill up the void caused by coal extraction. It has been observed that when the area of extraction is nearly equal to or more than, the square of the depth, the cracks due to depillaring extend upto the surface.
The roof fall which affects the surface is known as main fall and takes place long after the first roof fall (local fall) in the depillaring district. If the seam is at a shallow depth air leaks through the cracks to the coal buried in the goaf and is responsible for spontaneous heating. Such spontaneous heating has been the cause of fires in the areas near Jharia and Katras towns.
The main fall is expedited if-
(a) Area is near a fault plane, and
(b) If the upper seam has been depillared.
At Bhagaband colliery, Jharia field where the top seam is depillared, it is observed that during depillaring of bottom seam, main falls takes place much earlier than the period normally observed during depillaring of top seam.
Depillaring operations in the vicinity of faults need more than normal vigilance and precautions for roof control. Water seepage or emission of gas may take place near a fault. The line of goaf should not be parallel to the fault, but nearly at right angles to it.
Heavy roof fall and even premature collapse may take place if line of goaf is parallel to the fault. The unsupported roof may slide along the fault plane. It is a sound practice to leave some barrier of solid coal near the fault.
A line passing through all the corners of stooks under extraction at a time is called line of extraction. It is sometimes loosely called “line of face” or “line of goaf”. A line of extraction which is diagonal, i.e., nearly 45° to the dip direction is common. With such line of extraction each pillar contributes some portion of it towards roof support.
The step diagonal line of goaf is as shown in Fig. 12.3. In this case one solid stook intervenes between two stooks under extraction at a time. The line of goaf is more inclined towards the strike than in the earlier case. Where roof is bad, the presence of one solid stook between two stooks under extraction gives good support to the roof.
As stooks are extracted, the intervening stook may be subjected to heavy pressure and its crushing indicates roof weight. Steep diagonal line of goaf may be so arranged in some cases as to have two solid stooks in between the stooks under extraction.
A line of extraction that forms as arrowhead in the goaf. The pillar or stook at the arrowhead is under excessive pressure and is liable to be crushed.
Where conveyors are used in a depillaring district, it is necessary to have a straight line of extraction so as to reduce the number of conveyors and lead for manual loaders. In Raniganj field some mines using conveyors in depillaring district have experienced no trouble with line of goaf parallel to dip or strike doing to favourable direction of cleavage planes. A line of goaf parallel to the strike has the advantage that the goaf can be submerged in water if the depillaring is proceeding from dip to rise and the seam inclination is not mild.
6. Precautions against Fire During and After Depillaring
:
Where a roof fall takes place in the goaf after withdrawal of supports, roof coal and the coal of fenders, unextracted ribs and partially extracted stooks, is buried under the roof fall. The conditions conductive to spontaneous heating are then created and are present after the first roof fall as the heat of oxidation of the buried coal is not dissipated.
The period that elapses from the first local fall till the signs of heating in the goaf is called the incubation period. It varies from seam to seam. At Churi Colliery (North Karanpura), it is observed to be 3 to 4 months; in the Kajora seam in Raniganj coalfield it is nearly 4 months.
In one mine in Raniganj coalfield working Poniati and Koithi seam, Poniati is known to show signs of heating in 9 months but Koithi is quiet even after 2 years. The top seam in the Pench Valley Coalfield has an incubation period of 9 months; lignite at Neyveli, when stocked at surface is known to catch fire due to spontaneous heating in one month. It is observed that some seams are not liable to spontaneous combustion at all.
It is necessary to take steps to prevent spontaneous heating in the goaf and to prevent the spreading of heating or fire.
(1) Depillaring operations should be carried on in panels. Such panels may be formed during the development of the district by solid coal barrier all-round the district with only essential galleries for transport and ventilation, and the panel should contain only such number of pillars as can be extracted within the incubation period which, however, is not known during the development stage unless experience is available of the adjacent collieries practicing pillars extraction by caving in the same seam.
Where the panels are not formed or, if formed, contain such large number of pillars as cannot be extracted within the incubation period, it is imperative to form artificial panels of convenient size. Such artificial panels are formed by isolation stoppings built of brick in lime or cement.
Isolation stoppings in de. 1 gassy mines are minimum 1 m thick of brick in lime or cement. If heavy air blast is anticipated during depilating the stopping should incorporate a weak zone of say 0.3 m x 0.3 m which is not 1 m thick but only 250 mm thick in the middle of the stopping so that it gives way during the air blast and provides a vent to the pressure behind the stopping.
In deg. 2 and dcg. 3 gassy mines the isolation stopping should be explosion proof. Such stopping consists of a brick wall not less than 1.8 m thick, or of two walls, each 1 m thick, 4.5 m apart having incombustible material like sand or boiler ash in between.
It has been observed that in thick seams, fire behind an isolation stopping sometimes bypass the stopping through the roof coal or floor coal and affects the coal out bye of the stopping. It is therefore, necessary that an isolation stopping should be constructed from floor stone to roof stone.
(2) Depillaring operations should be conducted from dip to the rise, so that goaf can be submerged in water. If however, the seam has a mild gradient or if depillaring is conducted with a diagonal line of extraction, the goaf on the rise side is not drowned under, water.
Blackdamp which may be formed during depillaring in some seams tends to the dip areas in the goaf; and by blanketing the fallen coal it helps in preventing its oxidation.
(3) An attempt has to be made not to leave any coal in the goaf as far as practicable. This, however, becomes difficult unless the roof is good and permits complete extraction from stooks.
(4) In shallow seams, to prevent breathing of air to the depillared area through cracks to the surface, the cracks have to be blanketed with a layer of sand or earth nearly 1.2 m thick. Such blanketing is often done on the surface before depillaring commences. After the cracks are formed, it is dangerous to cross the cracked surface.
7. Precautions against Inundation during Depillaring
:
These may be summarised here as follows:
1. Enough allowance should be made for inaccuracy in the plans. Inaccurate mine plans have been the cause of accidents due to inundation in 90% of the cases during depillaring. All the features required to be shown in the plan under the Regulations should be clearly shown. A common omission is boreholes, as these are not conspicuous, and often forgotten after prospecting.
2. When depillaring zone is within 60 m of waterlogged area, advance boreholes have to be drilled. “Volsafe” machine for drilling at any angle is suitable for the purpose.
3. Depillaring below a water-logged area should be avoided. Streams, rivulets, etc. on the surface should be diverted if the expenditure is justifiable.
4. If overlying strata contain a water-bearing stratum, unless pumping capacity is adequate, caving method should not be adopted.
5. No depillaring operations should be conducted in an area which is likely to cause subsidence of the surface below the highest flood level of river, stream, or lake.
6. When depillaring in bottom seam is to proceed from rise to dip, the top seam may be dewatered in stages by advance boreholes from bottom seam with prior permission from the DGMS. As illustrated in Fig. 12.5 when depillaring takes place at A, advance boreholes may be made at C to dewater the top goaf.
