Mining Methods: Longwall, Horizon and Level Mining Methods!
Apart from the commonly practiced “bord and pillar” method of mining followed in this country, the other methods of mining adopted in some collieries are: 1. Longwall Mining Method 2. Horizon Mining 3. Level Mining or Semi-Horizon Mining
Room and pillar method of mining is not practiced in any of the collieries in India though it is the standard method adopted in American mines. Horizon mining a term wrongly used by some for a method of mining known by a distinct nomenclature, level mining, but the former term only covers a method of approach to the coal seams and is not a method of extracting coal. Where horizon mining is adopted, the method of mining may be longwall advancing, longwall retreating or any other method of coal extraction.
Method # 1. Longwall Mining Method:
Longwall method of working consists in laying out long faces (60-200 m long) from which all coal in working section of the coal seam is removed by a series of operations, maintaining a continuous line of advance in one direction and leaving behind the void (called goaf).
The roof over the goaf is partially or completely supported by walls of stone (called pack walls), sand or other material like crushed stone to prevent collapse of roof and only a small strip 3 to 6 m wide and parallel to the face is supported by timber or steel props, bars or chocks in a systematic manner. Alternatively the roof over the goaf is allowed to cave in but the roadways are secured by packwalls and chocks if they have to be used (as in longwall advancing method).
In longwall advancing extraction of coal commences from the vicinity of the shaft pillar and proceeds outward towards the boundary of the mine or panel. Approach to the face is by parallel roads, formed at a specified distance apart which is equal to length of the face.
The roof over the goaf may be supported by packwalls or sand stowing or allowed to cave in. In longwall retreating pairs of headings are driven in solid coal, certain interval apart, to a predetermined boundary where they are connected by a long roadway to provide a longwall face.
Extraction of coal then commences from the boundary and the coal face retreats towards the shaft. In this system goaf packing is not essential for roof support if subsidence of surface or strata above the seam is permitted.
Longwall method is the standard method of working in coal seams in Britain, Germany and other European countries. In India only a few mines have adopted it and that too in conjunction with hydraulic sand stowing, with very few exceptional cases of longwall with caving.
The roads at either end of the face are known as gate roads. Usually the coal transporting gate road is the intake airway and the other gate road is the return airway. The coal transporting gate road is called the haulage gate and the other gate, tail gate.
If rope haulage is used in the haulage gate, it serves for material supplies and also for coal transport. But if a belt conveyor is used in the haulage gate, the other gate road (tail gate) serves for material supplies and is called supply gate.
Fig. 13.1 shows a layout of single-unit faces and two short length double-unit faces. Where the gate belt conveyor (or other transporting medium) serves only one longwall face, the latter is known as single unit face; where the gate belt conveyor (or other transporting medium) is centrally situated to serve two adjacent longwall faces progressing in the same direction, the combined face is known as double unit face.
Fig. 13.2 shows a longwall advancing double unit face with hydraulic sand stowing. If hydraulic stowing is practiced the two longwall faces should be so staggered that stowing water of rise face does not flow to the dip face but is drained away by the central gate.
Longwall method is normally employed to work thin seams with thickness ranging from 0.6 to 2.4 metres lying at moderate depth from the surface, with inclinations from flat to 20°.
It has the following advantages over pillar and stall methods:
1. It is simple and offers concentration of work areas, being capable of giving the maximum yield per hectare of coal seam area. Concentration of work permits good supervision.
2. All the seam section is extracted in one operation, enabling the maximum extraction percentage.
3. Ventilation of working is rendered easier with simple and direct air routes.
4. The roof weight acting on the face assists in loosening the coal, yielding the greatest proportion of large coal. Cleats and slips in coal can be advantageously used to make easier winning and with proper attention to strata control techniques, friable and weak coal can also be won without much difficult.
5. Dirt bands in the seam can find useful purpose in packing of the goaf.
6. Floors liable to creep can be better controlled in this system as there are only a few roadways to be cared for.
7. Seams liable to spontaneous combustion can be satisfactorily worked by this system as conditions exist whereby all the coal which is a potential material of heating is removed.
8. It lends to mechanisation with the least capital cost per te of annual output. Where longwall advancing is adopted, it makes possible a quicker return on investment by enabling the mine to attain its optimum output within a short time.
9. It provides the most successful method for working beneath another worked-out seam.
The main disadvantages of the system are:
1. In longwall advancing method roadways are to be maintained in worked-out areas and entail substantial recurring maintenance costs. Sometimes there is convergence of roof in the roadways which are required to stand for long.
2. In the case of longwall advancing with caving a large expanse of goaf left behind constitutes a vast reservoir of firedamp, a potential source of danger.
3. In the case of longwall advancing with strip packing, if roadway packs are not well maintained against leakage, ventilation current may short-circuit through the goaf, which is both wasteful and dangerous (as it may cause incipient heating of small coal left in the goaf).
The longwall retreating system combines the advantages of pillar and stall method and longwall method. The roadways formed in the initial stages are supported by solid coal pillars and the property explored during the formation of roads – an advantage with pillar and stall working.
When the retreating longwall working commences the face offers the additional advantages of longwall method. Moreover spontaneous heating, if any that may occur in the goaf can be easily isolated. Ventilation planning is easier, leakages being very few. These combined advantages make longwall retreating a popular choice if longwall has to be adopted in a mine.
Application of the method therefore lies in working:
1. Thin seam (as thin as 0.7 m).
2. Seams with dirt bands.
3. Seams with tough roof which can be induced to bend gradually and settle on packs, or seams with weak friable roof which may cave in the goaf.
4. Contiguous seams, if solid packed faces are laid out causing little disturbance,
5. Gassy seams, which require meticulous planning of ventilation, and
6. Seams which are to be mechanised for large planned outputs.
As longwall method with caving requires a breaker row of props of high and uniform mechanical strength, the length of mechanical (friction) or hydraulic props available, limits the height of lift or section in longwall method. A high prop is unwieldy to handle, has bending tendency when under heavy pressure, and when the roof caves in, the prop is likely to be dragged in the goaf.
In longwall advancing the gate roads extend slightly beyond the face and the extended portion of the gate is called stable. Stables have to be provided where belt conveyor or rope haulage is used in the gate. Some types of machines used for cutting or cutting-and loading at the face require stables for their proper utilisation or reversal.
In longwall advancing stables are usually 8 m to 10 m ahead of the face. They enable the area ahead of the face to be explored in time for taking measures to deal with any irregularity without serious disruption of production. If stables are kept much ahead of the face their ventilation poses a problem. Formation of stables is obviously not necessary on a longwall retreating face.
Method # 2. Horizon Mining Method:
Horizon mining is a system of mining, applicable to inclined or undulating seams and also to relatively flat seams where these occurs in groups whereby all the coal seams are extracted between predetermined horizons, levels, or planes.
It involves driving main roadways horizontally (or almost so) through the measures or strata from the shaft at pre-arranged intervals of depth, and these road-ways form, as it were, the main arteries of the mine, through which coal is transported throughout the life of the mine, or of the horizon concerned.
At least two levels are driven at different horizons; lower level, called the haulage level, is used for haulage and serves as intake airway and the upper level called the ventilation or return level, is used as return airway and supply road.
Connections are made to each of the seams lying between these two levels and the portion of each seam intersected by the levels is divided into sections of suitable size either by staple or blind shafts or, in rare cases, by inclined roads. (Fig. 13.10).
Laterial drifts or roads, or simply laterials, are those roads driven parallel to the strike from the shaft and they may be sited in one of the coal seams or, more usually, in the strata below the lowest coal seam in the horizon concerned.
The term cross measure drifts, or simply cross-cuts, is used for all the approximately level main roads driven in rock at right angles to the line of strike, i.e., in the direction of the full dip or rise of the strata.
In general, the cross-cuts in the various horizons should be driven directly above one another. A network of these roadways, laterials and cross cuts driven at the same depth of horizon, constitutes a horizon or level. Vertical distance between horizons is 60-200 m.
Horizon mining is actually not a method of mining in the sense longwall or bord-and- pillar is, but is a method of lying out the workings and roadways in a coal seam and cross measure strata for speedy transport.
The actual method of mining may be longwall, bord and pillar or room and pillar though the method that has been normally adopted has been longwall advancing or longwall retreating as the countries that have first tried horizon mining and later developed it, were accustomed to longwall methods of mining.
The usual methods of transport are the conveyors on the faces and gates, spiral chutes in the staple shafts leading to haulage level, and locomotives in the haulage level. Seams are worked in descending order.
Layout for Horizon Mining:
Fig. 13.10 illustrates the system in its simple form, having regard to single, uniformly inclined seam lying to the dip of the shafts.
The first step is to drive main cross-measure drifts. XF and YE respectively, from the shafts in the direction of the full dip of the strata to intercept the seam at points F and E. The first (upper) drift XI7 forms the main return airway, and the second (lower) drift YE forms the main intake airway for the first area of coal to be worked, namely ABCD on the plan.
Ultimately, air will pass down the DC shaft, along drift YE, then ascension ally up the seam from E to F, and so to the UC shaft via drift FX. The vertical interval between the two drifts varies in different cases according to the dip of the bends, the number of seams to be worked, and other factors, but may range between 60 and 200 metres.
The next stem (in the particular case under consideration) is to drive lateral level roadways EC and ED in the seam (see plan) from point E, at right angles to the main drift YE. The roadway CED represents the main intake and conveyor level for the first pair of working faces, the loading point into tubs or mine cars being at E.
Thereafter, the seam may be opened up by one of the two methods:
1. The first method is to drive a main centre gate in the seam direct to the full rise from E, with two companion return airways which connect up with the main return drift at F. From these rise roadways other intermediate level gate roads GK, GL, HM, HN, FA, and FB (in succession) are turned away at right angles, so dividing the area ABCD into three steps, each 200 to 250 metres wide.
2. The second method is to drive the rising return airway and the intermediate level roads as before, but to dispense with the centres gate and drive vertical staple pits, RH and SG, upwards from the main intake drift YE to pierce the seam at, or near, points H and G.
In both methods, each strip of coal is worked in the usual way by longwall advancing along the strike of the seam, with the faces on the line of full dip and rise, the coal being conveyed down the faces to the loading gate at the lower end, namely to ED and EC for strip 1, GK and GL for strip 2, and HM and HN for strip 3. For strip 1, the coal is then conveyed along the bottom level to the loading point at E from which it is taken to the shaft by locomotive haulage.
In the first method, so far as strips 2 and 3 are concerned, the coal, after reaching G and H, is conveyed down the seam-confined centre gate to point E. In the second method, however, the coal, after reaching G and H, is delivered to a spiral conveyor situated in each of the staple pits and is loaded into tubs on the main intake drift at S and R.
It is generally considered better to adopt the staple pit arrangement which cuts through the strata and follows the shortest route from the seam to the main haulage road, thereby cutting out the centre rise gate which is subject to ground movement and is more costly to maintain, but local conditions will determine which is the better method in a particular case.
If stowing material is required at the faces, it is sent down the UC shaft and taken via the main return drift to point F whence it is transported by conveyors to the rise end of the faces.
When the area ABCD is exhausted, the lateral roadways ED and EC may be extended in both directions to open up further areas of coal, similar to ABCD, on either side between the same two horizons. The number of such panels of workings will obviously depend on the laterial distance to which the coal seam extends, or is to be worked.
When all the coal between the first two horizons has been extracted (or rather whilst extraction is still taking place) another main intake cross-measure drift ZT is set out from the DC shaft at a lower level, to intercept the seam at a point lying to the dip of E.
The lower are of coal between the second and third horizons are then worked in a similar way to ABCD, drift ZT becoming the new main intake and drift YE the new main return for that area.
Later, a fourth horizon may be opened up at greater depths if the reserves of coal justify the cost of development.
In practice, of course, each main cross-measure drift normally pierces a number of workable coal seams which can all be extracted in succession and in descending order between each pair of horizons. This is illustrated in the Fig. 13.11 (for two seams A and B).
The figure gives a more or less perspective view of two horizons (No. 1 being the upper horizon and No. 2 lower horizon). In this case CD and KL are the main cross-measure drifts from the shafts; DE and LM are the two corresponding laterals below the lowest seam; FG and NO are the cross- measure drifts driven backwards to reach the higher seam A and to intersect the lower seam B at H and P.
Seam A has been extracted and seam B is being worked. There are two advancing longwall faces in scam B, namely XS and YI’, each being on the line of full dip and rise. The level intake and loading gate in the seam is XY which is turned away in both directions from point P, whilst the return airway and materials road in the seam is ST at the top end of the faces and in No. 1 horizon.
The course of the ventilation from the DC shaft is along Nl. 2 main cross-cut and laterial, to point N; thence along the cross-cut NP where the air splits both ways and ascends along the faces to points S and T; and so to the UC shaft via the return cross-cuts and lateral in No. 1 horizon.
In case where the faces XS and YT are too long, intermediate level conveyor roads may be driven in the seam and the faces split up into shorter lengths.
The foregoing layout represents merely one method of development and extraction but there are other possible variations depending on the gradient of the seams, the incidence of faults, and other local factors. The general scheme of things to work a mine by horizon mining may be understood from the description and the figures.
General Considerations before Adopting Horizon Mining:
1. Large capital expenditure on shaft sinking and drifting is required before production starts. Interest on capital and depreciation of machinery as well as civil works is therefore heavy. To justify it the mine must have large reserves and production should be high over a long life. Production of at least 50,000 te per month from one pair of shafts may be considered the lower limit under Indian conditions and the life of the mine, not less than 30 years.
2. The property should be preferably virgin.
3. The reserves should be established by well organised prospecting and drilling programme.
4. The seam density i.e., the number and thickness of seams within the given vertical distance should be high.
5. The strata should be strong as each horizon requires shaft insert and long drifts that last for nearly 1/2 to 3/4th of the life of the mine.
Other factors which need to be considered before opening a mine apply in the case of horizon mining as well.
Advantages claimed for this system are:
(i) It provides the main road for efficient and adaptable haulage systems. A locomotive haulage, in its most efficient form capable of dealing with high outputs of the orders of 3,000 tonnes/day or more, is possible. Outputs are concentrated at a few loading and hoisting points, permitting mechanisation in transport and hoisting and achievement of large production rates.
(ii) It makes possible a highly efficient ventilation system, there being two separate independent roadways intake and return, without the possibility of any air losses and short circuit. Pressure difference between the intake and the return is easily maintained.
There is a further advantage in deep workings, in that, fresh air is not heated before it reaches the faces to the same extent as in in-the-seam mining development. Also emission of gas through the strata is usually much less and the fresh air does not get vitiated through its passage along the airway, an aspect which is of importance when high outputs of the order of 10,000 tonnes per day are to be obtained.
(iii) Maintenance cost of roadways is the least throughout the life of the horizons, as the roads are in stone.
(iv) It is easy to work several seams at a time.
(v) It is eminently suited for inclined (beyond 10°) and disturbed seams and for areas of high seam density.
(vi) These are stands explored geologically as the laterals and cross-cuts are driven in the initial days of mine life.
General considerations before adopting horizon mining, indicate the disadvantages of the system. High capital expenditure for development work and long gestation period are the main disadvantages. Any seam which is not worked before abandoning an upper level and staple shafts is virtually lost, as against the in-the-seam mining, where any unworked weak can still be worked at any time in the future.
Method # 3. Level Mining or Semi-Horizon Mining Method:
This is a system of development of thick, steeply incline d seams on the bord and pillar pattern in such a manner that excepting the “main dip” all the roads in the seam are level. The galleries are so laid out and the parting between adjacent horizons is so planned as to ensure galleries of one horizon to be either above or below vertically those of adjacent horizon to be either above or below vertically those of adjacent horizon.
The parting between the two horizons must be not be less than 3 m and the floor area of the pillar must not be less than that prescribed under the Coal Mines Regulations at the depth corresponding to the width of galleries. This method was adopted in the thick, steeply inclined Sirka and Argada seams in Karanpura field during development stage. The seams are nearly 22 m thick and the gradient approximately 1 in 2.
It is incorrect to call it horizon mining. The main advantage of this method is that all the galleries in a horizon are leveled. This adds to the efficiency of machinery and-workers and permits of high capacity transport form the face. Each horizon constitutes a panel which can be readily isolated from the other horizon in case of emergency. A property with extensive area on the strike without stone bands and without faults, special dip faults, is ideal for level mining.
Fig. 13.12 illustrates the manner of developing a seam by level mining method of working. The terms used for roadways are somewhat different as compared to the terms used in bord and pillar or longwall method. The approach to the seam which dips at 1 in 2 is by a haulage incline along the apparent dip.
A level roadway along the floor of the seam and extending upto the boundary of the mine is called floor lateral. A level roadway along the roof of the seam is called a roof lateral. A level roadway connecting these two laterials is called a crosscut. The crosscut may be at right angles to the laterals or at an angle forming rhombus pillars for transport by shuttle cars. If the seam is thick it may permit drivage of a mild lateral.
To ascertain whether the floor lateral and roof laterial are going along the floor and roof respectively, holes are drilled on the sides of the roadways by ordinary coal drills.
The ventilation system is simple. Vertically above the main haulage dip a gallery for return air is driven parallel to it in coal near the roof. The roof lateral of each horizon is joined to this main return airway which is connected to an up case shaft.
The depillaring of seams developed by level mining in the absence of stowing possess problems and so far no suitable method has been devised for it. Sirka seam at Sirka colliery which has been fully developed by level mining is going to be extracted by heavy earth moving machinery upto the quarriable limit of 1:3 (and perhaps up to 1:5 in future) by quarrying but the pillars formed beyond the quarriable limit may be split after permission from the D.G.M.S. and then abandoned.