DRIVAGE OF GALLERIES IN COAL

The methods commonly employed for drivage of galleries in coal are:

(a) mechanical method of drivage with coal cutting machines and drills.

(b) mechanical method of drivage with heading machines which cut and load the coal, like the continuous miner, coal auger, and road header.

The floor of the gallery should be parallel to the floor of the seam. This is important when the galleries are being driven in the middle or top section of the seam. Some distinctive band of shale, stone or bright coal in the seam helps to give an idea of the floor of the gallery, and serves as a guide.

Mechanical methods of drivage

Galleries are usually driven in pairs or sets of 3, 4 or 5. From considerations of ventilation drivage of minimum two galleries is essential unless the gallery has to go only a small distance. Drivage of galleries by coal cutting machines and coal drills is a common practice for quick progress of headings, large output and fast development of the area. When coal cutting machines are used the operations at the face require,

(i) giving a cut by the coal cutting machine,

(ii) drilling of shot holes.

(iii) charging with explosives and blasting,

(iv) dressing of roof and sides to render them safe.

(v) loading of coal into tubs, mine cars or conveyors.

(vi) dressing of sides, roof and floor to make the face ready for the next cut.

Blasting-off-the-solid (known popularly as solid blasting) was introduced in Indian Coal Mines in the early sixties with the appearance of P-5 explosives in the market. Prior to that the Coal Mines Regulations required that before blasting at a coal face a free face has to be created and this necessitated use of coal cutting machines for giving undercuts or over cuts at the coal face.

Skid mounted coal cutting machines were used in bord and pillar method of mining (mostly under-cutting machines) for a cut at the face to provide a free face before blasting. The introduction of explosives for "solid blasting" eliminated the need to have a "free face" resulting in gradual disappearance of coal cutting machines from the mines. In recent years, MAMC is manufacturing arc wall coal cutting machines for long wall and bord-and-pillar mining and it seems the coal cutting machines are likely to stage a comeback.

As the skid mounted machine has to pass in the space between the haulage track and the coal pillar, the galleries should be at least 4.0 m wide if double track (0.6 m gauge) is laid in the gallery and the delivery range of face pump should-be as close to the pillar as possible and preferably placed on large iron pegs in the coal pillar at about 1.2 m above floor.

The gate end boxes in the level gallery should be on the rise side to avoid damage to them by accidental slipping of the machine during flitting, an occurrence to be expected in seams steeper than 1 in 6. Length of the trailing cable Used for machines is restricted to 100 mm by the Electricity Rules and this limits the range of operation of the machine from the gate end box.

The electric coal drills are normally of 1.25 h. p., held by two' workers in hand during drilling. The hole size is 30 mm dia. and the depth may be 1.1 m to 1.5 m depending upon the depth of the cut as the hole must be at least 150 mm shorter than the cut under the Regulations if P—5 explosives are not used. In an 8—hour shift a crew of 2 workers can drill 50 to 60 holes, each 1.5 m deep.

The position, direction, depth and number of holes at a face are governed by factors detailed in the chapter on explosives and blasting.

Support

The coal pillars formed during drivage of galleries form the natural support. Where the roof is bad wide junctions should be avoided and level galleries on either side of a main dip should be staggered. Props should be avoided as far as practicable in roads having tub movements on gradients as they get dislodged by run— away or derailed tubs. In a thin seam a bar to support the roof at working places reduces the effective height and workers find it difficult to carry loaded baskets on head for tub loading. Roof bolting or roof stitching provides a good alternative for such situation.

An arc wall coal cutter (a) jib ready for cutting (b) jib at the end of a cut

Drainage

Drainage of water in the mine is effected by pumps which are designated as follows depending on the location of their installation.

1. Face pumps,

2. Stage pumps,

3. Main pumps.

Fig. 7.5 and 7.6 show the general arrangement for dewatering the dip faces. As the face advances the pump at the face also has to advance. For this reason the pump is usually small, of 5 to 15 h. p., 50 mm suction and delivery pipes, with capacity of 250 to 450 lt/min capable of developing only 15 to 30 m head, and is mounted on a trolley for ease of flitting. The controlling switch and pump starter also should preferably be mounted on the pump trolley. Centrifugal pumps are generally used but Roto pumps have greatly replaced them in recent years

.

The face sump is of a temporary nature and has a small capacity as the face has to progress daily. The sump serves one week where the face advances 9 m to 12 m per week. If the thickness of coal seam permits, the face sump may be formed by leaving in the floor a small ledge (called adkha in Hindi) 0.4 to 0.6 m high on one side of the gallery and 4.5 to 6 m out bye the face. Alternatively, in a thin seam the face sump may be formed by blasting the floor stone to give an excavation 2 m x 1.5 m x 1.3 m deep so as to have a capacity of 2500 to 3000 litres.

The pump discharges water into the sump of a semi-permanent pump on the out bye side, called the stage pump, which delivers water to the main pump. The sump of the stage pump may be formed by driving a short gallery S in a pillar if the latter is large. If the pillar is small a gallery driven to a length of about 5m  as shown in Fig.-7.6 may serve as a sump.

Small accumulations of water in a dip working at the face are bailed out by bailing majdoors with the help of buckets. The bailed out water is allowed to gravitate to the face pump of main dip or companion dipand from there it is pumped out. As a substitute for bailing majdoors small pumps operated by the coal drill are also available e.g. Rana Drill pump.

Each mine has a main sump to which all the water of the mine is pumped. Fig. 7.1 shows the main sump in an incline mine and Fig. 6.9 depicts the main sump in a pit mine. The delivery range is taken to the surface through a bore hole, through the incline (in the case of an incline mine) or through the shaft.

Because of the troubles in dealing with Water the main dip which has the pump is pushed ahead and the companion dip is driven only a few metres to avoid heavy bailing. When the main dip advances about 3 m in excess of a pillar length, level galleries are opened off it. From such level galleries companion rise galleries are driven to make connection with companion dips that have been driven some distance from the higher level.

In a mine having large make of water all the three headings need pumps and water from two headings, say A and C, is discharged into heading B. From there it is pumped out to the stage pump.

Transport

The common mode of transport in majority of our mines is rope haulage, though scraper chain conveyors, belt conveyors and locomotives have been introduced in some mines. The transport used for an advancing heading should be capable of providing transport facilities near the advancing in bye faces. Direct haulage suits well in this respect for the dip headings and the tail rope haulage for the rise headings. For level headings endless rope haulage is the common practice when the roadway is nearly 100 m or more in length, the shorter distances being covered by hand tramming before installation of haulage.

Two tracks, one for the load and the other for the empties, are always essential at the loading points at the face. The haulage track should be as near the face as possible for convenience of loaders and to avoid expenses on lead. When driving two or more headings a skid mounted machine has to pass in the space between the track and the pillars.

Scraper chain conveyors can be readily kept close to the face, within shovellable distance, by addition of pans. The scraper chain conveyor may deliver coal into tubs or into a belt conveyor. A belt conveyor is not fully utilised if employed for drivage of one heading or only one pair of headings due to insufficient coal and its extension is not as quick and simple as in the case of scraper chain.

Progress of headings

Due to water the advance of the dip headings is slow and the general tendency is to drive level and rise galleries which are free from water. Generally 2 cuts by the coal cutting machine per day in the main dip is treasonable performance. A weekly progress of 12 m or a monthly progress of 48 m is not difficult to achieve. A progress of 300 m per annum in dip headings is satisfactory with skid mounted coal cutting machines and rope haulages.

In a developing mine the aim should be to drive main headings,' particularly the dip headings. In a new mine which has no railway ^siding facilities, concentration should be on drivage of the dip headings instead of on production. To have coal production at a reasonable cost, it is advantageous to keep one development district on the strike with 5 to 6 headings. When the siding is available, or suitable arrangements for disposal of coal are made, it is easy to open districts along the strike and boost up the raisings.

Ventilation

The ventilation arrangements adopted when driving two or three headings have usually to be altered when developing a district. Ventilating air is normally coursed to the working places by stoppings, brattices and doors, but brattices and doors on haulage roads are inconvenient. Often the workers leave them in opened condition and their utility is then lost. Partitioning a roadway by brattice cloth is inconvenient as it reduces the width of the roadway and restricts the movement and speed of tub loaders. The brattice cloth partition, to be of any use, has to be extended upto the face. This is not always possible because of movement of coal cutting machines and coal loading. Auxiliary fans (F in Fig. 7.6) of forcing type with flexible ventilation tubing or ducts are good and T or V branches from these tubings can be connected. The tubing is suspended from the roof, or supported on long pegs driven in the sides, to avoid congestion of the roadway. Auxiliary fan should be so installed as to avoid recirculation of air.

In very gassy mines, drivage of rise headings may have to be stopped to avoid gas accumulation. Only level and dip headings may be driven.

(b) Mechanical methods of drivage of roads in coal with such sophisticated machines like the continuous miner, coal auger, tunnelling machines, etc. have been described in the chapter on bord and pillar development and also in Vol. 3 of this book.