DRIVAGE IN STONE (COAL MINES)

DRIVAGE IN STONE (COAL MINES)

A drift in coal mining terminology is a roadway in stone, and is driven for access to shallow seams, for passage from one seam to another, for transport, sand stowing and other purposes. Considerable drifting is essential for approach to a coal seam in horizon mining, and to cut down development period thereby saving interest charges on the capital invested in the constructional stage, drifting should be properly planned and speedily carried out. With hand held drilling machines and manual loading, speed of advance of a normal size (10 sq. metre) drift is about 5—6 metres a week, while a highly mechanised and carefully organised operation has recorded a progress of 100-120 metres per week in foreign countries.

Maintaining gradient & direction of the drift

Where a drift is to be driven to prove the throw of a fault and touch the seam on the other side, the drift should be driven, not in the proposed main haulage road through the fault, but in the companion road. This allows suitable grading of the proposed main haulage road once the fault is proved.

The surveyor has to mark permanent reference points by plugs in the roof at the starting point of the drift. Such reference points should be in the centre line of the drift and as the latter advances; master plugs are fixed in the roof after every 10 m along the centre line.

. The gradient line is marked on the side by red lead, 1m above the floor of the drift. Plugs are fixed on the grade line after every 3 m or 4.5 m. A template on which a spirit level may be mounted, is used to extend the grade line or check the gradient of the floor in day-to day practice. This is done by the overman or drift-in-charge. The surveyor checks up the centre line and the grade at frequent intervals, usually once a week, by taking levels of the rail track laid in the drift.

Drilling

For soft sandstone and shale the usual 1.25 H. P., hand held, 110 V. rotary coal drill can be used by

(i) replacing the gear box- to reduce the speed of drill rod from nearly 600 r.p.m. to approximately 240 r.p.m.

(ii) use of turbine section drill rod instead of the diamond section drill rod used for coal drilling. The diamond section drill rod wears very fast. The drill bits used for coal can be used for the soft stone also, though the life naturally is much less. The improvised drill, which normally has a rating of 20 to 25 minutes in coal, gets hot after use for 10 to 15 minutes and sufficient time should be allowed for its cooling down before re-use. Nearly 15 holes may be drilled m one shift of eight hours. The pull is only 1.2 to 1.5 m and with one handheld drill in conjunction with manual loading of muck into tubs, a weekly progress of nearly 5 m is possible in a drift of 4 m x 2 m cross section, 2-shift working.

For hard sandstone or similar rock, the same arrangement with eccentric drill bits works well, but the drilling is slower. More progress and accurate direction of holes can be achieved by the use of power feed or mechanical feed arrangement.

Drilling in hard strata is generally carried out by compressed air operated drills, usually with mechanical feed. The rate of penetration varies from 0.2 to 0.3 m per minute in hard rocks. One jack hammer drill, mounted on air leg and adopted for wet drilling is used in a drift of 4 m x 2 m cross-section. An air leg relieves the operator of the fatigue involved in holding the drill and keeping it pressed forward as it exerts an upward lift and a forward feeding pressure on the drill. An air leg does not increase the rate of penetration or feed and it is used for drifts upto2 m height (Fig. 7.8). Drilling rigs or jumbos have to be used for high speed and for large-sized drifts. The terms "jumbo" and "rig" are used synonymously. A jumbo is a portable carriage which has arms for mounting of 2 or more drills. The arms can be raised, lowered and slewed at any angle in position by hydraulic or air pressure and all the drill steels are placed in the carriage.

After the alignment and gradient are fixed, the drifting starts with drilling shot holes according to a specified form, called "drilling pattern", which consists of three or four groups of holes. The drilling patterns suitable for different types of rocks under different conditions of excavation in mining and the blasting methods are described in the chapter dealing with explosives and blasting. It may be mentioned as a broad guideline that burn-cut and wedge cut patterns are normally favoured for drifting in stone.

Blasting

All holes are charged and fired simultaneously with the use of milli-second delay detonators as the quantity of explosives is heavy. "Inverse initiation" is the general practice. "Sumpers" are connected td instantaneous detonators, "first eases" to delay detonators with delay upto 100 milli-soconds. The second easers and trimmers are similarly connected to delay detonators which give a suitable time gap of 100 milli-second or so, between successive rounds.

In a drift of 3.8 m x 3 m cross-section in sand stone the consumption of explosives per metre advance is 10 to 12 kg. It is less for shales. An average drift of 10 to 11 sq, metre cross-section in sandstones may require 30-40 shots with a total charge of 18 to 22 kg for a pull of 1.5 m.

Mucking

Mucking means removal of blasted rock and cleaning the face. Where economics does not permit of investment on equipment suited for high-speed, manual loading is carried out. The muck is packed in old galleries, not in use. When tubs are supplied at the face and men are restricted in their movement, an output of 2-4_m³ (broken) per shift per man may be expected. A double track to the face with steel sheet to traverse the tubs from empty to full track is the simplest arrangement.

Mechanical loading is a requisite when high speed and lower cost are to be achieved. The slusher loader, also called "scraper", and rocker shovel are more associated with drivage of drifts than other types of loaders.

A slusher (Fig. 7,10) is simply a main and tail rope haulage mounted on a wheeled carriage and having a ramp or slide at one end and a hopper for discharge of material at the other end. A return pulley for the tail rope is fixed at the face of the stone drift as shown in Fig. 7.10, The scraper bucket is open at the top and bottom and may be of V shape construction. It has a capacity of 0.5 to 1.5 m³. Slusher haulages underground are often used in metal mines and in stone drifts in coal mines. The slusher is clamped to rails during operation. It has a capacity of 30 to 40 te per hour and can operate on. gradients upto 1 in 3 and even steeper, but not on soft floor.

A rocker shovel has a compressed air powered bucket which is pushed into the muck, and then made to scoop the material with an upward-and-over motion for discharge into a tub or conveyor behind the loader. It may be mounted on crawlers, pneumatic tyres or wheels for travel on rails. A crawler mounted rocker shovel is best for a dipping drift (maxm. gradient 1 in 4) as the shovel, after loading, has to travel some distance for unloading into tubs. In a rising drift, at steep gradient a tyred rocker shovel, when loaded, raises problems of braking and control and a crawler mounted shovel has to be preferred. A rocker shovel requires adequate height of the drift which may not be possible in many cases. (Fig. 7.9)

Some of the machines used for mucking are now-a-days remote controlled. Such machines operate normally 4-6 m from a chute; e.g. Cavo 511 manufactured by Atlas Copco.

Ventilation

Drifting is an unproductive and much costlier work than drivage of roads in coal. In any case, it is not immediately paying. It is therefore a standard practice to drive only one drift, and not parallel drifts at close interval. The ventilation is always provided by an auxiliary forcing or exhaust fan. A small drift, maximum length 300 m, can be ventilated by a single fan, either forcing or exhausting. To avoid recirculation of air by the fan it should be placed in the path of the main ventilating air current (from which a branch is taken to the drift) at least 5 m away from the nearest corner of the drift. In_ long drifts, however, a combination of exhaust and forcing fan is preferred for efficient ventilation and speedy removal of fumes after blasting. Such fumes are drawn direct into the exhaust tube and the drift—air is unpolluted for persons travelling and working in the drift. The mouth of the exhaust tube should be within 2 m of the face and the forcing fan should be installed at least 10 m out bye the end of the exhaust tube to avoid recirculation of air {Fig. 7.12). The increasing use of diesel engines underground makes tremendous demand on ventilation systems that have to clear out dangerous or irritating exhaust gases. 200 m3/min of air is the accepted requirement of normal size drifts (say 4.2 m x 2 m) and where jumbos or drill rigs are used the standard requirement is estimated at 42 m³/min per drill. Exhaust or compressed air operated machines largely help in ventilation. I.L.O. (International Labour Organisation) recommends a quantity of 0.175 m³/s per m² of drift face area. In very hot faces upto 0.75 m³/s/m² have been used. DGMS circular no. 30 of 1973 requires the ventilation of drives exceeding 50 m in length to be such as to dilute the nitrous fumes produced by blasting to 5 p. p. m. and CO to 50 p. p. m. within a period of 5 minutes. This is however a stringent requirement in case of long tunnels. It would be more desirable in such cases to increase the permissible time of dilution or use an overlapping system of ventilation.

Working conditions in a drift are arduous; e.g. high temperature if ventilation is inadequate, rock dust produced by drills resulting into silicosis if inhaled over long periods in the absence of wet drilling arrangements, the foul and oil-charged exhausts of pneumatic machines. Noise of the drills is deafening as the sound echoes back and forth in the confined space and diesel e-ngines add to the uproar.

The arrangements for support, drainage, and transport are nearly the same as those followed when driving roads in coal.