Details

Types of Mine Supports: Timber, Iron and Steel Rock Mechanics Mining Technology

Type 1. Timber as Mine Support:

Timber is the most commonly used material for support in a mine as it is cheap and easily available. Dry and seasoned sal (Hindi-Sakhwa) props and bars are used in some parts of Maharashtra and Pench Valley coalfields where teak growth is plentiful. Sal props are available in lengths upto 9 m.

The size of sal props used for roof support are generally as follows:

A timber prop is sometimes called stull in metal mines.

Timber used in the mine is subjected to two main diseases during use:

i. Dry rot

ii. Wet rot

The diseased timber is soft and weak in strength. Moreover it warps and is subjected to attack by fungus leading to decay. Decayed timber forming part of mine supports has to be replaced frequently. The unseasoned timber (also called “green”) is prone to dry or wet rot.

Timber treated with chemicals in special kilns is normally not used in mines and the common method is to dry it in the open for a long period. This causes the sap in the wood to evaporate to a great extent. In the colliery store-yard the timber is so stacked that it is kept clear off the ground, free from dampness and permits air circulation to individual members. Turnover of the timber at intervals helps in maintaining the conditions which are not conductive to attack by dry or wet rot.

Setting Props:

A timber prop when erected in a mine to support the roof should yield slightly under the roof weight.

The timber prop is strongest when the load acts parallel to its length; a prop as such is almost unyielding but a certain yield is obtained by:

(a) Tapering it at the foot or top, or

(b) Providing a lid on the top of the prop as a compressible cushion between the roof and the end of the prop.

In flat seams the prop is erected vertical, and in inclined seams, axis of the prop should be normal to the dip of the seam. The prop then offers the maximum resistance to the roof. A prop which is so set that its axis is between the vertical and the normal to the seam is known as an underset prop. Erection of underset props is not common as it is not possible for the timber-man to judge whether it is underset or not.

Props should be set on solid floor and not on loose packing or debris except in case of emergency. If the prop has to be set on loose rock, loose coal, or sand pack, it should be placed on a flat base piece not less than 5 cms thick, 25 cms wide, and 0.9 metres long.

Such props on loose coal or rock should, however, be removed as early as possible and replaced by props erected on solid floor after cleaning up loose rock or coal. If the floor is soft, the base piece or foot lid mentioned here is used to prevent prop penetration in the floor. In our coal mines the floor is generally hard.

A timber prop is expected to support a roof area of about 0.4 m to 1 m all around.

The props are erected on hard floor in seams of varying thickness as follows:

(a) Roof Height upto 2.5 m:

Prop is held upright in the place where it is to be erected, a lid placed on top of it (not nailed), and a wedge is hammered between the lid and the prop to tighten the lid against the roof and the floor. Generally for this thickness, the lid and wedge are not used as two separate pieces, but the lid is so made (slightly tapering along the length), that it serves the purpose of a wedge also. If there is any crack or slip which needs support, the lid should be placed across the crack or slip.

A lid serves to distribute the resistance of the prop over a slightly wider area than its cross-section, to prevent penetration of the prop into the roof, and to indicate the load on the prop. It also helps easy withdrawal of the prop. Wedges are used to tighten the prop against the roof and floor. The lids and wedges should be at least of the same width as the diameter of the prop, of a minimum thickness of 80 mm and of a length of 0.5 metre.

(b) Roof Height of 2.5 to 4.5 m:

There are two methods:

(i) The position of the bottom end of the prop on the floor corresponding to the place in the roof to be supported is marked by a plumb bob suspended by a bamboo. The prop is held upright by 2 or 3 timber helpers. The timber man standing on a high stool or a ladder places the lid and a wedge is hammered on a position.

(ii) The above method requires a stool or a ladder to be carried from place to place for prop erection. The more convenient method is- a lid is attached to the prop by nails and a hole is made nearly 25mm deep in the floor where the bottom end of the prop is to remain after erection. The prop, laid on the floor with the bottom end in the hole, is made upright, the hole preventing the prop from slipping.

The prop is held in position by timber helpers’ and one timber helper levers up its bottom end by a crowbar bringing the lid in contact with the roof. A wedge is then hammered between the prop and the floor to tighten the lid against the roof.

(c) Roof Height above 4.5 m:

The method described in (b) (ii) is used for roof heights beyond 4.5 m also. The lid is fixed to the prop by nails. The prop is raised upright in its position of erection by helpers, with the help of a fork of the type shown in the or by a rope tied to the prop with a special knot which releases the rope when pulled. The prop is tightened up by the timber man by hammering a wedge at the bottom in the same manner as stated in (b) (ii).

In inclined seams, steeper than 1 in 5, it is usual to fit the foot of the prop into a stamp hole in the floor.

Tapered Props:

Tapered props are not much used in Indian mines. About 200 to 500 mm length of one end is tapered, and the other end is provided with a lid. Tapered end should be in contact with the hard surface. These props are used where the floor is hard and the roof, soft. With increasing roof pressure the tapered end burrs and provides yielding.

Timber Bars:

Bars act as beams. A timber bar is placed in the holes of minimum depth of 500 mm in the side of a coal pillar if the sides are strong. If the sides are weak, the bar is placed on vertical props. If the bar is circular in cross-section, the top end of the prop should be hollowed out to fit the contour of the bar.

Alternatively the bar may be flattened at each end so that the ends can rest on flat ends of the props. The bar should always be tight against the roof and should offer the maximum area of contact against it. The side of the bar towards the roof is chopped flat for this purpose. Flat wooden laggings are also used if the roof is uneven.

Safari Supports:

The conventional method of supporting galleries in coal mines is by means of wooden cross bars. For fixing these cross bars, holes are to be made in the coal pillars manually by crowbar. This is time-consuming and the whole operation of fixing one cross bar, this method takes about 2-21/2 hours.

Therefore the supports lag much behind the working face. For quick setting of the cross bars, the manual cutting of holes in the coal pillars is eliminated by drilling holes with the usual coal drills and a support, known as safari support, is installed to support the roof.This support consists of a pair of clamps of mild steel on which a cross bar is placed to support the roof. Each clamp consists of an angle iron frame to which semicircular m.s. bracket is welded as a seat for the wooden cross bar and in the angle iron two holes, 35 mm. dia. and 175 mm apart, are provided for two m.s. rods of 32 mm dia. 700 mm long.

The two m.s. rods of each clamp are inserted into the holes drilled in the coal pillar. The cross bar is placed in position over the two brackets and tightened against the roof with wooden wedges. The complete operation of setting the support is completed in 15-20 minutes for each cross bar.

The clamps can be easily recovered and used again for several times. The support stands the effects of blasting and the freshly exposed roof is supported in a short time after exposure. One of its applications is to support the split galleries in depillaring areas in thick seams and extract the floor coal, heightening the galleries upto 5 m. During extraction of stooks, wooden chocks are provided in the spilt galleries and later on the clamps are recovered completely for re-use.

Safari supports require strong coal pillars. In a thin seam, the effective height of the gallery is reduced which is a disadvantage where coal-loaded baskets have to be carried on head for tub loading.

Side Support:

Wooden laggings are placed tight between vertical props and pillar where the sides are weak and need support. Sometimes the timber set of prop and bar has to resist pressure from sides which tend to crush into the roadways. Notching is useful in such cases. The props should be set at an angle of 14° to 20° off the vertical and the feet well sunk into the floor.

An alternative method of resisting side pressure is to sink the props well into the floor and to reinforce the timber-set by an additional bar or stretcher, which may be nailed to the props. As this reduces effective height of roadway, its use may not be advisable in roadways of less than 2 m height, used by basket loaders.

Support by Wooden Cog, Chock or Chockmate:

A chock, cog or chockmate is a combination of sleepers above one another in a criss­cross manner. It supports a much larger stretch than a prop and is used in places where the roof is bad over a wide area and needs a substantial support. Cogs are also erected where main roadways have to pass through area having coal pillars of inadequate size. The term chockmate is generally used in metal mines.

Cogs are required under the Regulations at goaf edges, at junctions of splits and galleries in depillaring areas in bord and pillar workings, and at break-off line at the goaf on long wall faces.

Only rectangular sleepers, or alternatively, sleepers having their two opposite sides chopped flat, should be used. The minimum length of sleepers of a cog to support roof at a height upto 3 m may be 1.2 m but for a roof height in excess of 3 m it may be 1.5 m.

The sleepers should have a minimum roof at a height upto 3 may be 1.2 m but for a roof height in excess of 3 m it may be 1.5 m. The sleepers should have a minimum cross section of 100 mm x 100 mm.

The cog should not be normally erected on loose floor or debris, but on natural floor or on secure foundation. The floor area over which the cog is to be erected may be excavated for a depth of 25 to 50 mm and should be made nearly flat in seams of mild inclination.

Members of the cog are placed at right angles over the members immediately below in pairs. The chock is tightened against the roof by hammering wedges in it at a convenient height. When withdrawing a chock, withdrawal of such wedge loosens the chock and withdrawal of the sleepers becomes easy.

The chock should be tight against the roof and this may be tested by hammering for looseness at the uppermost sleepers.

In some cases a chock is erected in between four corner props.

Such corner props are generally not necessary except:

(a) Where the roof is very bad,

(b) At goaf edges of galleries

(c) At junctions of galleries and

(d) If the floor is steeper 1 in 5.

Where corner props are erected, one sleeper of the chock is always placed outside the prop. Withdrawal of the member helps dismantling the chock.

Support of a Roadway:

Where the roof of a roadway is bad over some distance bars resting in holes of coal pillars and tightened against the roof by wooden laggings may be erected at intervals of 2 to 3 m. If the coal pillars are not strong enough or the road is through a fault zone, the wooden bars are supported on timber props.

This is a common practice. Where the roof pressure is likely to be heavy bars may be supported on timber chocks. No props should be erected at such a place where they are likely to be dislodged by moving or derailed and runaway tubs. The bars may sometimes be placed on bricks walls constructed on solid foundation of coal floor with 150 mm layer of concrete at the base.  

Clearing Up Heavy Roof Fall:

Such occasions arise sometimes in mines. As a result of roof fall the haulage rope and cables may be buried and if the debris blocks up the roadway right upto the roof, ventilation is affected if there is no other path for ventilating air.

If the roadway is the only access to in bye workings men at the workings are stranded. It is therefore, necessary to clear up the roof fall speedily and only experienced timber men should be entrusted with the job. Power from the buried cable should be switched off.

In all such eases where the roof fall has to be cleared up work must always be done from a safe place and as the debris is cleared, supports should be set so that the place out-bye is always free from danger. In Fig. 9.12 sets of props and ban are erected at a, b, c after dressing loose roof rock and the approaches to the fall made safe.

Cogs are erected at the junction as shown at d, and bars placed over the cogs. Standing at the safe place near the cogs, the loose or hanging pieces in the roof are dressed down by the timber men with a button. The debris of roof fall is then removed by workers and packed up in the nearby galleries but where speed is essential, it may be packed along the sides of the haulage road itself.

Temporary props with thick and wide sole plate and top lids are erected on the debris to support the roof temporarily for safety of debris-clearing majdoors. The roadway is thus partially cleared so as to establish ventilation and to remove the trapped men. If the haulage rope is also freed by clearing the debris it is used for expediting the operation by loading debris in tubs.

The temporary supports are then replaced by permanent ones like cogs and bars for at least 15 m on all sides of the junction. If the place is important and life of the roadway justifies the expenditure, the place may be supported by brick-wall over which girders and corrugated galvanised iron (C.G.I.) sheets are placed. The cavity between the C.G.I, sheets and the roof is packed with boiler ash, or cogs may be erected over the C.G.I, sheets to support the roof above. This permanent measure, however, takes quite some time.

Systematic Timbering:

“Systematic timbering” is the term used for erecting supports in such a manner that the distances between supports are according to a specified pattern as laid down by the Manager and approved by the Directorate of Mines Safety.

Systematic timbering is essential in the district of bord and pillar workings where splitting of pillars or depillaring is going on, on every long wall working face, in every working in a disturbed or crushed ground (e.g. fault zone) and at other place where the D.G.M.S. may so direct.

The type of supports to be erected, whether cogs, props, or bars arc also specified in the order governing systematic timbering. In every case of systematic timbering, it is essential that additional supports shall be erected as and when necessary. Manager has to hand over copies of systematic timbering rules to all the supervising officials and has to post such copies at conspicuous place in the mine.

Withdrawal of Supports:

When props, bars or cogs have to be withdrawn it is prohibited by law to withdraw them by hammering. Suitable safety prop-withdrawer like the Sylvester prop-withdrawer has to be used.

It is an advantage to fix the chain C on the prop P G in such a manner before withdrawal that when the chain is pulled and is getting tightened, the prop receives a slight twist – on action which loosens the prop in its position.

In the depillaring areas, a long chain is often required in place of the chain C. To avoid such long length, a flexible wire rope with steel core (e.g. 15 mm dia. Coal-cutting machine haulage rope) having capels at both the ends may be used in conjunction with the usual 6 m long chain.

Where the roof is high, a suitable anchor prop may not always be available for operation of prop withdrawer. In such case a piece of rail is fixed in a half metre deep hole in the floor. It serves the purpose of anchor prop.

Alternatively, a strong bar fixed in the coal pillar may be used to serve the object. Care should be taken to see that the anchor prop or other props which provide safety to the timber men operating the prop withdrawer should not be dislodged by the prop under withdrawal when it is released.

Type # 2. Iron & Steel for Mine Supports:

Iron and steel are used in mines in the form of rigid and yielding props, beams and girders, reinforcement in concrete, corrugated sheets and roof/floor bolts. Discussed rails of 36 lbs. section or heavier greatly reduces fire risk in mines.

Steel props used in the miners are:

(a) Rigid props, and

(b) Yielding props.

An ordinary H section steel girder of suitable length with the web cut away and flanges turned over at one or both ends is the common type of a rigid prop. If the prop buckles in use it can be straightened by hydraulic pressure situated at the pit bottom.

Another timber core through major length and extending 25 to 40 mm beyond the steel pipe at either end. The timber core yields to some extent to the roof pressure and gives indication of roof weight.

Yielding Props:

Extensible non rigid props or yielding props are used in this country and are manufactured indigenously.

They are of Two Types:

1. Friction Props:

The common example is the FP3 series (Hibular friction prop) manufactured by MAMC, Durgapur.

2. Hydraulic Prop:

The familiar example is the MAMC – Downty prop, TU-40 prop of Usha Telehoist Ltd., etc.

The term setting load is used for that load which, by means of a setting device, is imparted to a yielding prop, purpose being to ensure that the support is firmly in place and can resist lateral pressure due to the passage of the face machinery.

In other words, setting load is the reaction of offered by the prop to the strata. It is applied to a prop before its yoke is tightened and it is considered essential that it should not be less than 4 te in a seam of moderate thickness (that is, above 1.2 m). The setting load does not affect the load-yield characteristic of the prop.

The yield load is that load on a prop at which the upper member begins to slide. Hydraulic props are specified by the yield load e.g. a 20-tonne prop, a 30-tonne prop, etc.

In the friction type of yielding props the yield load depends upon the force with which clamps or wedges are tightened. But clamping load is not the same thing as yield load.

Load-bearing capacity of a prop is the load at which an axially loaded prop reaches its elastic limit or at which it begins to buckle.

Characteristic curve or load-yield curve is a graph in which yield is plotted along the X- axis and load along the Y-axis and represents the behaviour of a prop under load.

Early bearing props are yielding props which accept the maximum load with a minimum yield. An early bearing prop (one which is capable of building .up of maximum load with a minimum of yield, thus reducing to a minimum both convergence and bed separation) should be used for a routine installation on the coal face while a late bearing prop should be used to stiffen the break-off line. It exerts a resistance which continues to increase with increase in yield. Hydraulic props are early bearing props.

Friction props rely upon the friction grip between two members, one telescoping into or sliding against the other.

If two bodies are held in contact and one moves with respect to the other, the force P for just moving the body in –

P = µ x Q

where µ is the coefficient of friction and Q is the normal force. If the upper member is held by two friction clamps, resistance to sliding, P (or in other words, the bearing capacity) of the prop, is = 2µ x Q.P can be increased by increasing p or Q or both.

In a friction prop Q is raised by means of a clamp which is tightened in various ways, or by compressing parts of the prop by the use of wedge-shaped upper member or by drag wedges which come into operation as the prop yields.

When setting the prop, first operation is to extend the prop to the length required in the position for setting and then it is tightened against the roof or bar by a setting wedge or claw attached to the prop for this purpose. The clamp is then tightened up and the setting device removed.

MAMC Friction Props:

The friction props type FP3 manufactured by M.A.M.C. consists of two seamless steel tubes of which the inner member is made captive to the outer member by means of a spring locking pin which prevents the inner member to come out completely from the outer member beyond the extended length.

The clamp unit which is fixed at the top of the outer member provides the necessary friction grip by hammering two locking wedges alternately. The inner member and the clamp unit are provided with special coatings which give the required frictional characteristics of the props and at the same time prevent from corrosion. In the closed position of the props adequate finger clearance has been given to avoid the injury to miner’s hand during operation.

The important features of these props are as follows:

1. These are early bearing props and accept the roof and load very quickly.

2. These have constant load yield characteristics.

3. The weights of these props compared to their nominal load are less in comparison with other types of props.

Friction props are not much favoured these days though they were adopted on the long wall face with sand stowing using Anderton shearter (for the first time in India) at Chinakuri colliery, and later at Gidi A colliery for the experimental method of extraction by inclined slicing with French collaboration. They were also used at Jeetpur colliery on long wall faces with sand stowing.

Hydraulic Props:

A hydraulic prop is simply a hydraulic jack. These props have been used at longwall mechanised faces in our country. A hydraulic prof can be set to take immediately three quarters of the maximum load (yield load), but when overloaded it will yield at the designed load after which the resistance is uniform and about 3/4th of the maximum.

A hydraulic prop basically consist of two oil-filled cylinders, the upper one telescoping into the lower one. A piston head is fitted to the lower end of the top member and this provides a seal with the inside wall of outer cylinder.

The piston (and the top members to which it is fitted) does to slide down, unless the load on it exceeds certain limits. The resistance to downward movement of the upper member is provided by the pressure of the oil in the cylinders and this oil pressure builds up by the operation of a pump at the time of setting up the prop in piston.

There are two ways of building up the oil pressure in the prop by a pump:

(a) Closed circuit system.

(b) Open circuit system.

In the closed circuit system a built-in pump is provided in the prop itself and forms an integral part of it. The pump is operated by an external detachable handle. In the open circuit system an external pump, serving a number of props from one central site, is connected to the prop by high pressure hose pipes and operation of the pump builds up the pressure of oil contained in the cylinders of the prop.

Non-return valves provided on the prop retain the oil pressure. The largest manufacturer of hydraulic props in the country. M.A.M.C. Durgapur, manufactures props of both designs, i.e., of the closed circuit system (Example- MAMC Duke hydraulic props) and also of the open circuit system (example-Salzgitter/MAMC hydraulic props).

The MAMC hydraulic prop consist essentially of an inner tube, a pump cylinder with oil, a guard tube, a release valve, a non-return valve, a main piston, a top extension fitting, and a pump and release shaft. The inner tube and pressure cylinder can be extended like a ram by hydraulic pressure. The pump can be actuated by an outside key or handle.

A large diameter steel tube connects the lower reservoir with a relief valve capsule. Action of the handle pumps the oil from the inner tube to the outer tube, thus extending the inner tube. After the prop has been so extended up to the roof, further operation of the pump handle provides the initial bearing pressure (or initial setting load) which is 5-8 tonnes.

The pumps handle is then withdrawn. In course of time when the roof pressure on the prop gradually increases the inner tube will not slide down till the load is 40 te, (in the case of 40-te prop) as the hydraulic fluid is compressed till that load is reached. When the load on the prop exceeds 40 te, a relief valve operates.

The relief valve is a capsule permitting adjustment and testing prior to insertion in the prop and it is set to the correct yielding pressure before the prop is assembled in the factory. During the yield, the oil pressure may be from 200 to 500 kgf/ cm².

The prop can be withdrawn easily by pulling the release shackle, which actuates a cam and lifts the valve assembly off its seating, allowing a free flow of oil back to the top reservoir. Withdrawal can be effected from a distance by attaching a chain to the relief valve shackle and pulling it.

A hydraulic prop can be tightened in a few seconds to any length within a wide range. The pump handle is the only tool required for operation and the prop can be released and withdrawn in a few seconds. The hydraulic oil used may be water with 10% suitable oil. The oil is DTE light oil, servo system 311 supplied by Indian Oil Corporation. In has anticorrosive characteristics.

M.A.M.C. manufactures 40-te props of its own design.

A precaution which must be borne in mind in connection with hydraulic props is that they must not be left lying on the floor, and the prop must be withdrawn before full closure i.e., before it becomes “solid”. On longwall faces having sand stowing, hydraulic props which were not used carefully, developed scratches due to sand rubbing on the inner cylinder thereby partially losing the oil-sealing capacity. Such neglect renders the prop ineffective.

Self-Advancing or Walking Supports:

These are open circuit type hydraulic chocks which, when already erected at a place, can be retraced by hydraulic pressure, pushed to the new site of erection by hydraulic shifting cylinders and erected by hydraulic pressure. The hydraulic pressure for chocks and for the operation of the rams is provided from a centrally located pump near the face.

A workman is not required to handle the supports during any of these processes, except for guiding the roof bars supported by the hydraulic props. The self-advancing supports are used in coal mines on prop-free front long wall faces and are introduced for the first time in India in Monidih colliery.

The walking ability of a powered roof support is provided as follows- There are two distinctive sets of one to four hydraulic props connected by a common roof canopy and floor base. The sets are connected at the base by a horizontal shifting cylinder.

The support moves or “walks” when one set of props or “legs” is lowered and the shifting cylinder is actuated, while the other set of legs remain firm against the roof. After the first set moves through a predetermined distance, approximately 0.6 metre, its legs are set against the roof.

The same operations are now repeated for the second set, such that the whole support is self-advanced through an approximate distance of 0.6 metre. The face conveyor is advanced by the double acting rams set between the supports, usually in alternate support units.

4-Leg and 6-Leg Canopy Supports:

A combination of 2, 4 or 6 hydraulic props on a common rigid base is manufactured by some companies to provide support which can withstand very heavy roof pressure. The 6-leg support has particular application in coal seams between 1450 mm and 2360 mm in thickness. The legs may be single telescopic or double telescopic. A 4-leg support is bulkier than 6-leg support and is available for yield load of 310 te, 590 te, and 728 te.

These supports provide a canopy to the armoured chain conveyor and the coal cutter or shearer mounted on it. Between two units of such supports there is a gap of only 70 to 150 mm so that the entire face is well supported and there is no exposed roof along the length of the coal face.

Such heavy support are retracted, advanced and are re-erected by hydraulic pressure with the help of rams, (one ram serving one support) by the workers standing under the adjacent support. Operation is effected by a single lever ‘Dead Man’s Handle’ type control valve.  

The support is attached to the armoured flexible conveyor (A.F.C) by a double acting hydraulic ram which gives a 6.3 tonne differential push load for conveyor advance, and 7.9 tonne pull load for support advance and has a 787 mm stroke.

Anti-Slew Equipment:

Such supports can be fitted with anti-slew equipment and provision is made for attaching brackets to the base for it. Sliding links act on a guide rail between two chocks to maintain alignment. The guide rails are attached to the conveyor between alternate chocks.

Shield Supports:

The shield supports provides a continuous cover all along the face by canopies placed side by side. In this respect it is like the multi-leg hydraulic chock support except for a little difference in construction at the rear side i.e., the goaf side.

The hydraulic powered chock has a Venetian blind type flushing shield suspended from the rear of the main canopy to protect the chock from the falling debris of the goaf side whereas a shield has a sloping rigid one-piece roof of adjustable inclination on the goaf side. This results in elimination of side loading on the legs.

The shield supports are with 4 legs (not with six) which react direct with the canopy. The base of the shield is of rigid construction incorporating fabricated sockets for locating the legs. The lemniscale linkage-formed by the shield structure and links limits the movements of the canopy in the face towards goaf. The linkage also resists forces exerted on the canopy by horizontal movement of the roof.

Where mining conditions are suitable, i.e. sound roof with minimal lateral roof movement, then a chock type support could be the answer at a considerable cost saving over a shield. One type of shield support marketed by Downty is 4-leg, 400 te sub-level caving shield support. It is suitable for sub-level caving in thick seams or for conventional operations in longwall installations.

Apertures are provided in roof canopy of the shield for drilling of large coal lumps and subsequent blasting. The sloping shield has an integral coal loading door which is hydraulically controlled from a safe position within the support and can be closed at any time during coal entry. The shield also protects the rear conveyor from falling debris.

A hydraulically operated agitator arms is fitted to the rear shield to assist the flow of coal, if it does not fracture readily. Hydraulically operated canopy side flamps effectively seal the gaps between the supports and further exclude the dirt and dust from entering the working area. The support provides a clear traveling way in front of the legs from which all major operations can be controlled.

Hydraulic shields are costlier than chock supports.

Steel Arches:

These are used for supports of permanent and semi-permanent roadways. Heavy section rails in two parts are suitably shaped in workshop to form an inverted U when assembled together. The two sections, after installation underground in the place of erection, are joined 6y fishplates and four bolts and nuts.

The legs are generally placed in holes made in the floor. Normally no sole plate or lid is placed at the foot of arches and the arch has no yielding property. Where a roadway has to be supported by a number of steel arches, struts are placed between adjacent arches to prevent lateral shifting. To keep arches in contact with the roof and sides wooden laggings are placed on the top and sides in the same manner as for timber supports.

In our mines steel arches are not erected in places where strata may have a tendency to descend, and such arches with yielding property are not required.

C.G.I, sheets are used in the mines mostly to cover bars of girders over which cogs may be erected for support of a high roof, or sand/obiler ash may be filled to pack up a cavity in the roof in a mine.

Roof Bolting:

The term roof bolting is applied to the practice of drilling vertical holes in the roof and fixing steel bolts into them in such a manner that the bolts grip the strata and support the immediate roof. The bolted roof strata behave as one thick beam capable of supporting not only their own weight but the weight of the strata.

Roof Stitching:

Roof stitching is a method of roof support which has been tried in our mines with encouraging results. In a gallery two slanting holes are drilled by the usual coal drill in the roof. The holes are about 1.4 m to 1.5 m deep.

To fit the hole a wire gauze (10 mesh) is made in the form of a tube open at one end. In this tube of wire gauze viscous cement mortar is filled up to three fourth length. The wire gauze is then inserted into the hole in the roof.

Old wire rope 18 mm to 22 mm diameter is cut into suitable length to cover the depth of the two holes and the distance between them. Each end of the wire rope is prevented from untwisting by typing it with binding wire. The wire rope is then inserted into the cement mortar of the wire gauze tube held in the holes.

As this is done, some of the cement mortar in the wire gauze is squeezed out of the container and it fills up the hole. The wire rope is held in position by hammering a wedge at the mouth of the hole. The assembly is allowed to set for 24 hours and thereafter the cement mortar becomes sufficiently hard so that the wire rope does not come out of the hole even when the sagging middle portion is pulled by hand.

The pairs of holes- for roof stitching are drilled at 1 to 1.3 m intervals and the roof stitching serves as a substitute for the conventional support by the set of timber props and bar. The roof gives an appearance as if it has been stitched.

Wooden laggings are placed between the roof and the sagging portion of-the wire rope. Where the roof is bad sometimes an extra holes (vertical) is drilled into the middle of the gallery and a wire rope inserted into it in the same manner as in the inclined holes. The principle of roof stitching is the same as that for roof bolting, viz., the immediate rocks are held together as a composite stratum of large thickness.

Roof stitching was tried at a number of coal mines as well as in the metal mines at Balaghat and Seetharama mica mine. This method of support was found to be not only effective but also the cheapest. At Ballarpur mines a fault zone, over 60 m length, was successfully supported with the ropes.

To make the technique effective it is recommended that:

(i) The face should not advance more than 2.4 m from the last tensioned rope,

(ii) Short ropes should be used in between long ropes for further reinforcement and to prevent bed separation, and

(iii) Ropes should be tensioned while using good wooden sleepers. Where the roof condition deteriorates immediately after blasting, roof stitching cannot be tried.

Plastic perforated sleeves, 35 mm dia. 1.2 m long, with 5 mm dia. perforations have been tried in place of the wire mesh sleeves by CMRS and the results were satisfactory. Preparation of the wire mesh sleeves at the mine takes time and the workers complain of the scratches on the hands due to handling of the wire mesh. With the plastic perforated sleeves tests conducted on coal roof indicated anchorage strength of 10 te and wire mesh sleeves also offered the same anchorage strength.

Wooden Downel:

It comprises wooden bolt bonded along its whole length with polyester resin. Immediate resistance to strata or coal movement is offered without the necessity of tensioning because the bolt is of wood and can easily be cut; cutting machine can operate freely. They are light, easy to use and positive results are quickly obtained with a minimum of equipment and supervision.

Bolts are manufactured from specially selected timbers which are straight grained, knot and resin free. One end is chamfered to 23 deg. taper to allow easy perching of the capsules, and the other end is left square to permit the attachment to a suitable box section spinning adaptor fixed into the chuck of a drilling machine. The surface of the bolt is left roughened to give good adhesion between the resin and the bolt.

Bamboom Bolting:

CMRS has experimented with a system of roof bolting known as bamboo bolting, as a temporary roof support in places such as splits in depillaring districts. A bamboo slightly longer than the depth of the hole for roof bolt is split longitudinally along the full length.

The two pieces are then tied together by a thin wire at 4-5 places after inserting a wooden wedge only partially at one end. Care is taken to see that the knots on the two pieces of the split bamboo are staggered. The wedged end is inserted into the hole and at the other end another wooden wedge is fitted.

The bamboo, with wedges at either end, is then hammered into position in the hole. The hammering pushes apart the two splits of the bamboo and breaks the thin wire. The bamboo splits grip the hole sides and the gripping effect is enhanced by the knots of the bamboo.

Sometimes a bearing plate is fitted on to the bamboo before inserting the wedge at the exposed end. A nut cannot be used for obvious reasons.