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

Height of gallery.

1.2 m to 1.8 m

1.8 m to 2.4

3 m and above

Diameter of prop at thick end

100 mm — 125 mm

150 mm — 175 mm

175 mm — 225 mm

A timber prop is sometimes called stull in metal mines.

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

(1)  Dry rot

(2)  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 conducive 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 under set prop. Erection of under set props is not common as it is not possible for the timber man to judge whether it is under set or not (fig. 9.1).

 

Leave 5 m on all sides of the boundary of the surface structure and make due allowance for angle of draw to know the underground area where pillars should be kept in tad.

After late RD Nath from an article on Roof Supports.

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 5cms thick, 25cms 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 {Fig. 9.7). 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 in 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 25 mm 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 4.5m:— 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 fig. 9.7, (v) 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 W 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 (Fig. 9.7).

Timber bars

Bars act as beams. A timber bar is placed in the holes of minimum depth of 500mm in the side of a coal pillar if the sides are strong (Fig. 9.7). 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 by this method takes about 2-2 ½ 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 stocks, wooden chocks are provided in the split 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 (Fig. 9.9). 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 (Dotted bar, in Fig. 9.9), 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 chock mate

A chock, cog or chock mate is a combination of sleepers above one another in a criss_-cross manner as shown in fig. 9.9. 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 chock mate 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 longwall faces.

Only rectangular sleepers, or alternatively, sleepers having their two opposite sides chopped flat, should be used. The minimum length of sleepers for 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 cross section of 100mm X 100mm.

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 the 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. Fig. 9.10 shows a method of supporting a wide junction by cogs and bars. 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 cases 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 bars 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 bunton. 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 operations by loading debris in tubs. The temporary supports arc then replaced by permanent ones like cogs and bars for at least 15m 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 a district of bord and pillar workings where splitting of pillars or depillaring is going on, on every longwall working face, in every forking 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 are also specified in the order governing systematic timbering. In every case of systematic limbering, 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 places in, the mine.