3. Make up the plan of the text.

4. Retell the text using new words and plan.

LESSON 18

1 True or false?

Shortwall cutters are usually used in thin coal headings.

Narrow headings in thick coal are driven with the aid of a coal-cutting machine with shearing bar.

Conveyors are used under the ripping and very near to the face.

The shutter isn’t controlled by a main-and-tail rope system operated by the face coal-cutter haulage drums.

Slusher is easily adopted for packing by using a system of guide pulleys at suitable points.

2 Read and remember new words.

Disturbance - порушення

Squeezing – роздавлювання, віджим

Absorb – поглинати

The extent – розмір, ступінь

Virgin area – нерозроблена місцевість

Shearing force – сила, що діє на вертикальний зріз

Downward force – спадаюча сила

Strip of coal – полоса (поклад) вугілля

3 Read the text and answer the questions. Support (part 1)

Disturbance of the coal measure strata will result in movement, and it is the combination of the mass or weight of moving stone and the rate of movement of that mass which creates the destructive forces which cause falls, floor lift and squeezing underground.

Supposing a mass of 100 tons moves 1 ft. per week due to a particular method of mining and it moves 1 ft. per day due to another method, then it is obvious that the second method causes the strata to move seven times as fast as the first. From the first principles of dynamics (the study of moving bodies) the force created by the second method is 7 X 7 = 49 times the force created by the first method. In each case the force has to be absorbed by the floor and underlying rocks and therefore in the second case they suffer much more compression, with more broken roof and sides and floor lifting or blowing. The floor needed to stop a mass of moving stone is generated in the floor by compression in the same way that hydraulic buffers, at a railway terminus stop a train if it is not moving too fast.

The extent of workings may vary from a single narrow road in a seam to a series of strips taken out side by side. The cases may be divided into:

1 ' a) Effect of one road driven in a virgin area. If the strata above the road could fall vertically by shearing off then it would be completely blocked. The downward force, however, is spread to the sides, and the immediate roof above the seam is subjected to bending and shearing forces which may cause it to collapse. The effect of these forces is again subject to depth and type of strata.

b) Effect of a number of roads driven in a virgin area. If the roads are driven far apart, each will have the same forces acting around it (as in the case of one road only) and can be regarded individually. As these roads are drawn nearer together the forces around the area are distributed in a different way. Each road affects its neighbour and finally the total force is spread right across the area so that the main force falls on the out­side pillars and only relatively small or secondary forces act around each road.

c) Effect of a wide face driven into a virgin area. The extraction of a wide strip of coal complicates the problem of roof movement.

Concerning the pressure arch, it should compare with a single arch bridge which carries the weight of the- bridge and the traffic on the foundation of the arch. Compare this arch with the arch of pressure formed first of all across a single road and then across a wide face. Two problems now arise:

1. The distribution of total pressure from all rocks above the workings.

2. The control of the weight inside the pressure arch.

The first problem is roof control, and it depends on a proper understanding of the nature and strength of the rocks in the working zone. The distribution of pressure depends on the mining engineer in the first place, because he decides the width and rate of extraction. He also decides the methods of support which control the rate of movement of the strata and the width of the pressure arch.