Underground Mining Methods

The most important types of bedded deposits are coal seams. It is possible to assume a coal seam of several feet thick extending in all directions horizontally to the bounda­ries of the mine. The sedimentary rock above the seam is usually of shale or sandstone and is termed roof. The rock below the seam is called floor.

If the seam occurred in mountainous country, then the access to the seam is by an adit, which would be called a drift. But if the surface terrain is horizontal, it will be necessary to sink two shafts from the surface through the seam. One of these would be a main working shaft and the other a ventilation shaft.

Where the shaft passes vertically through the coal seam, a shaft pillar of unmined coal is left to protect the shaft. All other workings will be within the seam of coal, so that all excavations will produce coal. In order to develop the mine, the seam of coal of, say, six feet thick and extend­ing horizontally to the mine boundaries will be divided into sections by extending main entries from the shaft pillar to the boundaries in each direction. The mine is now ready for the systematic removal of coal by one of the main mining methods: the roomvand-pillar or longwall.

It is necessary to say that there exists a number of mining methods used for the underground mining of bedded deposits.

Non-slicing methods of mining:

1. continuous (longwall): on strike; to the rise;

2. pillar mining: long-pillar method; pillar and stall method; long pillar method to the rise; shield mining method;

3. combined methods: room-and-pillar method; twin entry method.

Slicing methods of mining: horizontal slicing; in­clined slicing; diagonal slicing, top slicing, etc. Hydraulic mining. First let us consider the longwall method of mining which is one of the most important methods of mining coal. Unlike other methods of coal mining, the longwall method wins all the coal on the first working. Coal seams from 2 to 8 feet thick and from 0 to 30 deg. dip are worked. The roof must not be too hard. The characteristic of the method is a continuous working face and the absence of any develop­ment openings made in advance of production faces. The working face is divided into sections 40 feet in length. The sections may make a continuous face. The coal is removed by undercutting and blasting. In some cases the roof weight becomes excessive and must be controlled by timber cribs. The face is protected by three or four lines of props, which are placed from 3 to 4 ft centre to centre. As the face advances, the last line of props is removed and a new line is placed. Waste is thrown back into the gob. The waste may also be used for packwall and stowing.

In longwall retreating the face is mined from the boundary back toward the shaft. A cutting machine skims coal from the face and dumps it on face belt conveyer in the main entry. To support the roof near this long face, a line of specially designed powered roof supports, like massive hydraulic jacks, is maintained in proper position. As the face advances, the face conveyer and the powered supports are moved forward. At the same time the roof is allowed to collapse behind the line of supports in the goaf area.

At present longwall mining predominates, minimum losses being achieved through the employment of narrow-web techniques and widespread use of powered supports.

Room-and-pillar method is used principally for mining coal, but it can be applied to the mining of any mineral that occurs as a bedded deposit. Coal seams ranging from 4 to 12 feet thick and from 0 to 30 deg. dip and at a depth not greater than 500 feet can be worked by room-and-pillar. In this method the coal is extracted by driving rooms about 18 to 20 feet wide. This means that a pillar of coal about 40 feetwide is left between adjacent rooms. Crosscuts are made at right..І angles to the genera direction of mining. These crosscuts link the rooms. The unmined pillars about 40 by 60 feet to support the roof rock are of rectangular form. All the work carried out in driving rooms and crosscuts results in the production of coal.