**2.4 Room and pillar**

In India, underground mining is highly dependent on bord and pillar/room and pillar mining due to their adaptability, flexibility, and accessibility. "room and pillar" mining involves a sequence of activities that are performed to first enter and develop the mine and then progressively extract the coal. The continuous miner extracts the coal as it moves forward, loading it onto an attached loader/shuttle car', which, in turn, transfers the coal to the conveyor system. Bord and pillar method contributes 3−5% of total coal production and 65−75% of total underground production, whereas room & pillar mining, using continuous miners, contributes less than 1−2% of the total output and 20−25% of underground coal production [31].

**Figure 5.** *Basic layout of Wongawilli system of extraction.*

The depth at which the pressure of the superincumbent strata reaches nearly the crushing strength of coal may be considered the limiting depth for room and pillar mining. Generally, at depths greater than 2000 ft, longwall mining is usually adopted.

Longwall is not usually suitable in faulted areas, and a room and pillar system is adopted. As it is difficult to dispose of the dirt bands in the room and pillar mining, clean seams free from dirt bands are suitable for the adoption of the room and pillar method. In room and pillar systems, roadways are supported by solid pillars. Thus, the roads must be maintained in safe conditions without· using many artificial supports. The relatively smaller size of the group of workers encourages team spirit. Supervision and maintenance of discipline are facilitated, and work progress can be readily checked shift by shift [31–34]. In room and pillar work, the area is proved in advance by driving headings. In longwall, on meeting a fault, the whole output is lost. In room and pillar, on knowing the direction of the fault, the rest of the headings may be driven accordingly.

The deeper the seams, the greater the difficulty of working by room and pillar due to higher strata-pressure. In this method, the advance rate is sometimes slow because development takes more significant time. There is a danger of crushing during depillaring due to higher stresses of the superincumbent strata acting on the pillars.

This makes supervision difficult. If the mine is extensive, hauling lengths are greatly increased, involving increased haulage costs. In this system, the need for constant flitting of the machinery from· place to place is excellent. So, the number of idle hours of machinery in room and pillar will be greater than longwall. There is difficulty in the extraction of contiguous seams, especially during depillaring operations (without packing). However, this difficulty is reduced by working the seams in descending order. The roof control is comparatively difficult during the extraction of ·pillars when no packing is adopted. If pillars are weak, additional stresses may be thrown on the pillars in the depillaring area, which may cause the overriding of pillars or premature collapse.

If some pillars are left in the goaf, the goaf will not settle quickly, and this will produce an undulating subsided surface. In highly mechanized mines, a high standard of planning and organization and a larger staff of skilled technicians are needed to achieve maximum efficiency in mining operations.

#### *2.4.1 Split and fender*

Split and fender include pocket and wing, split and lift, and pocket and fender. Split and fender is the most practiced method in India because it can be implemented in shallow and deeper mines. In this method, initially, roadways are prepared with appropriate size for smooth movement of machinery employed, that is, continuous miner and shuttle car.

In this method, two pillars work simultaneously, one pillar under splitting and another under fender extraction. Unless the first pillar is completely extracted, CM is not allowed to start working on third pillar. A pillar is divided into two or three fenders by driving a split gallery in dip–rising direction into the pillar, depending on the pillar size and local strata conditions. Now the driven split gallery is supported by roof bolting with resin-grouted rock bolts and the installation of breaker lines; meanwhile, CM will be splitting the second pillar. The fender extraction is started by leaving a small portion of coal from the corner of the pillar; thus, slices are driven at an angle of 60 degrees with a suitable cut width depending on the fender size [12, 13, 35, 36].

A rib of 3 m coal is left against the pillar after driving the slices for stability purposes. The rib size may be 2 m or 3 m, and it is completely dependent on the pillar size and local strata conditions. After slices are extracted, a small portion of coal will be left against the goaf edge known as snook. Similarly, fender-B is also extracted.

If the caving in the goaf was delayed, induced blasting of the level junction was done. Before going for induced blasting over the junction of splits and level galleries after extraction of each fender, the snooks/ribs were further reduced or knocked down to facilitate the roof caving in the extracted area. The schematic manner of extraction in the split and fender depillaring method is shown in **Figure 6***.*

### *2.4.2 Christmas tree method*

Christmas tree is also known as the left-right method, fish and tail method, or twinning method. This method has been practiced in the Pinoura mine in the Johilla area (Umaria coalfields), South Eastern Coal Fields Ltd. In this method, coal is extracted with the help of continuous miner by cutting slices into the pillar from both level galleries and then extracting pushouts [37]. A straight line of extraction is being maintained and the sequence so followed is to facilitate the caving of the roof in a dip direction. The number of slices driven from both level galleries completely depends on the pillar size and the local strata conditions. The first slice is made most likely with a width of 3.50 m and a suitable length depending on pillar size by leaving a small portion of coal toward the goaf edge.

Similarly, two more slices are extracted in the same manner. The fourth slice will be extracted with a width less than the first. A similar manner of extraction is followed to extract other slices on the other side of the pillar, simultaneously driving the slices into the second pillar. Now, pushouts will be driven from the dip gallery, and the number of pushouts to be lifted is completed dependent on the pillar size and the local strata conditions [36].

**Figure 6.** *Split and fender method.*

In India, it is being practiced mainly at shallower depths due to its design, that is, extraction sequence, but it can be successfully implemented even at a depth of around 600 m. This method was successfully adopted at a depth of more than 600 m in the United States. It cannot be practiced for large pillars since the process involves driving slices where we provide supports only at rib pillars, which results in a considerable loss in extraction in case of large pillars and stability reasons. The schematic manner of extraction in the christmas tree depillaring method is shown in **Figure 7***.*
