::: Mining Geology :::

Generally speaking, there are two main methods of extracting coal from a seam. The first, and most generally used in the earlier years of coal mining, was the “room or bord and pillar” method of extraction and the second is the “longwall” method, which was used in the collieries of Cape Breton most recently.
1. Room and Pillar Mining
Once the coal was reached by means of a slope or shaft, levels were driven through the solid coal and headways and deeps branch out from these to permit the breaking off of rooms. The rooms ran parallel to the levels and at right angles to the headways.
Crosscuts were then made joining the rooms as they advanced which formed the pillars of coal which were so vital to roof support. With the use of doors and brattice, the air was directed throughout the working places and kept the mine ventilated and hopefully free from a dangerous gas build-up.
The rooms or working faces varied in width (12 feet to 21 feet) and were responsible for the chief output of coal.
A team of two men who completed hewing, blasting and loading coal into tubs or boxes drove each room forward. The room was undercut, most often at the bottom of the seam for a depth of three to five feet and from side to side of the face.
When the coal was undercut, it was won by means of a handpick or sometimes wedging or blasting. When blasting was required, boreholes were drilled into the coal face, charged with powder and fired. The system of blasting varied within collieries and depended upon the thickness of the coal seam and the quality of the coal and surrounding strata.
The dimensions of the pillars or blocks of coal that were left intact between the rooms and crosscuts formed an important part of room and pillar mining. The width and depth of pillars varied considerably because they were dependant upon a number of variables. The depth of the seam from the surface was one of these variables because with increased depth and pressure of super incumbent strata, the dimensions of the pillars must be enlarged in order to give sufficient strength to support the roof and prevent heavy lifting of the floor.
If the coal was soft or easily broken, the pillars needed to be larger. Also, if the stone forming the roof was soft or weak, the superficial area of the pillars was large to distribute the weight of the strata over a larger surface.
The angle or inclination of the coal seam was another factor that had to be considered. A larger pillar was required where the dip of the seam was steep. In many earlier operations, the pillars left were much too small, sometimes six feet by twelve feet. This was bad mining practice and often led to roof fractures, heavy water seepage and crushing.
Eventually, pillar sizes were increased to 20 by 30 feet in width and later still, under the Dominion Coal Company, pillars of 100 by 100 feet were left under heavy cover. Needless to say, it was better to make pillars stronger and larger than necessary rather than risk the mines and lives of the miners.
One important aspect of laying out a mine was the possibility of recovering or drawing the valuable coal left in these pillars. This was usually done when one area of the mine was worked out or when the life of the colliery was coming to an end.
2.Longwalling
As workings advanced, not only did the pressure increase due to additional thickness of cover and make itself evident on the roof, but side pressure on the ribs also increased. It then became economically impossible to maintain roadways in room and pillar sections. It soon became evident that some method must be found to neutralize these difficulties and reduce the cost of operations. Longwalling, therefore, became the only means of continuing mining at depth.
In the longwall method, the chief principle is to extract the whole seam in one operation and to allow the super incumbent strata to settle in the void know as the gob or the goat. Longwalling may be divided into two classifications – longwall advancing and longwall retreating.
Longwall Advancing
In this method of mining, the extraction of the seam starts at the shaft pillar and works its way from the shaft bottom. The coal is removed in a long length or face.
In early operations, these faces varied in lengths from 50 or 60 feet to 200 feet and finally 400 feet and more, as methods became more secure. Access to the surface was assured by the maintenance of two roadways, one on either side of the working face. One of these roadways was used as a haulage level and the other for ventilation.
As the workings advanced, these roadways were constructed by means of continuous six by six feet softwood, stone-filled packs on either side of the road from the floor to the roof. The stone was usually obtained from the roof – brushed from the roadways to make height for horses and coal tubs. Sometimes, the stone would be taken from the gob and used to form the packwalls necessary for road maintenance.
The coal was undercut by handpicks, hand shovelled into cars at the face which were then drawn through cross-gates to the main haulage level. Later, these walls were highly mechanized with machines cutting the coal and face conveyors taking the coal from the face to the main haulage.
In earlier years, temporary roof props were put along the face at regular intervals from two to four feet apart. The props were not arranged opposite each other in these rows, but alternating, and in this way a larger surface of roof could be supported. These props were eventually replaced by hydraulic roof supports that were mechanically advanced with the winning of the coal.
Longwall Retreating
In the longwall retreating system, the levels, haulage roads and airways are driven to their outer boundaries of the area to be worked. When these extremities are reached, the longwall faces are worked back in the direction of the shaft.
As the roads giving access to the shaft have already been constructed, the roof is allowed to fall completely as the face retreats and no roads are maintained through the gob or goat. The face is supported in the same manner as with the advancing method and the props are withdrawn as the face recedes.
This method of mining involves a high initial development cost and a small output until the mine is completely developed.