4.3.2: Underground Mining Methods

In room and pillar mining, the coal seam is mined in a checkerboard style, leaving pillars of coal to support the roof which allows for instant coal access with a relatively low invest, compared to long wall mining, however only utilizes the coal reserves between 50 and 75 percent. It is a mining method of its own right as well as the supporting technology to develop roadways. In order to prepare the coal face for a long wall operation, in room and pillar mining continuity is the key to profit. From the continuous miner to the continuous flow of material, the coal is cut by a continuous miner which delivers the product to haulers. They bring the product to feeder breaker units that prepare and deliver it onto the belt system while feeder breakers are moved only occasionally, the other equipment is in constant motion, so maneuverability, cableless operation, and maximum load capacities are vital. With a full range of battery or diesel-powered vehicles, caterpillar has an answer to every challenge in room and pillar mining. It all starts at the coal face with the right cutting technology. Today's continuous miners are designed to cut at highest efficiency while keeping dust levels to a minimum with water sprays and dust collectors. They are available for operations from as low as 70 centimeters up to a maximum of five meters. From production to the delivery point, it's just a question of volume and velocity. That's easy physics for our range of utility vehicles. Being equipped with rubber tires and an industry-leading capacity, they keep the circulation of product and material at a healthy and profitable level. The roof bolter, follows production on its tail to create a safe mining environment. Driving the bolts into the roof in a safe and efficient way, is most important in this real hands-on job, therefore ergonomic controls and easy material access are one of the most important features in our roof bolters. The scoops multi-purpose contoured bucket will carry equipment, serve as a multi-tool, or clean roadways and feeder sections. Extended battery life and a dual motor option make it versatile yet powerful. The scoop has often been called the miners Swiss Army knife. As a matter of fact it is a real workhorse too.

Cut and fill mining is a favorite choice for steeply dipping, and sometimes irregular or bodies, and preferred by mines that require the capability of selective mining and adaptability to variations in the rock mass. It is generally referred to as a small-scale mining method. Mining is carried out in horizontal slices along the ore body where the bottom slice is mine first. The excavated area is then backfilled and production continues upwards. Each production level is accomplished by drifting until the entire slice has been mined. The slice is then backfilled and the fill becomes the working platform from which the next level is mined. Backslashing is done for providing access to the upper slices within the stope. When a stope is completed, a new access drift from the ramp is created to continue the production within the upper stope. One of the advantages with cut and fill mining is the possibility to reuse waste for backfill material such as tailings sand from the processing plant or waste rock from development. To mine the ore in one of the slices, we first need to drill blast holes. The next steps are charging and blasting the ore and then ventilate the toxic blast fumes. The ore is then mucked out and dumped into an ore pass or on to a truck. Before continuing with the next round, the rock needs to be reinforced. How this is done is decided by the mind for each individual situation. The mining continues until the entire slice of the ore has been mined. Since the mining can be tailored to suit the shape of the ore body, it is possible to minimize dilution of waste rock. To get access to more production points, a second entrance can be opened at another level in the ore body and excavated in parallel. The equipment used for mining the ore is usually the same as what is used for development. As the ore body is mined, the Rock stresses increase in the pillar above the mined area. Cut and fill mining is regarded as a low productivity mining method, but the advantage is high selectivity with good ore recovery and low dilution.

Block caving is a large-scale mining method that allows for huge volumes of rock to be extracted efficiently. However the development time before production starts is longer compared to other mining methods. By drawing rock from the extraction level in the lower part of the mine, a gap is created. Absence of support for the overlying rock mass together with rock stress and gravity will cause the rock mass to cave. This minimizes drilling and blasting of ore but required the ore body to be large enough and the rock conditions to be favorable for natural breakage. To draw the first pieces of rock, to create this gap, the rock mass in the lower part of the ore body needs to be broken down into smaller pieces. To achieve this an undercut level is developed and blasted. Below, an extraction level is developed where or will be extracted throughout the life of that production area. Draw bells are created between undercut and extraction levels and become passages for caved rock. To avoid misfires, accurate drilling is crucial. Substantial rock reinforcement such as steel arches, sprayed concrete, cable bolts, rock bolts, steel mesh and straps are usually required due to several factors associated with block caving including extreme rock stress changes and a long production period. Rock is loaded from the draw points and can be dumped into ore passes connected to a haulage level or directly into a crusher. A variety of transportation methods can be employed for transporting ore to surface. The fragmentation of the ore and the crushing requirements are key factors influencing the choice of a method. The extraction of ore will sooner or later cause the surrounding rock to caves resulting in subsidence on the surface. Provided the rock breaks successfully and the ore can be extracted evenly at desired draw points, block caving is a high productivity method with low operating cost that allows a high degree of mechanization and capability of automation.

Sublevel caving is a large-scale mining method suitable for large or bodies with a steep dip and a rock mass where the host rock in the hanging wall will fracture under controlled conditions, therefore the infrastructure is always placed on the footwall side. Mining starts at the top of the ore body and progresses downwards in a safe sequence. It is a productive mining method where all of the ore is fragmented by blasting and the host rock in the hanging wall of the ore body caves. Once the production drifts have been excavated and reinforced, the opening raised and long hole drilling in ring patterns are completed. Minimizing hole deviation when drilling is crucial as it will affect the fragmentation of the blasted ore and therefore also affect the flow of the caving rock mass. Rock is loaded from the cave front after each blasted ring. In order to control dilution of waste rock in the cave, loading a predetermined extraction percentage of rock is done. Or if there is a significant difference in density between ore and waste rock, bucket weighing can be utilized for dilution control. When loading from the cave front, it is important for the productivity to keep the roads maintained in a good condition. Dumping the ore into or passes connected to the haulage level is an efficient way of transporting rock from the production points to the crusher. Each sub level features a systematic layout with drifts across the or body. Activities in the parallel production drifts are performed simultaneously in order to maintain a good process in the mine. Due to caving of waste rock into the Blasted ore, a certain degree of or loss and waste rock dilution comes with the method. Caving will sooner or later also cause subsidence on the surface. Sublevel caving is a productive large-scale mining method that enables safe and efficient use of mining equipment with good mine planning, there are also great opportunities for automation.

This animation is intended to demonstrate how a typical underground long war mind may be developed. The mind depicted uses the retreat line while mining method and is intended to be schematic only. The design of the mind has been simplified with clarity and the design and features presented by no means indicative of all long-haul operations. Thumbwheel mining is a method used to extract coal from scenes where surface mining is not viable. In this example. the shallow dipping coal seam shown is to be extracted using longwall mining techniques. The seam is four meters thick and is overlain by around 80 to 100 meters of overburden. The thickness of the seam is constant over the coal deposit. As a guide the proposed development of the mine has shown over line the same. As can be seen the majority of the development is kept within the scene to minimize costs. The first step in the development of a longwall mine is to provide access from the surface to the coal seam below. This can be achieved in several ways. However, in this case a decline is used. On the surface, a portal is driven down at around 10 degrees into the ground. This forms the start of the decline. The decline consists of two roads, one for the conveyor that will transport coal out of the mine, and the other for general access and fresh air intake. The decline continues from the portal down into the coal seam. The coal conveyor will be installed. Only after the conveyor Drive is nolonger needed for development access. Once the decline has been finished, the development of the main headings can commence. The main headings are shown in green. The main headings near the portal consist of six roads 35 meters apart and 5 meters wide by 3 and a half meters tall. These roads are joined by cross cuts every 75 meters or so. This pattern leaves behind a series of pillars that are used to support the roof in the roads. The main heading service a major transport routes for workers, equipment, coal, and ventilation throughout the mine. Further from the portal, where traffic will be less the number of roads in the main headings may reduce. The ventilation shaft, which is shown in blue will be sunk early in the mines development, the shaft serves as an exit rate for exhaust air out at the mine. The main headings will not be completely developed at the start of the mine life, but instead will be developed only as far as is needed as to provide access to the areas that are currently being mind. The areas that have been mined out are shown in mottled white. In longwall mining, the coal is mined in longwall panels. The longwall panels shown here are 300 meters wide by 2 kilometers long by 3 and a half meters tall. In this example, the heart of the longwall panel is a full height of the seam being mined, however other factors such as coal quality at different levels in the seam and the size of mining equipment may also influence the height selected. The length and width of the longwall panel may be restricted by stability of the ground in a region, by severe faulting, or by the ability of the roof to cave. From a recovery and productivity aspect, the panel should be as wide and as long as possible. The way the coal in these longwall panels is mined is covered in another animation, however at this stage it is important to realize that the sheer equipment used to extract the coal from the longwall panel will move from the rear to the front of the panel taking slices off the 300 meter working face as it proceeds. The working face is the name given to the side of the longwall panel that is currently being mined. As this example uses retreat mining, access must be obtained to the rear of the longwall panel prior to mining. This is achieved by the construction of gate roads either side of the panel. The gate roads consists of two drifts running the length of the longwall panel connected by cut throughs. Again, this leaves pillars of coal called chain pillars to support these openings. The gate road next to the previously mined longwall panel is called the tailgate while the other is called the main gate. The tailgate of the current panel was the main gate of the previous. At the start of the block of longwall panels, both gate roads have to be developed, however as in this case, the previous longwall panel has already been developed and only the main gate must be driven. At the rear of the longwall panel, a starter drift is also excavated. The starter drift defines the working face and provides a space for the sheer and other equipment to be installed. At the other end of the panel, a barrier pillar is left behind this pillar will not be extracted and serves to support the main headings the pillar protects the main headings from the high stresses associated with the caving of overlying rock that occurs when the longwall panel has being mined. It is desirable for development to be around two or three longwall panels ahead of the panel currently being mined. Although this may be difficult to achieve in reality. This is done for several reasons; one, production is not slowed by development; two, methane in the longwall panels is allowed to diffuse out before mine it begins, and three so that as soon as one long will panel has finished being mined, the equipment can be moved to the next without being delayed. If development occurs too far ahead of mining however, there will be increased costs associated with the maintenance of the gate road openings.