4.3.2: Underground Mining Methods

Room and pillar is a common mining method for mining ore bodies with a dip less than 50 degrees. Variations of the method are used, depending on rock conditions and the ore body dip and thickness. When mining the ore, a grid system of open rooms together with a system of pillars to support the overhead rock mass is created. The mining sequence can be optimized to accommodate rock conditions and ventilation demands. The ore is usually mined out using the same drill-and-blast technique as in conventional drifting, where the drift becomes the room between the pillars.

The process itself includes several sequential operations, where the number of activities varies depending on ground conditions and design criteria. The key to success in room and pillar mining is establishing optimum room and pillar sizes to recover the maximum amount of ore and leave the smallest number of pillars behind without compromising safety.

Room sizes and pillar dimensions are based on rock conditions and predicted rock stress changes that may occur when mining. To ensure safety in areas with a larger roof span, rock reinforcement, such as cable bolts, is sometimes required in addition to rock bolts. Accurate drilling and good blasting practice minimize overbreak and blast damage in the pillars. This helps to keep the pillars strong and prevent pillar failure.

As the ore is being mined out, roadways for ore transport and infrastructure are created inside the production area. Working at several production points enables efficient equipment use. Various transportation methods move ore to the surface. A well-planned room and pillar mine is flexible to changing commodity prices and can quickly respond to changing market conditions.

Room and pillar is a productive, flexible mining method that comes with a certain degree of ore loss for both large- and small-scale operations.

Sublevel stoping is a commonly used method in large-scale mining. It is a versatile, productive method that is primarily used for large ore bodies with a steep dip, regular shape, and defined ore boundaries. 

There are different variations of the method depending on the shape of the ore body. However, the method is based on the principle of blasting out large stopes, which are usually backfilled to maximize recovery of the ore body.

Drifts are created through the ore body to enable mining of stopes between sublevels. The ore body can be divided into primary and secondary stopes, where the primary stopes are mined first. The secondary stopes are left as pillars until the adjacent primary stopes have been mined and backfilled. Stabilized fill in the primary stopes prevents sidewalls from caving in when secondary stopes are mined.

Opening raises are required to accommodate the initial blast and startup of the stoping process. If rock conditions allow, it is common to drill the entire stope before charging and blasting. Once ore from the first section has been blasted and mucked out, enough room is made to blast larger sections of the stope.

One of the advantages of backfilling is the possibility to reuse waste for backfill material, such as tailings sand from the processing plant.

The excavation sequence continues according to plan. Before it is time to excavate the secondary stopes, cable bolts are often installed above the stope. This will provide additional support to the rock mass, helping to prevent larger blocks from falling into the excavated stope.

Productivity is maintained at a high rate since it is possible to mine ore on different levels and stopes at the same time. Sublevel stoping is a productive, large-scale mining method that enables a multitude of production points in operation at the same time, offering good productivity and low dilution.

Cut and fill mining is a favorite choice for steeply dipping and sometimes irregular ore bodies, and is 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 mined 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 to provide access to the upper slices within the stope. When a stope is completed, a new access drift from the ramp is created to continue production within the upper stope.

One of the advantages of 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 ventilating the toxic blast fumes. The ore is then mucked out and dumped into an ore pass or onto a truck.

Before continuing with the next round, the rock needs to be reinforced. How this is done is decided by the mine 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. The 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 requires 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 ore 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 the 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 cave, 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 costs that allows a high degree of mechanization and capability of automation.

Sublevel caving is a large-scale mining method suitable for large ore 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 raise and long-hole drilling in ring patterns is 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 productivity to keep the roads maintained in good condition. Dumping the ore into ore passes connected to the haulage level is an efficient way of transporting rock from the production points to the crusher.

Each sublevel features a systematic layout with drifts across the ore 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 ore 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 longwall mine may be developed. The mine depicted uses the retreat line while mining method and is intended to be schematic only. The design of the mine has been simplified for clarity, and the design and features presented are by no means indicative of all longwall operations.

Thumbwheel mining is a method used to extract coal from seams 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 overline the same. As can be seen, the majority of the development is kept within the seam 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 no longer 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 crosscuts 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 headings serve as 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 be reduced. The ventilation shaft, which is shown in blue, will be sunk early in the mine's development; the shaft serves as an exit route for exhaust air out of 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 to provide access to the areas that are currently being mined. 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 the 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 the 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 shearer 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-metre 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 on either side of the panel. The gate roads consist 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 shearer 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 been mined. It is desirable for development to be around two or three longwall panels ahead of the panel currently being mined. However, 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 mining begins; and three, so that as soon as one longwall 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.