Stop Mining Flat Deposit
In the previous topic you were introduced to the different methods adapted to mine underground deposits. Well the basic elements are common to most underground mining operations, there are many variations in the theme. In the next three topics we will see how these methods can be adapted to mining deposits in different geologic situations. Let’s start by looking at stope mining in flat deposits. In the previous topic we were introduced to strata form and stratabound deposits which are common from minerals such as coal and iron ore. Where these occur the flat deposits that is sloping at around less than 20 degrees. These lend themselves to a particular stall called stope mining. What is common today is that the mining progresses from one part of the deposit to another mostly within the same plain. What is different is the size and the shape of the pillars used to support the roof and the methods used form them. Stope mining and flat deposits are distinguished on the basis of the stall or the stake being merely described as rooms, stalls, or boards in the way in which these are created and arranged. Common classifications include room and pillar, board and pillar and pillar and stall. The most common arrangement for stope mining in flat deposits is the room and pillar arrangement where the roof is supported by square or rectangular pillars on a more or less rectangular pattern. The profitability of a room and pillar mine depends on optimization of the size and spacing of the pillars so that the greatest portion of coal can be removed without compromising the stability of the pillars. The mechanics you were introduced to in module two is used by the mining engineer as a basis for determining this arrangement which depends on the strength of the pillar rock, the height of the pillars and the spanning ability of the roof. Since stopes and temporary structures they only need to remain stable for as long as the miners are working within them. This means that the size of pillars is chosen so that they are only just large enough to hold up the load of the roof, therefore, the chance of a pillar failing is relatively high. If a pillar fails the load it carries is redistributed to its neighbors and this increases the chance of they too could fail. If a pillar fails there is the chance that a chain reaction of pillar failures could be in place that could eventually cause the entire mine to collapse. To safe guard against this; mining engineers usually divide the mine into a number of extraction areas separated by larger continuous pillars called barrier pillars which prevent the collapse in one area spreading to another. Rooms are extended and pillars are fallen as the mining progresses to its furthest extent and it is important that these pillars remain stable until the mine reaches its furthest extent. However, as lots of valuable material is left behind in the pillars and as the stability of pillars is no longer anywhere near important once the mining has reached its fullest extent it is common that a second phase of mining takes place as the mining operations retreat from the furthest extent back to the access point. This is done by rubbing the pillars and it can involve shaving sleds from the outer places or splitting them by cutting through them. The process usually involves the use of temporary support and at the end of the process it is expected that the roof will collapse. Retreat mining can be dangerous however, if the roof refuses to collapse progressively or if the roof collapses suddenly with little working warning. On way of increasing the recovery in room and pillar operations is to remove the pillars and replace them with artificial support. These are typically referred to as stalls or props and timber is commonly used for the purpose. In flat deposits, it is usually possible to place stalls in isolation between the roof and the floor. In most steep inclined working it might be necessary to use more elaborate arrangements including vertical and horizontally arranged stalls to achieve support stability. The extent to which stalls can be used to replace pillars depends upon the amount of vertical stress to be carried, the strength of the stall and the height of the rooms. It is uncommon to use stalls to support rooms higher than 4 meters. In the next topic we’ll take a look at how stoping is carried out in deposits that aren’t flat.
Stop Mining Inclined Deposit
Stoping can be carried out in both flat and inclined deposits although the geometric arrangements needs to be varied according to the conditions.
As a general rule, man and machinery work more safely and more efficiently on flat horizontal surfaces.
So, in the flat deposit considered in the previous topic, stoping conveniently perceives laterally across the deposit removing it from between the roof and the floor.
By contrast in an inclined deposit, stoping techniques are modified to remove the ore in a series of horizontal slices at different levels one above the other and on the other side by the hanging and foot walls.
Further, for flat deposits the stopes are generally arranged side by side but for inclined deposits steps may either be side by side or arranged one above the other or both depending upon the inclination of ore body.
In the this way, the pillars may be either vertical called rib pillars which are between adjacent stopes or horizontal called sill or crown pillars separating the stope from the ones above and below it.
In steep deposits the ore extents both above and below a given level and so stopes can be developed either upward or downward. Stopes worked upward are called overhand stopes. Stopes worked downward are called underhand stopes.
Overhand stopes involve added difficulty in working in the roof and drilling out of the head but they have big advantage that gravity hides in rock break-out. Underhand stopes are easier to prepare but rock must be removed against gravity. Production stoping usually adapts overhand methods to utilize gravity.
Two common stoping methods commonly used to mine vertical deposits are shrinking stoping and sub-level stoping. Shrinking stoping gets its name from the phenomenon whereby if we take a volume of rock and we break it up, the volume of the broken rock is larger than the volume of the intake rock that we start up with.
In the context of underground mining this means that if we could break rock out from the rock face into an open stope then hole created by breaking up the rock ends up being smaller than the volume of the stope that we’ve filled with the broken rock hence, making the stope appear to have shrunk. Well overhand stoping is gravitational advantageous it doesn’t involve the difficulty of accessing an increasing high roof as the stope is extended. The alternative to this is to mine in a series of short stopes but this requires the leaving of many crown pillars and a lower recovery of the ore.
Shrinking stoping achieves roof accessing in tall stopes by collapsing the ore body in successive increments and using the macro-pile created as a platform to access the roof at higher levels for drilling and blasting.We can see the basic steps of the shrinking stoping operation in the diagram of shrinking stoping applied to a vertical ore body. To begin, parallel thorough and extraction drives are constructed through the base level in the mine. These are linked by sloping cross-cuts. Flaring reel holes are drilled into the roof of the thorough roof to define a shoot structure and these are blasted to break up the rock in the shoot, allowing it to drop through the thorough roof.
Hence, the thorough roof becomes the drill point for the collapsing wall. An amount of the built up broken in the thorough roof is removed to create a cavity between the top of the mac and the roof conveniently high to allow access to drill and blast equipment to access the roof.Then another portion of the roof is block drilled and blasted raising the roof and heading to the mac part. Another amount of built up broken ore is then removed from the ore pass to reestablish a convenient working hoist from the top of the mac to the new roof.
The process is repeated until the rock is broken up to the desired stope pipe. At this point, the stope is full of mac this can then be moved progressively while the next stope is developed. In the alternate method of sub level stoping the ore body is mined in a series of segments one above the other in the ore body. The segments are separated by crown pillars and when necessary rib pillars each segment is mined in series of sub levels using the underhand approach not similar to the shrinking stoping approach.
It differs however, in the way that access is gained to the ore at high levels in the roof. The steps in sub-level stoping are illustrated in the following diagrams. To begin parallel access and thorough roof are driven across the sill pillar. These are linked by stoping cross-cuts. Overhand stoping begins by drilling and blasting of the accessed roof to form an ore pass.
The ore above this is accessed from a higher drift allowing drill and blast excavation of the next sub-level of the ore body. This is repeated for successively high sub-levels extending the stope upward until the crown pillar is reached. The mine ore is mac out of the ore pass into the transport roof through the cross-cuts at the base of the stope as each sub-level is worked. When a stope is fully extracted and new stope is developed. Yet another approach to mining is deep deposit is we support stoping or cut and fill stoping. Here access to an ever rising roof is achieved by back filling the stope with waste as it is mined. In this way, the ore body is removed in a series of horizontal slices with back filling used to maintain a convenient working pipe to the roof as the stope is extended up towards its desired height. The presence of filling this stope also provides support for rib pillars allowing the thickness to be reduced. In the next topic, we will learn how there are advantages and disadvantages if the requirement to maintain open stopes doesn’t apply.