Handout3_Underground Mining Methods

Handout 3 – MIN 454The next stage in mine development is to actually open the mine. Typically, coal is mined by one of two methods, room-and-pillar (or bord and pillar, in British terminology) mining and longwall mining. The method of mining is chosen based on several factors (to be discussed in the next few handouts). We begin with room-and-pillar mining. The first part of this handout presents the nomenclature.Room-and-pillar mining involves opening a series of tunnels in parallel (called entries) with tunnels going across them (called crosscuts) at regular intervals leaving solids blocks of coal in between (Figure 1). The crosscuts can be perpendicular to the entries or be at an angle. The tunnels themselves (be it the entries or the crosscuts) are called rooms (locally, i.e. in a give spot) while the solid blocks of coal are called pillars. Pillars are left to support the rooms. During mining, these entries are extended all the way up to the end of the deposit after which, the pillars are removed in retreat. Removal of the pillars (called pillaring) from an area concludes mining in the area (the roof collapses very quickly after a pillar is removed, thereby reducing the stress).Figure 1. Basic geometry of room-and-pillar workings.Entries are generally named from left to right, facing inby. Inby and outby are terms frequently used in mining to describe location relative to the observer. Inby is towards the direction of advance, while outby is in the opposite direction (generally towards the exit). In Figure 1, A is inby of B, while B is outby of A.Mine Plan Format: If you are seeing a room-and-pillar mine plan for the first time, you will notice that in the mine plan given to you, rooms are depicted as lines, unlike in Figure 1. To help you make the transition to mine plans, Figure 1 has been redrawn in a typical mine plan format in Figure 2 (not in the same scale, however).Figure 2. Figure 1 redrawn in mine plan format.Before opening a mine, few parameters need to be decided on. These are presented next. Number of entries: The example mine plan that was given to you uses a 7 entry system (ask me if you do not have the mine plan). This is evident from the 7 entries for mains, sub-mains and panels. Mains, as the name suggests, are an important part of the mine. They form the skeleton or backbone for the mine, allowing access to various parts of the mine. Every ton of mined coal passes through the mains. Due to their long life and pivotal role in coal hauling, mains usually have pillars larger in size than in other areas of the mine. Sometimes, the same is true for sub-mains also. Sub-mains are the system of entries that branch off mains, giving access to various portions of the mine. Sub-mains may be absent in small mines. Panels are entries that branch off the sub-mains. Mining is generally done off the panels. However, mine geometry may sometimes necessitate driving panels off mains. Also, the distinction between mains and sub-mains, and sub-mains and panels may not always be clear.The decision to drive a certain number of entries in a panel should consider the following:-The purpose of the panel. For longwall development, typically 3 or 4 entries suffice. It is difficult driving anything lower than three entries since belts, air-return and track/humantransportation each (typically) require a separate entry. Additionally, congestion can be anissue for fewer entries; for example, even with three entries, there is very little room forequipment parking (for repairs or routine daily maintenance). Figure 3 shows a 4 entrylongwall development section. Note how congested it is.-When the number of entries are small, there are only a few available working faces. This usually results in delays. This is because mining is cyclical (including continuous mining).Cutting is followed by bolting and servicing (clearing the floor, advancing ventilation androck dusting), and therefore, any delay in any of the activities affects the others. When thereare many working faces available, an activity can be shifted to another location without anyinterruption. For example, if the bolter is down in entry#1, the miner could work in entry #2rather than wait for the bolter to be repaired.-More entries mean more space. Therefore, machines can maneuver easily and quickly, saving time. In case of shuttle car sections, more entries may mean different routes forloaded and empty cars, thereby reducing travel times.Figure 3. Congestion in a four entry longwall development section. Return Air Intake Air Check curtain Door Stopping Section Belt Point Feed C D- The more the entries, less is the air resistance. Therefore, volume of air flow is higher than with fewer entries. However, with too many entries, section ventilation may be difficult andprone to leakages (too many curtains).- One disadvantage of many entries is that it slows down development and therefore, pillaring. Hence, cash flow may be small for a long time.- Too many entries can also be cumbersome and uneconomical. Section ventilation andmanagement becomes difficult in a very spread-out section.The number of entries in a panel is usually 3 to 7; with 3-4 entry systems being primarily forlongwall development. In the mine plan given to you, it was decided to use bridge sections and,therefore, the mine was planned with 5 entries (since any less number of entries causes congestion with bridges). Many times, when the number of entries is large, two sets of equipments (i.e. two sets of miner-bolter-car combinations) are used. These sections are termed super-sections.Angle between entries and crosscuts : The angle between crosscuts and entries depends on the machinery. Bridges and ramcars, for example, require oblique angles, while shuttle cars requireperpendicular angles. The angle between the panel entries and main/sub-main entries in the mine plan given to you is oblique to reduce spillage in conveyor transfer points. Conveyor spillage is high when the direction of flow takes a sharp turn (Figure 4). Therefore, one must plan oblique angles wherever the conveyor flow direction is expected to change.Figure 4. Spillage at transfer points. Higher spillage LowerspillageEntry/crosscut height and width : The mining height is dependent on geology (seam heights) and panel requirements. Typically, as long as it can be economically justified, a height comfortable for humans, is preferred. However, many times heights are also justified based on current equipment and personnel. For example, I have known mines to drive certain entries higher to accommodate longwall shields. Sometimes, entry height is determined by roof control; for example, a thin layer of slate is better mined down instead of being supported.The width is primarily dependent on equipment needs. Most entries are 16-20 feet wide.Pillar sizes : These are determined by ground control and systems engineering. The ground control aspects are discussed at length in the next handout. As far as the systems aspect go, as pillars become longer, travel times from one crosscut to another becomes longer. It impacts the haulage cycle times (shuttle or ram car) since when one car is in an entry, others have to wait until the entry is cleared. For larger pillars, such idle waiting times can be long. While driving longwall panels, there is always a conflict between driving longer pillars (reduces total amount of cutting) and reducing idle waiting times by reducing pillar lengths.Mining Progression : Figures 5 through 8 show the progression in mining activity during the life of a mine. This discussion assumes a pure room and pillar mine (i.e. no longwall panels). The first stage (Figure 5), development, involves driving the mains all the way to the end of the property. The second stage may involve further development, especially in large deposits. In Figure 6, panels are being developed on the flank, while small sections are being developed off of the mains. In the next stage (Figure 7), the sections off of the main are extracted or pillared, while panels continue to beingdeveloped. At this point, the production levels in the mine are very high due to the presence of pillaring sections. Mines like to have few pillaring sections as they are very productive. Most mines remain at this stage for a long time, i.e. most mines have both pillaring sections and development sections for a majority of their lifetime. Mining progresses this way (Figure 8) till the deposit is completely mined.i. Panels developed on the flanks PANEL SECTIONii. Sections developed off of the mainsFigure 7. Stage 3: Development and ExtractionDevelopment around a paneli. Extraction in some areas (dashed lines indicate pillaring or extraction)ii. Development in othersPillar extraction(sectionsdeveloped earlierbeing extracted)Figure 8. Stage 4: Repeat of Stage 3Mining slowly retreats to the portalCaveats: Many times, however, due to financial reasons, a mine may start developing panels as soon a possible, without waiting to drive to the end of the property. This is especially true of longwall panels, that take about a year to develop. In Figure 9, advance of the mains is delayed, while the longwall panel is developed. The main reason for doing this would be to start the revenue stream quicker.A possible problem with the above is that in the future when panels are developed farther from the mouth of the mine, travel to these panels require that one pass by the older sealed off panels (Figure 10). While sealed off panels are generally safe, they can prove hazardous as seen in the Sago mine accident in 2006.Production Cycle: There are two types of production methods in room-and-pillar mining, continuous mining and cyclical mining. Cyclical mining consists of drilling, blasting, mucking or loading and supporting. As these actions occur in sequence or cycles, this type of production is called cyclical. As drilling and blasting is becoming rare in coal mines, and due to time constraints, only continuous mining will be discussed in this course.Continuous mining assumes non-cyclical production. This is attained by the use of a continuous miner that cuts the coal while simultaneously loading it into a conveyor or a shuttle car, thereby, eliminating the need for drilling, blasting and mucking cycles. However, in reality the term continuous mining is a misnomer. Given the continuous miner, and assuming a good haulage system, one can theoretically mine continuously. However, mining is not continuous. This is because mining laws require ground control and ventilation measures to be undertaken after each cut. For example, no opening can be unsupported for more than 40 feet. This means that any time the depth of the cut reaches 40 feet, one must stop cutting and allow roof supports to be installed. Therefore, cutting is only one aspect of a cycle of activities. The cycle usual consists of:-cutting coal-install roof supports, usually roof bolts-extend ventilation using ventilation devices such as brattices or tubes-service the face (scoop it) including rock dusting-pump water, if necessary-survey, to ensure advance in the proper direction-periodically one must re-locate the power center so that the machines can advance-also periodically, one must advance the static conveyor.The last two need not interfere with the production cycle when a mine only has two shifts of production per day and one shift of maintenance. These can then be accomplished during the maintenance shift. Any backlogs in roof support is generally also accomplished in the maintenance shift. Surveying is not necessary after each cut though foremen measure the advance from each cut using a tape.Note that miner-bolters are equipment that allow bolting, while cutting coal, thereby not requiring that mining be stopped for roof bolting. Additionally, these machines also have a built-in rock duster. Therefore, when using miner-bolters, coal mining can be very close to continuous mining.Equipment: The typical equipment requirements for room-and-pillar mining are (per section):-continuous miner-roof bolter-haulage machinery such as shuttle cars and bridges-utility equipment – a scoop and a lo-trac-conveyorsThe various types of continuous miners, roof bolters, haulage machinery such as shuttle cars and bridges, and conveyors are described in the textbook. The capacities of the machines, however, are a little outdated.Some recent numbers (these change constantly) on machine output are:Miner loading rate : 9-18 tons/minShuttle car payload for 6ft seam: 8.2 tonsShuttle car speed : 91-114 m/minDischarge capacity : 810 tons/hrPanel belt size : 1.07Bolting capabilities : 1-1.5 mins/boltBolting per row : 5-6 minsUsing these numbers, let us compute the average time to make a cut. Let us assume a 12 m long cut, 6 m wide and 2 m high. The density of r.o.m. coal is: 1.342 tons/m3 i.e. 1.342/0.91 short tons per cubic meter.Tons/cut = 12x6x2x(1.342/0.91) = 212.4 tonsTime for the miner to cut = 212.4/9 = 24 mins (from miner loading rate)=26212.4/8.2=Carloads/cut=13trips/car2cars,Assuming1.5mins=forTraveltimecar(assume given) Loading time for the car = 1 min (from miner loading rate)=2.5minscartheCycletimeforTotal time required by each car = 2.5 x 13 = 32.5From the above, it becomes clear that the miner will have a wait of about 8.5 minutes (32.5 – 24). If another car is added:=26/39=Tripspercarmins2.5Time=Cycle22.5mins==2.5x9timeTotalIf the above time estimates were accurate, we have a good miner-car match. No miner wait time (due to cars) is expected. However, addition of another car will probably add to the car cycle time due to congestion and higher utilization of resources such as feeder (discharge hopper) or miner.The miner-bolter match should also be examined. If the bolting requirements are 4 bolts per row and rows 1.5 m apart:# of rows/cut = cut length/dist. betw. rows = 12/1.5 = 8† = = 2x1.25 = 2.5 mins Time per rowTram time from row to row = 0.5 minsTotal time for row = 2.5+0.5 = 3 mins=cutmins24boltTimetoaTherefore, the miner and the bolter are perfectly matched. In real life, however, one would design it so that the bolter had a cycle time much less than the miner. This would insure there were no delays due to bolter.†Twin boom ATRS has two drilling booms. Therefore, each operator drills half the required number of holes in parallel to the other operator. Therefore, in the example, time is accounted for only 2 holes.Note: In the above example, we did not take into account the tram time between cuts or the time to service the face. We are assuming that the miner and the bolter have the same tram time and that there is an additional face that the scoop services, while the bolter is bolting the fresh cut. In a four entry system, typically only one free face is available and, therefore, bolting and servicing have to be completed, while the miner is cutting the face. The ventilation also determines cycle time. A four entry longwall development section is presented next, where it becomes clear that miner-shuttle car-bolter times are not the only factors that determine cycle time.Figure 11 presents a 4 entry longwall development section. As in most modern mines, the section has two miners (one for each side, left and right) and two bolters (one for each side, left and right) in addition to four shuttle/ram cars. Note the section ventilation. In the left side, the intake air travels from #2 entry to #1 entry before returning, while in the right side, it travels from #3 entry to #4 entry before returning. Since miners cannot work downstream of a continuous miner (due to high dust levels), it precludes #4 entry being bolted or serviced, while #3 entry is being mined. Similarly, when #2 is being mined, #1 entry has to be idled (as shown in Figure 11, where the bolter is simply parked). When the miner is in #1 entry, #2 can be bolted and serviced (except, when #2 is being rock dusted, the miner in #1 would have to be briefly stopped). Same applies to #3 and 4 entries.Miner cutting #4Roof bolter pinning#3Return Air Intake Air Check curtain Door Stopping Section Belt Point FeedC DMIN 454: Underground Mining MethodsTherefore, as far as equipment utilization goes, only the miner is being utilized on one side when the inside entries (#2 or 3) are being mined. The bolter is effectively idled since space limitations require it to be parked in the outside entries (#1 or #4 entry), which gets dusted out. The scoop can hopefully be kept active doing other work. Lets look at a longer cycle to see how the actual run might be.Considering only the right side (the analysis applies to right side since it’s a mirror image), when the miner is done cutting #3, it moves to #4. While it is cutting #4, #3 gets bolted and serviced. If everything works perfectly, #3 will be ready when the miner gets done with #4. But so what? Can the miner go to #3? No, because #4 now needs to be bolted and serviced, which means, you cannot dust out #4. Therefore, the miner will have to wait, so that #4 can be bolter and serviced. Hopefully, during this idle time, some maintenance is done on the miner (oil and bits changed, general cleaning done besides other routine maintenance).In any case, these issues are not taken into account in computations done previously. Therefore, one must take into account the entire cycle when estimating section productivity. In my experience, I have seen management very often over-estimate section advance rates since idle times such as from moving the return up, or those due to belt and power moves, or inbuilt cycle delay time (like the one presented earlier) are not incorporated into time studies.References and Recommended ReadingSpecial on Appalachian Coal, 1995, Mining Engineering, SME Publication, December.Ganguli, R, 1997, Multiseam Mine Design for Buffalo Creek and Winifrede Seams, Report submitted to Arch of Kentucky, July.Stefanko, R., 1983, Coal Mining Technology, Theory and Practice, SME Publication.Instructor: Dr. Rajive Ganguli, University of Alaska Fairbanks。