MEMORANDUM

TO: DR. KAREN THOMPSON FROM: MOLLY JEAN MCGEE SUBJECT: TECHNICAL DESCRIPTION: OVERHAND MINING DATE: FEBRUARY 16, 2013 The following explains the audience, purpose, and placement of a technical description of the process overhand mining. Audience The audience for this description is an employee of a mining company using overhand mining techniques. The audience in the description has some basis in the mechanics of mining. It is to be expected that some in the target audience are greenhorns to mining. In which case, the audience would be advised to consult the company’s mining terms handbook when confused by terms used in the description. Purpose Mining companies utilize different techniques for extracting ore bodies. When a person is hired onto a mining company, they must attend an MSHA (Mine, Safety, and Health Administration) course for that specific mine. In this course, new employees learn about the mine of which they are a new hire. If a mine utilizes an overhand mining technique, a technical description of this process would be useful as a reminder of how this technique is executed. Placement This technical description would likely be placed in an orientation packet for new hires of a small vein, hardrock mining company, or within the pages of materials handed out during a site-specific MSHA course.

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Overhand Mining

Overhand Mining Overview Overhand mining is a method of extracting ore bodies using a cut-and-fill technique. A cut and fill technique is most often used in the case of steeply dipped ore bodies or non-uniformly shaped deposit1. In the cut and fill method, an ore body is mined horizontally in “cuts”. After a cut is completed, the mined out area is backfilled with “sand”, a concrete-like mixture that is made

from mine tailings from the mill, or a sand/cement (more typical in underhand mining).  Error! Reference source not found.2 above shows a raise with the final 3D images of each cut. Following a cut and fill pattern, this raise will continue until it has broken through to the next level or it is no longer following a sufficiently graded ore.

Making the Cut A cut is made by drilling, blasting, and mucking several “rounds” outward from the raise (ladderway that progresses upward towards the next mine level) or entrance ramp (drift made for diesel/rubber-tire equipment to make it to the work-headings).    Figure 23 shows a jumbo drill beginning on the next cut of the stope. The final length of each cut is dependent on the grade of the ore body. When a geologist determines that the cut is no longer on the ore body, the cut is finished and ready to be filled. The drill, blast, muck, pattern is laid out in the subsections to follow.

                                                               1 http://www.miningknowhow.info/glossary‐of‐terms/cut‐and‐fill‐mining 2 Image Courtsey of US Silver & Gold, Galena Mine 3 http://www.ece.gatech.edu/research/labs/bwn/wusn/fig/cut_fill.jpg 

Figure 1 Raise with stope cuts 

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    Figure 2 Overhand mining process

Drill The more modern method for drilling rounds is to use a jumbo (See Figure 34). A jumbo is a drilling machine that typically has multiple hydraulically (or pneumatically) operated arms, called booms that contain the drill. In older workings, where large diesel vehicles are not feasible, a jackleg drill is utilized (See Figure 45). The face of the stope is drilled at the angle and in the pattern set to optimize the effectiveness of the blast by the engineer overseeing the given area. Each round drilled is typically 9-12 feet into the face of the rock, depending on the capability of the drill. The type of working, raise or rubber-tire, determines the width of the stope. A typical raise stope is less than 9 feet wide, whereas a stope running diesel equipment is more than 12 feet wide.

  Figure 3 Double Boom Jumbo Drill                                            Figure 4 Miner Drilling with Jackleg 

Blast Ore is extracted from the rock by the use of explosives. The process of blasting begins with loading each drill hole with explosives/blasting agents. ANFO (Ammonium Nitrate/Fuel Oil) is most commonly used, as it is far more stable than explosives like dynamite. Water gel is also a common blasting agent. The detonator for these blasting agents is a “non-el”, which is a non-electric detonator. Non-el’s are the commonly used detonator now because its predecessor, the                                                                4 http://www.kramer.ca/images/stories/2013_Cat_Bucyrus_images_Jan_18_2013/cat%20md2060_full%20jumbo%20drill.jpg 5 http://elementsunearthed.files.wordpress.com/2013/01/jack‐leg‐display.jpg  

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electric detonators, was prone to being set off unintentionally by any stray electrical current. The non-el is attached to a primer cord, which when ignited by an electric charge, sets off the round. Because ground is highly unstable immediately after blasting, the loaded round is detonated at set blasting times to better insure the safety of all workers.   Muck Mucking is the task of removing the blasted rock from the previous shift’s round. Before removal, the rock pile is sprayed with water to settle out particles still suspended from the blast. The rock is taken from the area as either waste rock or ore based on the recommendations of the geologist in charge of the heading. The rock is removed to its respective pass (waste or ore) via a slusher or a LHD (load, haul, dump) to await transport out of the mine for processing (See Figure 56 and Figure 67).

Filling the Cut The end result of a cut is a void of rock. To reduce the stresses in the rock and create a platform on which to begin a new cut, the void area must be filled with material. The material used from overhand mining is most commonly “sand”. The sand slurry is composed of water and coarse materials from the tailings of the mine’s mill. The sand to water ratio by weight is typically 60:40 and 70:30. This sand mixture is pumped to the stope as backfill material via a pipe network that runs from the surface, down the shaft of the mine, and outward to the headings. The sand cannot be poured without the cut being prepped. A wall must be put up to prevent sand from pouring out of the cut as it is being filled. In addition to a wall barricade, the entries to the area must be barricade as well to prevent water and sand from blocking haulage ways. The slurry’s water content settles out mostly from decanting during the pour; however, drainpipes are sometimes installed to aid in the drying process. The fill is allowed to sit a couple days (typically 2-4) until the water drains and a new cut can safely begin on top of the previous cut8.

                                                               6 http://www.miningartifacts.org/Minnesota‐MInes.html 7 http://www.britannica.com/EBchecked/media/1530/A‐25‐ton‐loading‐hauling‐dumping‐machine‐used‐in‐the 8 Baker, Steven J. “Assessment of Hydraulic Backfilling in Metal Mines within the State of Idaho”. Idaho Department of Water Resources. October 1986. 

Figure 5 Miner mucking previous round with slusher  Figure 6 Miner mucking previous round with LHD 

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The sand/water mixture is not the only fill material utilized, but it is one of the more economical choices. Utilizing the tailings from the mill as a backfill for the cuts minimizes the amount of material that the mine must handle as contaminated/hazardous material on the surface. It also reduces the need to purchase material with which to backfill. Additional backfill options include waste rock and a sand/cement mixture. The sand/cement mixture is not often used for the entire backfill on an overhand cut. However, it is used as a final pour for the overhand fill in a stope with rubber-tire equipment, as it provides a more solid area on which to begin the next cut.