Underground Hard Rock Mining PDF

218 Underground hard rock mining 3.3 Mine access and development 3.3.1 Introduction to shaft and decline access The main underground development openings are designed so that ore bodies are safely, easily and efficiently accessible for men and materials. The development would also need to ensure tha...

218 Underground hard rock mining 3.3 Mine access and development 3.3.1 Introduction to shaft and decline access The main underground development openings are designed so that ore bodies are safely, easily and efficiently accessible for men and materials. The development would also need to ensure that the ore can be moved to the surface for processing effectively. Access can be typically categorized into three, depending on their importance: Primary or main openings (e.g., shaft, adit or slope/decline/ramp) Secondary, level or orebody openings (e.g., haulage, drift or entry and ore-pass) Tertiary or lateral or panel openings (e.g., internal ramp or crosscut) The construction of underground openings is a specialized operation, time consuming and costly. Mine development has therefore become increasingly mechanized and efficient in order to reduce costs and time to develop and start mining a deposit. The most critical decision is the mine access method from surface and the location. A number of other decisions related to the primary development openings for a mine must also be made early in the mine planning stage and include the type, number, shape and size of the main openings. Factors that influence this decision include: The depth, shape and size of the deposit The surface topography The geological and hydrological conditions of the orebody and surrounding rock The mining method and the production rate The climate The local infrastructure available Relevant regulations Sustainability and environmental considerations The size of the main access development depends mainly on: Ventilation requirements The tonnes that must be hoisted per day The size of mobile equipment to be used The underground personnel required The provision of services (electricity, water, compressed air, etc.) Primary underground development openings can also be used for exploration purposes. Those openings, if driven in advance of full-scale mining, can provide valuable additional and detailed exploration information and afford suitable sites for future exploration drilling and sampling. Obviously, excavations driven for exploration purposes can be utilized to develop the deposit; some shafts, adits, declines and drifts would later serve to accelerate the opening up of a new ore deposit. Underground mines are used for accessing and exploiting ore bodies that are not close enough to surface or that have already been mined from surface and are still viable at depth, but uneconomic for continued surface mining. Underground hard rock mining 219 Figure 3.15 General layout of an underground mine (Hamrin, 1982, 2001) The infrastructure of underground mines is obviously more complex than that of surface mines (open-pits or strip mines). A typical layout of an underground mine is shown in Figure 3.15. A main feature of an underground mine is often the shaft (or shafts), which is a vertical or subvertical access to the underground workings, or depending on the depth and extent of the deposit, it can be an inclined tunnel, called a decline or ramp. Both types of access developments, shaft and decline, can be present in the same mine, as shown in Figure 3.15. In mountainous terrain, the underground orebody can be accessed from the slope of a hill (or a mountain) using horizontal or gently inclined tunnels, called adits. The surface entrance to a decline or an adit is called a portal. Generally, horizontal excavations are developed slightly up-dip to facilitate gravity drainage away from the advancing excavation. Underground mines usually have at least two independent access systems because of safety considerations for the workforce underground if one becomes inaccessible for some reason, and to facilitate ventilation. In many countries, this is required by law. This can be achieved by using various combinations such as a ramp for equipment, personnel and intake air and a shaft for transporting ore out of the mine and for upcast (waste) ventilation. There are generally three methods of accessing an underground mine: shaft, adit and decline or ramp. The shaft often remains the mine’s most critical infrastructure, and downward development is frequently via ramps to allow access for the mobile equipment. A decline ramp from surface can facilitate easy machine movement and 220 Underground hard rock mining Figure 3.16 Decision tree for main access transportation determination (De La Vergne, 2003) the transportation of people and materials. It can also be used for ore transportation by truck or conveyor, eliminating the need for hoisting shafts, but that is an economic decision as shafts are typically more effective and cheaper for moving especially large tonnages to surface especially from deeper mines. The selection of whether a shaft or ramp is more effective as the primary access depends mainly on the orebody depth and planned production rate. De La Vergne (2003) proposed the decision tree as shown in Figure 3.16. 3.3.2 Shafts, raises, adits and ore (rock) passes 3.3.2.1 Shafts A shaft is a vertical excavation (but could be inclined too, although this option is seldom used now as it is less efficient) in which elevators are used to transport people, equipment and ore (and sometimes waste) in and out of the mine. Shafts are always the option used where the deposit is located at depth. Shafts are generally used for the following functions: To access an orebody To transport men and materials to and from underground workings For hoisting ore and waste from underground To serve as intake and/or return airways for the mine (ventilation) To provide a second egress as required by mining law Access to nuclear waste storage Hydropower generation Access underground civil structures such as basements and underground rail stations or road tunnels Underground hard rock mining 221 Most shafts are divided into a number of compartments, each with a different use, by brattice walls or steel works fitted with conveyance guides. For example, one major compartment could be used for moving people and equipment, a second with two skips for taking rock to the surface and other compartments for service infrastructure (typically pipes and cables) and space for ventilation. The main factors to establish the shaft size are the monthly tonnage requirement and the ventilation needed. At the mine planning stage, it is useful to include some excess rock handling and ventilation capacity in case future mining production increases. The orebody size, grade and mining method will determine the rate of mining, and thus the tonnage (ore and waste) to be hoisted, the size of the workforce and the material to be moved efficiently in any given shift. Once a shaft is excavated to its final depth, typically some distance below the economic depth of the orebody for ore handling and water handling, the shaft is equipped with all the necessary steel work, etc. to guide the shaft conveyances and hold the pipes and cables for the services needed underground. Figure 3.17 shows a cross-section through a typical shaft. Shafts are often essential for underground mines, and their location is determined based on detailed surface topography and infrastructure, the orebody, geology, rock mechanics and environmental assessments. The location must be changed where adverse geotechnical conditions are identified in the originally planned site. Proximity to the orebody, strata conditions and water-bearing structures are the major parameters that govern the ultimate location of shafts. The decision to locate the shaft is critical because the process to develop a shaft is very expensive and relatively slow. The vertical shaft must be well located with respect to the ore deposit and to be able to handle production needs. The correct diameter and configuration of the shaft will provide optimum operational efficiencies. The shaft can be rectangular, circular or elliptical in profile, although almost all hard-rock underground mines have circular section shafts because this shape generates a better geometry for airflow and is naturally more Figure 3.17 Sections of a typical shaft layouts (Kempson et al., 2015) 222 Underground hard rock mining stable from a stress concentration point of view. At great depth, however, an elliptical profile can be used if one of the principal horizontal stresses is significantly larger than the other. Elliptical shafts were designed as an alternative to large circular shafts by simply adding half-moons along the main axis. This had the effect of reducing the circular excavation and therefore the cost of sinking the shaft and could be useful under certain high horizontal stress conditions (where σ2 >> σ3). Most shafts constructed in the early 1900s were of a rectangular cross-section mainly because they were easier to equip with square or rectangular conveyances using timber beams and were not so deep that stresses caused instability issues. Blasting a square or rectangular cross-section was, however, problematic and this slowed down the rate of sinking. Circular shuttering for concrete lining is easier to move when doing concurrent lining resulting in faster work progress during sinking operations. This is an important aspect when it comes to the cash flow for any mining project. The shape and size of equipment to be taken down a shaft are also considered in the calculation of the final shaft dimensions. This process is equally applicable to ramps (declines), the main difference being that the ramp is equipped to rather handle trackless equipment and possibly be fitted with conveyors instead of the skips and cages. Inclined shafts can also use monorails. Determining the rate of mining can be summarized as follows: Identify possible mining methods Define standard mining blocks (stope or panel size) per method Calculate steady state conditions per level Define steady state inputs/outputs requirements per level Determine minimum access dimensions to cater for equipment and ventilation Calculate development requirements to get to steady state Simulate full level production from start of block to orebody extremity Determine the maximum number of levels that will operate simultaneously Estimate shaft size required to cater for the sum of the requirements of the maximum number of working levels Do economic analyses using, for example, the net present value (NPV) and internal rate of return (IRR) Decide on optimum mining layout and shaft configuration Ensure an adequate and achievable rate of investment is achieved; if not, repeat the process 3.3.2.2 Raises Raises are steeply inclined openings linking the mine sublevels at several vertical elevations and are often developed within the orebody. They are normally placed near or in the stopes employing specialized cyclic or continuous operations. Specific applications of bored raises are transfer of material, ventilation, personnel access and ore production. Inclination varies typically from a minimum of 55° to the vertical, which is the lowest angle of repose of blasted rock. They have variable cross-sections from typically 2 to 30m2. Since manual excavation of raises is a potentially more hazardous activity, raise boring is frequently utilized for developing ventilation raises, ore passes and rock fill passes. It provides safer and more efficient mechanized excavation of circular raises Underground hard rock mining 223 up to 6-m diameter because this method eliminates the need for explosives, and keeps personnel remote from the actual rock excavation. RAISE BORING Raise boring is the procedure of mechanically boring a vertical or inclined shaft between two or more levels (one of which could be the surface). Access must be available at the start and end of the excavation. In conventional raise boring, a downward pilot hole is drilled to the target level by the raise boring machine, where the bit is removed and replaced by a reaming head (Figure 3.18). The development of directional drilling has made raise boring long shafts even more feasible for longer excavations as the pilot hole can be kept correctly aligned and positioned. After the pilot hole is complete, a reamer head is fitted and the machine then reams back the hole to final diameter, rotating and pulling the reaming head upward towards the start of the pilot hole (where the raise boring machine is securely located). The cuttings fall to the lower level and are removed by any convenient method using remote control for safety considerations. The capital cost of a raise boring machine is relatively high, but the return on investment is good particularly for longer length excavations. Advantages of raise boring are that miners are not required to enter the excavation while it is underway, the excavation process is continuous, no explosives are used, a smooth and more stable profile is obtained and manpower requirements are reduced. Safety is, however, the most important advantage. Figure 3.18 Raise boring process (Atlas Copco, 1997) 224 Underground hard rock mining ALIMAK RAISING (www.alimak.com/industry/mining/) Alimak raising is another excavation alternative to raise boring that is still safer than conventional raise development. For ore-passes (rock-passes, box-holes), short raises and short shafts, a semi-mechanized system can be used especially if only bottom access is available. The excavation must be vertical or steep dipping to use this system, which was developed in 1957 by Alimak. It was initially developed (and still used) for high rise building construction. Typical mining applications include: Ventilation raises Stope slot raises (for longhole open stope first production, for example) Ore and waste pass systems Raise bore rehabilitation Ventilation shafts Manway raises Emergency ladder way installations Alimak Raise Climbers are available with air, electric or diesel/hydraulic drive units. One of the longest shaft driven in one step was 1,050m long in Norway. The platform can be any shape and size. The largest work platform supplied so far measured more than 30m2. The raise climber system is basically a movable working platform that runs along a monorail beam that increases productivity and improves safety when compared with other conventional blasting systems. As the excavation is developed, the monorail track is lengthened by adding sections to it and bolting them to the excavation sidewall. Using the system involves five production steps (as shown in Figure 3.19): Scale (make safe from the protection of the personnel cage fitted with a protective canopy) Drill (conventionally from the cage) Charge the holes (then remove the cage from the excavation using the monorail, which has a drive-toothed section) Blast (remotely) Ventilate and remove the broken ore from the bottom of the excavation Figure 3.19 The Alimak raise development production steps Source: Vikay Mining Equipment Underground hard rock mining 225 Step 1 Drilling Drilling is undertaken from the drill deck on top of the raise climber, which is sized to suit the size, shape and angle of the raise. Step 2 Charging-up When drilling is complete, the face is charged with explosives. Step 3 Blasting The Alimak climber is then lowered to the bottom of the raise and into a station for protection before the blast is triggered from a safe location. The rail is left in place and protected (only the top is really exposed to the blast directly). Step 4 Ventilation Step 5 Scaling (removing lose and unstable rocks) The Alimak system provides for efficient post-blast ventilation and a powerful air/water blast effectively dislodging loose rock from the freshly blasted face, making it ready for re-entry. 3.3.2.3 Adits An adit is a near-horizontal excavation that is used in mountainous areas or from highwalls where the orebody is located near or above the access (valley) floor (Figure 3.20). They are typically developed slightly up-dip at about 4° so that any water will drain out of the adit under gravity. Developing an adit is the same process as developing any horizontal tunnel and is an option only where the topographic relief is considerable. In this access opening, the ore and waste can be taken out of the mine at minimal operating cost. The traditional method of developing adits is to drill and blast the face, load the material into a haulage device (after the area has been cleared of dust and fumes) and then provide support and ventilation to the newly advanced face. Thus, drilling and blasting are the standard excavation method for adits and most Figure 3.20 Footwall adits development, generalized (Zhang & Wang, 2016) 1. Adits 2. Haulage drift 3. Ore pass 226 Underground hard rock mining Figure 3.21 Decline development (Zhang & Wang, 2016) 1. Decline 2. Crosscut 3. Sub-level drift 4. Ore-body hard-rock horizontal development. TBMs can be used depending typically on the length and the economics. The other main exception to the use of drilling and blasting are underground mines in relatively soft rock such as coal, salts and limestone where the rock can be removed without the need for blasting using continuous miners or road headers. A decline or ramp is an access tunnel usually developed at a low slope angle from the horizontal (