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Sub Level Caving

Sublevel caving techniqueSimplicity and low cost are the essence SUBLEVEL CAVING TECHNIQUE SIMPLICITY AND LOW COST ARE THE ESSENCE Author: Partha Das Sharma, B. Tech(Hons. ) in Mining Engineering, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ 1.

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Introduction – Sublevel caving is usually carried out when mining of the orebody through an open pit method is no longer economically feasible. Mining now proceeds underground, underneath the open pit. At first, both a raise and a network of tunnels are made.

At different sublevels, jumbos are used for long hole drilling, drilling directly upwards into the roof. These holes are then charged with explosives and blasted. As the roofs cave in, the rock from the ground surface will cave in to the underground as well. Scoop trams or Load Haul Dump (LHD) vehicles transport the muck, loosened rocks, to an ore pass where the rocks are lifted to the surface. Drilling and blasting takes place at different underground levels of the mine at the same time.

As the blasted rock, muck, is continuously transported to the ore pass, more blasting will encourage the roof to cave in to the void and further into the drift. This is repeated until blasting, caving and transporting depletes the entire orebody. 2. Modern day Sublevel caving – The sublevel caving technique, as the method is applied today, the whole quantity of ore between the different sublevels is broken using ——————————————————————————————————Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ 1

Sublevel caving techniqueSimplicity and low cost are the essence controlled drilling and blasting. Sublevel caving is in many respects simple. It can be used in orebodies with very different properties and it is easy to mechanize. In sublevel caving, ore is developed from a series of sublevels spaced at regular intervals throughout the orebody. Mining begins at the top of the orebody. A series of ring patterns is drilled and blasted from each sublevel. Broken ore is mucked out after each blast and the overlying waste rock caves on top of the broken ore. This technique is inexpensive, highly mechanized and yields a large amount of muck.

It is normally used in massive, steeply-dipping orebodies with considerable strike length. Since dilution and low recoveries are unavoidable, sublevel caving is used to mine lowgrade, low-value ore-bodies. However, studies are going on in various levels to eliminate the shortcoming of the system. The designs which are used and the measures which can be taken to eliminate the disadvantages are less understood. Possibly the modern version of the method was developed in the iron mines of Sweden. Over the past few years, the scale of sublevel caving has increased markedly with LKAB being a leader in this regard.

Today, with the continuing push to increase mining scale, a fundamental question is whether the gravity flow principles which served as the design basis for the small-scale sublevel caving mine designs of the past can be applied at much larger scales or whether some other approach is required. In fact, in design and operating practices, sublevel caving is among the most advanced of all mining methods. Sound engineering is indispensable to its conduct. The reason is related to the complexity of the caving action and the necessity of controlling it. The increased sublevel interval requires diligence in drilling, loading and blasting practices.

The advent of improved drilling technology has made increased sublevel intervals possible in the sublevel cave mine. Hydraulic tube rod drill rigs have made long ——————————————————————————————————- 2 Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ Sublevel caving techniqueSimplicity and low cost are the essence up-holes accurate with less than 2% deviation. Apart, blasting pattern used has also plays very important role.

Proper fragmentation and blasting efficiency is evaluated by measuring the percentage of muck which is able to pass the designated blast size. 3. Description of sublevel caving method – An underground mining method used in large, steeply dipping orebodies where the wall rock is of an incompetent nature. Parallel drives are developed in the ore on equally spaced levels. The ore is then drilled and blasted using longholes between levels, often in a circular or fan pattern, on retreat. The weight of the rock is used to assist breakage. The upper levels are mined ahead of lower levels.

Broken ore is extracted from the drive before the next blast takes place. The wall rock caves in when the ore is extracted. As discussed, in sublevel caving, the hanging wall is allowed to cave in as the ore is drilled, blasted and removed. The wall caving is induced by mining the ore in controlled sequence, usually from hanging wall to footwall and from top to bottom. Sublevel caving is often used in conjunction with surface mining. Sublevel caving extracts the ore via sublevels, which are developed in the orebody at regular vertical spacing. Each sublevel features systematic layout with parallel drifts, along or across the orebody.

In the wide orebody, sublevel drifts start from the footwall drive, to continue across, reaching the hanging wall. In the orebody lesser width, sublevel drifts are branched off in both directions, from a centre crosscut drive. ——————————————————————————————————Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ 3 Sublevel caving techniqueSimplicity and low cost are the essence Development volume to prepare sublevel caving is extensive, compared to other mining methods.

However, development is mainly drifting to prepare sublevels. Drifting is a simple and routine job for the mechanised mine. Development of sublevels is done efficiently, in an environment of multiple faces on one sublevel available to drill rigs and loaders. A ramp connection is needed to connect different sublevels, and communicate with main transport routes. Ore passes are also required, at strategically locations along sublevels, for LHD-loaders to dump ore from sublevels, to be collected and transported on the haulage level below. —————————————————————————– ————————Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ 4 Sublevel caving techniqueSimplicity and low cost are the essence Longhole rigs drill the ore section above the drift, in a fan spread pattern. Longhole drilling is a procedure which is done independent of other jobs, often well ahead of charging. Thus, drilling and charging-blasting longholes can be timed to suit the mine’s production schedules.

Blasting on each sublevel starts at the hanging wall, mining then retreats toward the footwall. Miners aim the cave to follow an approximately straight front, and adjacent drifts mined at similar pace. A section through the cave to show upper sublevels one step ahead of sublevels underneath. Blasting the longhole fan breaks the ore volume covered by the fan-pattern. As the cave is filled with fractured rock, most of the fresh ore remains in the cave, while some caves into the drift opening. Mucking out with LHD-loaders creates a cave pattern of ore and waste from above.

Loading continues until the operator decides that waste dilution is too high, and stops the mucking, and transfers to a nearby drift heading with a fresh cave. In the meantime, the empty heading is occupied by the charging team, to charge next ring of longholes. Ore handling involves mucking out at the cave, transport on sublevels and dumping into ore passes. Waste dilution and ore losses are drawbacks for sublevel caving. Waste dilution varies between 15 and 40%, ore losses from 15 % to 25 %, depending on local conditions. Dilution is of less influence for orebodies with diffuse boundaries, where the host rock contains low grades minerals.

Parameters directly influence flow behaviour have been found to include the geometry of the extraction layout and drives, sublevel height, blast ring design, material characteristics of the blasted and waste material, and draw control methodology. In most of the sublevel caving system blasting parameters dominate in association with recovery when compared to drawpoint and geological parameters. Generally, sublevel caving method employs use of fan drilling using long and smalldiameter holes, between sublevels or drill drifts to undercut and blast the ore-zone. An initial slot is developed at the wallrock, and vertical uphole fans are rilled in a diamond pattern from sublevel in sequence. Generally, eight holes, inclined 80 to 85 degrees toward the slot, are drilled. Hole diameter average 51 mm, Burden and Spacing vary from 1. 2 to 1. 8 m and 1. 5 to 1. 8 m respectively. Blasting is performed against broken waste rock as wall rock caves. Generally, specific-charge range from 0. 3 to 0. 4 kg/ton. 4. Below are some salient features of Sublevel caving technique: Application * Weak walls and strong ore preferred though weak ore can be mined too. * Steep dip: A vertical dip is best, while dip>60 is fine too.

Possibility of loss of ore in non-vertical steep dips Considerable loss of reserves in flat dips * Preferably, the ore and the rock should be easily separable * Surface should be amenable to caving (not an inhabited or the watershed area etc) ——————————————————————————————————Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ 5 Sublevel caving techniqueSimplicity and low cost are the essence Development * Significant. Almost 20% of the ore is mine during development * Sublevels are established generally at 7. -12. 2 m vertical intervals and about 10. 7 m horizontal intervals The vertical interval is dependent on the drilling accuracy and the dip of the orebody The horizontal and vertical spacings affect the eccentricity of the cave * The size and shape of the production drift affects the draw Drift should be as wide as possible Should give good support to the back and the brows If the back is arched, the draw is mostly at the center and none on the sides If the back has to be arched for ground support, the drifts should be closer Ore remnants left behind (due to being out of reach of LHD’s) increase with height.

Therefore, drift height should be as small as possible (usually about 3-3. 2 m). * Slot raises are driven at the hanging wall end of the production drift all the way up to the next level * Haulage levels driven in waste In wide orebodies, transverse layouts may be used (ore widths should be > 12-15 m) Here, the production drifts are perpendicular to the strike Recovery better than longitudinal layouts Haulage drift in waste (footwall), about 9 m from ore contact. This distance is maintained so that blasting does not occur too close to it.

Diamond drilling is done to obtain ore boundaries so that the haulage drift is neither too close nor too far away. o o * Ramps may be driven at 15 -18 , to production levels to provide access * Long, fan holes drilled 70-80 degrees forward (about 8 holes Production totaling 100 m in a ring) * Side holes, if drilled, should be steeper than 70 degree Side holes reduce length of holes driven up from the level below If flat holes drilled to the sides, however, the blasted material from the side holes cannot be extracted from the working level.

Therefore, the neighboring fan hole blasted material does not get enough room to expand * Brows should be supported if necessary If brows collapse, ore floods the drift and covers some rings Also, the loader has problems loading as the ore does not stack high If brows are uneven, the ore funnels down the high spots ——————————————————————————————————- 6 Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/

Sublevel caving techniqueSimplicity and low cost are the essence If sloughing or high brows are noticed from development, it may be decided to blast more than 2 rings just to advance through the area * If drilling is not accurate, a bridge/arch may be left in the stope Re-slotting may become necessary * Powder factor high as blasting is always against blasted muck (almost twice that of blasting against an open face) * LHD’s used for mucking * Good ventilation necessary as all working faces are dead ends * Productivity is quite high (about 36 ton/miner-shift) Comments * High dilution from caved waste.

Ore losses occur as well since not all mined ore can be recovered. * Ore/waste flow cannot be predicted accurately prior to mining. * Probably the most economical when mining in weak strata * Development openings are not kept open for the entire life of mine. Once a level is extracted, the development openings are consumed. * Safe, since all mining activities are in small, protected openings. * High degree of mechanization possible. 5. Discussion on ore flow, drilling and blasting for efficient workings: * Fragmentation is of key importance to sublevel cave stope.

Proper fragmentation of the ore column resulted in improved efficiency for the other mine operations and was critical to recovery. * Underground observations indicate that the draw-points must be wide enough to account for inconsistent blast results and provide regular flow. * The draw-point width determines the width and flow strength of the central channel which is important to achieving high recovery of the ring. * The middle holes of the ring are fired first and can make first use of the swell volume offered by the underlying sublevel drift. * The central holes are drilled subvertical, fairly parallel, and relatively close to one another.

The result is a relatively high and uniform specific charge compared to the other holes in the round. Thus, one would expect the best, most uniform fragmentation. * The ore material in the central part of the round can make the best use of the effect of gravity in directing it to the drawpoint. * All of the material in the fan is drilled and blasted. * Because of the fan geometry, the amount of explosive/unit volume and hence the fragmentation varies throughout the fan. * The ore material in the centre part of the fan and the lower part of the fan has a much higher specific charge than that at the boundaries of the ring. Furthermore, the “cave” which lies in front of the blasted slice is an eclectic mixture of waste rock and ore remnants. * Its mobility varies with location and with time (it changes with the extraction geometry). ——————————————————————————————————Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ 7 Sublevel caving techniqueSimplicity and low cost are the essence * Finally, most rock materials upon being blasted would like to bulk (swell) of the order of 50%. In sublevel caving, it is the sublevel drift located at the bottom end of the fan which is the primary provider of swell space for the ore in the ring. * It has been observed that, increased amounts of oversize in the ring decreases recovery, proving the importance of proper fragmentation for sublevel cave operation. * Recoveries could be less than 60% if the ring is composed of 30% oversize. The oversize in the draw-point reduces the mobility of the ore, thus preventing high recovery. * Effective blast patterns also take into consideration the amount of benching. High powder factors are also required to reduce benching but may create excessive blast damage and over-break. * Practical solutions, such as, the blast pattern, blasthole size, primer sequencing, powder factor and degree of fragmentation are to be considered in order to be efficient. 6. Design parameters and some preliminary design rules – Design parameters in sublevel caving are largely a function of caving mechanics, the branch of rock mechanics related to the breakage and collapse of consolidated materials in place and their flow downward by gravity.

Although, the ore has to be drilled and blasted, the overlying rock comprising capping or hanging wall is undercut and caves. Extremely careful controls must be exercised in drawing the ore to avoid excessive dilution. Draw control is the practice of regulating the withdrawal of ore in the sublevel crosscuts so as to optimize the economics of draw. Preliminary design rules: * Sublevel drift size (width (Wd) and height (Hd): determined based on equipment. * Sublevel interval (HS): the theoretical maximum value is based on the ability to drill long, straight holes.

This, in turn, is based on the hole diameter (D). The actual limit is based on recovery and dilution considerations which are due to managing ore/waste pulsation. * Hole diameter (D): based on the available drilling equipment and the ability to charge long holes. * Spacing of the sublevel drifts (Sd): Sd = (2. 4 to 2. 7) Wd * Ring spacing (Burden B): Based upon the damage radius (Rd): B = 2 Rd Where: Rd/rh = 20 ( Peexp/PeANFO)? ( 2. 65/? rock)? ? Rd = damage radius (m); rh = hole radius (m); Peexp = explosion pressure for the explosive; PeANFO = explosion pressure for ANFO = 1600 MPa; ? ock = rock density (g/cm3); 2. 65 = density of typical rock (g/cm3). * Hole toe spacing (ST): based upon the burden ST = 1. 3 B. * Spacing for parallel holes (SP): based upon the burden SP = B. * Front inclination: 70–80 degrees (forward). Example: If it is assumed that: D = 115 mm; Drift dimensions: 7 m wide by 5 m high; Explosive: emulsion (Pe Exp = 3900 MPa); Rock density = 4. 6 g/cm3; Sublevel interval: 25 m based on drilling ability and control of pulsation. ——————————————————————————————————Author: Partha Das Sharma, E. ail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ 8 Sublevel caving techniqueSimplicity and low cost are the essence One finds that the remaining dimensions are: Sublevel drift spacing: 17–19 m; Burden: 2. 7 m; Toe spacing (fanned): 3. 5 m; Toe spacing (parallel): 3 m; Front inclination: 80 degree selected. 7. Experience from LKAB, Sweden – Sub-level caving (SLC) is an important mass mining method, used at LKAB. The caved rock or debris at the SLC interface reduces the fragmentation and the swelling of the blasted ring and it dissipates the explosive energy.

These phenomena may immobilize the blasted ring, causing ore losses. There are two major factors that influence the mobilization of the blasted ring, fragmentation and swelling of the blasted material. The caving process is influenced by also the stiffness of the waste rock, which is dependent in some way by the compaction that the blast ring contributes to. By using the acoustic impedance between the blasted material and the confining debris, a relationship for both fragmentation and compaction have been found depending on material, specific charge and physical properties of the debris.

Regression analysis has been used for both tasks, where the two statistical hypotheses clearly have good agreement with actual data i. e. the prediction models can forecast both the fragmentation and compaction for this set-up with reasonable accuracy The results can be comparable with confined blasting in large scale, this both that it have representative design parameters that fulfil in many ways the scaling laws and the second is that it can be linked to other comparisons between large-scale and small-scale, where the similarities have been shown.

LKAB’s Malmberget – The Malmberget mine consists of about 20 orebodies, of which ten are currently mined. Most of the deposit consists of magnetite ore, but non-magnetic hematite also occurs. The present main level of the Malmberget mine is at a depth of 1000 meters. About 14 Mt of crude ore is extracted from the orebodies each year. Development – The first stage of mining is drift development. A drift is a tunnel that is driven into the rock. Development involves construction of new areas of the mine where ore can be extracted.

A development project begins with construction documents prepared by the mine planning department. Each year, the mine planning department orders development work on the basis of the forecast demand for products as well as current knowledge of the status of the orebody. A development drift goes right through the orebody. Drifts are driven with electrichydraulic drill rigs. For each charge, as many as 60 holes are drilled. Each hole is about 5 meters deep. When all the holes are drilled, they are charged with explosives. The charge is blasted at night. The loose ore is then hauled out by a front loader.

This procedure is repeated until the entire development drift has been driven. The drifts can be up to 80 meters long. If necessary, the walls and ceiling of the drift are reinforced with rock bolts and/or shotcrete. When development is complete, i. e. , when several drifts have been driven in the same area, the next stage of mining can begin; namely, production drilling. ——————————————————————————————————Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. ordpress. com/ 9 Sublevel caving techniqueSimplicity and low cost are the essence Production drilling – Slices of ore are drilled up with remote-controlled production drilling rigs. From their control rooms, the operators (drillers) operate several drill rigs out in the production areas via remote control. The rig drills upwards into the ore, forming fan-shaped patterns of holes. There are 10 drill holes in each series. They are normally about 40-45 meters deep. The holes are straight, so that subsequent charging with explosive and blasting can be done efficiently.

When a pattern of holes has been drilled, the rig is moved back three meters, then drilling of the next pattern begins. About 20 of these patterns will be drilled in an 80-meter drift. Once drilling is completed in the entire drift, the holes can charged with explosive. Blasting – A robot injects explosive into the drill holes in one pattern. The explosive is manufactured by LKAB’s own explosives company. Blasting is done every night. Each round brings down about 10,000 tonnes of ore. When the blast has been ventilated, loading with wheeled loaders can begin.

Then, the next pattern is charged, etc. The procedure is repeated until the entire drift has been mined out. 8. Conclusion – Caving methods have become the underground bulk mining methods of choice and expected to continue in the foreseeable future. The growing popularity of caving methods around the world is largely due to the very low production cost and the intrinsic safety associated with this mining approach. It is often the only viable mining method for some of the lower grade massive orebodies that are becoming too deep for open pit mining. Caving of strong rock masses has ecome accepted practice even though there are still some challenges to be resolved. Strategically, most medium and large mining companies are operating or planning to operate a caving mine. Codelco’s El Teniente mine in Chile and the LKAB Kiruna Mine in Sweden are among the largest and most famous caving operations in the world. Australia’s leading caving operations include Rio Tinto’s Northparkes mines, BHP Billiton Nickel West’s Perseverance Mine and Newcrest’s Telfer and Ridgeway gold mines. References: * Kvapil, R , “Subleve l Caving”, Underground Mining Methods Handbook. “d Edit ion, Vol. 2. , H. L. Hartman, ed. , SME. Littleton, CO, 1992, p. 1789. * Geddes, P. J. , “Swedish Drilling and Blasting Practice”, Trans. Int. Min. Metail. , 1 986, Sect. A. , V. 95, p. 204-207. * Alatalo. R. , Heden, H. and Ronnback, L. , ‘large Scale Sublevel Caving in LKAB Malmberget Mine”, Proc. Int. Sym. Large Scale Underground hfining, Nov. 1985, V. 7. Lulea, p. 139- 145. * Baase, R. A. , Diment, W. D. , and Petrina, A. J. , “Sublevel Caving at Craigmont Mines Ltd. “, Underground Mining Methods Handbook, W. A. Hustrulid, ed.. SME, New York, 1982, p. 898. Chatterjee, P. K. , Just, G. D. , and Ham, G. I.. ‘Sub-level caving simulation of 3000 pillar-recovery operation at Mount Isa mine. Australia”. Tram inst . min. & Met.. Sect. A. , V. 88.. Oct. 1979, p. Al47-Al55. ——————————————————————————————————- 10 Author: Partha Das Sharma, E. mail: [email protected] com, Blog/Website: http://miningandblasting. wordpress. com/ Sublevel caving techniqueSimplicity and low cost are the essence * Cox, LA. , “Latest Developments and Draw Control in Sublevel Caving”, Trans.

Int. Min Metall. , 1967, V. 76, p. A149-159. * Hustrulid. A. , “Sublevel Cave Mining at LKAB Kinina Mine”. iNCO presentation Aug. 1995. * Bull, G. and Page, C. H. (2000) Sublevel caving – today’s dependable low-cost ‘ore factory’, in Proceedings MassMin 2000, G. Chitombo (ed), 29 October to 2 November 2000, Brisbane, Australia, Australasian Institute of Mining and Metallurgy, Melbourne, pp. 537–556. * Hustrulid, W. and Kvapil, R. (2008) Sublevel Caving – past and future, in Proceedings 5th International Conference and Exhibition on Mass Mining, MassMin 2008, H. Schunnesson and