AU2012378771A1 - Hydraulic foliating of ore bodies exploited by block or panel caving mining methods - Google Patents

Abstract

A method of mining to increase the ore fragmentation in block caving, using the strain relief cracks surrounding the cave's back, produced during the caving progression. Intermixed (TOR) and (FF) boreholes are down drilled from top of the block up to undercut drifts. Once block undercut and ore drawn start, with each newly cave back, strain relief cracks tangential to the cave back appear, detected through breakable cables installed inside the (TOR) boreholes and connected to: a TOR (time-domain reflectometry) instrument, an oscilloscope and monitor screens. In tempo with each newly cave back. The cracked bands are hydraulically foliated through the (FF) boreholes. Foliation ascending from the undercut groove's back, in parallel layers as thin as feasible covering most of the block in height; transforming it - during the caving's upward progression - in fragile flagstones.

Description

1 HYDRAULIC SLICING OF ORE BODIES EXPLOITED BY BLOCK OR PANEL CAVING MINING METHODS TECHNICAL FIELD (0001) The invention relates to degrade deep ore bodies, exploited by Block or Panel Caving mining methods (generically named: block caving), slicing them hydraulically to increase the ore's fragmentation. With at hand state of technology, the efficiency and productivity of those mining methods are adversely constrained by the coarser ore incoming from drawpoints. BACKGROUND (0002) Geotechnical context.- Deep orebodies are constituted by healthy rocks practically unaltered -, non-permeable, with few natural fractures; geomechanical very competent (harder and tougher), denominated: hard rock or primary ore. Because of the further deepening of these orebodies, they are beneath a large gravitational stress component, generated by the overlying rockmass weight with consequent increment of the stress field. Additionally, in several ore deposits, there are lateral residual stresses (tectonic), which can be higher than the gravitational field. Besides previous factors, the combined effects of those stresses, if they are not properly known or respected, generate situations and conditions of great structural instability will be generated by the redistribution action of the stress field, induced by: the excavations of the galleries, and the imperfect undercutting and caving process. (0003) Coarse ore.- The main subjects are the new problems appearing, as mining exploitation progresses to greater depths, because the fragmentation of the ore is increasingly coarse. Many boulders have some cubic meters size. It enforces the building of large and expensive underground infrastructures and to utilize inefficient material handling systems. The poor ore fragmentation is the main cause of the serious weakness affecting the block caving mining, such as: * Early dilution entrance * High production costs * Low productivity. * Very low draw ore rate per unit area. * Require considerable capital investment (CAPEX) in: Infrastructure, development and equipments.

2 * Main inconvenience is that at present it is unfeasible to apply to the extractive process modern bulk material handling systems known as: Continuous Mining. (0004) Material handling systems.- To handle the coarse ore, exploitation systems are applying large fleets of loading and transport equipments of big size, with alternate and intermittent operational cycles, frequently exposed to long interferences caused by: / Re-handling the big boulders. / Hydraulic hammering and/or secondary blasting / Hang-up (arching) of boulders into drawpoints. / Environmental contamination with noxious gases and dust. (0005) In order to overcome inefficiencies, extra capacity and speedy equipments have been introduced larger in size, capacity and tasters. Nowadays, in production, LHD (load, haul and dump) equipments are operated at some underground mines outfitted with buckets of up to 10 m 3 capacity and trucks with payload capacity up to 80 t. The introduction of larger equipments has increased productivity, but such upsurge has been marginal and not proportional to major capital investments and operating costs associates. Enlargement the LHD's buckets allowed to handle bigger boulders, relocating now the problem in ore passes and/or require installing underground crusher plants feeding by LHD units. The LHD technologies applying to extractive ore process are inefficient because its cyclical and intermittent performance. The overall efficiency of LHD equipments hardly exceeds 10%. In fact, the time feeding the ore passes is around 10% of each cycle. Moreover, each LHD requires a large area to operate, and in that space it is mutually exclusive with other LHD units, impeding the extraction of more tonnage per unit area. Besides, the tendency to use greater capacity equipments brings, as a consequence, a structural weakening of the underground openings. Large excavations are required to hold those equipments, which - to assure their stability demand strong and expensive fortification by the high pre-existing stresses field, plus that rearranged by the exploitation itself. Nowadays, because of the coarse ore, it is just unthinkable to replace the LHD units by new material handling technologies that allow higher extraction ore rates through some continuous extractive system. (0006) Cavability.- Sometimes, the caving process of a block situated in an undisturbed rockmass (first caving) progresses upwards at a lesser speed than the broken ore's extraction rate, due to: High stress fields; the great toughness of primary rocks and the few planes of weakness or scarce discontinuities. As a consequence, occasionally, a 3 self-supporting arch is formed in the cave back, stopping the caving process, forming a stable vault, reaching horizontal dimensions between 1,500 up to 10,000 m 2 and more. The vaulted ceiling keeps on suspension at a height of several meters, and is practically inaccessible to induce its collapse. If there are no pre-existing drifts located over and close to the vault, they must be excavated urgently to overcome the crisis. This disturbance has already happened in several mines, such as: Urad (USA) 1968, Codelco-Andina (Chile) 1971, Codelco-El Salvador (Chile) 1999, Northparkes (Australia) 1999 and others. It provokes severe consequences, such as: long disrupting production; expensive reactivation and high risks for the personnel, inasmuch as when collapsing the air displacement (air blast) can be an extremely violent and devastating event. As a preventive measure, some mines permanently operate at a very low ore extraction rate, thus augmenting the already enunciated inefficiencies. Those lower extraction rates demand still greater production areas plus the respective associated infrastructure such as: ventilation, accesses and so forth. In expert's opinion, the main factors that determine Cavability of a first block, in an undisturbed zone, are: o Strength of the rockmass o Major geologic structures o In situ stress field o Water occurrence o Induced stresses during the undercutting and caving process o Undercut's geometry. (0007) Each one of those factors, once characterized and properly weighed through detailed studies, allow defining - for a particular portion of a rock mass - the Modified Rock Mass Rating (MRMR). This index - in a simplified interpretation -, establishes 5 categories or classes of rockmass. With the support of this parameter, it is intended to express - quantitatively -, in undisturbed zones, the more or less degree of complexity or the geomechanical quality of the rockmass - to act in response to the requirements that ensure a block's Cavability. Empirical evidences, deduced from several successful blocks caved at the first intent, have attempted to formulate a virtuous relationship among: o The geomechanical factors of Cavability, already enunciated. o The minimum extension of the area to be caved. o The basal geometry to cave.

4 (0008) Such relationship is expressed as the hydraulic radius, for a portion of the specific rockmass and is used as a criterion to predict its Cavability. The hydraulic radius formula is a concept adopted from hydraulic engineering. Corresponds to the proportion between the flow's cross area divided by the perimeter of that area. Formula stipulates that to a smaller MRMR - related to a very competent rockmass -, a bigger hydraulic radius is required, which means a larger undercutting area. At the first block caved at Northparkes mine, the Cavability predictions, based on MRMR and associated to the hydraulic radius, were not fulfilled. This is explained by the difficulty to assign numerical parameters to nature, represented, in this particular case, by the complexity of the rockmass and, in addition, by the unpredictable stress field redistribution induced by mining. (0009) Dilution control.- The daily ore Draw Chart is the only operative existing tool to control the dilution. This tool assumes that - inside the block under exploitation - the drawing and caving rates are alike and that the ore drawn from each draw point comes from the ore positioned, when in situ, vertically over the draw point. Problems arise when - as usual - the draw rate is higher than caving rate, generating air voids between the top of the broken ore column and the cave back, making possible the erratic collapse of big boulders, distorting the verticality of the flow. This issue favors the formation of chimneys connecting with anticipation the overlying waste with the caved ore. (0010) Extent production areas required. - At some mines, with Andesitic rock, extraction rates of 0.5 t/m 2 /day have been adopted as a design criterion. The lower the extraction rates the larger the required production infrastructure. Already a large part of its areas will be in production while others shall be under development to replace the exhausted ones. At present state of technology, each one of those areas consist, spatially, of several levels (four to five), interconnected by vertical and inclined openings, such as: ore passes; chimneys for ventilation and drainage; ore transport galleries; shafts equipped with elevators and ramps for personnel displacement and distribution of supplies, etc. Several mines are producing in ranges of 40,000 to 140,000 metric tons per day. To extract those tonnages - assuming a caving rate of 185 mm/day -, the areas required, in production only, as a minimum are 80,000 m 2 and 280,000 M 2 , respectively, plus the areas under development to replace those being exhausted. Attending the magnitude and difficulties represented by excavation and fortification of the extensive underground infrastructure and service facilities, construction start up at 5 least two to three years before undercutting the blocks. This signifies huge capital expenditure (CAPEX) needs with subsequent considerable financial cost. (0011) Instabilities.- In extreme underground environments, characterized by higher in situ stresses with a complex geology and geotechnical (lithology and structures) as, for example, El Teniente mine (G. Diaz and P. Tobar - MASSMIN2000), different instabilities are manifested in the rockmass, as follows: - Slabbing and over-excavation of galleries.- This is the most common type of instabilities in horizontal and vertical openings. Along horizontal galleries it is associated to rock falls, mainly during the excavation stage. - Formation of structural blocks.- It corresponds to major over-excavations and it is characterized by the formation of great wedges that move along relevant structural features and the limits of caved areas. They also appear in large-scale galleries or at their intersections. - Collapses .- Corresponds to the gradual failure of the rockmass over an extensive area, usually in the Extraction Level, with observed damage at the crown pillars, whose maximum expression is the total closing of the affected galleries lessening temporarily the total production area. - Rock burst. - This is the most complex and risky phenomenon of instability, mainly due to the rupture and the sudden and violent displacement of the rock that could affect extent grounds. (0012) Measures adopted to face instability situations normally are quite expensive and not often successful. Causes are: Compactness and heterogeneity of the rockmass its, the large magnitude of the existing stress field and the complex and instability effects of its rearrangement, because of the intense disturbances caused by mining activities Research and development for new solutions: (0013) Technologies for pre - fracturing rockmass. - Due to Caving method's inefficiencies - because of the coarse ore fragmentation -, particularly in relation with the ore handling systems, there exists consensus about the convenience of intervening the rockmass in situ, prior to undercutting the block; fracturing it, previously, as much as being technically feasible. A block previously fractured should deliver smaller ore fragments at drawpoints. At present - in Chilean mines - the following technologies are being tested: Pre - fracturing in situ of the rockmass by massive blasting. - The intensive use of explosives applied in confined massive blasting (without free faces) 6 inside the block is at an experimental stage. The idea is to take advantage of the shock waves interaction, generated by several explosive charges located in different drill-holes, and activated sequentially by programmable electronic initiators of delays of microseconds. The main disadvantage of this technique is the low yield of the enormous amount of energy delivered by explosives, due to: o The full confinement of the explosive. It is blasted without free faces. o Rockmass elasticity absorbs most of the shock waves. The scant information that has been disclosed, about the massive blasting tests, points out that in the Extraction Level, indeed an additional fragmentation over the "natural" one is appraised. However, this technology is still not efficacious enough to improve significantly the LHD unit's efficiency. Hydraulic fracturing. - Some mines are testing the hydraulic fracturing of blocks, according to concepts disclosed in the USA patent of invention N" 6,123,394 titled "Hydraulic Fracturing of Ore Bodies". The hydraulic fracturing tests have shown some decrease in quantity and size of boulders. Nevertheless, improvement is far from generating fragments PIoo s 1.5 m., allowing increasing the efficiency and productivity of the material handling systems. And, mainly, this technique does not yet allow replacing, in the Production Level, the LHD fleet by new technologies that move toward continuous mining neither to replace yet, at the Haulage Level, trucks and trains by belt conveyors. From the Patent of Invention No 6,123,394, are highlight those features that we consider as weakness or not in agreement with the empirical block caving practice: V This tool does not allow orientating the new fractures. Direction of the new hydraulic fractures depends strictly on the in situ stress field. Normally the new fractures open against the minimum stress vector as. When the difference of the in situ stress magnitude between vectors - minimum and maximum - a1 and as is small, orientation of the new fracture is unpredictable. / According to mentioned patent, it is necessary to have at one's disposal an air gap underneath the fracturing ore, so that it can collapse there, and 7 to be thus able to continue fracturing the block in ascent. As is known, to dispose - permanently - an empty void there, is not a programmable goal. / The fact that this tool would allow creating sets of new fractures of I to 10 meters distance, permits anticipating that the ore's fragments are going to be large, that the only option to handle the ore at Extraction Level is with LHD units. / The mentioned invention, mainly, is under favorable conditions, a corrective tool to solve non-caving situations, but not the coarse ore problems. In 1997, at the Northparkes hang up of the first caved block, this corrective tool proved its efficacy. (0014) Crushing Plants installation.- In order to solve the problems associated to coarse ore vertical transfer, the Northparkes mine (Australia) installed crushing stations at the Extraction Level, outside the orebody's "footprint". This solution allows using big LHD units to handle the coarser boulders. This approach looks fair for small and narrow ore bodies, with short transport distance for the LHD fleet. In ore bodies with a large "footprint", the LHD's inefficiencies will increase, because of the longer transport distance and the LHD queues at crusher stations due to the noteworthy reduction of dump points. Relevant antecedents: (0015) Technological status.- At present, the intermittent and cyclic mining production operative processes, maintain stagnate the block caving mining's efficiency. It is necessary to develop new mining methods, essentially: Continuous, low costs, reliable and high productivity. (0016) Comminution during the extraction process.- Important gravitational forces are generated within the downstream ore flow to drawpoints, such as: Friction, bending, compression and shears; causing to broken ore an important level of fragmentation. The gravitational Comminution process was theoretically calculated by W. Hustrulid (MASSMIN 2000) based on the Bond his Third Comminution Theory. The Hustrulid analysis shows that in the case of a boulder, 1 ton weight, descending 100 meters by gravity, the potential energy released is equivalent to blast that boulder with 300 grams of ANFO explosive. That expected high yield of fragmentation doesn't happen, due to: - The high toughness of the rock - The fragmented material in motion contains a great amount of air gaps (swell), reason why boulders in their descent, notwithstanding being 8 affected by important mechanical requests, are under cushioned conditions. e The boulders' tendency to be rounded confers to them, structurally, a great resistance to compressive and shear stresses. e The most probable is that many of the new fragments come from the blunting of the protuberances generated when the caving process of fragmenting the original rockmass. Anyway there are no doubts about that the gravitational Comminution contribution during the extraction process to the ore's fragmentation, it is quite significant and it will be more efficient if the in situ ore is previously transformed into structurally fragile pieces. (0017) Strength of the rocks.- It is well known that rocks are quite more breakable under tensile stress than under compressive stress. Moreover, thin rock slabs are - in their minute dimension - rather fragile under shear stress and even so more under bending stress (another tensile form). Next table shows the remarkable relationship of the strength of a rock of a Chilean mine, in Mega Pascal: Rocks Type Parameters Andesite Diorite Compressive strength (C) 124.3 140.9 Tensile strength (T) 4.2 6.1 Ratio 29.5 23.1 (C/T) Table 1.- Compressive and tensile rock's strength ratio (0018) The Panek Cracking Effect. - The cracking phenomenon, was disclosed by Louis A. Panek (1981), taking place in a rockmass around an active caving volume previously extracted by block caving exploitation. In the zone surrounding an active cave, ground movement was detected. The displacements reveal the development of a zone of deformation - by strain relief - surrounding the active cave inside which the ground tends to migrate, developing extension cracks, tangential to the cave.

9 BRIEF SUMMARY OF THE INVENTION (0019) The main mining target and its hindrance.- The deeper, compact and tougher primary ore, are demanding new technological approaches to improve its fragmentation and reach an acceptable productivity and lower costs. It can be obtained, ideally, through the implementation of continuous ore handling systems. Because of the coarser ore, after four decades signed by extraction and handling the ore through the LHD equipments, the block caving mining method is showing severe signs of stagnation. (0020) Functional boreholes net.- From a new Slicing/Fracturing Level, located at the top of the blocks, two interlaced nets of vertical boreholes - (TDR) and (SF) -, are down drilled connected with the undercut drifts of the undercutting level. The (SF) boreholes are used to foliate hydraulically most of the block. The (TDR) net of boreholes are equipped with sensors to detect the appearing of the extension cracks disclosed by Panek. Inside the (TDR) boreholes there are breakable cables connected to a TDR (time - domain reflectometry) instrument. The TDR signals are sent to a computer with capacity to display captured information on monitors, showing the data, in vertical sections or in 3D indistinctly (0021) Slicing the blocks.- Hydraulic fracturing is the tool used for gradually slicing/fracturing the blocks. It will be applied starting from cave back of the undercut groove and progressing upward step by step, slicing the neighboring belt as appear in the cave back's vicinity ribbon new extension cracks created during each cave back's upward progression. The hydraulic slicing will be extremely favored, because the uncaved ribbon zone close to each new cave back is strain relieved, with many extension cracks developed tangentially to each new cave back. The purpose of the hydraulic slicing of the block, ascending in successive parallel layers - from the undercutting groove's ceiling and at tempo with the caving's progression - is to divide the block progressively, in all its height, in innumerable layers as thin as possible. It will be carried out as follows: - Slicing will progress in ascending order and ideally very horizontally, starting from the ceiling groove undercut's vicinity, supplying in this manner the genetic rockmass deficit of that kind of fractures. - The block will be foliated simultaneously with the new extension cracks appearing due to the ascent of each cave back.

10 - Process in height will have to be managed, ideally, in successive parallel planes, maintaining through the hydraulic slicing, the cave back's profile as even as possible. (0022) Weakening the rockmass. - Because of slicing the block it will be structurally transformed into a set of thin and fragile flagstones. As a result, the block gradually will be taken to a geomechanical status notably less tough than the original. (0023) Extra gravitational fragmentation.- The potential energy variation's contribution to ore fragmentation will be quite efficient, inasmuch as the block, once foliated, remain divided into several thinly, slender and structurally fragile flagstones. (0024) Relax of the in situ stress field. - As a result of the multiple fractures sequentially created by hydraulic fracturing, it will produce an effective relax of the original high stress field. Relax will progress gradually at controlled rate, attenuating any seismic activity and suppressing the rock burst risk. The Slicing/Fracturing process: (0025) Efficacious undercutting of the block.- Undercutting the block's base is an intrinsic step in these caving methods. In this process, special attention must be given to avoid eventual pillars. If any TDR sensor discovers a pillar, that pillar must be removed immediately, slicing it hydraulically through the surrounded (SF) boreholes and, simultaneously the ore should be drawn by the drawpoints located closer to the pillar, up to reach the same elevation of the cracks detected in the neighboring (TDR) boreholes. (0026) Simultaneity between ore extraction and hydraulic slicing/fracturing.- After undercutting the block, the ore drawn at the entire block's base stars from the several drawpoints of the Extraction Level, as uniformly as possible.. (0027) The evolution of the extension cracks is monitored through the TDR sensors and displayed in vertical sections - coincident, for instance, with the (TDR) borehole's axis - and an overall picture could be displayed in 3D, in order to know and visualize exactly the profile geometry of the entire vault's ceiling. This tool will allow managing the draw ore chart and the slicing/fracturing sequence in order to raise the ceiling caving vault evenly in thinner incremental advances, inducing it to adopt a typical dome or inclined plane configuration (0028) As soon as new extension cracks are detected in any (TDR) borehole, through the contiguous surrounding (SF) boreholes - situated at the same level of the new cracks - one must start, ascending, the hydraulic slicing/fracturing. The new fractures 11 must be extended as much as possible and simultaneously start the drawing of the foliated ore, paying attention to maintain the cave back's shape even. The foremost, should be to maintain at the minimum the air gap between the just caved and broken ore and the recently cracked ribbon ore. (0029) Monitoring the progress of caving and slicing - As soon as the cave back adopts a dome form or an inclined plane shape - evidenced by displaying in 3D of TDR instrument's signals from the monitored (TDR) boreholes -, in order to optimize the block's slicing it is convenient to draw the ore evenly, trying to generate simultaneously cracks in several nearby detection (TDR) boreholes and slicing them immediately, in strict coordination with the daily draw-chart. (0030) The signals of the TDR instrument should be monitored in 3D to outline the ceiling's shape of the uncaved ore, and when any sector of the vault's ceiling is backward, it must be corrected immediately by hydraulic slicing/fracturing it, through the nearest (SF) boreholes. (0031) During the caving / slicing / extraction process, it is foremost to keep at a minimum the air gap between the new cave back and the top of the broken ore. (0032) Slice's benefits.- Consequences of integral progressive block slicing, the following factors will be substantially modified: - Structurally, the rockmass is gradually transformed into many thin and fragile flagstones. - The original stress field's magnitude will be gradual and significantly reduced, because - as a result of the hydraulic slicing - there has been a controlled process of energy release. - Geomechanical, the original great toughness of the rockmass, represented by the MRMR index, has been weakened. (0033) End products.- At last, progressively, most of the block results intensively foliated, in pieces as thin as can be possible. This is the main step of the invention. The intensive slicing, leaves the block conditioned for the next productive Extraction/Fragmentation step. Due to slicing, the gravitational forces contribution to fragmentation, during the down ore flowing to drawpoints, will be highly efficacious.

12 BRIEF DESCRIPTION OF THE DRAWINGS (0034) FIG. 1 is a schematic illustration of the two vertical interlaced boreholes nets. They are down drilled to all the height of the block, from a level located at the top of blocks. Boreholes are regularly spread and connected with the lower undercut galleries. (0035) FIG. 2 shows an optional arrangement of the two kinds of boreholes: / The (TDR) boreholes are for detecting the strain relief cracks appearing during the upward progression of the cavity ceiling. / The (SF) boreholes will be used to slicing/fracturing hydraulically the block. (0036) FIG. 3 shows cave back progressing upward as the ore is drawn, and in its upper vicinity the extension cracks disclosed by Panek appear. (0037) FIG. 4 shows schematically how the slicing/fracturing accompanies progressively and closely on course the advance of the caving up to such a block height that is compatible with the safe disassembly and recovery of the slicing equipment from the Slicing/Fracturing Level. DETAILED DESCRIPTION OF THE INVENTION (0038) Continuous Mining: The foremost target.- To confront the increasing mining problems which are displayed by deeper ore deposits, containing more and more stronger rocks, in USA a committee integrated by five scientific institutions, published the report: Evolutionary and Revolutionary Technologies for Mining" (2002). The document outlines the main investigation and innovation subjects that will have to be urgently developed for the mining industry in the next future. The report concludes that, to reach a Material Handling Continuous Mining System - such as those applied in coal mining -, innovative developments in ore fragmentation and material handling are mandatory. Particularly, among several recommended themes to investigate, the non explosive rock fragmentation was highlighted. (0039) There are great expectations about continuous mining because of its potentiality. Some of the expected beneficiated areas are: e Productivity increases e Reduce the number of operation units. * Intensify automation.

13 (0040) In order to fulfill those expectations it is needed at first to solve the ore's coarse fragmentation, because that is the main impediment to apply a bulk continuous material handling system to block caving mining. (0041) Purposes of the invention are: Increase the present drawing ore rate by improving fragmentation of the ore by means of slicing/fracturing the block - gradually -, reducing the ore to a size apt to be handled at a continuous flow. * Impede formation of major wedge blocks to avoid structural instabilities. e Increase the block's Cavability, through a progressive, controlled and intense process of slicing/fracturing in situ of the rockmass. - Relieving, controlled and gradually, the original stresses of the rockmass to moderate the seismic activity and neutralize rock bursting. - Reduce the dilution, because the finer the ore, it will constitute a barrier to overlying waste. (0042) Relevant aspects of the invention: " Modification of geomechanical parameters of the blocks- The best way to create ideal conditions inside the rockmass - referred by Panek and Kendorski -, in order to get certainty in block's Cavability and boost the ore fragmentation, it is weakening the rock mass in - situ. In this new method of mining, the block is incrementally weakened in situ simultaneously with the progression of the caving. " Redistribution of stress field.- The noteworthy geomechanical effect looked to be taken advantage of - and utilize as the caving progress - is the intensive and favorable redistribution of the in situ stressed field in the just created undercut groove's upper back vicinity, and afterwards, in around the multiple and subsequent new cave backs accompanying the upward caving progress. The caving process generates, in the cave back vicinity, a very special condition: The in situ stress field, over each new cave back vicinity results radically redistributed, with the main stress vector tangent to cave's back and the magnitude of the new perpendicular stress vector being of low magnitude. " Applying the Panek Cracking disclosure.- The main effects sought to use with advantage, are the extension cracks created by caving progression as disclosed by Panek.

14 " Slicing/Fracturing and Drilling Level FIG. 1. - A Drilling Level 1 it is developed at the top of the blocks. Each gallery coincides, vertically, one to one, with the undercutting drift galleries. From there two kinds of interlaced holes 2 are drilled vertically down and connected to the Undercut Level drifts 3. The Extraction Level 4 is shown too. " Functional Drill Holes FIG. 2.- From the Drilling Level drifts, two interlaced kinds of drill holes are drilled and connected with the undercut drifts. They are spread regularly: o One set of drill holes is the Slicing/Fracturing (SF) boreholes 5. They are used to fracture the band hydraulically, one of whose faces is the cave's back, exactly the zone strain relieved by the upward caving progression. Each borehole will contain an assembly of tubes and seals to fracture the rockmass connected to a high pressure hydraulic central pumping system. The fracturing sequence can be commanded, in each (SF) borehole, at remote control by a computer programmed to optimize the best global sequence. o The (TDR) boreholes 6 are destined to detect the strain relief cracks appearing during upward caving progression. Break-detection cables, connected to a TDR (time - domain reflectometry) instrument and to an oscilloscope (and any other useful instrument), are installed inside these boreholes. This device is described in the paper "GROUND MOVEMENTS NEAR A CAVING STOPE" (Panek, 1981). o Optionally additional boreholes can be drilled to install TV cameras or laser distance meters to scan the cave back's evolution. (0043) Sequence of the mining method: " Step 1. - Undercutting the block. FIG. 3 The undercut of a block's base is intrinsic to block caving mining method. It can be seen how the groove after undercut is full of broken ore 7. Close over the undercut groove's back - because of the strain relief - the extension cracks 8 appear. " Step 2. - Slicing the block hydraulically, FIG.4. After undercutting the block's base, instead of starting to draw the ore - as usual -, will start immediately slicing the block hydraulically, initiating it in 15 the undercut groove's back vicinity, just in the freshly created zone of strain relief disclosed by Panek. As soon as new extension cracks 9 are detected, in any (TDR) borehole, through the contiguous surroundings (SF) boreholes - at the same level or closer to level of the new cracks -in ascending sequence, the hydraulic slicing/fracturing should start, creating as many extended fractures as possible and initiating now the drawing of the foliated ore, paying attention to maintain the cave back's shape as even as possible. It will guarantee the structural stability of the of Slicing/Fracturing Level, during great part of the process of having sliced in height. The ascending slicing, starting from the undercut groove's ceiling, will go forward gradually and uniformly, strictly together with the new extension cracks, in the smallest increments than the hydraulic fracturing technology can do it. The smaller the increments, the thinner the slabs 10 and consequentially, the smaller the final ore fragments in the drawpoints. The entire band showing extension cracks in the screen monitors must be foliated endeavoring to level the cave's back, to conform a dome shape as smooth as possible. It will guarantee it the structural stability of the Slicing Fracturing Level, during great part of the process of having sliced in height. The air gap 11, between the top of the broken ore and the vault's ceiling, must be maintained strictly at the minimum, in order to provide a long life, safety and stability to Slicing/Fracturing Level and impeding an irregular cave back shape or an erratic caving. Step 3. - Extraction/Fragmentation In the block caving mining method, this is the productive step properly such. Ore is extracted by gravity, from several drawpoints at the Extraction Level 4. The block, because of the hydraulic slicing, was turned into many slender flagstones - structurally fragile - those that during the extraction are exposed to triturating due to the significant gravitational forces induced in the flowing column, process known as: secondary fragmentation. By effect of the high requests exercised by gravitational forces, acting within the caved mass during the ore's descent, the weak flagstones will be intensively broken. The acting forces mainly are of: Compression, 16 flexion and shear. The requests easily surpassed the lower resistance to flexion and shears of the plate and also shears resistance of the discontinuities, thus slender flagstones will fragment with relative easiness, with the favorable plus that, in this kind of deposits, the pre existing vertical fractures are prevalent over the horizontal ones. Summarizing, due to slicing effect the gravity ore flow's contribution to fragmentation will be very efficacious. The advantages: (0044) Hydraulic slicing/fracturing process.- Because this tool will be applied into a strain relieved zone, it will have the following advantages: - Lower initial propagation fracture pressures and, therefore, the slicing process will be very low energy consumption. - Fast fractures propagation. (0045) Finer ore fragmentation.- A remarkable advantage, of this method of mining, will be to obtain a fragmentation of the mineral rather finer than that obtained with the present state of technology. The new method of mining allows slicing, intensively, each band recently cracked. The block, because of the hydraulic slicing, is turned into many slender flagstones - structurally fragile -, those which during the extraction are exposed to the significant gravitational forces induced in the gravity flowing column, process known as: secondary fragmentation. (0046) Controlled relief of high stress fields.- During the slicing step a significant, very gradual and, mainly, a controlled relief of the stored up energy contained by the bulk mass, will take place. An additional contribution - besides the slicing- is the water used in the hydraulic slicing process. Water will act like a sort of a poor lubricant. The gradual relaxation of the block will avoid seismic activity and put away the rock burst possibility. (0047) Ore Extraction Rates.- The ore finely fragmented will allow reaching ore draw rates quite superior to present ones. Higher rates of ore extraction will reduce, significantly, the productive area's extension, improving the Cash Flow and the economical yield of the project. With the ore intensely fragmented it will be feasible to implement an efficient continuous ore handling system. (0048) Cavability assured performance.- The pre-conditioned block, by hydraulic slicing will, experience a significant reduction in its MRMR quality. From an initial class 2 or 3, it will probably descend to a class 4 or 5. As a consequence its hydraulic radius will be reduced significantly.

17 (0049) Dilution control.- The finer the ore becomes an effective barrier against the dilution caused by the intrusion of the low grade or barren overlying waste. Each band with new extension cracks must be foliated as soon as is detected, maintaining at a minimum the air gap between the top of the broken ore and each new cave back. This practice avoids developing chimneys in the uncaved ore connecting the broken ore with the overlying waste or low grade ore. (0050) Disarrangement of significant structural mobile blocks.- The hydraulic slicing disintegrates all these structures transforming them- chew to chew - in numerous flagstones preventing the formation of elements such as large movable wedges. Slicing immediately the new extension crack zone, minimizes the space where any important structural mobile block could collapse. (0051) Tight draw control.- Monitoring, in line, the TDR signals, together with the hydraulic slicing of the new extension cracks, will be a powerful tool to manage the draw. It will allow: - Draw the ore evenly throughout the production area. e Maintain in any event, at the minimum, the distance between the top of the broken ore and each cave back. This practice assures the stability of the Drilling/Slicing/Fracturing Level during most of the block's life. To get this, the hydraulic slicing/fracturing should be done immediately when any new extension cracks are detected. (0052) The closer the distance between the top of the broken ore and the cavity's ceiling: - Prevent the instability and mobilization of big structures, such as huge wedges. - Optimize the slicing/fracturing process. * Avoid the funneling connection with overlying waste. (0053) First step towards Continuous Mining.- As previously mentioned, the main goal for the future mass mining is to achieve - through research, maturity and innovation in several key areas - a continuous method of mining. Among those issues the in situ rock mass's fracturing and finer ore fragmentation is emphasized. This invention properly solves - through hydraulic slicing - the rockmass fracturing and fragmentation. Since now it will be possible to approach the next step to reach a Continuous Mining System.

18 REFERENCES: 1. - Louis A. Panek; Geotechnical Factors in Undercut-Cave Mining; Underground Mining Methods Handbook. 1981. 2. - Louis A. Panek; Design and Operation of Caving and Sublevel Stoping Mines; The Society of Mining Engineers of AIME. - U.S.A., 1981. 3. - W. Hustrulid; Method for Selection Large-Scale Underground Mining; MassMin 2000, PROCEEDINGS. - Australia 4. - Robert G. Jeffrey; Hydraulic fracturing of ore bodies; Invention Patent Nr. 6.123.394. - U.S.A., 9/2000. 5. - Evolutionary and Revolutionary for Technologies Mining; National Academy Press. U.S.A., 2002. 6. - G. Diaz and P. Tobar; Panel Caving Experiences and Macrotrench; MassMin 2000, PROCEEDINGS. - Australia 7. - F. S. Kendorski; Cavability of ore deposits; Mining Engineering. - U.S.A., 1978 19 CLAIMS What is claimed is: 1. A method of mining to degrade primary orebodies, to be mined by caving methods (block or panel) improving the ore fragmentation, consisting in: 1.1. - Prior to undercutting, from a level located at the top of each block, a net of vertical boreholes, connected with the galleries of the Undercut Level. Two types of evenly intermixed boreholes - (TDR) and (SF) - are regularly spread covering the area to cave. The (TDR) boreholes are for detecting the strain relief cracks appearing contiguous to cave back during the upward caving progression. Inside the (TDR) boreholes break-detection cables are installed, connected to a TDR (time - domain reflectometry) instrument, an oscilloscope and to monitor screens. The (SF) boreholes will be used to Slice/Fracturing the block hydraulically; wherein they are connected to a central pumping system. 1.2. - After undercutting the block's base through an ample groove, simultaneously with initiating the ore drawing, the hydraulic slicing/fracturing of the block can start up at the border of the groove's back; taking advantage of the surrounding vicinity reordered stress field, manifested by the strain relief cracks appearing in the band over the undercut groove's back and detected through the breakables cables of the (TDR) boreholes and displayed online on monitors screens. 1.3. - Through the (SF) boreholes the hydraulic slicing/fracturing advance during the upward caving's progression at tempo that TDR instrument detects the turn up of newly strain relief cracks. Foliation is done from the central pump station commanded by remote control. 1.4.-Foliation should cover, ideally, the overall height of the block, in successive planes. Consecutive slicing fractures should be, mutually, as close as feasible. The block, once sliced, is transformed into innumerable and fragile thin flagstones. 2. A method of mining to slice massive primary orebodies, according to claim No 1, wherein the hydraulic slicing/fracturing transforms the block in a set of slender and fragile flagstones, increasing the block's Cavability. 3. A method of mining to slice massive primary orebodies, according to claim No 1, wherein the process of hydraulic fracturing induces - inside the block - an intense, 20 gradual and controlled relief of the in situ stress field, attenuating seismic activity and preventing rock burst. 4. A method of mining to slice massive primary orebodies, according to claim No 1, wherein the slicing avoids formation of any mobile structural rockmass blocks (wedges). 5. A method of mining to slice massive primary orebodies, according to claim No 1, wherein during the ore extraction phase, consequences of the gravitational dynamics friction, compression, and mainly shears and bending -, consubstantial to gravitational ore flow, the slender and weakened flagstones will be extra fragmented in smaller pieces and the ore can be drawn at elevated rates leaving them ready to be handled by continuous ore handling systems.

Claims (5)

1.1. - Prior to undercutting, from a level located at the top of each block, a net of vertical boreholes, connected with the galleries of the Undercut Level. Two types of evenly intermixed boreholes - (TDR) and (SF) - are regularly spread covering the area to cave. The (TDR) boreholes are for detecting the strain relief cracks appearing contiguous to cave back during the upward caving progression. Inside the (TDR) boreholes break-detection cables are installed, connected to a TDR (time - domain reflectometry) instrument, an oscilloscope and to monitor screens. The (SF) boreholes will be used to Slice/Fracturing the block hydraulically; wherein they are connected to a central pumping system. 1.2. - After undercutting the block's base through an ample groove, simultaneously with initiating the ore drawing, the hydraulic slicing/fracturing of the block can start up at the border of the groove's back; taking advantage of the surrounding vicinity reordered stress field, manifested by the strain relief cracks appearing in the band over the undercut groove's back and detected through the breakables cables of the (TDR) boreholes and displayed online on monitors screens. 1.3. - Through the (SF) boreholes the hydraulic slicing/fracturing advance during the upward caving's progression at tempo that TDR instrument detects the turn up of newly strain relief cracks. Foliation is done from the central pump station commanded by remote control. 1.4.-Foliation should cover, ideally, the overall height of the block, in successive planes. Consecutive slicing fractures should be, mutually, as close as feasible. The block, once sliced, is transformed into innumerable and fragile thin flagstones.

2. A method of mining to slice massive primary orebodies, according to claim No 1, wherein the hydraulic slicing/fracturing transforms the block in a set of slender and fragile flagstones, increasing the block's Cavability.

3. A method of mining to slice massive primary orebodies, according to claim No 1, wherein the process of hydraulic fracturing induces - inside the block - an intense, 2 gradual and controlled relief of the in situ stress field, attenuating seismic activity and preventing rock burst.

4. A method of mining to slice massive primary orebodies, according to claim No 1, wherein the slicing avoids formation of any mobile structural rockmass blocks (wedges).

5. A method of mining to slice massive primary orebodies, according to claim No 1, wherein during the ore extraction phase, consequences of the gravitational dynamics friction, compression, and mainly shears and bending -, consubstantial to gravitational ore flow, the slender and weakened flagstones will be extra fragmented in smaller pieces and the ore can be drawn at elevated rates leaving them ready to be handled by continuous ore handling systems.