Fragmentation in sublevel caving (SLC) is vitally important. Both gravity flow and any downstreamprocesses are affected. Fairly coarse fragmentation may lead to larger draw bodies (isolatedextraction zones) and hence potentially higher primary ore recovery and depressed/delayedwaste rock inflow from above. Fewer flow disturbances are expected by mitigating oversize. Inaddition, it requires less boulder handling and reduces wear and possible hang-up problems inorepasses. To assess the present-day functionality of large-scale SLC, a multiyear, comprehensivemeasurement program was initiated. It covers the main elements for SLC, namely blast function,fragmentation and gravity flow. The present paper focuses on fragmentation measurements. Animage acquisition system was used to document the drawpoint and the load-haul-dump (LHD)bucket for each mucking cycle. At the beginning, four buckets (about 70 t total) were sieved tovalidate the results of both 2D and 3D image analysis techniques. The fundamental and specificproblems are discussed herein. At the moment, fragmentation of the SLC rings is evaluated usinga quick rating system and a 2D fragment delineation software. The results enable a descriptionof fragmentation and its variation during mucking but also – combined with the gravity flowmeasurements – conclusions on the fragmentation for different parts of the SLC ring. Possibleinfluences on flow disturbances and ore recovery/dilution are investigated. The recent findingsallow a better understanding of breakage and flow and support future process improvements.

The flowability of the caved rock masses in the sublevel caving (SLC) mining method is of greatimportance. During the blasting process of a SLC-ring, the caved rock mass in front of the ring iscompacted. As a result of this compaction, the mechanical properties of the fractured rock masseschange, for example void ratio, angle of internal friction and structure of the compacted material.The changed mechanical properties of caved rock mass and recent blasted material can influencethe mobility of the material, which could result in blockage of the flow path of the material.The identification of the involved mechanisms in this process can be done after a series of testsin different scales. However, there is no available equipment for dynamic measurements duringblasting, ie velocity and displacement of the burden and in the caved masses. This study focuses onthe development and laboratory evaluation of a measuring system that can be installed in a borehole.The first trial was carried out in a pillar, where two blastholes had been drilled (9.5 m) parallel withthe drift and two measuring holes (16.5 m) were perpendicular to the drift. Two measuring systemswere installed in the measuring holes. Some preliminary tests were carried out to find suitableinitiation procedures for the emulsion explosive. Applicability and robustness of the developedmeasuring system were also evaluated. In addition to these tests, an expandable cement mixture wasdeveloped to firmly anchor the measuring systems in the boreholes. The measuring system consistsof an anchor, armoured cable in a pulling tube and a protective case. The anchor is equipped withuniaxial and triaxial accelerometers. A potentiometric measuring system for displacement up to 2 mis installed at the end of the pulling tube and connected to an extendable spiral cable. All sensorsare connected to a data acquisition system (DAS). Both measuring systems have their own DAS toacquire data in case of failure of one of the systems. The measuring system was tested in laboratoryconditions with impact velocity up to 10 m/s. It was proven to be functional and gave reliable results.

Sublevel caving (SLC) is a highly productive mining method with the major disadvantages of irrepressible ore loss and dilution. The confined blasting situation of SLC rings is commonly regarded to have a significant impact upon the material flow characteristics and hence on the overall performance. The initial conditions for the ore flow after blasting are unknown, as both the blasted geometry and the fragmentation itself are normally hidden. However, when opening a new drawpoint and in hang-up situations an inspection of the actual conditions is feasible. For this purpose, a remotely operated 3D photogrammetry system was custom built and used to reconstruct the ring front and/or cavity as a geo-referenced mesh model with colour information. Various blasting situations have been observed and this allowed a deeper study of the i) broken geometry and height, ii) over- and underbreak and their effects on subsequently blasted rings, iii) interaction effects between adjacent holes/rings, iv) drilling accuracy based on identified boreholes and v) mapping of geological structures and their influence on the blast result. The capabilities of the 3D image acquisition system to evaluate the blasting results are demonstrated with an example, in which a series of blasts were surveyed. Monitoring under hang-up situations, has then revealed the actual effects from confined blasting and gravity flow related issues. The 3D photogrammetry system is now increasingly being used to study hang-ups. By revealing and quantifying hitherto inaccessible information, the system has proven to be a valuable tool in increasing the understanding of both SLC breakage and flow.

Blasting in SLC (sublevel caving) takes place in a semi-confined situation. Blasted material swells while the caved material compacts, and also, to a lesser extent, fills parts of the void volume of the production drift. Several analytical and empirical models have been developed in the past. However, understanding of the interaction of semi-confined blasting conditions, SLC blast design and rock mass characteristics on rock breaking performance is rudimentary. Instrumentation of the blasted burden with various sensors and study of the dynamics of burden movement against confinement is therefore important. Such measurements are very scarce and thus different systems have been tested both in laboratory and in the field. Main focus was thereby the development of measuring equipment that could be scaled up to full-scale SLC blasting and installed behind the rings. In the field tests, blastholes were drilled in a pillar, parallel to a cross cut drift in the LKAB Kiruna SLC mine. The burden constraint was achieved either by filling the drift of 7.0 x 5.2 m (width x height) with rock masses or reinforcing the drift wall. The tests were instrumented with different redundant sensors, which were installed in holes drilled from a parallel cross cut. The most promising one was a concept, which combines the initial movement recorded by an accelerometer designed to minimize zero-shift with the final displacement recorded by a fibre photoelectric sensor (‘fibre-optic zebra gauge’). For the filled drift wall blasting test the maximum velocity varied between 16 - 32 m/s with a compaction in the range of 4 - 5 %. A gap was verified to exist between the intact pillar and the blasted material. By comparison, the velocity for the unconfined situation yielded a considerably increased velocity (42 - 47 m/s). In addition, ‘Blo-Up’, a component within the Hybrid Stress Blast Model (HSBM), was used to model the blasting results. The ob-jective was to show that reasonable predictions of fragmentation and burden movement under confined conditions can be made. An unconfined reference experiment was used to calibrate Blo-Up and forward predictions of the confined case were made. In both cases, Blo-Up accurately reproduced the burden velocity and displacement observed in the experiments. The suggested measurement concept could be used for further measurements of confined burden movement in filled drift wall blasting tests and under controlled situations. As mentioned the final objective is instrumentation of the burden in SLC production rings.

Blast function, fragmentation and gravity flow are core elements for sublevel caving (SLC). For the ore recovery to be as high and the waste rock dilution as low as possible all three elements have to work as planned. In the past some boundary conditions were changed: The scale and layout of SLC changed tremendously and production advanced to greater depths. As a consequence, blast performance and material flow are believed to have changed. The difficulties in simulating SLC relate to the physical scale as well as the broad range of time-scales involved, which results in a large number of unknowns and uncertainties. The present thesis contributes with the development of new measuring techniques, full-scale tests and analysis of results in all the above three core elements. Based upon that a better understanding of breakage and flow is obtained and this supports future process improvements.A remotely operated 3D photogrammetry system was designed and used to study the blast results from SLC rings in opening and hang-up situations. Various blasting situations were observed and this allowed a deeper study of the i) geometry of blasted cavity, ii) over- and underbreak and its effect on subsequently blasted rings, iii) interaction effects between adjacent holes/rings, iv) mapping of geological structures and its influence on the blast result and v) drilling accuracy based on identified boreholes. An attempt to identify malfunctions and categorize breakage problems was made. Some of the identified problems for initiation were unexpected and require a closer examination to increase blasting efficiency and to mitigate nitrate leakages. Potential improvements for blasting initial openings and scaling-up of rings are presented.Confined drift wall blasting tests in which the blasted burden was instrumented with various sensors to study movement and breakage were carried out. The main focus was the development of measuring equipment that could be scaled-up to full-scale SLC blasting and installed behind the rings. Results from an unconfined and confined blast situation are presented. A new measurement system to measure burden movement was developed, which combined the initial movement recorded by an accelerometer designed to minimize zero-shift with the later slower movement recorded by a photoelectric sensor (“fibre-optic zebra gauge”). In addition, time domain reflectometry (TDR) proved to be a reliable method for the verification of burden breakage (over- and underbreak).Fragmentation characteristics for blasted ore loaded from a specific ring were studied by sieving. For comparison lab-scale data from crushing, grinding and blasting, and also historical full-scale data from the mine, were analyzed. This data confirms that the material follows the NBC criteria (NBC = “natural breakage characteristics”) in the fines region (0 – 10 mm). However, a relative flattening of the sieving curves due to “selective” breakage in the mid-range (25 - 75 mm) and increased amount of fines was observed. The main conclusion is that magnetite behaves like normal hard rock from a fragmentation point of view despite this flattening deviation from Swebrec distribution behaviour as this is likely to be caused by the internal flow mechanisms in the SLC process.Gravity flow and the effects from confined blasting in hang-up situations were studied with the 3D imaging system and allowed the extension of conceptual models for breakage and flow. Evidence is given that gravity flow was disturbed and occurred in shallow, crescent-shaped flow zones. The front part of the burden was fractured, but immobilized. For the subsequent blast this meant that swell and compaction was limited to that zone with only marginal dilation and disaggregation of the burden at the sides. The operating figures for the blasted rings with observed flow zones indicated a disturbance-free extraction. Inflow of caving debris might occur either from above or in front when the loader penetrates into the flow zone of the previous ring. As a result of flow disruption in this narrow channel a temporary hang-up might evolve and the flow scheme be altered so that material is then loaded from previous blasts. With continued extraction, the cavity enlarges and finally becomes unstable, collapses and both previously “frozen ore” and waste rock enters the draw point.At the end a pivotal question remains: Are the recent observations of disturbed flow an inherent part of blasting SLC rings? This raises also the follow up question: Can we expect an undisturbed gravity flow if all borehole charges and rings detonate and break as planned? With continued investigations an answer seems to be obtainable in the near future. It is also recommended that the methods developed in this thesis be used in instrumented confined blasting tests in full-scale rings and be combined with gravity flow measurements.

Sprängsalvors funktion, fragmentering och rasflöde är viktiga delprocesser i skivrasbrytning. För att malmutbytet skall bli så stort som möjligt och gråbergsinblandningen så låg som möjligt måste alla tre delprocesser fungera enligt plan. Förutsättningarna har ändrats med tiden. Brytningsskalan har ökat betydligt under årens lopp, utformningen av raskransarna har ändrats och brytningen sker på allt större djup. Svårigheten att modellera skivrasbytningen beror både på den fysiska skalan och på de olika tidsskalor som ingår i förloppet, vilket medför stora osäkerheter och även okända storheter. Denna avhandling bidrar med utveckling av nya mätmetoder, fullskaleförsök och analys av resultat från alla tre delprocesserna. Detta leder till en bättre förståelse av sönderbrytningen och rasflödet vilket möjliggör en framtida förbättring av hela processen.Ett fjärrstyrt system för 3D-fotogrammetri har konstruerats, byggts och använts för att studera kaviteter vid såväl öppningar som ”häng” (stopp i rasflödet) i skivrasbrytningen. En kvantifiering och kartläggning har gjorts av i) den utsprängda kavitetens geometri, ii) bakåtbrytning och ofullständig lossbrytning, iii) hur brytningen samverkar mellan närliggande spränghål och skivraskransar, iv) hur strukturerna påverkar sprängningsresultatet och 5) felborrningen baserad på identifierade kvarstående borrhål. En ansats att identifiera felfunktioner i lossbrytningen och kategorisera problem har gjorts. Problemen med initiering av laddningarna var oväntade och de behöver undersökas närmare för att öka salvornas effektivitet och att minska kväveförluster. Förslag ges till förbättringar av sprängning av de första hålen och efterföljande kransar i en skivrasöppning där kransarnas höjd ökar snabbt.Sprängning av ortväggar mot mothåll från inlastade rasmassor har utförts. I och bakom försättningen har olika instrument installerats för att mäta rörelser och sönderbrytning. En viktig utgångspunkt har varit att instrumenten skall kunna användas i fullstora skivraskransar där de installeras från baksidan. Resultat från måttligt och stort mothåll presenteras. För att kunna mäta försättningens rörelse under fältförhållanden har ett nytt mätsystem utvecklats. Det kombinerar en accelerometer med minimal nollpunkts-förskjutning för registrering av det transienta initialförloppet med en fotoelektrisk givare (s.k. fiberoptisk zebragivare) som mäter det långsammare efterföljande förloppet. Dessutom används s.k. TDR-teknik, dvs. ingjutna koaxialkablar genom vilka pulser skickas och där reflektioner från olika störningar längs kabeln registreras. Denna teknik ger en god uppfattning om var gränsen för försättningens lossbrytning går.Styckefallet hos sprängd magnetitmalm från en utlastad skivraskrans har studerats genom siktning. Som jämförelse har data från malm som krossats och malts i labbskala samt historiska data från raskransar analyserats. Data styrker att malmen i finområdet 0 - 10 mm har s.k. NBC-egenskaper (NBC = ”natural breakage characteristics”). I mellanområdet 25 - 75 mm uppstår en relativ avflackning i siktkurvan jämfört med en s.k Swebrec-funktion, förmodligen pga selektiv neddelning under rasflödet, och en större andel finmaterial bildas. Trots avvikelsen från ”Swebrec-beteendet” hos siktkurvan är ändå slutsatsen att magnetiten uppträder som gråberget ur fragmenteringssynpunkt.Rasflödet och effekterna av sprängning mot mothåll (rasmassor) har studerats med 3D-fotogrammetri under hängsituationer. Detta har gjort det möjligt att förbättra befintliga sönderbrytnings- och flödesmodeller. Resultaten visade att flödet varit ”stört” (lokaliserat) och skett inom smala månskäreformade kanaler. Försättningens främre del hade spruckit sönder men inte brutits upp, dvs kunde inte börja rasa. För efterföljande raskransar blir svällningen av försättningen och åtföljande kompaktering av mothållsmaterialet i rasmassorna begränsad med mycket liten dilatation och uppluckring av sidodelarna av försättningen. Produktionsuppföljningen (lastade bergmängd mm) för kransarna med stört flöde visade inte något onormalt innan hängen uppträde. Inflöde av gråberg från rasmassorna sker normalt uppifrån eller framifrån om lastmaskinen kan penetrera rasmassorna tillräckligt långt in. I en hängsituation har flödet uppifrån genom kanalen upphört. Hänget kan vara tillfälligt och när lastaren börjar lasta i rasmassorna från föregående krans kan det kollapsa och tidigare ”frusen” malm och gråberg rasa ner i lastorten.En viktig fråga kvarstår: Är det störda flödet som observerats en integrerad del av rasflödet efter sprängning av skivraskransar? Och följdfrågan: Kan vi förvänta oss ett ostört rasflöde om alla spränghålen i en skivraskrans detonerar och bryter upp försättningen som planerat? Svar på dessa frågor kan fås inom en nära framtid om undersökningar fortsätter. Det rekommenderas också att mätmetoderna som utvecklats i denna avhandling används i instrumenterade sprängförsök i storskaliga skivraskransar (under mothåll) och kombineras med mätningar av gravitationsflöde.

Disturbance free fragmentation and flow in sublevel caving (SLC) are desirable as they could lead to an increase in ore recovery and a decrease in waste rock loading and nitrates leakage. This requires a reasonable understanding of the actual rock breakage in semi-confined ring blasting situations, where both the blasted geometry and the breakage itself are hidden. Sometimes there is a chance to inspect the actual conditions, such as in the opening of a new drawpoint and in hang-ups in which cavities exist. For these situations a lightweight mobile camera crane has been constructed for use in the LKAB Kiruna mine. A 3D photogrammetry system has been constructed, which enables the reconstruction of the ring front/cavity as a geo-referenced mesh model with color information. It is remotely operated from a safe distance behind in the drift. The paper first reviews the current drill and blast practice. Then it presents the design, construction and application of the 3D image acquisition system. A case study shows the measurement application and also the evaluation possibilities. So far the system promises to be a valuable tool in increasing the understanding of SLC breakage and flow.

Este artículo estudia la volabilidad de una roca a través de la piedra crítica, definida ésta como la piedra máxima para la que se rompe completamente la roca desde el barreno a la cara libre. El procedimiento aplicado consiste en perforar y disparar un barreno inclinado desde la cara libre de forma que la piedra aumente con la profundidad a lo largo del barreno y aplicar métodos fotogramétricos y de análisis en 3D para determinar las características de la zona excavada: área excavada, sobreexcavación y anchura a varias profundidades, mediante la obtención de secciones planas de la excavación. La piedra crítica se asocia al mínimo de un factor de forma de la sección excavada. Se han realizado once ensayos de volabilidad a escala real en anfibolita, magnesita y caliza y tres a escala reducida en mortero de magnetita. La piedra crítica define el límite superior que en ningún caso deberá superarse en la práctica. No obstante, para iguales parámetros de voladura, la definición de piedra óptima dependerá del resultado perseguido; si éste es el de optimizar la fragmentación con vistas a la reducción de finos, la piedra óptima será aquella para la que el área de la sección excavada es máxima, que coincidirá con un consumo específico mínimo. El estudio de las áreas excavadas para la piedra óptima obtenidas experimentalmente es igualmente útil para cuantificar interacciones entre barrenos adyacentes y por tanto para deducir el espaciado de barrenos. Finalmente se analiza la influencia de las características geotécnicas de la roca sobre el área de la zona excavada y las piedras críticas y óptimas.

An experimental investigation of blastability has been performed in small- and full-scale by continuous critical burden tests: A single inclined borehole is drilled into the rock face thus eventually exceeding a technically feasible burden. A useful definition of the “critical” burden Bcrit can be made by the study of the shape of the breakage area and it is defined by the maximum burden with complete breakage from the hole to the surface and by the shape factor SF that approaches a minimum at the same time. It has been shown that Bcrit is primarily a function of the specific charge and the prevailing rock conditions. Concerning the optimization of drilling and blasting patterns, an “optimum” burden Bopt can be found by systematic analysis. At this burden the specific charge shows a minimum and this has a positive influence on the size distribution of blasted rock, i.e. less fines. The full-scale experiments have shown that at smaller burdens the energy is sufficient for considerable breakage sideways whereas with increasing burden the shape of breakage becomes generally narrower, depending on the actual orientation of the strata. Bopt can be found at the section where the maximum area is obtained under the condition that Bcrit is still not exceeded and over-break ≥ 0 as well as 0 ≤ SF ≤ 1 at the same time. The study of experimentally derived breakage areas at Bopt can be useful in quantifying interactions between adjacent holes and thus be used in the design of spacing. Within operational possibilities the influence of the local rock conditions on the shape of the breakage area should additionally be considered in designing a blast. Considering the technically feasible drilling precision a drill and blast pattern optimized by using the described approach should lead to technical as well as economic benefits.

The objective of this investigation constitutes a detailed study of the fragmentation characteristics for blasted magnetite ore from a specific ring in the sublevel caving (SLC) mine in Kiruna.For comparison lab scale data from crushing and grinding as well as blasting plus historical full-scale data from the mine were analysed. The present data confirms that the material basically follows the “Natural Breakage Characteristics” criteria within the fines region. The complete fragmentation distributions yield an extremely large variation for the parameter x50. The shapes of the complete sieving curves do not entirely fit the Swebrec function since they are flatter, roughly over the range 25-75 mm. The relative flattening of the sieving curves may be due to “selective” breakage in the mid-range, which would increase the amount of fines. This resembles the behaviour in autogenous grinding, i.e. the self-breakage of large fragments createspebbles that grind the mid-fractions to fine, and also an effect attributable to secondary fragmentation in block caving. Several different mechanisms are conceivable to account for an altered fragmentation mechanism. From the present large SLC layout one could expect more comminution occurring in the debris flow, due to longer flow paths. On the other hand, the present samples were taken before 10 % draw, which speaks in favour of relatively short paths, unless the draw has been extremely uneven. Despite this flattening deviation from Swebrec distribution behaviour – which is likely to be caused by the internal flow mechanisms in the SLCprocess – the main conclusion is still that magnetite itself behaves like waste rock from a blasting point of view.