The Santo Niño ore body at Pinos Altos Mine has been in production since 2009 using both open pit and underground mining methods. Between 2015 and 2017 an extensive study was conducted to evaluate extraction of the crown pillar which remained between the open pit and underground mine workings. The project evaluation considered numerous factors influencing mine safety, operations and economics. This case study paper will describe the geomechanical aspects of the project evaluation and execution.
The Santo Niño ore body is one of several mineralized zones at Agnico-Eagle Mines’ Pinos Altos property. Pinos Altos is located in the state of Chihuahua in northwestern Mexico within the Sierra Madre mountain range (Fig. 1).
Gold and silver production at Pinos Altos started in 2008 with the extraction from the Santo Niño and Oberon de Weber open pits. The Pinos Altos Complex produces over 2Mt per year from the main Santo Niño orebody and nearby satellites. Milling and heap leach are used to treat the underground and open pit ores. Underground mining was initiated simultaneously with the open pit production at the Santo Niño orebody with the first pyramid extending from Level 20 to Level 16 (Fig. 2). The mining strategy consisted of excavating and backfilling the upper most stopes up to Level 16 before the open pit reached its final depth. The recovery of the crown pillar between the pit bottom and Level 16 was not fully detailed in the initial mine planning. Following the technical and economic evaluations in 2015-2016, the crown pillar ounces between the pit bottom (elev. 2023 masl) and the top of the underground mining (elev 2000 masl) were incorporated into the Life of Mine Plan and contribute to the replacement of higher grade ore to the mill following depletion of the large Santo Niño open pit.
Initial thoughts on crown pillar recovery focused on simply mining the open pit in the usual 7 m benches down to the top of backfill on Level 16. Open pit mining is less costly than underground extraction and geotechnical studies focusing on open pit slope stability indicated that deep slope instability was not expected with the removal of the crown pillar. Nonetheless, further analyses demonstrated that there were advantages to recovering the crown pillar from the underground; economic evaluations showed that resource recovery could be increased which made underground recovery more profitable (60% by open pit versus at least 85% from underground), slope deformations could be reduced hence the risk of localized slope instability was decreased and exposure of personnel in the pit ramp could be significantly reduced. A thorough risk analysis and review process was implemented by a team of key personnel from pit and underground operations, mine and corporate technical services, health and safety and mine management. The objectives of the exercise was to develop measures to control identified risks to personnel and machinery while optimizing the recovery of the reserves. This paper documents the geomechanical analyses and monitoring measures implemented to support the design and recovery of the high grade Santo Niño crown pillar.
Studies have shown that the roof in coal mines is nearly twice as likely to fail at intersections as at entries due to enlarged roof spans, stress redistribution and other factors. However, the relative stability of intersections in a mine varies, and an improved understanding of what factors impact roof instability can lead to efficient, proactive mining practices that both enhance stability and mining efficiency.
A study was undertaken at an underground longwall coal mine to develop a model to predict the stability at the intersections of entries and cross-cuts. The statistical analyses relied upon observational and measured data regarding the development of tension cracks, seepage, intersection geometry, mining practices, geological attributes and geomechanical factors at 783 intersections. Each intersection was given a NIOSH roof. Common data mining techniques, such as multivariate linear regression, multinomial logistic regression, decision trees and probabilistic neural nets, were considered and evaluated to establish correlations and associations between stability and the other variables. Two of the more successful techniques were Decision Trees and Multinomial Logistic Regression. The analyses showed that a number of factors impacted stability:
• Overburden thickness (impact on stress concentration magnitudes)
• Initial opening area (impact on stress concentration magnitudes)
• Sulfur content (depositional environment & impact of mechanical properties, leaching effects)
• Intersection type (impact on stress concentrations)
• Gob distance (impact on stress concentrations)
• Presence or proximity of a particular sandstone (impact on mechanical properties)
• Roof bolt type and diameter (impact on reinforcement)
Some factors showed no statistically significant relation to stability. These included:
• Total area
• Supported area
Seepage was a more problematic variable in assess. Seepage is associated with instability, but it is not clear whether seepage occurs as a consequence of instability, for example, through the creation of tension cracks, or whether the intersection has been excavated where water already exists and has degraded the rock prior to excavation, leading to greater instability.
Seepage appears related to the geology of the roof rock mudstones, not the sandstones. Tension cracks in the intersections occur preferentially where the Sulfur content is below 0.9%. The Sulfur may relate to mechanical differences in the roof rock, and where the Sulfur is low, the roof is less stable, there are more tension cracks, and seepage is greater. This suggests that the weaker mudstones may be more prone the development of tension cracks, not because these rocks are more brittle, but possibly weaker.
2-way and 3-way intersections appear to have less seepage than 4-way, although very few intersections overall have any seepage. This is similar to the roof stability, in which the 2-way and 3-way are more stable. 2-way and 3-way intersections have fewer-than-expected tension cracks, while 4-way have more than expected. These results suggest that four-way intersections are weaker and more prone to tension crack development, also suggesting that increased seepage results from weaker roof conditions, rather than the converse. Thus seepage appears to be mostly a consequence of instability rather than a cause and is higher where there are 4-way intersections, mudstone geology and external loading factors like depth of cover and proximity to gob that promotes crack development. The study also indicated that a better understanding of the mudrock facies could reduce uncertainty.
As land resources decrease, commodity prices increase and technology evolves, deep sea mining is becoming a viable and sustainable alternative to meet the increasing demand for minerals. Successful deep sea mining operations are built on sound identification of the resource, proper selection of equipment, a thoughtful production plan and good project management. The four key activities can be further optimized by analyzing how the spatial variability in ore body properties impacts the final mining operation.
To address this problem MTI Holland, the R&D institute of IHC Merwede, has developed a simulation framework, which makes use of advanced statistical interpolation techniques to model the spatial distribution of geotechnical and ore grade properties. The software captures the spatial correlation of each parameter and calculates the best estimate and corresponding uncertainty distribution at each unsampled location. The resulting collection of spatial distributions is subsequently inserted into physical models, which translate the geological parameters into financial or operational performance indicators. These models, the so called utility or transfer functions, can be used to compute for example cutting forces, power requirements, bearing capacities and cash flows. Contrary to geological properties, these performance indicators are convenient during decision making.
The main advantage of this simulation framework lies in the fact that the spatial variability and uncertainty is propagated through the whole equipment design and mine planning process. As such, the whole procedure results in a risk robust decision, adding value to the project. During a number of business cases, the framework proved to be successful in assisting the equipment design as well as in developing sound mining strategies.
Siefert, Matthias (Department of Mining Engineering Montanuniversity Leoben Austria) | Mali, Heinrich (Department of Geo-Sciences Montanuniversity Leoben Austria) | Wagner, Horst (Department of Mining Engineering Montanuniversity Leoben Austria) | Frommer, Thomas (RHI Veitsch-Radex GmbH & Co)
An inter-disciplinary geotechnical study has been carried out in an Austrian magnesite mine. The paper describes the how geological data, in situ and laboratory tests as well as subjective, empirical, geophysical and numerical methods have been used to identify critical areas in the mine.
In einem österreichischen Magnesit Bergbau wurde in den letzten Jahren eine interdisziplinare geotechnische Studie durchgefuehrt. Der Vortrag beschreibt den Einsatz subjektiver, empirischer, geophysikalischer und numerischer Methoden zur Identifikation geotechnischer Problembereiche
Ces dernières annees des etudes geotechniques interdisciplinaires sont effectuees dans une mine souterraine de magnesite en Autriche. Ce rapport trace l'application des methodes subjectives, empiriques, geophysiques et numeriques pour identifier les zones problematiques dans la mine
The need for a methodology of geotechnical risk assessment of Austrian underground mines was formulated by the Austrian mining Inspectorate. The Veitsch Radex GmbH & Co as owner of one of the largest Austrian underground operations participated in the development of this procedure. The geotechnical investigations started in 2000.
(Figure in full paper)
General description of the investigated mine
The study mine is located in the eastern part of the Austrian Alps 150 km SW of Vienna. The sparry magnesite deposit is located in the Hackensteiner Formation of the Silurian/Devonian Laufnitzdorf Group which is a part of the Graz Paleozoic Thrust system (Fig. 1). The massive mineral body has a length of approximately 2 km, and a width of 150 m to 500 m. The thickness varies between 50m and 200 m. The general angle of dip of the deposit is ~ 25° to the south and opposite to that of the mountain slope. The overburden varies between 0 m up to 1,000 m. The tectonic regime is dominated by two steep fault systems trending in ENE-WSW, and NNE-SSE directions. These systems displaced parts of the mineral body for distances of a few meters only. Host rocks of the magnesite are anchimetamorphic slates rich in organic material, siltstones, sandstones, lydites, limestones and metatuffs of poor to very poor mechanical properties. Mining activities started at the beginning of the last century, and a remaining lifetime of 20-30 years is estimated. The mining method is post pillar mining using uncemented backfill. The pillars are rectangular in cross-section with a width of ~5 m and a length of ~15 m. In a first step a 7 m high opening at the deepest point of a mining area is excavated. Afterwards backfill is placed to a height of 3.5 m. The backfill is used as a working level for the next 3.5 m mining slice. Depending on the geometry of the deposit up to 26 slices have been mined resulting in pillar heights ranging from 7m to more than 90 m.
(Figure in full paper)
In the first step a full 3D computer model of all excavations was created, Figure 2. All available geological information, drill core data, geometry of the deposit, geostatistical block model etc. were added to this model.