ABSTRACT: Roof failures in the longwall headgates mostly occur outby the T-junction area within the front abutment pressure zone. The roof in this area could fail in the form of cutters, sagging, or collapse. Although front abutment pressure, high horizontal stress, and adverse roof geology are primary risk factors that cause headgate roof failure, other mining factors such as panel orientation, pillar sizes, roof support and operational parameters also contribute to the instability of a headgate. To prevent headgate failure during longwall mining, it is important to understand the causes of roof falls, risk factors, and effective risk mitigation measures. This study is based on the headgate roof support experience in a longwall mine in the Pittsburgh seam. The occurrence of roof falls in two longwall panels and the geotechnical solutions are described in detail. Numerical modeling was conducted to investigate the effect of the longwall retreat direction on stress concentrations in the headgate. Causes and risks of roof falls in the longwall belt entry, and mitigation measures for roof-fall risks are discussed. Through diligent geologic reconnaissance, proper roof support design, and proactive risk management, roof falls in longwall headgates can be prevented.
Longwall mining is the primary underground coal mining method in the United States, and currently accounts for more than 60% of the underground mine production. The longwall headgate, as a passageway for the longwall crew, intake air, material supplies, and coal belt transportation, is critical for both safety and continuous production of the longwall panel. A roof fall in the longwall headgate would not only result in substantial interruption of production but potentially cause injuries or fatalities. Rehabilitation of failed roof in the headgate would also expose miners to the risk of injuries. Due to the importance of the longwall headgate to longwall production, roof support in the headgate has been carefully designed to accommodate both abutment pressure and horizontal stress. However, because the roof stability in the longwall headgate is influenced by many factors associated with both geology and operational parameters, roof falls still occur, though very infrequently, in the headgates of U.S. longwall mines. While bold typeface has been used in this template example to denote emphasis for critical instructions, bold should not be used in a final submission.
Rockfalls are a relevant hazard in quarrying and open-pit mining as well as in roads with rock cuts. The Spanish Association of Aggregate Producers reported that the most common cause of fatalities in their quarries comes from rockfalls. With the aim of having available methods to quantify this phenomenon in mining, the existing rockfall hazard rating systems for roads such as the RHRS and the RHRON were reviewed to conclude that these methods could not be applied to the working conditions of an open pit mine or a quarry. This lead to develop a new technique able to assess rockfall in quarries and open pit mines, named ROFRAQ based on empirical observations and a probabilistic approach that assumes that an accident occurs as a result of a sequence of events. Different events are rated, and the product of these ratings represents an estimate of the probability of accidents due to rockfalls. In this paper will be presented two particular case studies to quantify the risk of rockfall in mining operations with this new developed method, to find out that a low risk exists, and that simple protective measures can contribute to a safer environment.
Rockfalls are a relevant hazard in quarrying and open-pit mining, as well as in road with rock cuts, particularly in mountain areas with unprotected steep slopes. Records from the Spanish Association of Aggregate Producers reported that the most common cause of fatalities in their quarries come from rockfalls, as it is also reported in other parts of the world, especially in Australia.
Recent research studies in the field of rockfall engineering tend to focus on particular detailed modelling issues (Frayssines & Hantz, 2009; Chen et al., 2013, Thoeni et al., 2014), identification of potentially unstable blocks (Mavrouli et al., 2014) as well as calibration of parameters (Wyllie, 2014). However, not so many research studies focus in the application of rockfall control and protective measures in quarries and mining fields (Giacomini et al., 2013; Chai et al., 2015), where a large rate of accidents is recorded every year.
It is highlighted that in an active construction environment, such as quarries or open pit mines, the potential for mechanical initiation of a rockfall will probably be one or two orders of magnitude higher (Hoek, 2000), than in those natural slopes where climatic and biological initiating events take place (Alejano et al., 2013).
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.
Slope stability in open pit mining industry is one of the important issues at this time given that most mining companies in Indonesia to increase production. As a result, the mining companies to do the widening and deepening of the excavation. Increasingly wide and deep open pit excavation is done, then of course the greater the risk that would arise, or the increasing uncertainty in the factors that influence the stability of open pit slopes. The factors that lead to the risk of slope kelongsoran include physical and mechanical rock properties, groundwater conditions, rock mass characterization, as well as the existing structure on the rocks. This paper described risk approach of slope failure based on probability of failure (PoF) and the consequences on a case study mineral X in Indonesia. PoF is determined from input parameters and FoS distribution, while the consequences is based on the results of field observations. Certainly the results of this approach can provide a decision on the slope stability conditions, thereby reducing the greater effect of risk of slope failure.
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.