Goulet, A. (Université Laval) | Grenon, M. (Université Laval) | Morissette, P. (Agnico Eagle Mines Ltd) | Woodward, K. (Australian Centre for Geomechanics) | Wesseloo, J. (Australian Centre for Geomechanics)
Strainburst refers to local small seismic events generating shallow spalling with violent ejection of fragments in an active development heading. This rockburst category may affect worker safety and mine productivity. This paper conducts a preliminary study investigating properties of large-scale geological features, mine operational context, and both aseismic and seismic responses generating strainbursts. Using the LaRonde mine as a case study, key parameters influencing strainburst occurrence and severity are defined and highlighted. The distance to a lithological contact and the orientation of the drift are parameters that affect strainburst potential and severity at LaRonde. The analysed bursts were seismically triggered or self-initiated. The analysed seismic events generating strainbursts had a local magnitude of −0.7 ± 0.5 on average and were located between 3 and 58 meters from the damage. Finally, strainbursts mostly occur within five days after a development blast.
Mining in deep, hard rock mines poses many challenges for engineers. An increasing engineering challenge associated with higher stress conditions at great depths is the potential for strainbursts, which can have considerable adverse effects on mining activities. In this study, strainburst refers to local small seismic events that generate shallow spalling with violent ejection of fragments in an active mine development heading (Ortlepp, 1992).
Many authors have studied and described strainburst mechanisms (Ortlepp, 1992; Ortlepp and Stacey, 1994; Kaiser et al., 1996; Kaiser and Cai, 2013). However, strainburst parameters such as time of occurrence and its link to more remote seismicity are uncertain and not fully understood, leading to difficulties in designing and implementing risk management strategies. Seismic risk management should rely on all available data at the mine to help understand rockmass responses to mining. Seismic and aseismic responses to mining are influenced by many parameters, such as contrasts in rock mass competency and stress conditions, which are partially controlled by the proximity to large-scale geological features such as faults, lithological contacts, and foliation. Hence, it is of great importance to characterize geotechnical environments and both seismic and aseismic responses properly in order to better understand strainburst occurrences.
This paper presents the development of a geotechnical model based on the integration of geological and geomechanical characterization data along with seismic events. This model provides useful tools for understanding the rockmass response to mining. More specifically, this paper focuses on determining seismic parameters and the properties of large-scale geological features that affect strainburst occurrence and severity. Properties of large-scale geological features refer in this case to the distance from the damage to geological contacts and faults, and the angle of interception of the damaged drift with the foliation. Analysis of the seismic sources includes the quantification of the distance from the damage to the linked seismic event, the magnitude of the linked seismic event, and the time of occurrence of the event in relation to the previous blast in the sector.
The study uses strainburst data from the LaRonde mine, a deep seismically-active hard rock underground mine located in the mining district of Abitibi-Témiscamingue, in Quebec, Canada. As mining progressed deeper, this mine experienced an increased frequency of strainbursts, providing a unique opportunity to demonstrate the applicability and usefulness of the developed methodology.
Seismicity in the mining environment is controlled by factors including stope and development blasting, the presence of geological features, and stress conditions. The Goldcorp Eleonore mine is located in the James Bay region, Quebec, Canada. It’s 800-metre depth makes Eleonore a relatively shallow mine when compared to other seismically active Canadian mines. Despite the mine’s depth, seismicity is a geotechnical hazard that may be arguably attributed to a particular stress regime and complex geology. An improved understanding of the seismic responses following blasting can decrease seismic risk and is beneficial to mine planning and productivity. Seismic responses to blasting were spatially delineated using a density-based clustering approach. The spatial characteristics of clusters were assessed using Principal Component Analysis (PCA). The best-fit planar representation of seismic event clusters was identified. The orientation of the best-fit planar representations was then compared to the mine’s local jointing to investigate the causative seismic source mechanism for these events. The results of this study show that seismicity is linked to local jointing, and in particular to the different structural domains.
Blasting is a significant factor in triggering seismic events at mine sites (Vallejos and McKinnon, 2008). Seismicity is defined as a stress wave resulting from inelastic deformation or failure in the rock mass (Hudyma and Mikula, 2002). Seismicity in the mining environment is controlled by factors including stress, geological structures or rock mass weakness, and mining activities. This induced seismicity—directly connected to mining operations—is associated with formations of fractures at stope faces and with movement on major discontinuities (Gibowiicz and Kijko, 1994). Fig. 1 illustrates the different source mechanisms of induced seismicity in an underground mine environment (Hudyma et al., 2003). Many authors have observed that well-located seismic events exhibit strong spatial clustering (Leslie and Vezina, 2001; Dogde and Sprenke, 1992; Kijko et al., 1993). Hudyma and Mikula (2002) hypothesized that clusters of seismic events represent a separate seismic source mechanism. Limited quantitative means of assessing a seismic source mechanism have been presented in the scientific literature especially in the case of clusters of seismic events. A better understanding of induced seismicity source mechanisms in underground mining can help to optimize mining operations, reduce delays and production losses. Understanding the main source mechanism in mines is essential to a better prediction of the rock mass response to mining.
Uranium deposits in the Athabasca Basin are normally related to graphitic faults which typically behave like thin conductors. Slingram-style time-domain electromagnetic (TDEM) methods are commonly used in the exploration of the uranium deposits in the Athabasca Basin. A finite-volume time-domain (FVTD) method that is designed to model the Slingram-style electromagnetic (EM) surveys in parallel is presented. Numerical experiments show that this method can reproduce results that were previously presented in the literature. Results are also shown for a real data-set from the Athabasca Basin.
Presentation Date: Thursday, October 18, 2018
Start Time: 8:30:00 AM
Location: 213B (Anaheim Convention Center)
Presentation Type: Oral
Turner, R. (Montana Tech of The University of Montana) | Becker, R. (Montana Tech of The University of Montana) | Fitzgerald, J. (Montana Tech of The University of Montana) | Sandau, B. (Montana Tech of The University of Montana) | MacLaughlin, Mary (Montana Tech of The University of Montana)
The rock masses at the sites represent different types of geology and also different degrees of geotechnical characterization. The intent was to augment the field data with data collected by mapping features on the 3D photogrammetric model. These case studies are presented as examples of discrete fracture network generation. The 3D photogrammetric models were created using digital photos taken using a standard terrestrial digital camera or still frames extracted from video recorded using a camera mounted on an unmanned aerial vehicle (UAV). The software packages used to build the models is Bentley's ContextCapture and Autodesk's Recap. The discrete fracture network models were created using Golder's FracMan software.
ABSTRACT: Canada has a long history of underground hard rock mining. As mines deplete current resources, new technologies enable deeper deposits to be extracted economically. With increased depth, Canadian mines typically experience an increase in both quantity and severity of mining-induced seismicity. A Canada Wide Seismic Monitoring Survey was used to gather data pertaining to the current state of seismic monitoring in Canadian hard rock mines. This paper presents the initial survey results with a focus on: microseismic monitoring systems and data quality.
Seismic monitoring has existed in Canadian mines since the 1980's. Over the past four decades, significant advances in on-site microseismic and macroseismic monitoring have enabled technical and operational personnel to integrate the use of seismic data into daily operations. When a seismic event or rockburst occurs, details regarding size and location are available within minutes for communication with underground personnel.
For some Canadian mining operations mine seismicity has been a concern for a decade or more. Other sites are only now beginning to deal with workplace safety concerns resulting from dynamic rock mass failure. Seven of the twenty-five seismic monitoring systems currently operating in Eastern Canada have been installed within the past five years (Hudyma et al., 2016). In 2014, the Ontario Ministry of Labour released a list of mining related hazards ranked by an expert group of industry and worker representatives. The occurrence of a rockburst was risk ranked number one overall (Ontario Ministry of Labour, 2014).
The future of Canadian mining is deep. More than ten Canadian hard rock mines are currently operating at depths greater than 1500 metres below surface. Of these, four are currently mining more than 2000 metres below surface. With increased depth, typically comes elevated stress and an increased potential for seismicity and rockbursting.
The Canada Wide Seismic Monitoring Survey was designed to gather data pertaining to the current state of seismic monitoring in Canadian hard rock mines. The survey was provided as a Microsoft Excel workbook, to better enable site personnel to quickly and easily complete and electronically return the survey. Figure 1 provides an example of the survey format.
ABSTRACT: Mining at greater depths can lead to stress-induced failure, especially in areas of high horizontal in-situ stress. The induced stresses on the boundary of an opening are known to be in a biaxial state. Biaxial testing of intact rock mostly requires sophisticated loading systems making it expensive and time-consuming. This study investigated the impact of biaxial stress state on the failure response of intact rock using a simple and cost-effective design. The apparatus consists of two platens that apply biaxial compressive stress on a cubical specimen when placed inside the uniaxial testing frame. The platens apply equal loading in both directions (σ1 = σ2) until specimen failure. In addition, a confining device was used to perform separate biaxial tests under constant intermediate principal stress (σ2 = constant). The failure modes and peak strength of sandstone specimens were compared with other biaxial devices to validate this design. Uniaxial tests were performed on both cubical and standard cylindrical samples. The biaxial strength of 50.8 mm cubical specimens was found higher than its uniaxial strength at any level of confinement. A quadratic polynomial, based on regression analysis, provided a good fit to the data points at failure. Spalling characterized the failure mechanism at free faces followed by shear failure of mid-section. In biaxial tests, shear planes at mid-section rotated with the increase in the intermediate principal stress.
Excavation at deeper depths often creates adverse stress conditions around an underground mine opening. In addition to in-situ stresses, the excavated opening produces induced stresses that may initiate fractures causing failure of the rockmass. Three mutually perpendicular principal stress components (σ1 > σ2 > σ3) can represent the stress state in the earth which is usually in a polyaxial state. However, on the boundary of an unsupported excavation, a biaxial stress state exists where σ3= 0 and σ1, σ2 ≠0. In addition, the minimum principal stress is smaller compared to other two stresses in the region (within the pressure arch) around the vicinity of the opening (Cai, 2008). Therefore, the constitutive behavior of rockmass under biaxial stress state plays a vital role in the design of underground structures. Mohr-Coulomb and Hoek-Brown are two most prevalent failure criteria used in tunneling and mining operations. Both failure criteria are known to exclude the influence of intermediate principal stress which makes them inappropriate in case of biaxial loading. For instance, these criteria are unable to explain the mechanism behind ‘‘onion-skin’’ fractures, observed in hard rock mines and deep civil tunnels (Cai, 2008). Hence, a generalized failure criterion that accounts for the effect of intermediate principal stress is essential for rocks.
Zhu, G. L. (Masdar Institute of Science and Technology) | Sousa, R. L. (Masdar Institute of Science and Technology) | Zhou, P. (China University of Mining and Technology) | Yang, J. (China University of Mining and Technology)
ABSTRACT: An innovative approach to the gob-side entry retaining non-pillar mining is being used to increase the coal seam re cycling rate and productivity in China’s coal mining. The retained entry with a sidewall formed by gob caved-in filling rocks is unique in this method and the stability of caved-in material is critical to ensure efficient and safe mining activities. In this paper a numerical investigation on the stability of the gob-side entry is conducted using a discrete fracture network (DFN) model developed by the Massachusetts Institute of Technology (MIT), GEOFRAC, in combination with discrete element modelling (UDEC). The proposed method is applied to a case study of a gob-side retained entry in an underground coal mine in China. Fracture traces are measured along the gob-side wall of the entry, and statistical methods are used to estimate the fracture intensity and the mean fracture areas, which are the key inputs to GEOFRAC. Fracture networks generated by GEOFRAC estimate the rock blocks in the filling body, and simulations with UDEC are done to evaluate the stability of the gob-side entry. Two models are developed, one considering the generated fractures and the other considering no fractures within the gob-side filling. The results show the effects of considering the fractures in the filling body on the distribution of displacement and field stress in the gob-side entry zone. Also, the stability under the mining impact loading, due to periodic roof caving, is simulated, providing the basis for the optimization of the design of the entry support.
Coal is one of the most significant energy resources, covering about 30% of energy consumption worldwide. China is the largest producer and consumer of coal in the world. Despite, China’s reduction of coal consumption in the past few years, and its target to reduce coal to 58% of total energy consumption by 2020, coal remains an important source of energy, and China is still one of the largest coal producer in the world. Fig.1 shows China’s annual coal production in recent years.
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.
ABSTRACT: Longwall Top Coal Caving (LTCC) is a thick coal seam mining method which uses the Single Pass Longwall (SPL) method for extracting the lower section while the upper section (top coal) is mined by means of caving. The mining height in LTCC, compared to other methods, may result in roof instabilities such as caving and weighting. The understanding of roof instabilities involved in LTCC, however, is limited and the applicability of conventional rock mass stability assessment systems into LTCC can be questioned. This paper presents a systematic review of the LTCC-associated roof instability mechanisms and the applicability of widely-applied rock mass classification systems into LTCC. The study confirms that the vertical stress redistribution caused by LTCC is in general similar to that caused by SPL; the predominant failure mode in roof rock mass can be either shear or tension while the controlling failure mechanism in top coal is shear; the movement of immediate and main roofs in LTCC is similar to that in SPL; conventional rock mass classifications can be applied in assessing LTCC roof stability however their sensitivity to coal stability is low. The paper's findings can assist engineers in better applying and managing LTCC operation.
The Longwall Top Coal Caving (LTCC) method divides a thick coal seam (whose thickness is greater than 4.5 m) into two sections: the lower or cutting and the upper or top coal. The lower section is extracted by conventional Single Pass Longwall (SPL) method and the top coal is recovered by means of caving through face support (Fig. 1). LTCC has significant advantages compared to other methods in extracting thick coal seams, as reviewed in Le et al. (2017a). In this paper, the roof strata to be studied are limited to the immediate roof and main roof. The immediate roof is the portion of the strata lying immediately above top coal. Immediate roof can fail and cave immediately, or with little delay as support advances. Above the immediate roof, the strata in the lower portion of fractured zone is called the main roof. Main roof can fail, but normally will not cave, and can still transmit horizontal force through broken strata (Fig. 1).
ABSTRACT: A routine method of evaluating temporal and spatial changes in relative rock mass stress levels based on microseismic information was developed by NIOSH researchers in collaboration with U.S. Silver Corporation’s Galena Mine. The method is based on the Energy Index concept of Aswegen and Butler (1993), but was modified to evaluate the “running average” of inferred stress levels in a given seismogenic region for a selected time window. The new Average Scaled Energy Index method proved to be informative and efficient in providing daily information to mine operations personnel on the changing nature of stress in the rock mass due to mining and provided a standardized means to evaluate and compare conditions in different parts of the mine. It was found that underground mining crews easily understood the concept and adopted the method into daily operations. This paper discusses the initial evaluation and implementation of the modified method at the Galena Mine and provides three examples from different areas of the mine having distinct geologic and seismic characteristics to demonstrate its effectiveness.
Mining-induced seismicity has been monitored almost continuously at the Galena Mine in the Coeur d’Alene mining district of Idaho, U.S.A., since 1968. Daily seismic hazard analyses of selected mining areas started in the late 1980s using multiple methods, including magnitude relationships, decay rates, and energy release rates. One of the difficulties experienced at the mine was the time required to complete the daily seismic analyses for each of the different mining areas due to their unique seismologic and geologic parameters, which required a high degree of familiarity by the ground control staff. The method was not sustainable under these conditions with both new ground control staff and mining crews lacking the requisite knowledge of the specific rock mass regions, particularly when they were moved into unfamiliar parts of the mine.