Nickel Rim South Mine of Sudbury Integrated Nickel Operations (Sudbury INO), A Glencore Company, has challenging ground conditions related to: complex geological structure; depth of the current orebody (from 1,160-1,710 m below surface); the mine layout where two sills were required; the draw point pillar geometry; and the type of ground support that has been used during the project ramp up phase. In order to manage the risk associated with mining while these conditions exist, sophisticated and state of the art tools have been used to predict the ground response to mining activities. The overall mine layout and rock mechanics issues are discussed. Analysis techniques using high resolution seismic monitoring, non-linear numerical models, cad/database linked models to assess geotechnical hazard, and high end microseismic analysis software are shown as examples.
The PT Freeport Indonesia Grasberg mine in Papua, Indonesia is composed of the Grasberg open pit and four underground mine operations. The Deep Mill Level Zone (DMLZ) is one of the currently active block cave underground operations. The DMLZ orebody presents a number of significant engineering challenges and technical risks when compared to other operating block caves. Due to its mining method and the depth of the operation (+1,500 m), mining-induced seismicity was identified as one of the key factors that will affect the rock mass stability. The seismic system is one of the main tools that is used to track and understand the rock mass response for this cave propagation. This paper presents initial results from the DMLZ, and includes details about the rock properties, seismic system optimisation using blast data, and seismic analysis examples to help aid in the safety and productivity of the block cave operation.
The majority of seismic hazard assessment tools are solely based on statistical analyses of several seismic source parameters such as event rate and time, and seismic moment. These analyses are often applied to the entire mining area which can impact the accuracy and reliability of the hazard assessment tool in each zone. Experience has shown that mining geomechanical risk is complex and its mitigation needs a broad understanding of other geotechnical factors such as rock mass properties, geological structures, mining method, stress regime, etc. Since all the contributing parameters and their impact are not entirely understood, it is critical to apply a range of geotechnical/geomechanical analyses in correlation to each other and quantify the changes in the rock mass behaviour. The goal of this paper is to develop a seismic hazard assessment tool calibrated for each geotechnical domain within the mine. To develop the tool, we incorporated mine geotechnical and geological data, seismic source parameters, and tomography analyses from a hard rock underground mine in North America. There exist several sub-vertical faults and one horizontal structure in the mine which create clear contrasts in rock mass behaviour across the structure. The results show good correlation among the different datasets, and a calibrated seismic hazard tool has been developed that provides ongoing updates to the mine operation.
The orebody at Eleonore Mine (Eleonore) consists of multiple lenses of narrow thickness. Owing to ground stability issues, the capacity of the support was increased and the mine sequence was changed successfully in 2016. In 2018, mine-scale geomechanical numerical analyses were conducted in the continuum code FLAC3D to better understand the conditions leading to these improvements and further optimise the sequence. Locations of falls of ground and damage, blast hole performance and a micro-seismic database were used to calibrate the model, and different future mining sequences were analysed. The models helped demonstrate that persistent shallow dipping joints subject to high horizontal stress put a lot of demand on bolts in the back of excavation; they are likely to be the main source of energy release as they are sheared peripheral to the top and bottom of the stopes. The narrow-mined width and good rock strength involve limited stope interaction, resulting in highly stressed remnant stopes and limited impact of the sequence.
The National Research Council of Canada has developed a novel ultrasound based Rock Bolt Sensor (RBS™) to monitor load, deformation, and integrity of rock bolts. Several RBSs were deployed to monitor bolts in the roof of an intersection of two drifts and with another region of monitoring tens of metres away in the roof of the drift. The intersection spanned two rock types with different stiffness characteristics. The region was also covered by an extensive seismic system with a mixture of triaxial geophones and both triaxial and uniaxial accelerometers. Using three weeks of recorded data, the loading on 12 bolts was monitored during the production of several nearby stopes and analysed alongside the recorded seismicity. A significant increase in loading on the bolts in the roof was recorded when production occurred within 40 m of the instrumented intersection despite several other stopes producing more significant seismicity following blasting. The load was also taken up asymmetrically, with more substantial changes in load occurring in the bolts in the greywacke. During the study period, several large seismic events occurred within 500 m of the study area. Seismic moment tensor analysis was conducted on several of the seismic events to estimate the mechanism of failure and, using seismic stress inversion, the orientation of the observed fractures was estimated and the directions of principal stress. There was a strong increase in the load on the bolts located in the intersection, across the two different formations, several hours after a large seismic event occurred on that contact several hundred metres away. This work showed a connection between loading on the bolt and large changes in the local stress field due to nearby production and significant seismicity along structures in the region. In addition to monitoring the loading of the rock bolts, the Rock Bolt Sensor can be utilised to better understand the coseismic and post-seismic deformation within the mine and improve models of the local stress field.
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