Mining Technology: Transactions of the Institutions of Mining and Metallurgy

Mass mining systems such as block caving and sublevel caving have been applied in the successful extraction of large orebodies for more than a century. Although the fundamental concepts have largely remained the same, both the designs and particularly the equipment used have seen remarkable changes over the years. Effective development drifting has played, and continues to play, a key role in the successful application and operation of caving methods. With the continuing search for an introduction of more cost effective extraction systems, ‘engineered’ drifting will play an even more central role in the future. An important ingredient in this regard is the acceptance of the concept of smoothwall excavation and then its whole hearted introduction and use by mining companies. To assist in this process, the central part of this paper will deal with practical smoothwall blast design for development drifting. Experience and knowledge gained while conducting an extensive research and development programme on cautious blasting in drifting by the National Institute of Occupational Safety and Health (NIOSH) will be highlighted. The paper concludes with some ideas regarding future developments.

This paper supplements a keynote address prepared for the 2010 Second International Symposium on Block and Sublevel Caving. The author gauges the cave mining industry's position (practice and theory) since the publication of what he considers a seminal paper on ‘Cave mining – state of the art’, hence ‘16 years after’. For clarity, this paper is not meant to be a critique or an endorsement of the cave mining design rules as presented by Laubscher's 1994 paper, as well as an endorsement of a selection of emerging and noteworthy cave mining design and optimisation approaches. The latter methodologies are expected to gain significance as cave mining enters into a new phase of large scale operations at greater depths which the author and others describe as super caves. In this next era of cave mining, improved knowledge and incorporation of the governing physics and fundamentals of the associated caving process or phenomena will become even more important for cave design and performance prediction and, therefore, reliability. The paper starts by providing a snap shot of contemporary caving designs (and practices), highlighting achievements made by the cave mining industry since the introduction of mechanised caving in primary ores. It concludes by listing the developments made in the last 16 years as well as the current and future challenges. The author concludes that contemporary designs and the number of design rules in Laubscher, while still remaining in use, may be reaching their limit when applied to large scale operations or super caves and they need to be supplemented by a number of the emerging techniques. The attributes which make super caves unique are presented. The opportunity is to continually improve and to test these emerging methods. Finally, it should be noted that the paper is written from the author's perspective as a mining research engineer and more recently (from 1997 to current) as the technical director of two international industry collaborative projects; the International Caving Study (ICS) and the Mass Mining Technology (MMT) projects. These projects focus on improving the understanding of the underpinning fundamentals associated with the caving of strong rock masses or primary rock (i.e. rock mass characterisation, caving mechanics including seismicity, preconditioning, gravity and disturbed flow, primary and secondary fragmentation and confined blasting), mainly at moderate depths and stress environments. The original intent of the ICS was to critique, improve and/or supplement a number of the design rules as presented in the Laubscher seminal paper, given the move towards caving of strong rocks or primary rocks. The MMT then put more emphasis on the study of caving fundamentals.

Goldex is a sublevel open stoping mine near Val-d'Or in northwestern Quebec. The orebody will be extracted in stages, using a novel mining method, resulting in a single stope with the dimensions of 450 m on strike, 250 m in height and up to a width of 120 m. A microseismic monitoring system was designed to monitor the profile of the large stopes and the extent of overbreak around the stopes. This paper discusses the design of the seismic system, and the results of the seismic monitoring with regard to observations of stope performance. In particular, the paper discusses how the seismic source parameter Apparent Stress can be used to identify areas of active rock mass fracturing.

The Grace Mine, located in southeastern Pennsylvania, USA, was owned and operated by Bethlehem Steel Corporation during 1951–1977. During this time, iron ore was extracted using the underground panel caving mining method that has resulted in significant surface subsidence. Upon recovery of the water table after mining, a lake has formed over much of the subsided area. Before redevelopment of the abandoned mine site for residential and light industrial usage, an investigation of the subsidence zone of influence, and the potential for further subsidence has been undertaken.

Block and panel caving is one of the most suitable methods for exploitation of large massive ore bodies where a high rate of extraction is required. However, many authors have studied the rate of extraction and concluded that it may be limited by the ability to prepare mines at high rates. Different approaches, to increasing the rate of extraction, tend towards the introduction of some kind of continuous drawing and materials handling on production levels, which apparently are close to reaching a practical solution. However, corresponding studies to match the rate of mine preparation to high extraction rate operations have not been discussed or published to any great depth. This paper describes some layouts and constructive techniques compatible with rapid mine preparation demand. A significant reduction of time in mine preparation, in both underground drifting and construction, is expected as a consequence of the introduction of non-blasting excavation methods and using precast concrete modules to build up underground mine construction. The main advantage of non-blasting drifting is the predictability of finishing underground drifts, such that it is possible to standardise all the components of single underground mine constructions (i.e. drawpoint and dump points). The solution requires adapting and developing both new equipment and new layouts, because neither standard full face boring machines nor current layouts for conventional block/panel caving are suitable to high speed development of caving levels. The layouts discussed focus on conventional load–haul–dump units and on mechanised continuous drawing system arrangements. In each case, the general arrangement presented has been developed by JRI Ingeniería S.A. as part of its technological innovation policy. Finally, an evaluation of the impact upon scheduling and costs is presented in comparison with conventional methods. New approaches are also mentioned about other issues such as logistics and commissioning, with new actors in the business, which may produce additional benefits for project and operation management.

Planning and implementing massive underground mines such as block caves at increasing depths is a process that can extend over many years and carries a considerable level of risk. This paper examines the time that it has taken some major underground mines to plan and implement new mines at depth and the implications of this extended process. Geotechnical concerns are a major source of uncertainty as mines move to greater depths. Gathering, interpreting and using the geotechnical data to plan and implement deep mines add considerably to the time and uncertainty of the planning process.

Reservas Norte (RENO) is one of the panel caving sectors of El Teniente mines, owned by Codelco Chile. The sector has experienced mine induced seismicity for many years. The work presented in this paper focuses on seismic activity recorded between the period from January 2004 to July 2008. The interpretation of the seismic data revealed that the sources of elevated seismic hazard (large events) at RENO during this period could be attributed to four major geological structures: Falla G, Falla F, Falla C, Falla N1. In particular, the seismic response of the four structures to undercut blasting activities is examined in detail. The use of numerical modelling has shown that it is possible to simulate this response after calibrating the model against the cumulative seismic moment released by the faults, as mining advances towards them. This calibrated numerical model can then be used to forecast future seismic responses. The main product of this work is a tool that can be used to rank different undercutting rates and geometries in terms of seismic hazard.

In the past, measurement of ore flow in block and sublevel caves has been performed with passive ‘markers’ (often made from steel pipes) embedded into the orebody. Extracting passive markers along with the ore is labour intensive and often requires many years of commitment. The Smart Marker System uses hardened radio frequency identification technology to automate the marker detection process, allowing the measurement and analysis of underground ore movement to be carried out without affecting production draw rates. This paper presents the results from block and sublevel cave testing of the Smart Marker System from late 2008 and throughout 2009. Block cave testing was carried out at Rio Tinto's Northparkes E26, and sublevel cave testing was done at Newcrest's Telfer Mine. The test results demonstrated the successful use of the automated system in the underground production environment and provided high resolution, real-time extraction data suitable for the analysis of underground orebody movement. This paper also addresses the use of real-time marker data in the analysis of back-break, ore flowrates, dilution entry and rill detection.

Hydraulic fracturing can be used for preconditioning in block cave mining. Microseismic monitoring is one of the techniques used to determine the orientation, extent and time evolution of the hydraulically-generated fractures. Data are recorded continuously at significantly higher frequencies than currently used in routine seismic monitoring in mining environments, and the data analysis is accordingly more complicated. Microseismic monitoring was performed as part of the hydraulic fracturing programme at Ridgeway Deeps Block Cave. The seismic data that are recorded during a typical fracture session have been described in detail. Frequency filtering and time domain methods are used to isolate the individual seismic events. A straight-ray method is used to estimate source location. A group of seismic events is then relocated with a double-difference method and the locations are used to describe the hydraulic fractures. Several hundred events from several different fracture sessions were located using this technique. For many fracture sessions, the locations are clearly planar, and estimates of strike and dip could be obtained. Fracture planes from consecutive fracture sessions were compared. The time evolution of individual fracture sessions was analysed and used to provide estimates of propagation rates. Criteria have been discussed for applying the technique to other hydraulic fracturing programmes.