The possibilities of applying the technology of ore caving in caved-in deposits

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The main drivers for the selection of a method and flowchart for the development of ore deposits are mining-geological conditions of occurrence of ore bodies and host rocks, their geological structure, strength, deformability and stability.

On the basis of the detailed and thorough study of these indicators, it is possible to assess the choice of mining technology, determine parameters of main structural elements of the accepted development system and decide upon solutions that ensure efficiency, reliability and safety of operations as well as integrated development of indicated and inferred subsoil reserves.

This technical article reviews the development of the Zhezkazgan field over the last 80 years and its innovative approach to recover ‘lost’ ores in room and pillar operations after long-term intensive operation of the Zhezkazgan field resulted in quality and quantity depletion of recoverable reserves.

Its operator, Corporation Kazakhmys, is the largest in CIS countries in terms of mining volumes and technical level of mining production.

The Zhezkazgan field has run for 80 years, and the main underground development system at the Kazakhmys subsidiary Zhezkazgantsvetmet’s projects is the room and pillar process.

Compared to other underground development systems, the room and pillar method accounts for 85% of the operations.

Development of the reserves of flat deposits is carried out in two stages: Stage I – excavation of chamber reserves; and Stage II – excavation of pillars using the caving of the overlying strata moving backward from the deposit.

Despite numerous advantages, the downside of the room and pillar method is considerable ore losses of between 30% and 40% as well as accumulating cavities that result in the complicated technology of stoping, caving of pillars and roofs of the chambers with the outcrop onto the daily surface.

The total area of the worked out space at the Zhezkazgan field, caved or at the stage of irreversible deformation processes, amounts to nearly 48% of the area of the whole worked out space and the share of pillars with long-term operation life.

In recent years mining operations at the Zhezkazgan field shifted from the Central ore field to its sides within the Annensk and Akchiy-Spassk mining areas. The mining and geological conditions of these areas considerably differ for the worse. According to mining technical and geomechanical conditions of these areas, up to 80 % of ore reserves are concentrated in inclined (15-45°) and multiple overlain deposits while the ore and rock massifs are characterised with increased fissuring and water saturation.

During the transition to inclined deposits, the use of the traditional underground technology of development of field sections mastered during the decades results in the reduction of efficiency and often leads to formation of weakened and unstable sections that indicatedly complicates the conditions of repeated development of pillar reserves.

 

Analysis shows that in recent years’ operation there has been a reduction of productivity coupled with an increase of production cost for the hard rock miners in Zhezkazgan, which in turn caused a reduction in efficiency of the applied traditional mining technology using the room-and-pillar system. This decline in productivity required a transition to more progressive mining technologies and methods for the development of temporarily inactive recoverable and unrecoverable reserves with the purpose of extension of the field’s operation life.

Currently, the level of reserves recovered using the underground development system amounts to 36% of copper ores.

However, in the worked out panels and deposits there is a considerable amount of temporarily inactive unrecoverable reserves – written off as losses – of prolific ores, concentrated in safety and support pillars, rooves, the soil of chambers, and at edges of deposits  as well as in caved areas which amount to nearly 607 million tonnes of ore.

Almost eight per cent, close to 80 million tonnes of ore, is located in the rib, panel and barrier pillars. Copper content in these pillars, especially of those abandoned at the earlier stages of development, is almost twice that of the content in the remaining recoverable reserves.

Development of these trapped reserves is essential to solve the issue of building the ore base without additional capital investment, as well as for the extension of the operational life companies located in the Zhezkazgan field.

This is crucial as in recent years mining operations at the Zhezkazgan field shifted from the central ore field to its sides within the Annensk and Akchiy-Spassk mining areas, where geological conditions differ for the worse.

According to mining technical and geomechanical studies of these areas, up to 80 per cent of ore reserves are concentrated in inclined (15-45°) and multiply overlain deposits, where ore and rock massifs are characterised by increased fissuring and water saturation. 

In the transition to inclined deposits, the use of the traditional underground technology in the development of field sections excavated over decades resulted in a reduction of efficiency, while degradation over and often time leads to formation of weakened and unstable sections that indicatedly complicates the conditions of repeated development of pillar reserves.

Geomechanical issues at the development of the Zhezkazgan field became as important as issues related to the accumulation of the large volume of worked-out areas and increase of the deformation of rocks around stopes; the reduction of stability of structural components of the development system; and appearance of negative manifestations of mine pressure seriously affected the safety of mining operations.

Mining pressure at the field depends on a wide range of geological and mine technical conditions of occurrence and field development as well as on significant unevenness of distribution of stress of a virgin massif.

In the course of experimental studies of indicated stress field, it was established that the direction of the maximum principal stress is close to the meridian stress; the direction of intermediate principal stress is close to latitudinal stress; that of the minimum principal stress is close to the vertical one.

The value of the maximum principal stresses exceeds by two to seven times the vertical component (γH) that proves the significant role of a tectonic (horizontal) component.

At present, the geomechanical situation at the Zhezkazgan field has considerably changed, with new effects and phenomena which were not previously observed.

According to the data from Kazakhmys’ geomechanical department, it was established that at various stages of extraction nearly 12.1 per cent of the area is formed into rib pillars.

The total area of the sections where conditions of support of the overlying strata do not conform to the designed ones equalled 20 per cent of all undermined areas. In terms of  operational life, of 84 panels out of 547 panels at the Zhezkazgan field, approximately 14.5 per cent, is more than 30 years. At that, 74 panels are referred to as weakened. In 84 panels there are 7,151 pillars, of which 1,457 (20.4%) are referred to as weakened, and 123 (17.2%), as collapsed .

The ‘ageing’ effect on rib pillars caused by the passing of time as well as the long period of support of the worked out area, resulting in a reduction in their strength, are the main reasons for gradual worsening of geomechnical situation at the Zhezkazgan field.

In recent years the growth of a number of partially and completely collapsed rib pillars is accelerated.

During the whole mining period in the Zhezkazgan field, there were nearly fifty thousand rib pillars created, however during the 2000s, the percentage of stable pillars reduced from 75% to 61% due to the doubling of the number of completely collapsed pillars, which rose from 13% to 26%.

The level of partially collapsed pillars remains between 12 and 13%.

According to the geomonitoring data, signs of increased mine pressure were recorded, which included audible ‘clicks’; systematic chips of rib pillars; the collapse of certain rib pillars; failures of ‘bridges’; and seismic manifestations.

These geomechanical processes occurring at the Zhezkazgan field [the collapse of rib pillars; strata movement; caving of an overlying strata and failure onto the earth surface; air percussions; and inferred earthquakes] are common for room and pillar operations.

Despite being typical for similar site, Kazakhmys decided these ongoing factors required the creation of new methods of mining and development to utilise the remaining resources with minimum ore losses both at the fields under development and those planned for the future development.

Solving this issue was one of the most urgent concerns for the operator, along with the creation of safe flowcharts and rational procedure of development at the repeated mining of indicated and inferred reserves from the subsoil of the caved area.

Therefore, a strategy for the underground development at the Zhezkazgan field was needed to develop and implement a more efficient technology for mining the pillars to recover this lost material and justify safe flowcharts and procedure for the repeated mining of reserves from the subsoil of the caved area using geoinformation technology and integrated solution of economic, technological, and geomechanical problems.

(Image:The process of rib pillar destruction – a) initial form, b) condition after 30 years)

The focus of the solution was through the analysis and assessment of the existing mine technical and geomechanical data and site physicality.

At the beginning of the 2000s, the operating mines recorded extensive weakened areas, the caving in of mines over large areas accompanied by inferred earthquakes on the surface and air percussions in mines, with large-scale caving of overlying strata occurring at some sections of mine’s fields in 2004 and 2006, with the outcrop onto the daily surface.

By 2011, at Zhezkazgan’s underground mines there was approximately 152.1 million cubic metres of unfilled cavities, 60.5 million cubic metres of cavities filled with solid, hydraulic, dry stowing, stored rocks as well as under the caved rocks of the overlying strata.

There was also an accumulation of large volume of worked out areas, with losses of recoverable reserves in pillars of various types, roofs, edges of deposits, increase of stress in the rock massif around stopes, reduction of stability of structural elements of the development system,  all these adversely affect mining safety.

There was the estimation that during the transition of second working to the lower levels, to a depth of more than 200 metres, subsidence rarely occurs at the surface.

However from 1977 through to 2005, 17 subsidence instances still occurred.

As a result after heavy excavation, of the Annensk mine area in the Zhezkazgan field in 2004 and 2006, along the contour of the caving a zone of joint movement troughes formed, running approximately 2 km across and 500 to 600 metres wide. 

In the Annensk area movement trough zone, there is thought to be recoverable and unrecoverable reserves as well as reserves in the pillars, all told amounting to over 25 million tons with a grade of 0.67-0.96% of copper ore .

In view of some positive experience in the ore mining in the contour of the movement trough at underground mines, these reserves can be partially developed.

While the development of ore reserves in the zone with caved and weakened massifs is quite complicated and dangerous, in the area of the Annensk ore mine some reserves were still able to be mined, and from 2005 to 2006, 813,000 tons of copper ore were extracted.

While there was experience gained at these underground mines, the repeated development of reserves in specific zones of caving of the Zhezkazgan field resulted in unfavorable mining technical conditions and movement of the rock massif.

In view of the formed conditions at this field, for the repeated development of reserves in the caved areas with the movement trough it is proposed to apply the effective underground technology with ore caving with the purpose of the development of the remaining reserves of the caved Annensk mine area and overlapping close caved deposits of the Zhezkazgan copper-ore basin.

The ore caving technology is a spontaneous development of the process of separation of lumps from the massif on the roof of the undermined space of certain area as a result of mine pressure and under the action of the proper weight of the parts of ore massif and a discharge of ore from funnels of the developed mine on the bottom of the block.

With the purpose of repeated underground development using the technology of ore caving with high level of reserve extraction from collapsed and weakened areas of Zhezkazgan as well as the potential to apply this technology at the development of similar type of indicated and inferred reserves of the caved area developed using the room and pillar system, for the first time the author tested the idea in 2012 at the International Scientific and Practical Conference

On 28 January 2013, the proposed idea for the CIS countries to develop the underground combined technology for repeated development of remaining reserves of the caved areas of the Zhezkazgan field was presented at the Moscow Report at the Moscow State Mining University (MSMU) at the International Scientific Symposium XXI Miner’s Week-2013, held jointly by MSMU and IPKON RAN, where positive feedback was obtained regarding further research of this problem and its implementation in production.

It is worth noting that the application of the ore caving at the development of sections of reserves of close overlapping deposits of the caved area of Zhezkazgan, the leading design institute of Corporation Kazakhmys has presented the basic charts of the development system with ore caving at the development of flat and inclined deposits of the caved area of the Zhezkazgan field with production capacity of 4 millon tons per year. However, the flowcharts of the development using this technology in production conditions of the Zhezkazgan field are not tested, and in order to determine the efficiency of its application it is required to carry out an industrial test.

In addition, there is no scientific method of justification of parameters for the use of this technology for repeated extraction of pillars ensuring the assessment of economic efficiency, rational and integrated completeness in underground mining of similar types of reserves from the subsoil of Zhezkazgan copper sandstones developed using the room and pillar system.

To solve this problem the Mining Institute after D.A.Kunayev, under the leadership of the author, has carried out the research for the development of the method and new options of the resource-replenishing technology of repeated underground mining ensuring the rational and integrated development of indicated and inferred ore resources in the Zhezkazgan region.

To this end, the problems to be solved are: assessment and systematisation of the indicated and inferred reserves of the copper field; development of the method and new options for the resource-replenishing technology in underground mining ensuring the rational and integrated development of indicated and inferred reserves from the subsoil of copper fields; and creation of the process regulation and the use of up-to-date geoinformation systems (GIS) and calculation method for the technical and economic efficiency.

The major solution for the aforementioned problems is through the use of up-to-date GIS technology.

 Its practical value is through its capacity to model aligned wire-frame geoinformation models of caved, developed blocks and blocks under development in the Annensk mining area of the Zhezkazgan field using the Gemcom Surpac software as well as the utilisation of the optimied geoinformation models to support recommendations regarding further feasibility and development of the remaining indicated and inferred reserves in the movement trough of the caved in area.

The innovation for the development is through method and new options of the resource-replenishing technology this creates for continued mining of the deposits, ensuring the rational and integrated development of the subsoil and the method of calculation of technical and economic efficiency along with the drawing up of the process regulation for its application in complicated geomechanical conditions at the repeated underground mining of similar reserves of copper fields undermined using the room and pillar system.

In conclusion, it is worth noting that in the current market conditions there is a need for new non-traditional methods of mining and development of reserves with minimum ore losses.

As such, the results of the research will be focused on drawing up the process regulation for the development and application of these new methods for accessing previously ‘lost’ ore, increasing the resource base, and extending mine life through:

– Proposed process solutions, through the mining of indicated and inferred reserves in complicated mine technical and geomechanical conditions of the Zhezkazgan field undermined using the room and pillar system in mining the indicated and inferred reserves of the copper field;

– Economic efficiency via the extraction of the remained indicated and inferred reserves with repeated mining from the caved in regions of the Zhezkazgan field, with 65-70% level of extraction instead of writing it off as lost ores; 

– The wider results of the research and their impact on the development of the science and technology contributing to the development of the research of the scientific applied problems of the mining science at the repeated underground mining of indicated and inferred reserves as well as the possibility of their application in complicated mine technical and geomechanical conditions thanks to the example of the Zhezkazgan underground mines.

An added social effect is the fact that at the involvement of indicated and inferred reserves the proposed technology ensures long-term employment for workers and engineers of the Zhezkazgan mines.

 

D.K.Bekbergenov is a candidate of technical sciences, Head of the “Integrated Subsoil Development” Laboratory at the D.A.Kunayev Mining Institute, in Almaty in the Republic of Kazakhstan; G.K.Jangulova s a candidate of technical sciences, Head of the “Integrated Subsoil Development” Laboratory at the D.A.Kunayev Mining Institute, in Almaty in the Republic of Kazakhstan, and Kassymkanova Kh.M. is doctor of technical sciences, professor of the "Cartography and Geoinformation" Department at the KazNU, in Almaty in the Republic of Kazakhstan.