Codelco Norte Division, CODELCO, Chile
A. Karzulovic & Assoc. Ltd., Santiago, Chile
Edwin T Brown
Julius Kruttschnitt Mineral Research Centre, University of Queensland, Australia
The benchmarking exercise carried out as part of the International Caving Study Stage II(ICS-II) and reported in a companion paper, concluded that there are currently no geotechnical tools available with which to define the likelihood of vertical caving propagating through ore columns having differing heights, stress fields and rock mass qualities. This paper presents the results of analyses of the effects of several aspects of geometry, stress field, and rock mass strength onvertical cave propagation. Using the results of a series of parametric two-dimensional numerical analyses, a Caving Propagation Factor is defined and design charts developed to estimate the likelihood of vertical cave propagation during the initial engineering stages of projects involving a transition from open pit to underground mining by caving methods.
The essential conceptin mining by block or panel caving is to take advantage of gravity by undercutting the base of the mineralized column to induce caving of that column. Therefore, the first geomechanical consideration must be to evaluate whether the rock mass to be mined will cave naturally under gravity alone, or if induced or augmented break-up of part or all of the column of mineralized rock will be required.After the feasibility of using a caving method of mining has been verified, the second geomechanical consideration must be to determine the size of the undercut required to initiate the caving process based on the rock mass quality and the stress field in the mining sector.
Current practice is that this determination is usually, but not invariably, based on empirical correlations between thegeotechnical quality of the rock mass, expressed in terms of the MRMR index (Laubscher 1993) and the hydraulic radius of the caved area, HR. However, this correlation must be used with caution and, preferably, as a basis for the development of a correlation adjusted to the local conditions at each mine.
Once the area required to initiate caving has been determined, the next step is to evaluatewhether the caving will propagate vertically to connect with the ground surface, or whether there is a risk that cave propagation will be arrested with the cave eventually failing suddenly, potentially creating an air blast. Some examples of operations that have experienced caving arrest are listed in Table 1.
|Table 1: Some Cases of Caving Arrest |
|Mine|Caving Arrest |T |Air |References |
| |Size | |Blast? | |
|Jenifer, |43 m ( 85 m |3 |Yes |Obert and Long |
|California, USA | | | |(1962) |
|Crestmore, |3,250 m2 |(?) |No(?) |Long and Obert |
|California, USA | || |(1958) |
|Ertsberg East, |75 m ( 120 m |(?) |No |Julin (1992) |
|Indonesia | | | | |
|Inca Oeste, |90 m ( 125 m |6 |Yes |de Nicola and |
|Salvador, Chile | | | |Fishwick (2000) |
|Northparkes, |195 m ( 180 m |23(?) |Yes |van Asand |
|Australia | | | |Jeffrey (2000) |
|Rio Blanco, |3 blocks of |6 |Yes |Godoy and |
|Andina, Chile |60 m ( 60 m | | |Carpenter (1973) |
|Ten 4 Sur D, |75 m ( 20 m |3 |No |Pasten and Cuevas|
|Central Sector, | | | |(1999) |