CLASSIFICATION OF UNDERGROUND MINING SITES BY MEANS OF A RISK INDEX

This abstract describes the first results of a research program aimed at defining a new risk index to classify areas near abandoned underground mines. The index represents the level of geomechanical instability phenomena forecast. As known, such phenomena are typically referable to discontinuous subsidence, like sinkholes, chimney caving, crown holes, pillar collapse, plug subsidence. To define the risk index, the results of a previous research, that has identified about three thousand mines producing between 1870 and 2004, were considered. The risk analysis is based on assessments resulting from the knowledge of many parameters derived from information on mining methods adopted for each mineralization, position and geometry of stopes and mechanical characteristics of orebody, surrounding country rock and overburden. In many cases (i.e. exhausted mines for many years) documentation is often not available or totally lost. As a consequence, it was impossible, for all three thousand mines, to carry out the risk analysis based on the evaluation of potential instability phenomena induced by underground cavities because of partial or total lack of main data for most of them. Taking into account that open pits and mines which are still working or converted into museum and so reclaimed, are excluded from this research, only 819 mines have been studied from the point of view of instability risk. On the basis of the risk index, a classification of identified sites is suggested by this research in order to select areas to keep under control. In applying this classification system, the sites are divided into three risk classes (high, medium, low risk). Due to the large number of considered mines, the present research program plans to control only high risk ones. The proposed index, RI, is obtained from six coefficients, CFi, depending on following parameters: depth and width of underground openings, duration of exploitation and time from site’s abandonment. &#61656; CF1 is correlated to depth cavities and can take two values: o 1,2 for mines that started near the surface and over the years evolved into underground; o 1,0 for deeper underground mines. &#61656; CF2 represents the rheological effects and is related to the time of site’s abandonment (Tabb, in years) as follows: It is assumed that the relationship between CF2 and Tabb is exponential and the rheological effects are maximum fifteen years after site’s abandonment (CF2 = 1 for Tabb = 15). &#61656; CF3 depends on the exploitation’s duration (Tc, in years) and on the mining claim’s extension (E, in 104 m2) according to the following relation: The use of logarithm is necessary because of not complete reliability of data. &#61656; CF4 can take two values (1,5 or 1,0). The highest score (1,5) is assigned to salt mining sites (typically halite and potassic salts) because instability phenomena often occurred in this kind of mines (for example, Timpa del Salto mine in Calabria). For the other mineral deposits the score is equal to 1,0. &#61656; CF5 considers the geometric configuration of orebody. It is assumed: o CF5 = 1,2 for tabular deposits; o CF5 = 1,0 in other cases. &#61656; CF6 is related to population density (DP) as follows: DP is the weighted mean of: o population density of the municipality including the mine (weight 0,75); o average density population of neighboring municipalities (weight 0,25). Furthermore, the logarithmic scale is used to make up for the variability of DP from one situation to another. By imposing a range of variation from 1 to 100 and a linear relationship, on logarithmic scale, between standardized and absolute values, Standardized Risk Index (SRI) values of sites are determined with the following relation: The risk levels are defined as follows: • low (SRI &lt; 10); • medium (10 < SRI &lt; 70); • high (SRI > 70). According to the proposed classification, it results that 105 sites (high risk sites) should be keep under control.