Strapping of pillars with cables to enhance pillar stability

Pillar design for underground mining is typically done using empirical formulae or numerical modelling. Practical experience and recent literature, nevertheless, illustrate the shortcomings of these design approaches. Ongoing monitoring of pillars is therefore recommended to minimize the risk associated with these designs. In a mine where a large number of pillars are cut, different pillar strengths can be expected owing to variations in rock mass strength from area to area. This problem is compounded by the fact that in mining environments the pillars are not always cut according to the prescribed dimensions. Although the original design using either empirical methods or modelling may predict stable pillars, unstable pillars will be encountered in reality. Very conservative designs with large factors of safety may circumvent this problem, but this approach is uneconomical. A possible solution to localized stability problems may be to enhance the strength of a few unstable pillars. It may even be hypothesized that reinforcing a few critical pillars may prevent ‘pillar runs’ on a much larger scale. Rockbolting, strapping of pillars, and pillar shotcreting have occasionally been used in the past as possible solutions. It appears that these have not been successful in all cases and large collapses have occurred in spite of the pillar remedial work. The authors investigated the strapping of pillars by conducting laboratory tests on cabled rock specimens. The results were qualitatively compared to actual attempts of pillar reinforcement available in the literature, as well as additional observations in an old haematite roomand- pillar mine in Spain. Based on these results, the value of steel cabling and mesh wrapped around pillars to improve stability is demonstrated. Some cases where this approach will not be successful are also discussed.The Spanish Ministry of the Economy and Competitiveness for partial funding, awarded under Contract Reference No. BIA2014-53368-P, partially financed by ERDF funds from the EU.http://www.saimm.co.za/journal-papersam2017Mining Engineerin

In situ stress measurement near faults and interpretation by means of discrete element modelling

The design of underground caverns of significant size and of complex geometry is often accompanied by the use of numerical models able to estimate the range of stresses and strains induced by the excavations and, consequently to evaluate the stability of the caverns. In this context, it is convenient to estimate the natural state of stress of the rock mass, the mechanical properties of the rock matrix and the discontinuities and to carry out in situ measurements to calibrate the model. In the Carrara basin (Italy) there is a huge number of underground excavations that need to be monitored and, for this purpose, a series of models and in situ measurements have been performed. This study aims to point out the difficulties involved in understanding the in-site measured stress. A series of tests were conducted in Carrara, in an underground marble quarry site, focusing to study the complex tectonic environment in the area. A series of comparisons between 3D DEM numerical models has been performed in order to understand the influence of the presence of the main faults on the in situ state of stress. Some final comments regarding the variability of stress fields in faulted rock masses are provided