The goal of this study is to improve the analysis of the stability of pillars and to answer the question of the choice of the pillar strength using an empirical approach. The pillar strength depends mainly on two factors: the uniaxial compression strength and the pillar geometry. We have shown in this study the importance of geometry of pillars by comparing calculations with and without the effect of W/H ratio (slenderness). The sensitivity of the parametric study based on the empirical relationships converges to the same resistance of the pillar despite the difference between the coefficients of these empirical relationships. The methodology was applied to case studies (France). The application of these relationships can help to explain the collapse of Clamart mine (Paris). Generally, and in the absence of data, it will certainly be more prudent to use the relationships available taking into account the effect of uncertainty.
The room and pillar mining method is conventionally used for shallow mines. The method consists in practice to create rooms separated by small parts of the deposit, deliberately not exploited (pillars). The depth is controlled by the deposit; the extraction ratio should be selected according the bearing capacity of pillars. The pillar function is to ensure local and overall stability of the surface and underground mine. The stability of mines should be checked for short-term, long-term, local (isolated pillar) and large scale.
Several methods (approaches) are used to design the pillars, from the simplest to the most sophisticated one. The design is generally based on empirical methods from lessons learned, sometimes with dramatic consequences due to large collapse. Geologists and engineers nowadays have more scientific approaches to assess the stability of these structures. They are based on analytical and numerical calculations, sometimes in 3D and use sophisticated models with numerous parameters used to evaluate the short and long term pillar behavior.