Deterministic and probabilistic numerical modelling of fractured rock masses of underground exploitations

Abstract : The study of the mechanical behaviour of a rock mass can be performed by considering as a discontinuous medium composed by rock blocks isolated by rock discontinuities. The mechanical behaviour of the global system is due to the local movements of blocks subjected to a certain state of stress and ruled by the rock mass mechanical features. Mechanical features of the rock mass are determined by the mechanical and the geometrical features of the rock matrix and rock joints that can be directly measured by experimental tests and in situ surveys. Laboratory and in situ measurements can be affected by scale effects, and in situ surveys can be carried on only on exposed rock faces. Consequently the mechanical behaviour must be calibrated by the comparison with in situ measurements. In a discontinuous medium, the localisation of the measurement devices strongly influences the measurement results and consequently a deep knowledge of the rock mass structure is required. For this reason a deterministic reconstruction of the rock mass that takes the real joint localisation into account represents the best possibility to obtain numerical previsions comparable with the in situ measurements whilst, the more common practice to reconstruct the rock mass on a statistical basis, can determine local behaviour in disagreement with the monitored one. A comparison between a deterministic and probabilistic rock mass tessellation is described in the paper together with the consequent discontinuous models. Geometrical rock mass models is carried on by means of a specific code (RESOBLOK) whilst the mechanical rock mass behaviour is studied by a 3D code (3DEC). The paper reports an application to a rock mass excavated for the extraction of blocks utilised in the ornamental stone industry.