The effect of microstructure on the behavior of an underground excavation

• Historical limestone underground quarry. • Panoramic photos with high-resolution X Phase Pro S2 360, 3D camera. • Low tensile strength marl limestone. • Micromechanical model using Linear Elastic Fracture Mechanics theory. • Flac2d numerical approximation of the room and pillar method. The room-and-pillar mining method is an ancient technique used in the extraction of underground deposits. Even today, it remains one of the most widespread and productive methods of underground mining, where the abandoned pillars ensure the stability of the opening. The mechanical behavior of the pillar plays an important role in its sizing and the estimation of ore recovery. In this paper, the rock material from a historical ancient underground quarry in Gortyna, Crete, Greece, was numerically investigated. This material, composed of limestone, that exhibits pronounced layering and is very soft, allowed the ancient Romans to excavate underground tunnels with a total length of >2.5 km. A high-resolution 3D X Phase Pro S2 360 camera, was used to capture panoramic photos in both horizontal and vertical planes. These photos were used to estimate the dimensions of the chambers and the central pillar of the accessible underground area of the site (the large underground quarry has been closed). The effect of micro-cracking of the rock on the behavior of the room and pillar excavation method was investigated using Linear Elastic Fracture Mechanics (LEFM) theory. For this purpose, a numerical model was created using the FLAC2D software to estimate the effective elastic parameters of the material based on its density and the arrangement of cracks within it. This analysis revealed that the presence of microcracks introduces an anisotropy similar to that of a transversely isotropic material. The observed anisotropy results in larger deformations in the main chamber crown than those obtained using average elastic properties from experimental, literature, and field investigations of limestone rock.