ABSTRACT
This study investigates how the stability of the roof cavity dug in the Neapolitan Yellow Tuff is influenced by the spatial variability of the rock uniaxial compression strength and by the presence of horizontal joints. The geometrical schemes and the loading condition analyzed by Evangelista et al. (2000) were reproduced in the 2D finite difference Code FLAC to calculate the factor of safety. The variability of the resistance was introduced by assigning a Mohr-Coulomb constitutive model to the rock in which the friction angle is constant, and the cohesion is defined by a Gaussian distribution calibrated on the data of several uniaxial compression tests performed on tuff. Following the Monte Carlo simulation, fifty analyses were performed on each geometrical scheme, in which the cohesion was assigned randomly to the mesh elements. The presence of the discontinuities was introduced by modelling a 0.5 m thick horizontal layer in the roof with a resistance reduced with respect to that of the intact rock according to the Barton (1977) model for the rock joints. The analysis was repeated by changing the position of the less resistant layer along the roof thickness. The results allowed to generate stability curves expressed as a function of the cavity width normalized to the roof thickness and on the ratio between the rock uniaxial compression strength and the acting vertical stress. The variability of the rock strength and the presence of joints reduce the safety of the roof cavity. Such effect is more pronounced as the position of the discontinuity approaches the bottom of the roof.
