One of the most relevant problems when modeling rock masses derives from the fact that rock masses are natural materials, so their physical and mechanical features have to be assessed with the help of non-straight- forward experimental procedures and cannot be a priori accurately defined as it can be the case for other construction materials. Rock masses are discontinuous, heterogeneous, anisotropic, non-elastic, scale-affected, complex materials. In the 80s Hoek & Brown proposed and later refined a procedure to homogenize rock mass behavior and derive equivalent continua properties of rock masses, but this approach is not suitable in all cases. The choice to model a rock mass as continuous or discontinuous is usually made in the light of the jointing of the rock mass, for instance comparing the average joint spacing or blocksize in the rock mass to the span of the underground excavation. To further study the appropriateness of the approach in this work we have carried out a comparison of continuous and discontinuous models of tunnels in 6 types of rock masses of growing geotechnical quality by means of standard codes FLAC and UDEC, following the development by Ferrero et al. (2004). In these models stresses and displacement distribution have been compared to analyses the representativeness of the derived stress-strain behavior around excavations by means of continuous and discontinuous approaches.

1. Introduction

Rock masses can be often considered as a discrete medium due to the pervasive occurrence of joints and other of discontinuities of different nature and origin. Discontinuities usually control the mechanical behavior of rock masses, particularly in comparison to most other engineering materials; so they could largely influence the choice of modelling scheme to be adopted for any design approach. Discontinuities can be considered in an explicit way by adopting a discontinuous approach or in an implicit way by using an equivalent continuous approach (Jing 2003).

Among the methods involving a continuous medium scheme, and with specific reference to domain methods, mostly Finite Element Method (FEM) and Finite Difference Methods (FDM) are applied. Within these methods, the discontinuity presence is considered in an implicit way (equivalent continuum).

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