A new approach to obtain tunnel support pressure for polyaxial state of stress

Nowadays the numerical approaches to the tunnels designing are widely integrated to the traditional technologies (Empirical, Analytical and Observational Approaches). The numerical suites have, in fact, the advantage of an intrinsic simplicity of use and the ability to solve problems that, because of the complexity and plurality of the factors at play, cannot be easily managed through the analytical and empirical methodologies.Unfortunately the numerical software applications, commercially available for geo-mechanical purposes, have the limitation of using only the most famous constitutive models. This study sets the target of extending the numerical applicability of a constitutive model different from the commonly used one to increment the choice of model codes available for geotechnical numerical suites.In this paper a general methodology to extend the applicability of the Polyaxial Strength Criterion, introduced by Singh et al., to any numerical application is explained. The present procedure does not require any specific compilation of numerical constitutive model and takes advantage of the bi-dimensional Mohr-Coulomb model already present in every numerical suite.The Polyaxial Strength Criterion is a tri-dimensional constitutive model, introduced in 1998 for the analysis of severe squeezing in underground excavations. The present approach has shown high coefficients of correlation with the observations in many cases of tunnels in high squeezing conditions in Himalayan region.

► A new approach to analyse underground openings using polyaxial state of stress is proposed. ► The effect of intermediate principal stress is included in the Mohr-Coulomb Theory. ► The modification is suggested without any alteration in the original failure criterion. ► The suggested method can be easily implemented in many tunnel support analysis.