New Approach to the Design of Tunnels in Squeezing Ground

Abstract

The study of the mechanical behavior of discontinuous rock masses is a rapidly growing subject. Much research has been reported in the literature concerning the estimation of the tunnel support pressures incorporating the real behavior of a rock mass using different types of constitutive models and rock parameters. These studies have favored the introduction of various analytical solutions for the easier geomechanical cases but have not been converted into numerical models suited for commercially available software and therefore could not be used in real and more complex engineering applications. The notable constitutive models used in common practice in rock mechanics are the Mohr-Coulomb and Hoek-Brown failure criteria. Most of the commonly available software supports only these two failure criteria. The difficulties faced in possibly implementing a new constitutive model into the numerical suites already available are so large that the studies using innovative constitutive models result in a mere academic exercise. In this paper, a new methodology is described for the application of the polyaxial constitutive model, which is characterized by the direct influence of all the principal stresses in the resistance of a rock mass. The methodology applied in this research uses the format of the Mohr-Coulomb model using the equivalent angle of friction and cohesion of the rock mass surrounding the tunnel. The equivalent parameters of rock mass resistance, which are directly derived from the common Mohr-Coulomb parameters, are also influenced by the intermediate principal stress σ2, in accordance with what was suggested in the polyaxial strength criterion, and by the approach chosen for quantifying the uniaxial compressive strength σcr of the rock mass. To demonstrate the practical applicability of this constitutive model, it has been used to predict the squeezing of rock observed and measured at three different instrumented sections of a tunnel.