Anisotropic behaviour of stratified rock masses in tunnelling

Abstract

This paper investigates tunnel excavation through stratified rock masses from the engineering geological behaviour to the rock mass properties quantification and finally to the study of tunnel response, based on numerical analyses results. Initially the spectrum of the engineering geological behaviour of stratified rock masses in tunnelling is delimited and the critical failure mechanisms according to rock mass structure are described. Rock mass simulation as an equivalent isotropic geomaterial through the widely used characterisation systems in most cases cannot lead to a realistic prediction of the distribution and the values of total displacements. In addition, the complete and accurate simulation of all discontinuities networks involves high uncertainty. Therefore in the numerical analyses carried out, based on an already applied approach, the stratification planes, which contain less uncertainty than the secondary discontinuities and affect significantly the behaviour of tunnel, were simulated as separate elements and the rock mass parts between them as an isotropic material. Additionally, using simple rock mechanics principles, an approach for the quantification of the rock mass properties involved in the analyses is described, which tries to obtain the equivalence between the stratified rock mass and the sum of the distinct rock mass elements (stratification planes and internal rock mass). The numerical analyses depict the mechanism of convergence development in stratified rock masses and the differences between isotropic, anisotropic and transversally isotropic approaches are clearly demonstrated. Based on the results of the numerical analyses the incorporation of the stratification planes leads to an increase of the convergence mainly due to the bending of the rock mass strata where the stratification is tangential to the tunnel section. This increase depends on the GSI value of the reference rock mass and the discontinuities surface conditions.