Time-Dependency and Long-Term Strength of Rocks - Limitations, Interpretation, and Application in Finite Difference Continuum Models

Abstract

Many constitutive models have been developed to aid the engineer or scientist in the analysis of material yielding and deformations in the field of rock mechanics. Many of the classical constitutive formulations used are based on instantaneous shearing, or block rotation, but special considerations need to be made when ductile or brittle time-dependent instabilities might arise. These time-dependent effects lead to an accumulation of strains, or even delayed rupture, that cannot be captured using classical constitutive models. The research in this thesis provides an overview of several constitutive models developed for the analysis of time-dependent deformations both at the laboratory and excavation scale. This thesis explores several aspects of time-dependent creep and rupture behaviour for ductile and brittle rocks, respectively. Emphasis is placed on how the different creep material models behave in a numerical model as well as exploring their respective limitations and the effect of boundary and loading conditions on their deformation behaviour. This thesis also further develops the understanding and scientific approaches taken to the testing and interpretation of long-term strength data for brittle rocks. A new proposed methodology and error analysis technique is proposed for analyzing and curve fitting time-to-failure data based on a lower boundary strength limit related to the crack initiation strength threshold. Observations made about the limitations of constitutive creep models and time-to-failure data are used to employ a new numerical model applied at the laboratory and tunnel scale. The model is shown to be able to effectively account for confinement effects in brittle creep as well as capture a pseudo-tertiary creep stage based on simple curve fitting techniques. The work presented in this thesis advances the understanding of the applicability of time-dependent constitutive models as well as provides the necessary framework for practical numerical implementation when analyzing brittle rock failure at the tunnel scale, an important factor for future deep geological repository selection.