The application of grain-based distinct element modelling technique is investigated, with the aim of improving prediction of the extent and nature of the excavation induced damage zone (EDZ), a critical part of the design process and safety assessment for deep geological repositories (DGR) for nuclear waste. Relative calibration requirements for the micro-mechanical properties are outlined to generate macro-response in laboratory test settings and near excavation walls. The primary focus here is to adapt the phenomenological result of the role for friction mobilization in small-scale models to obtain proper in situ properties to be used in large-scale tunnel models. For this study, the in situ strength envelope of Cobourg limestone is obtained by employing triaxial compressive tests. Brittle failure mechanism around a circular tunnel in massive rockmass is then investigated and guidelines are suggested for delineating EDZs. A modified longitudinal displacement profile (LDP) approach is proposed for 2D analysis of brittle failure mode.

1 Introduction

As mining and tunneling operations are advancing deeper into high stress regimes in hard rock, the problem of brittle failure becomes a significant issue. In such challenging projects like deep geological repositories (DGR) for nuclear waste disposal in crystalline rocks, it is of great importance to develop tools to aid in the simulation and prediction of the excavation damage zone (EDZ). The concept of EDZ zonation was first conceived for the purpose of underground nuclear waste repositories in the early 1980's (Kelsall et al. 1984) and further developed in recent works (Siren et al 2015, Perras and Diederichs 2016). The zonation was done based on the fact that density and connectivity of fractures decrease moving radially outwards from the surface of the excavation. There are three zones associated with stress induced damage as illustrated in Figure 1. The nearest zone to the excavation is the highly damaged zone (HDZ) dominated by interconnected visible macro-fractures. Moving outwards, the inner EDZi, with connected but isolated micro-damage, makes a gradual transition to the outer EDZo, with only partially connected to confined micro-damage. Beyond the EDZs is a stress and/or strain influence zone that involves only elastic change, the excavation influence zone (EIZ).

This content is only available via PDF.
You can access this article if you purchase or spend a download.