Abstract

A tunnel in the region of Toronto, Canada, situated within the Georgian Bay shale and limestone formation, was simulated using the hybrid finite-discrete element method (FDEM). The model was calibrated based on input parameters that adequately reproduced standardized laboratory rock tests. Through the simulations, various short-term excavation mechanisms were explored including the influence of rock support, rock mass heterogeneity and stress field anisotropy. Qualitatively, the models served as a method of quantifying risk in the case that unexpected ground conditions are encountered during construction. Using FDEM, there was value added to the analysis compared to finite element method (FEM) simulations. Fractures in the rock mass were simulated to determine the mode of failure, extent of fracturing and the influence it had on stress distribution. Excavation induced failure of the rock mass was dominated by shearing along bedding planes in the shale.

1.
Introduction

This work is based on a tunnel in Toronto, Canada planned for construction using an earth pressure balance (EPB) machine. Portions of the proposed alignment of the tunnel are to be constructed in the Georgian Bay formation, which consists dominantly of layered shale with thin strata of limestone. In this study, the integrity of the tunnel will be analyzed using a two-dimensional (2D) calibrated hybrid finite-discrete element method (FDEM) model. The main advantage of the numerical method is the ability to capture the transition of a continuum to a discontinuum; thus it is capable of simulating realistic deformation and fracturing of geomaterials (Mahabadi et al. 2012; Lisjak et al. 2014a, 2015). Various short-term excavation mechanisms were explored including the influence of rock support, rock mass heterogeneity and stress field anisotropy. As a result of simulating excavations with different variables, a quantification of risk associated with unexpected ground conditions can be made and discussed.

2.
Finite-Discrete Element Method (FDEM)

FDEM was initially developed by Munjiza et al. (1995) to overcome the limitations of continuum and discontinuum numerical modelling methods, namely finite element method (FEM) and discrete element method (DEM), respectively. FDEM combines both numerical methods to capture the progressive failure and/or damage processes of a continuum, thus transitioning the body to a discontinuum. The method is based on micromechanical models, and thus macroscale observations are emergent behaviour.

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