Abstract

Overbreak can be a significant challenge in tunnelling and is usually assessed on a project-by-project basis. This paper attempts to quantify tunnel overbreak in jointed rock masses by producing a predictive model that practitioners can utilize at the preliminary design stage. Discontinuum numerical analysis of a circular-shaped tunnel is performed using the combined finite difference – discrete element method to generate a database of rock masses prone to overbreak. The Q-system, rock mass quality index, is used to classify the rock masses that are expected to overbreak in various tunnelling environments. The results are scrutinized to identify trends between overbreak and: i) Q, ii) average joint spacing and, iii) in-situ stress conditions. It is observed that overbreak increases when Q ≤ 4 and is uniform when Q > 4. A predictive model is generated through multiple regression analysis and can be used to identify and highlight the maximum likely overbreak or scenarios where overbreak is expected to occur.

Introduction

The need to understand and quantify overbreak in tunnels and underground excavations is of significant importance to engineering projects. Overbreak has been seen to pose a substantial problem during the excavation of hard rock tunnels and caverns, leading to unsound designs at the preliminary stages of the design primarily due to the heightened likelihood of cost overruns, increased construction times, reduced safety and stability, and poorer performance [1]. Hence, it is becoming increasingly important to quantify the overbreak expected during the construction of an underground excavation to account for this during design. Although there have been numerous studies considering the formation, causes, and effects of tunnel overbreak, there has yet to be much significant focus on the relationship between overbreak and a commonly used and relatively simple classification system, such as the Q-system.

Understanding the rock mass behaviour is particularly important in tunnel design to confirm that the parameters that define the rock and the discontinuities are appropriate while ensuring sufficient support requirements. A rock mass behaviour is typically described by the behaviour of the intact rock and the discontinuities. Geological descriptions describing the rock mass behaviour are commonly qualitative; however, for design and support, quantitative data must be input into numerical models.

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