1 Introduction

Understanding progressive rock failure and monitoring the stability of underground excavation zones, such as tunnels and underground mines, are critical to their operational safety. Many tunnels and underground structures are constructed under high-stress conditions, and stress redistribution may occur during and after the excavation. Stress redistribution generates energy imbalance in the rock mass, resulting in damage of the rock that impact the stability of underground structures (Chang and Lee 2004). Therefore, evaluating the progressive failure and damage mechanisms of rock at different stress levels are of great importance to geo-hazard assessment and operational safety of underground structures.

In this study, we address this problem using an experiment at the laboratory scale, coupled with ultrasonic tomography (UT) and numerical simulation. A time lapse two-dimensional (2D) UT observation was conducted on a granite slab under uniaxial compression. This test was then reproduced using the combined finite-discrete element method (FDEM). The entire deformation and failure processes were studied using this combination of technologies at the macroscopic and microscopic scales.

2 Material and method
2.1 Sample and compression test

The rock sample investigated in this study was a coarse-grained granite (Fangshan granite) slab 220 mm long, 110 mm wide, and 30 mm thick. This granite consisted of three main mineral phases: Feldspar (67%), Quartz (23%) and Biotite (10%), with an average grain size of 2.6 mm. The sample was tested by applying a uniaxial stress and a UT survey was conducted initially (0 MPa) and at intervals of 20 MPa applied stress.

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