Abstract

Fracture propagation in brittle rock is very fast and highly dynamic. Typically this process consists of fracture initiation, propagation and termination. Growth of micro-fractures is conceptually and numerically well established, however, current practices to model fracture propagation in rock employs slow evolving static regimes that do not represent the true nature of fracture propagation in the laboratory or the field. This paper presents a newly developed numerical approach using Micro-Brittle Dynamics theory to model the propagation of fractures through rock in real time. This work presented here is based on a newly developed Dynamic Rock Fracture Model (DRFM2D) and validated against laboratory experiments.

1. Introduction

Current advances in computing technology have enabled solutions of many complex geotechnical problems. However when a presence of large fractures dominate strata behaviour, these solutions may not be correct. Available numerical software can be generally divided into three groups: Elastic, Inelastic Time-Dependent Implicit and Inelastic Time-Dependent Explicit. These are briefly discussed in Table 1.

Whilst available Inelastic Time-Dependent Explicit Methods are currently used to simulate ground failure, they are in most cases not employed correctly. Most researchers will use the Quasi-Static formulation and slow evolving material damage to resolve highly dynamic fracture phenomena, consequently they are not capable of capturing true real time dynamic brittle effects occurring within the rock mass.

New development of a 2-dimensional dynamic rock fracture model (DRFM2D) in FLAC2D [1] challenges the use of numerical methods, demonstrating superior computational efficiencies towards accurately modelling the dynamic brittle fracture propagation through rock in real time. Our currently developed DRFM2D model is focused on the practical application of fracture dynamics in engineering. An overview of the natural physical processes and the governing constitutive laws is outlined here.

2. Micro-Brittle Dynamics (MBD)
2.1. Micro-Scale Dynamic Response

The dynamic action of the smallest finite mass in a continuum model represents the discrete dynamic response of a single element. In general, mechanical stress-strain response of any brittle or ductile rock past the elastic limit can follow one of four behaviours; Strain Hardening, Elastic-Plastic, Strain Softening or Perfectly Brittle. These rock failure types are illustrated in Fig. 1a. In Micro-Brittle Dynamics (MBD), where brittle materials are unable to absorb plastic deformation energy prior to fracture, the dynamic micro-scale interactions exhibit an ideal brittle failure response. This occurs at the elastic strength limit taking the brittle failure path curve in time as illustrated in Fig. 1b.

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