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Project Details
Funding Scheme : General Research Fund
Project Number : 17201419
Project Title(English) : 3D cracking behavior of rocks under true triaxial stress conditions: Mechanistic modeling and investigations 
Project Title(Chinese) : 岩⽯於真三軸加載下的三維斷裂⾏為：基於機理的模擬與研究 
Principal Investigator(English) : Prof CHOI, Clarence Edward 
Principal Investigator(Chinese) :  
Department : Department of Civil Engineering
Institution : The University of Hong Kong
E-mail Address : cechoi@hku.hk 
Tel : 63880285 
Co - Investigator(s) :
Dr Choo, Jinhyun
Prof Wong, Louis Ngai Yuen
Prof Wong, Teng-fong
Panel : Engineering
Subject Area : Civil Engineering, Surveying, Building & Construction
Exercise Year : 2019 / 20
Fund Approved : 748,300
Project Status : Completed
Completion Date : 31-12-2022
Project Objectives :
To develop a mechanistic computational model that can reliably and efficiently simulate tensile and shear fractures in 3D rocks
To validate the newly developed model of rock fracture using experimental data
To elucidate cracking processes and mechanisms in 3D rocks under a variety of true triaxial stress conditions
Abstract as per original application
(English/Chinese):
Realisation of objectives: The three objectives of the project have been fully achieved. First, we have developed a novel mechanistic model that can simulate mixed-mode (tensile and shear) fractures in 3D rocks. At the core of this work is the double-phase-field method, which allows one to simulate and naturally distinguish between complex tensile and shear fractures in rocks. Second, we have validated the newly developed model of rock fracture using an extensive set of laboratory test data from Co-I Wong and other studies in the literature. The validation shows that the model can well reproduce rock fracturing processes, in both qualitative and quantitative manners. Lastly, using the newly developed model, we have investigated how cracking processes and mechanisms in 3D rocks under a variety of true triaxial conditions.
Summary of objectives addressed:
Objectives Addressed Percentage achieved
1.To develop a mechanistic computational model that can reliably and efficiently simulate both tensile and shear fractures in 3D rocks.Yes100%
2.To validate the newly developed model of rock fracture using experimental dataYes100%
3.To elucidate cracking processes and mechanisms in 3D rocks under a variety of true triaxial stress conditionsYes100%
Research Outcome
Major findings and research outcome: The project has produced two main outcomes: (i) a technology for computer simulation of 3D rock fracturing processes in digital rock specimens, and (ii) new physical insights into rock fracturing processes under true triaxial conditions. The first outcome is based on the double-phase-field method that the project team has developed (Fei and Choo, 2021). This method enables one to simulate the complex topology of 3D tensile and shear cracks without any algorithms. Furthermore, it allows one to distinguish between tensile and shear fractures without any tools, which is highly attractive for studying the nature of rock fractures. The second outcome has been attained using the developed simulation technology. We have systematically investigated how the intermediate principal stress controls the 3D cracking behavior of rock specimens with preexisting flaws (Sun et al., 2023). Our results show that both the orientation and magnitude of the intermediate principal stress exert dominant control over the cracking pattern and peak stress. Also, the effects of the intermediate principal stress magnitude manifest differently depending on the orientation of principal stresses with respect to the flaw. In addition, we have investigated energetic size effects on the compressive strength and cracking behavior of flawed rocks through high-fidelity simulations of mixed-mode fracture in quasi-brittle materials (Choo et al., 2023). We have found strong size effects on both the uniaxial compressive strength and cracking patterns. These findings of this work provide important insights into how the intermediate principal stress controls rock fracturing processes and how they manifest differently across scales. In terms of publications, we have produced 7 journal papers (6 published, 1 in preparation) and 1 conference paper.
Potential for further development of the research
and the proposed course of action:		 The research would be developed further as follows. First, the computational cost of the simulation methodology needs to be reduced dramatically. Even though the current methodology allows one to simulate 3D rock fracturing processes reliably, its computational cost is quite high. To make the methodology more practical, it is necessary to decrease the computational cost significantly. Another line of important future work is to shed more light on the effects of the intermediate principal stress on 3D rock fracturing processes. Particularly, it is recommended that one studies how the intermediate principal stress controls rock fracture in real-world 3D rock masses at the field scale.
Layman's Summary of
Completion Report:		 Crack initiation, growth, and coalescence in rock masses are responsible for the majority of failures of natural and engineered rock systems. Therefore, many rock engineering problems require us to analyze, predict, and control these rock fracture processes. Numerous studies have investigated the mechanisms and processes of cracking from pre-existing flaws (discontinuities) in rock masses. However, under true triaxial stress conditions, which are typical of rocks in the field, very little is understood about the mechanisms and processes of 3D fracturing. This knowledge gap is rooted mainly in our inability to characterize 3D rock fracturing processes under true triaxial stress conditions. To tackle this limitation, this project has developed a new technology for computer simulation of 3D fracturing processes in digital rocks. Using the new simulation technology, the project has produced new physical insights into 3D rock fracturing processes under a variety of true triaxial stress conditions. The outcomes will help address many rock engineering problems including the stability of rock caverns.
Research Output
Peer-reviewed journal publication(s)
arising directly from this research project :
(* denotes the corresponding author)
Year of
Publication		 Author(s) Title and Journal/Book Accessible from Institution Repository
2020 Fan Fei, Jinhyun Choo*  A phase-field model of frictional shear fracture in geologic materials  No 
2021 Fan Fei, Jinhyun Choo*  Double-phase-field formulation for mixed-mode fracture in rocks  No 
2021 Jinhyun Choo*, Ammar Sohail, Fan Fei, Teng-fong Wong  Shear fracture energies of stiff clays and shales  No 
2022 Fan Fei, Jinhyun Choo*, Chong Liu, Joshua A. White  Phase-field modeling of rock fractures with roughness  No 
2022 Yupeng Jiang, Yidong Zhao, Clarence E. Choi*, Jinhyun Choo*  Hybrid continuum–discrete simulation of granular impact dynamics  No 
2023 Jinhyun Choo*, Yuan Sun, Fan Fei  Size effects on the strength and cracking behavior of flawed rocks under uniaxial compression: from laboratory scale to field scale  No 
Yuan Sun, Fan Fei, Louis Ngai Yuen Wong, Jinhyun Choo*  Intermediate principal stress effects on the 3D cracking behavior of flawed rocks under true triaxial compression  No 
Recognized international conference(s)
in which paper(s) related to this research
project was/were delivered :
Month/Year/City Title Conference Name
Houston (Online) Phase-field modeling of mixed-mode fracture in rocks with discontinuities: From laboratory scale to field scale  55th US Rock Mechanics/Geomechanics Symposium 
Other impact
(e.g. award of patents or prizes,
collaboration with other research institutions,
technology transfer, etc.):
  SCREEN ID: SCRRM00542