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A modeling study of the micro-cracking behavior of brittle crystalline rocks under uniaxial loading considering different scenarios of grain-based models

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ARABIAN JOURNAL OF GEOSCIENCES 2021年第4期14卷 1-22页

作者： Wang, Song Zhou, Jian Zhang, Luqing Han, Zhenhua Zhang, Fenxiang Chinese Acad Sci Inst Geol & Geophys Key Lab Shale Gas & Geoengn Beijing 100029 Peoples R China Univ Chinese Acad Sci Coll Earth & Planetary Sci Beijing 100049 Peoples R China Chinese Acad Sci Innovat Acad Earth Sci Beijing 100029 Peoples R China Beijing Univ Technol Key Lab Urban Secur & Disaster Engn Minist Educ Beijing 100124 Peoples R China BGI Engn Consultants Ltd Beijing 100038 Peoples R China

The grain-based model (GBM) in two-dimensional particle flow code (PFC2D) is widely used to simulate the deformation and brittle failure of crystalline rocks. However, the original GBM consisting of the parallel-bond (PB) and the smooth-joint (SJ) produces unrealistic micro-cracking processes in which excessive microcracks are distributed along the interfaces of mineral grains under uniaxial loading. To solve the problem, this research presents a discussion of the mechanical behavior of six GBMs composed of different contact models including PB, SJ, and flat-joint (FJ). After the uniaxial compressive and direct tensile tests of numerical granites, it is found that the different contact types in GBMs have a limited influence on the tensile strength (TS), elastic modulus (E), and Poisson's ratio (nu) but have a great impact on the uniaxial compressive strength (UCS) and microcrack pattern. The UCS calculated by the PB/FJ GBM is higher than the actual value. Microcrack distributions simulated by the FJ/SJ and FJ/PB GBMs show that numerous mineral grains are cut across under uniaxial loading instead of their interfaces. While the FJ/SJ and FJ/PB GBMs may well reproduce the brittle failure process of crystalline rocks, it remains to be seen whether they will be applicable to other rock mechanics issues, including triaxial compression, thermal cracking, and hydraulic fracturing.

关键词： Grain-based model particle flow code Numerical modeling Crystalline rock Micro-cracking evolution

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A three-dimensional grain-based model for studying the microscopic fracture behaviour of granite

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COMPUTERS AND GEOTECHNICS 2023年 159卷

作者： Quan, Junsong Rong, Guan Xu, Lida Chen, Zhiheng Wuhan Univ State Key Lab Water Resources & Hydropower Engn S Wuhan 430072 Peoples R China Wuhan Univ Key Lab Rock Mech Hydraul Struct Engn Minist Educ Wuhan 430072 Peoples R China

Grain-based models (GBMs) are widely used to analyse the micromechanical characteristics of granite. However, the classical two-dimensional (2D)-GBM has some shortcomings: poor matching of mineral composition and grain size distribution to the rock, small number of intragranular microcracks, and inability to represent the three-dimensional (3D) structure of the grains. Therefore, we propose a new method to establish a 3D-GBM in the particle flow code (PFC). The PFC3D-GBM parameters are obtained by calibrating the experimental data, such as the elastic modulus, Poisson's ratio, tensile strength, compressive strength, Hoek-Brown (HB) strength criterion, and fracture characteristics. The fracture mode of granite is analysed on the basis of the experimental and simulation results. In addition, the fracture characteristics and microscopic fracture mechanisms of granite are analysed from the perspective of mineralogy. The results reveal that the mineral composition and the grain size distribution of PFC3D-GBM match well with those of the rock, and the model can simulate the macroscopic and microscopic fracture characteristics of granite. This provides an effective way to study the relationship between microscopic parameters and macroscopic behaviours in a 3D model.

关键词： Granite Grain-based model Three-dimensional Microscopic fracture mechanism particle flow code

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Numerical investigation of interactions between hydraulic and pre-existing opening-mode fractures in crystalline rocks based on a hydro-grain-based model

GEOENERGY SCIENCE AND ENGINEERING

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GEOENERGY SCIENCE AND ENGINEERING 2023年 229卷

作者： Wang, Song Zhou, Jian Zhang, Luqing Han, Zhenhua Kong, Yanlong Beijing Univ Technol Minist Educ Key Lab Urban Secur & Disaster Engn Beijing 100124 Peoples R China Chinese Acad Sci Inst Geol & Geophys Key Lab Shale Gas & Geoengn Beijing 100029 Peoples R China Univ Chinese Acad Sci Coll Earth & Planetary Sci Beijing 100049 Peoples R China Chinese Acad Sci Innovat Acad Earth Sci Beijing 100029 Peoples R China

Hot dry rock (HDR) is deep geothermal reservoir rock predominantly containing granite, whose physical and mechanical properties are governed by heterogeneous crystal structures. However, the fluid conductivity of intact granite with ultra-low permeability cannot be reformed using existing techniques. Complex fracture networks must be created by connecting hydraulic fractures with natural fractures developed in reservoirs. In this study, we introduce the wall barrier method into the novel hydro-grain-based model (hydro-GBM) to build a prefractured granite model at the mineral grain scale. This model enables investigation of the effects of the stress environment, mineral spatial distribution, and fluid injection rate on hydraulic fracturing characteristics. Microcracks around the inner tips of pre-existing fractures reproduce the initiation, propagation, and coalescence behaviors similar to experimental results. Mineral spatial distributions induce random changes in the propagation paths of hydraulic fractures. Increasing the fluid injection rate delays the deflection of hydraulic fractures and extends the propagation distance along the major axes of pre-existing fractures. The average activity level and proportion of large seismic events are reduced by injecting low-rate fluid. In summary, our results reveal the interactions between hydraulic and pre-existing fractures and provide a valuable reference for constructing an efficient enhanced geothermal system (EGS) for deep reservoirs.

关键词： Hydraulic fracturing particle flow code Pre-existing fracture Grain-based model Microcracking behavior Acoustic emission

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Failure behavior of horseshoe-shaped tunnel in hard rock under high stress:Phenomenon and mechanisms

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Transactions of Nonferrous Metals Society of China 2022年第2期32卷 639-656页

作者： Hao WU Guo-yan ZHAO Shao-wei MA School of Mines China University of Mining and TechnologyXuzhou 221116China School of Resources and Safety Engineering Central South UniversityChangsha 410083China

A particle flow code(PFC) was first applied to examining the mechanical response of a horseshoe-shaped opening in prismatic rock models under biaxial compression. Next, an improved complex variable method was proposed to derive the stress distribution around the opening. Lastly, a case study of tunnel failure caused by rock burst in Jinping Ⅱ Hydropower Station was further analyzed and discussed. The results manifest that a total of four types of cracks occur around the opening under low lateral confining stress, namely, the primary-tensile cracks on the roof-floor, sidewall cracks on the sidewalls, secondary-tensile cracks on the corners and shear cracks along the diagonals. As the confining stress increases, the tensile cracks gradually disappear whilst the spalling failure becomes severe. Overall, the failure phenomenon of the modelled tunnel agrees well with that of the practical headrace tunnel, and the crack initiation mechanisms can be clearly clarified by the analytical stress distribution.

关键词： horseshoe-shaped tunnel fracture behavior rock burst stress distribution complex variable method particle flow code

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Characterization of a fractured rock mass using geological strength index: A discrete fracture network approach

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TRANSPORTATION GEOTECHNICS 2023年 40卷

作者： Singh, Jaspreet Pradhan, Sarada Prasad Vishal, Vikram Singh, Mahendra Indian Inst Technol Roorkee Dept Earth Sci Roorkee 247667 India Indian Inst Technol Dept Earth Sci Mumbai 400076 India Indian Inst Technol Roorkee Dept Civil Engn Roorkee 247667 India

Rock mass contains siginificant heterogeneity due to the presence of structural discontinuities. The failure in the rock mass may cause a serious concern to road and rail transportation. Rock mass characterization is a first step towards preliminary investigation for road or tunnel excavations. Different field-based methods have been developed to characterize the rock mass conditions. The Geological Strength Index (GSI) is a simple and commonly adopted method with wide applicability in rock engineering. Traditional approaches are limited to 2D exposures for mapping purposes, but block formation or joint intersection is a 3D parameter. The advancement in computational techniques led to significant involvement of numerical modeling techniques such as those backed with discrete fracture network (DFN). The remote sensing techniques render the data with high precession po-tential not accessible with conventional methods. The stochastic DFNs generated based upon the statistical distribution of the input parameters can represent the natural fracture system in 3D. The developed synthetic fracture network can be used to examine the rock mass characteristics. This work addresses the incorporation of the Discrete Fracture Network (DFN) in the estimation of the Geological Strength Index (GSI) of the rock mass. The work compares the results of DFN generated using aggregate and disaggregate approaches in block size distribution (BSD) and rock quality designation (RQD) measurements for a fractured slope. The calculated BSD and RQD using DFN and field-estimated joint condition parameter are used to estimate GSI of the rock mass. A machine learning based python GUI tool was developed to find GSI from block volume and joint condition parameters. The prediction of GSI from input parameters using machine learning has led to systematically digitizing the standard GSI chart.

关键词： Joints particle flow code Discrete Fracture Network Geological Strength Index Photogrammetry GUI tool Rock mass

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Study on mechanical properties of soil-rock mixture of various compactness subjected to freeze-thaw cycles

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Research in Cold and Arid Regions 2021年第5期13卷 450-462页

作者： Zhong Zhou HaoHui Ding WenYuan Gao LinRong Xu School of Civil Engineering Central South UniversityChangshaHunan 410075China

The soil-rock mixture,a collection of soil particles and rock blocks,is inherently heterogeneous and anisotropic due to significant particle size and material strength *** study conducts triaxial tests on soil-rock mixture samples of various compactness subjected to varying freeze-thaw cycles.A mesoscopic simulation is carried out by particle flow code(PFC)to analyze the effects of freeze-thaw cycles on the mechanical properties of soil and rock *** results show that the mechanical properties of the soil-rock mixture under freeze-thaw cycles are greatly affected by the initial *** general,when the degree of compaction is higher,the influence of freeze-thaw cycles on the soil-rock mixture is *** stress-strain curves of the samples with different compactness demonstrate strain-softening *** freeze-thaw cycles greatly influence the failure strength of the samples with a higher degree of compaction but have little impact on the samples with a lower degree of *** the microscopic level,during freeze-thaw cycles,the pore volume in the highly compacted sample is too small to accommodate the volume expansion from ice crystal formation,causing significant strength loss among the soil and rock particles and deterioration of the macroscopic properties of the soil-rock mixture.

关键词： soil-rock mixtures freeze-thaw cycle degree of compaction particle flow code

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Study on Axial Tensile Strain Rate Effect on Concrete Based on Experimental Investigation and Numerical Simulation

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MATERIALS 2022年第15期15卷 5164页

作者： Sun, Bi Chen, Rui Ping, Yang Zhu, Zhende Wu, Nan Harbin Inst Technol Shenzhen Shenzhen 518055 Peoples R China Shenzhen Water Planning & Design Inst Co Ltd Shenzhen 518001 Peoples R China PowerChina Ecoenvironm Grp Co Ltd Shenzhen 518102 Peoples R China Hohai Univ Minist Educ Geomech & Embankment Engn Key Lab Nanjing 210098 Peoples R China Guangzhou Univ Guangzhou Univ Tamkang Univ Joint Res Ctr Engn St Guangzhou 510006 Peoples R China

The material of concrete is a three-phase composite material composed of an aggregate, a mortar and an interface transition zone (ITZ). Based on this characteristic, the axial tensile test of mortar, the interface and concrete specimens under intermediate strain rate was carried out in this paper. The sensitivity of these three materials to strain rate was compared and analyzed. The numerical simulation of the axial tension of the concrete materials was studied. The following conclusions are drawn: in the axial tension test, the rate of sensitivity of the specimen interface is the strongest. With the increase in strain rate, the tensile strength and elastic modulus of concrete specimens increase but the effect of the ITZ decreases. The low tensile strength of the ITZ leads to its failure in concrete. The parallel bond strain energy and the dissipated energy of specimens increase with the strain rate. When the strain rate is higher (greater than 1 x 10(-2)), the increase rate of the dissipated energy is greater than that of the parallel bond strain energy. The results of this research can provide the corresponding basis for the safety evaluation and the stability analysis of concrete engineering in the range of intermediate strain rate.

关键词： concrete interfacial transition zone (ITZ) tensile strain rate meso-strength parameters particle flow code intermediate strain rate

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Numerical studies on micro-cracking behavior of transversely isotropic argillaceous siltstone in Longyou Grottoes under three-point bending

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THEORETICAL AND APPLIED FRACTURE MECHANICS 2022年 122卷

作者： Wang, Song Zhou, Jian Zhang, Luqing Han, Zhenhua Zhang, Fenxiang Fu, Yan Yang, Zhifa Beijing Univ Technol Key Lab Urban Secur & Disaster Engn Minist Educ Beijing 100124 Peoples R China Chinese Acad Sci Inst Geol & Geophys Key Lab Shale Gas & Geoengn Beijing 100029 Peoples R China Univ Chinese Acad Sci Coll Earth & Planetary Sci Beijing 100049 Peoples R China Chinese Acad Sci Innovat Acad Earth Sci Beijing 100029 Peoples R China BGI Engn Consultants Ltd Beijing 100038 Peoples R China Inst Longyou Grottoes Quzhou 324400 Peoples R China

Longyou Grottoes, the underground projects with a rich history of thousands of years, are one of the key cultural relics to be protected in China and are actively preparing to apply for a world cultural heritage site. However, due to the shallow buried argillaceous siltstone with low-strength bedding planes, some cavern roofs experienced apparent breakages or large-scale collapses. Clarifying the roof deformation and failure mechanism is essential for the long-term protection of the historical relic and sustainable development of tourism. Productive previous work proved that three-point bending tests of rock samples could reproduce stress states of cavern roofs. Therefore, three-point bending numerical models with various bedding plane structures for Longyou argillaceous siltstone are created in this study to investigate the rock mechanical behaviors. The simulation results show that as bedding mechanical properties decrease, the bending strength (BS) has a slow to fast decrease, indicating that timely protection in Longyou Grottoes before the BS drops sharply is conducive to preventing roof failures. This declining trend is further accelerated when the bedding dip angle changes from horizontal to vertical. Dip angles, mechanical properties, and spacings of bedding planes control fracture modes of models. Moreover, the fitted Gutenberg-Richter-type relationships between acoustic emission (AE) moment magnitudes and logarithmic AE events present high average activity levels and high proportions of large AE events in both bedding planes and rock matrix for models with strong, dispersedly distributed, and near-horizontal bedding planes. In summary, the simulation results reveal the impacts of transverse isotropy in surrounding rocks on roof bending failures, which may provide a valuable reference for the reinforcement and protection of Longyou Grottoes.

关键词： Longyou Grottoes Three-point bending test Micro -cracking behavior particle flow code Transverse isotropy

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Meso-Mechanical Characteristics of Granite with Natural Cracks after Mud Acid Corrosion

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ENERGIES 2022年第3期15卷

作者： Tong, Rongchen Liu, Hejuan Liu, Jianfeng Shi, Yingkun Xie, Lihuan Ban, Shengnan Chinese Acad Sci Inst Rock & Soil Mech State Key Lab Geomech & Geotech Engn Wuhan 430071 Peoples R China Univ Chinese Acad Sci Beijing 100049 Peoples R China Chinese Acad Sci Inst Rock & Soil Mech Hubei Key Lab Geoenvironm Engn Wuhan 430071 Peoples R China Sichuan Univ Coll Water Resource & Hydropower Chengdu 610041 Peoples R China Shandong Hispeed Co Ltd Jinan 250014 Peoples R China

Most of the discovered high-temperature geothermal energy systems are often related with granite that is characterized by natural faults, fractures and cracks of different size. However, the porosity and permeability of the granite matrix is very low, greatly limiting the efficiency of heat extraction in granitic rock. Chemical stimulation, which is regarded as one of the most important means of reservoir stimulation, has consequently received more and more attention. In this paper, a Triassic granite obtained from the eastern region of Liaoning Province in China was reacted with three different concentration of mud acid solution (8% HCl + 1% HF, 10% HCl + 2% HF, 12% HCl + 3% HF) and the resulting microstructure changes studied by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). The experimental results show that the number of micropores in the granite increases after chemical corrosion by mud acid solution. A higher mud acid solution concentration results in a much higher pore volume. Triaxial compression tests on the granite before and after chemical corrosion were carried out to study the effect of acidification on the mechanical characteristics of granite, showing that the peak stress and elastic modulus of granite decreases 25.7% and 16.5%, respectively, after exposure to mud acid solution (12% HCl + 3% HF) corrosion for three weeks at room temperature. The particle flow program PFC2D based on discrete element method was used to investigate the mechanical response before and after the chemical corrosion. Considering that the granite is rich in microcracks, the study is simplified by considering them all grouped into one main closed fracture. The influences of main crack inclination angle, crack length, friction coefficient and confining pressure on the mechanical response were investigated. Under the triaxial compression loading state, wing cracks appear at the initial crack tip, then secondary cracks begin to appear. The sensitivity ana

关键词： geothermal energy development granite mud acid solution particle flow code characteristic strength

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Rock grain-scale mechanical properties influencing hydraulic fracturing using Hydro-GBM approach

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ENGINEERING FRACTURE MECHANICS 2022年 262卷

作者： Kong, Lie Ranjith, Pathegama Gamage Li, Qiuyi Bing Song, Yuqi Monash Univ Dept Civil Engn Melbourne Vic 3800 Australia Western Univ Dept Civil & Environm Engn Spencer Engn BldgLambton Dr London ON N6A 5B9 Canada

Rock cracking is of key concern to many geological applications. On a grain scale, rock fracturing depends on not only the external load but also the mechanical properties of the mineral grain and grain boundary. In this study, we investigated the effect of rock grain-scale mechanical param-eters on fluid-driven cracking behaviours and discussed the identification of micro mechanical parameters in the grain-based model. A coupled hydro-grain-based DEM model (Hydro-GBM) is used to reconstruct rock microstructures and simulate hydraulic fracturing. We analyzed the influences of the main micro-mechanical parameters of mineral grain and grain boundary and explored the responsible micro-mechanisms. Results including crack initiation pressure, break-down pressure, partitions of intragranular and grain boundary cracks in tension/shear are pre-sented in detail. Then, based on the parameter analysis, some issues in identifying micro parameters in existing DEM simulations are discussed. We proposed a formulation to determine contact friction angle, which could eliminate the long-standing mismatch of shear cracking be-tween experiment and simulation. We also suggested the importance of calibrating micro results in grain-based modelling. The presented study systematically revealed the effects of rock grain-scale properties on hydraulic fractures and could provide valuable references to the selection of micro mechanical parameters in future modelling.

关键词： Distinct element method Hydro-GBM Micro mechanism Calibration Hydraulic fracturing particle flow code

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