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Experimental and numerical study on mechanical and fracture behavior of rock-like specimens containing pre-existing holes flaws

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EUROPEAN JOURNAL OF ENVIRONMENTAL AND CIVIL ENGINEERING 2022年第1期26卷 299-319页

作者： Zhao, Zhenlong Jing, Hongwen Shi, Xinshuai Han, Guansheng China Univ Min & Technol State Key Lab Geomech & Deep Underground Engn Xuzhou 221116 Jiangsu Peoples R China Nagasaki Univ Grad Sch Engn Nagasaki Japan

Laboratory tests and numerical simulation are conducted to study mechanical behavior of rock containing two pre-existing holes in this paper. First of all, rock-like specimens containing two pre-existing holes were prepared to investigate mechanical and acoustic emission characteristics in the uniaxial compression. Different geometries were obtained by changing the angle of the center line of holes with respect to the direction of axial loading. Experimental results show that the stress-strain curves present obvious fluctuation, and evolution process of acoustic emission signal can be divided into four stages. There are three kinds of failure modes: only shear failure, mainly shear and auxiliary tensile failure, mainly tensile and auxiliary shear failure. Then, parallel bonded mode (PBM) based on the particle flow code in two dimensions (PFC2D) were established to study strength and failure modes in the biaxial compression. Finally, process of cracks initiation, propagation and coalescence, and fracture mechanism were further discussed. The most important conclusion reached is that the deformation and failure process of rock mass containing holes defects can be divided into two stages: rock bridge penetration and overall instability, which are sequential.

关键词： Pre-existing holes fracture mode acoustic emission progressive failure process particle flow code

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Unloading Response Characteristics of Cross Fault Caverns: Effect of Fault Angles

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GEOTECHNICAL AND GEOLOGICAL ENGINEERING 2022年第3期40卷 1061-1073页

作者： Shang, Yong-Hui Xu, Lin-Rong Li, Yong-Wei Cent South Univ Sch Civil Engn Changsha 410075 Peoples R China Huanghuai Univ Inst Architecture & Engn Zhumadian 463000 Peoples R China Cent South Univ Natl Engn Lab High Speed Railway Construct Changsha 410075 Peoples R China

The research of unloading response characteristics of caverns with weak structure has important engineering value for the development and construction of underground space. In this paper, firstly, the cross fault cavity model is established based on particle flow code (PFC) software, considering different fault dip angles. The parallel bond model (PBM) is used in the cavern model and the smooth joint model (SJM) is used in the fault model. Then, the mechanical, acoustic emission (AE), energy response characteristics and failure modes of the cavern model with different fault angles are explored. The research result show that the fault angle has a great influence on the stress distribution and failure modes of surrounding rock. The existence of faults does not change the overall law of AE evolution curve, that is, the number of AE hits increases first and then decreases. However, the existence of faults changes the maximum number of AE hits and the duration of AE. When the cavern is unloaded, the strain energy of the model first decreases and then increases.

关键词： Caverns Unloading response particle flow code Acoustic emission Energy Failure modes

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Rules of overburden crack development in coal mining with different ratios of rock-soil strata conditions

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ARABIAN JOURNAL OF GEOSCIENCES 2022年第6期15卷 1-13页

作者： Lian, Xugang Zhang, Yanjun Liu, Jibo Deng, Weinan Guo, Junting Cai, Yinfei Taiyuan Univ Technol Sch Min Engn Taiyuan 030024 Peoples R China Guizhou Univ Engn Sci Coll Min Engn Bijie 551700 Peoples R China Tiandi Sci & Technol Co Ltd Coal Min & Designing Dept Beijing 100013 Peoples R China State Key Lab Water Resource Protect & Utilizat C Beijing 102209 Peoples R China

After coal seam mining, the overburden movement is transmitted to the surface to form cracks. As a main factor in the damage inflicted by mining subsidence on ground environments, ground cracks always threaten the safety of people's production and life. There are obvious differences in the laws of surface movement and deformation with different geological and mining conditions. In particular, ground cracks are more likely to occur in the loess area. Therefore, it is particularly important to study the relationship between mining ground cracks and related factors. In this paper, the particle flow code is used to simulate the discontinuous deformation of strata, especially the cracks caused by underground coal mining;and the four cases (a to d) with three factors-the RLD (ratio of the loess layer thickness to the mining depth), the RRM (ratio of the overlying rock thickness to the mining thickness), and the RHD (relative height difference at the surface)-are considered. The results show that when the RLD =16.67%, RRM =40, RHD =11, the surface mining damage is light. With the RLD, RHD, and RRM increase, surface mining damage gradually worsens. When the RLD = 83.52%, RRM = 3.75, and RHD =121, the surface mining damage is most serious. RLD, RRM, and RHD have a crucial influence on the ground cracks formed in underground coal mining. Crack width (d) and depth (h) satisfy the equation h = A root d + b, and coefficient A is large, which provides a reference for the dynamic prediction of cracks. The change in stope stress and the distribution of the internal force chain of the overburden reflect the movement and deformation of the overburden and surface mining failure. This research goes towards preventing ground hazards caused by underground coal mining and plays a positive role on land ecological restoration.

关键词： Cracks particle flow code Mining subsidence Movement and deformation of strata

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Investigation of macro-micro mechanical behaviors and failure mechanisms in cemented tailings backfill with varying proportions of fine

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CONSTRUCTION AND BUILDING MATERIALS 2024年 438卷

作者： Yang, Liu Kou, Yunpeng Li, Guangbo Chen, Miao Zhu, Gengjie Song, Zepu Wang, Pengtao Shandong Gold Min Technol Co Ltd Backfill Engn Lab Yantai 261400 Peoples R China Shandong Key Lab Deep Sea & Deep Earth Met Mineral Yantai Peoples R China Shandong Univ Sci & Technol Coll Energy & Min Engn Qingdao 266590 Peoples R China Northeastern Univ Key Lab Minist Educ Safe Min Deep Met Mines Shenyang 110819 Peoples R China

This study investigates the impact of tailings characteristics, particularly fine tailings, on the efficiency and effectiveness of mining engineering backfill operations. Contrary to the common underestimation, fine tailings significantly affect the mechanical properties and failure mechanisms of Cemented Tailings Backfill (CTB). Through a series of experiments examining various particle size distributions and employing advanced Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) techniques, this research clarifies the role of fine tailings in modifying the mechanical strength and failure patterns of CTB. Adding 20 % fine tailings optimally enhances both the early and long-term strength of backfill materials. With aging, adding 30 % fine tailings shows a rapid strength increase from 7d to 28d, with the 28d strength slightly lower than that of specimens with 20 % addition, but the specimens demonstrate better ductility after reaching peak strength. As the content of fine tailings increases, crack propagation in the specimens shifts from radial to axial, simplifying the fracture mode from complex to singular. Consequently, the fracture mechanism changes from ductile to brittle, and then reverts to ductile. Moreover, using particle flow simulation and moment tensor analysis to study the failure processes indicates that while increased fine particle content boosts the material's initial strength, it ultimately leads to a simpler crack network and volumetric expansion as the primary failure mode. In terms of energy conversion mechanisms, as the content of fine particles increases, the energy first increases and then decreases. During the elastic stage and the early stage of the plastic stage, the energy is mainly converted into strain energy and PB bonding energy, which affects the elasticity of the sample. In the later stage of the plastic phase, the energy is mainly converted into dissipated energy, leading to the failure of the sample.

关键词： Fine tailings content Fracture mechanism particle flow code Weibull distribution Moment tensor

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Research on the Macroscopic and Microscopic Failure Mechanisms and Damage Deterioration Patterns of Granite under Unloading Paths

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INTERNATIONAL JOURNAL OF GEOMECHANICS 2024年第11期24卷

作者： Liu, Heyi Liu, Lipeng Wang, Xiaogang Pei, Jiangrong Chen, Tiannan China Inst Water Resources & Hydropower Res State Key Lab Simulat & Regulat Water Cycle River Beijing 100038 Peoples R China

Accurate analysis of the deformation characteristics and the damage destruction mechanism of a rock mass is a prerequisite for the evaluation of the stability of the surrounding rock in tunnel engineering. This paper proposes a combination of numerical simulation techniques based on microstructure analysis and physical model experimental methods, which allows for the microscale interpretation of macroscale experimental phenomena and provides new insights for further summarizing the instability and failure patterns of rocks under unloading paths. To investigate the macroscopic and microscopic failure mechanisms as well as the damage deterioration patterns of granite under unloading conditions, physical model tests were conducted using stress paths of conventional triaxial and constant axial pressure unloading confining pressure. The experiments encompassed unloading paths, and the associated mechanical responses were finely simulated using the particle flow code method coupled with digital image processing techniques. The results reveal that at lower unloading rates, granite predominantly undergoes shear failure, with the destabilizing mechanism attributed to the formation of an "X"-shaped conjugate failure surface under the influence of tension-shear coupling. As the unloading rate increases, tensile forces progressively take precedence, leading to more pronounced brittle failure characteristics in granite. The ratio of the confining pressure reduction to the initial confining pressure at the point of specimen failure increases with the unloading rate and decreases with the initial confining pressure. Faster unloading rates correspond to a more rapid increase in Poisson's ratio, and the unloading path primarily influences the lateral strain variation during the initial stages of unloading. Additionally, under unloading conditions, the internal friction angle of granite increases, while the cohesion decreases. The impact of unloading rate and path on cohesion becomes

关键词： Unloading paths Macroscopic and microscopic failure mechanisms Damage deterioration patterns Digital image processing particle flow code

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Study on Static Mechanical Properties and Numerical Simulation of Coral Aggregate Seawater Shotcrete with Reasonable Mix Proportion

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MATERIALS 2024年第10期17卷 2353页

作者： Peng, Yuxuan Yu, Liyuan Li, Wei Zhang, Tao Ma, Linjian Wu, Dongyang Wu, Changan Zhou, Linjie China Univ Min & Technol State Key Lab Coal Resources & Safe Min Xuzhou 221116 Peoples R China Army Engn Univ PLA State Key Lab Explos Shock Prevent & Mitigat Nanjing 210007 Peoples R China Yunlong Lake Lab Deep Underground Sci & Engn Xuzhou 221116 Peoples R China

This study aims to explore the static mechanical characteristics of coral aggregate seawater shotcrete (CASS) using an appropriate mix proportion. The orthogonal experiments consisting of four-factor and three-level were conducted to explore an optimal mix proportion of CASS. On a macro-scale, quasi-static compression and splitting tests of CASS with optimal mix proportion at various curing ages employed a combination of acoustic emission (AE) and digital image correlation (DIC) techniques were carried out using an electro-hydraulic servo-controlled test machine. A comparative analysis of static mechanical properties at different curing ages was conducted between the CASS and ordinary aggregate seawater shotcrete (OASS). On a micro-scale, the numerical specimens based on particle flow code (PFC) were subjected to multi-level microcracks division for quantitive analysis of the failure mechanism of specimens. The results show that the optimal mix proportion of CASS consists of 700 kg/m3 of cementitious materials content, a water-binder ratio of 0.45, a sand ratio of 60%, and a dosage of 8% for the accelerator amount. The tensile failure is the primary failure mechanism under uniaxial compression and Brazilian splitting, and the specimens will be closer to the brittle material with increased curing age. The Brazilian splitting failure caused by the arc-shaped main crack initiates from the loading points and propagates along the loading line to the center. Compared with OASS, the CASS has an approximately equal early and low later strength mainly because of the minerals' filling or unfilling effect on coral pores. The rate of increase in CASS is swifter during the initial strength phase and decelerates during the subsequent stages of strength development. The failure in CASS is experienced primarily within the cement mortar and bonding surface between the cement mortar and aggregate.

关键词： coral aggregate seawater shotcrete particle flow code mix proportion static mechanical properties orthogonal experiment

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An iterative method to improve the calibration accuracy of flat-joint models: Catch-up penalty algorithm

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SIMULATION MODELLING PRACTICE AND THEORY 2024年 134卷

作者： Yang, Zilong Hu, Yong Xu, Mingxu Tian, Jiyu Pang, Hao Liu, Xiangyang Jilin Univ Key Lab CNC Equipment Reliabil Minist Educ Changchun 130022 Peoples R China Jilin Univ Sch Mech & Aerosp Engn Jilin 130012 Peoples R China South China Univ Technol Sch Mech & Automot Engn Guangzhou 510641 Peoples R China

Parameter calibration is a critical step in accurately modeling using the discrete element method (DEM), but the time-consuming and complex calibration process limits the practical utilization of DEM. Herein, a catch-up penalty algorithm was proposed to simultaneously adjust multiple micro parameters of the flat-joint model through iterations. The effect of micro parameters on macro parameters was investigated by conducting 64 sets of orthogonal tests in PFC3D and analyzing the results by ANOVA. Regression analysis was used to establish the preliminary formulas for directly obtaining initial values of micro parameters and the trend equations for deriving iterative formulas. Based on the preliminary and iterative formulas, the calibration process for the algorithm was proposed, in which the micro parameters of each iteration can be calculated, thereby reducing researchers' dependence on the experience. The calibration capability of the algorithm was verified on four types of rocks, and the results showed that the average calibration error between the simulation results and the target values was reduced to within 5 % after six iterations, proving the reliability and applicability of the algorithm.

关键词： particle flow code Flat -joint model Calibration process Orthogonal test

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Experimental and numerical investigation of the fracture behavior in granite specimens with two co-planar side flaws under biaxial loading

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THEORETICAL AND APPLIED FRACTURE MECHANICS 2024年第PartA期133卷

作者： Wu, Rui Fan, Xiang Lin, Hang An, Honglei Feng, Wenwen Chen, Siwei Changan Univ Sch Highway Xian 710064 Shaanxi Peoples R China Xian Key Lab Geotech Engn Green & Intelligent Tran Xian 710064 Shaanxi Peoples R China Cent South Univ Sch Resources & Safety Engn Changsha 410083 Hunan Peoples R China Shanxi Transportat Planning Survey & Design Inst C Taiyuan 030032 Shanxi Peoples R China

It is well known that the structural plane plays a decisive role in the mechanical properties of rock mass. To better understand the cracking mechanism and stress characteristics of co-planar intermittent double-flawed granite specimens with various dip angles (Abbreviated as flawed specimens), two co-planar flaws were prefabricated on both sides of the granite, and biaxial compression tests were conducted. The failure characteristics and local microstrain were recorded by the camera and strain acquisition instrument. The evolution law of mechanical parameters of flawed specimens was analyzed. Simultaneously, a two-dimensional particle flow code (PFC2D) was used to establish a biaxial compression model of flawed specimens. The crack initiation and propagation of the specimens were explained by the crack hot spot, the evolution law of crack, and the evolution law of the number of microcracks. It is found that (a) as flaw dip angle (alpha) increases, peak strength and elastic modulus initially decrease and then increase, with reductions ranging from 17.6% to 46.1% and 15.6% to 48.2%, respectively. (b) Increasing lateral stress (sigma 2) triggers a shift in failure mode from a tensile-shear mixed failure to a shear failure. The cohesion (c) and friction angle (phi) of flawed specimens were notably smaller compared to those of intact specimens, with reductions correlating with the alpha. (c) Flawed specimens exhibit pronounced contact force concentration and stress concentration at the flaw tip. The analysis of the microstrain confirms stress concentration at the flaw tip, leading to failure. However, various sigma 2 induces the differences in the magnitude and extent of tip stress.

关键词： Flaw dip angle Biaxial compression test particle flow code Stress distribution Failure characteristic

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Creep-fatigue interaction of rock salt using discrete element simulation

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COMPUTERS AND GEOTECHNICS 2024年 176卷

作者： Zhao, Kai Wong, Louis Ngai Yuen Univ Hong Kong Dept Earth Sci Hong Kong Peoples R China

The rock surrounding underground salt caverns hosting compressed air energy storage (CAES) systems is subjected to simultaneous creep and fatigue loads. A proper design considering the mechanical response and damage evolution of rock salt under such loading conditions is crucial for ensuring long-term stability. In addition to the conventional laboratory creep-fatigue tests, numerical simulation can serve as a complementary and validating method for unveiling the underlying mechanisms of interaction occurrences. The integration of these methods offers significant value in advancing our understanding of the meso-mechanical properties of rock salt. This study delves deep into the creep-fatigue interactions of rock salt using discrete element method (DEM) simulations. By establishing a DEM model of rock salt, that employs a hybrid contact model of Burger's and linear parallel bond (LPB) to represent creep and damage behavior, respectively, we unveil the S-shaped evolution of the strain-time curve, encompassing initial, steady, stable damage, and acceleration deformation stages. Our findings highlight that stress intervals significantly reduce fatigue and creep life compared to pure creep and fatigue conditions. Furthermore, we demonstrate the effectiveness of a bilinear cumulative damage rule in describing the creepfatigue life of rock salt under uniaxial conditions, with longer stress intervals leading to increased cumulative crack numbers and earlier crack initiation times. Ultimately, our numerical salt model exhibits an inclined shear crack and numerous micro-cracks surrounding the macro-shear crack, aligning with fracture modes observed in laboratory tests and advancing our understanding of rock salt behavior in CAES system.

关键词： Creep-fatigue interaction Rock salt Crack evolution Life predication particle flow code

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Analysis of tensile failure mechanism in intact, foliated, and cracked rocks using distinct element method: Influence of anisotropy

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ENGINEERING FAILURE ANALYSIS 2024年 166卷

作者： Singh, Piyush Kumar Pradhan, Sarada Prasad Vishal, Vikram IIT Roorkee Dept Earth Sci Roorkee 247667 India Indian Inst Technol Dept Earth Sci Mumbai 400076 India

A thorough numerical analysis was conducted to understand the failure mechanisms and behaviour in intact rocks, foliated rocks and rocks with pre-existing cracks using the Brazilian tensile strength tests. The dominant anisotropy in rocks, especially foliated rocks like phyllites, slates and schists, as well as sedimentary rocks like sandstone, limestones and shale, leads to complex failure mechanism. The presence of cracks leads to anisotropy causing different mechanisms of failure and variation in tensile strengths of rock mass. Numerical simulations were conducted based on experimental studies of foliated phyllite and dolomitic limestone from the Tejam Group, Lesser Himalaya. The variations in failure behaviour and strength of foliated rocks were studied for samples at different foliation angle using the particle distinct element model. The simulation results show that with increase in foliation angle to the loading direction (from 0 degrees to 90 degrees), the peak tensile strength of rocks remains almost same for specimens with theta = 0 degrees to 30 degrees and then increases linearly till 90 degrees. The influence of cracks as anisotropy was also studied by changing crack length (from 5 mm to 30 mm) and angle (from 0 degrees to 90 degrees). The development of failure patterns in rocks with pre-existing cracks leads to a better understanding of the failure modes and makes it possible to interpret failure behavior, pattern, and strength parameters. With increase in crack length, the tensile strength of rocks decreases. Overall, this study depicts the influence of anisotropy and microparameters on failure modes and behavior in Brazilian tensile strength tests on foliated and pre-cracked rocks.

关键词： Tensile strength Fracture analysis Anisotropy Distinct element model particle flow code

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