Research

19 pages, 9514 KiB

Open AccessArticle

Study on the Influence of Specimen Size and Aggregate Size on the Compressive Strength of Rock-Filled Concrete

by Xiang Li, Yufan Zhang, Tao Yang, Haimei Liao, Lei Yu, Yunke Liu, Guoji Wang, Yinghong Zhao and Haoyang Qiao

Appl. Sci. 2023, 13(10), 6246; https://doi.org/10.3390/app13106246 - 19 May 2023

Cited by 1 | Viewed by 1207

Abstract

Rock-filled concrete (RFC) technology is a new type of mass concrete construction technology, which consists of two basic components: the force transfer frame formed by large-size rock accumulation and the matrix formed by self-compacting concrete (SCC) filling. Its unique construction method also distinguishes [...] Read more.

Rock-filled concrete (RFC) technology is a new type of mass concrete construction technology, which consists of two basic components: the force transfer frame formed by large-size rock accumulation and the matrix formed by self-compacting concrete (SCC) filling. Its unique construction method also distinguishes RFC from ordinary concrete in terms of its force characteristics. In this paper, RFC is considered as a composite material consisting of aggregate and SCC; based on the realistic failure process analysis (RFPA) method, the effects of specimen size and aggregate size on the compressive strength of RFC were studied. Firstly, RFC cube specimens were prepared and uniaxial compression tests were conducted. During the preparation process, in order to eliminate the influence of factors such as shape, spatial distribution state, and volume share of aggregates on the compressive strength, aggregates of different sizes were set as spheres and arranged in simple cubic stacking; then a numerical model of RFC with different specimen sizes and different aggregate sizes was established for uniaxial compression numerical simulation experiments to analyze the variation law and failure pattern of the RFC compressive strength. The results indicate that the compressive strength of RFC exhibits a significant size effect and follows a negative exponential function distribution law; with the same volume fraction of aggregate, the smaller the aggregate size, the higher the compressive strength of the RFC will be, and this increasing trend gradually levels off. Based on the findings of this study, it is recommended that the size effect and the reduction of aggregate size on dam strength be taken into account in the design of RFC dams. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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19 pages, 11932 KiB

Open AccessArticle

Case Study and Risk Assessment of Water Inrush Disaster in Qingdao Metro Line 4

by Yongjun Zhang, Weiguo Zhang, Huangshuai Xia, Bin Gong, Fei Liu, Jiahui Zhang and Kai Liu

Appl. Sci. 2023, 13(6), 3384; https://doi.org/10.3390/app13063384 - 07 Mar 2023

Cited by 2 | Viewed by 1535

Abstract

Water inrush is one of the most frequent and catastrophic hazards in tunnel engineering, and poses serious threats to the safety of engineering and personnel. This paper presents a case study of a water inrush and ground collapse in the Qingdao Metro Line [...] Read more.

Water inrush is one of the most frequent and catastrophic hazards in tunnel engineering, and poses serious threats to the safety of engineering and personnel. This paper presents a case study of a water inrush and ground collapse in the Qingdao Metro Line 4, which caused a cave-in with the diameter and depth of about 30 m and 6 m, respectively. Based on the field data and numerical modelling, the causes of the disaster were analyzed. A numerical model was used to analyze the changes of surface settlement, vault settlement and water pressure during the tunnel excavation. The results of the study indicate that the cause of this disaster was the failure of the tunnel vault surrounding rock caused by the weakening of the tunnel surrounding rock and water pressure, which in turn triggered the water inrush in the tunnel and caused a large volume of surface collapse. As the tunnel was excavated from the slightly weathered area to the strongly weathered area, the vault settlement increased, and the influence zone expanded towards the surface due to the continuous decrease in the strength of the surrounding rock. In particular, a negative pore water pressure zone was formed in a certain area around the tunnel during the water inrush. The negative pressure zone caused the surrounding groundwater to converge here, leading to an increase in the amount of water inflow, which also increased the scope and scale of the impact of this disaster. A risk assessment method for water inrush in tunnels is proposed. According to the geological and engineering characteristics of Qingdao area, the evaluation index system of tunnel water inrush risk was established. An RBF neural network was improved by gray correlation analysis and a PAM clustering algorithm to establish the tunnel water inrush risk assessment model. Comparing the evaluation data with the actual data, the prediction data of a traditional RBF neural network and a BP neural network, the accuracy and reliability of the model were verified. This study has value in reducing the occurrence of water inrush in a composite formation tunnel. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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18 pages, 7880 KiB

Open AccessArticle

Study on Shear Mechanical Properties and Fracture Evolution Mechanism of Irregular Serrated Rock Discontinuities

by Xinpeng Li, Dong Wang, Yujing Jiang, Hengjie Luan, Sunhao Zhang, Changsheng Wang and Jiankang Liu

Appl. Sci. 2023, 13(4), 2444; https://doi.org/10.3390/app13042444 - 14 Feb 2023

Cited by 1 | Viewed by 1123

Abstract

To analyze the shear characteristics and mesoscopic failure mechanism of irregular serrated rock discontinuities, a great deal of interview samples of irregular serrated structures were made by 3D printing technology, and laboratory shear tests were carried out on them under different normal stresses. [...] Read more.

To analyze the shear characteristics and mesoscopic failure mechanism of irregular serrated rock discontinuities, a great deal of interview samples of irregular serrated structures were made by 3D printing technology, and laboratory shear tests were carried out on them under different normal stresses. At the same time, PFC numerical simulation software is used to establish relevant models to study the evolution of microcracks and the distribution characteristics of the force chain on the rock discontinuity during the shear process. The results show that the shear mechanical properties of irregular serrated rock discontinuities are affected by normal stress, undulating angle, and undulating height. The shear strength increases with the increase of normal stress and undulating height, and decreases with the increase of undulating angle. The numerical simulation results show that the irregular structural surface cracks under different undulation angles, which first start at the near force end serration root on both sides and further evolve to the adjacent serrations, while the irregular structural surface cracks under different undulation heights, which first start at the serration root with the lowest height and expand to the adjacent serrations. At the same time, the number of cracks increases with the increase of normal stress and the force chain is mainly distributed near the sawtooth surface. The force chain is more concentrated near the near force end sawtooth and at the tip and root of the rest of the sawtooth. At the same time, the direction of the force chain is approximately perpendicular to the force surface of the sawtooth. The research results are helpful in further understanding the shear mechanical properties and differences of irregular serrated rock discontinuities. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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18 pages, 56283 KiB

Open AccessArticle

Influence of Normal Stiffness and Shear Rate on the Shear Behaviors and Acoustic Emissions Characteristics of Artificial Rock Joints

by Yujing Jiang, Xinpeng Li, Changsheng Wang, Hengjie Luan, Sunhao Zhang, Gang Wang and Pu Wang

Appl. Sci. 2023, 13(2), 1189; https://doi.org/10.3390/app13021189 - 16 Jan 2023

Cited by 2 | Viewed by 1371

Abstract

Understanding the asperity damage behaviors of joints during shearing is critical for evaluating the stability of deep underground engineering structures. In this paper, we prepared plaster joints and used them for direct shear tests under different normal stiffness (0–7 MPa/mm) and various shear [...] Read more.

Understanding the asperity damage behaviors of joints during shearing is critical for evaluating the stability of deep underground engineering structures. In this paper, we prepared plaster joints and used them for direct shear tests under different normal stiffness (0–7 MPa/mm) and various shear rate (0.5–20 mm/min) conditions. The effects of normal stiffness and shear rate on mechanical behavior and AE characteristics were studied. With the increase of normal stiffness, the damaged area of the surface of the joint and the weight of the damaged, rough body basically show a linear increase. With the increase of the shear rate, the peak shear stress and the final shear stress of the joint are non-linearly decreased (the decrease rate at the shear rate of 0.5–5 mm/min is much larger than that at the shear rate of 5–20 mm/min), more local cracks appear on the surface of the joint, and the dilatancy of the joint slightly decreases. More than 60% of the acoustic emission signals in the shearing process of the joint are concentrated in the post-peak phase. With the increase of normal stiffness, the cumulative number of acoustic emission impacts and cumulative energy both increase. With the increase in shear rate, the accumulated acoustic emission impact number decreases, and the accumulated AE energy tends to increase when the shear rate is 0.5–5 mm/min and decreases when the shear rate increases to 5–20 mm/min. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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21 pages, 8028 KiB

Open AccessArticle

Research on Fault Activation and Its Influencing Factors on the Barrier Effect of Rock Mass Movement Induced by Mining

by Yanhui Guo, Luo Luo and Chuangye Wang

Appl. Sci. 2023, 13(1), 651; https://doi.org/10.3390/app13010651 - 03 Jan 2023

Cited by 14 | Viewed by 2177

Abstract

For the study of the driving forces behind fault activation and its influencing factors on the barrier effect of rock mass movement under the influence of mining, the discrete element numerical simulation software 3DEC was used for the analysis of the impact on [...] Read more.

For the study of the driving forces behind fault activation and its influencing factors on the barrier effect of rock mass movement under the influence of mining, the discrete element numerical simulation software 3DEC was used for the analysis of the impact on the distance to mining area from fault, the buried depth of the upper boundary of the fault, the dip angle of fault, the size of the mining area and the thickness of the fault zone respectively. The results show that the mining areas are closer to the fault as distances decrease, the burial depth of the upper boundary of the fault increases, and the size of the mining area increases, the fault is easier to activate, and fault activation has a stronger barrier impact on displacement field and stress field propagation. When the fault is cut into the goaf, the difference of rock displacement in both directions of the fault increases when the dip of the fault increases, and the fault is more susceptible to instability and activation. The barrier strength grows with the increase of the thickness of the fault fracture zone. The results of this study have important implications for the guard against and control of deep mining-related fault activation disasters. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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21 pages, 7174 KiB

Open AccessFeature PaperArticle

Research on the Mechanism of the Passive Reinforcement of Structural Surface Shear Strength by Bolts under Structural Surface Dislocation

by Yinfeng Tang, Donghai Jiang, Tongxu Wang, Hengjie Luan, Jiangwei Liu and Sunhao Zhang

Appl. Sci. 2023, 13(1), 543; https://doi.org/10.3390/app13010543 - 30 Dec 2022

Cited by 1 | Viewed by 979

Abstract

In order to study the local deformation of an anchor bolt and the improvement in the shear strength of a structural surface under the misalignment of an anchorage structure surface, FLAC^3D software was used to simulate granite, sandstone, and coal specimens with [...] Read more.

In order to study the local deformation of an anchor bolt and the improvement in the shear strength of a structural surface under the misalignment of an anchorage structure surface, FLAC^3D software was used to simulate granite, sandstone, and coal specimens with anchorage angles of 90° to analyze the damage of the anchoring agent and the changes in the local axial and shear forces of the anchor bolts with the misalignment of the structural surface. The results show that the anchor bolt near the structural surface had significant local characteristics with the misalignment of the structural surface; that is, the length of the local deformation area of the bolt was approximately equal to the length of the damaged area of the anchoring agent, and the stress on the anchor bolt was in a coupled tensile–shear stress state when the bolt reached the yield state. For the fully grouted bolts, it was this significant local feature that made the shear strength of the structural surface increase rapidly under a small shear displacement so that the structural surface reached a stable state. The improvement in the shear strength of the anchoring structural surface was caused by the misalignment of the structural surface. This is referred to as the passive improvement of the shear strength of the anchoring structural surface, which is the mechanism of the bonding section anchor to control the shear displacement of the structural surface and realize the stability of the rock mass. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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15 pages, 5497 KiB

Open AccessArticle

Study on Mechanical Characteristics of Deformation and the Failure of Gas-Containing Coal in the Wuhai Mining Area of China under Different Gas Pressure Conditions

by Yejiao Liu, Hui Xing, Zeyu Duan, Chaoyun Yu, Zhichao Tian and Ting Teng

Appl. Sci. 2022, 12(19), 10139; https://doi.org/10.3390/app121910139 - 09 Oct 2022

Cited by 1 | Viewed by 974

Abstract

The mechanical properties of gas-containing coal and rock mass play important roles in controlling the occurrence and development of coal and gas outbursts. The gradual increase in mining depth will change the failure mechanism of gas-containing coal and rock mass. In order to [...] Read more.

The mechanical properties of gas-containing coal and rock mass play important roles in controlling the occurrence and development of coal and gas outbursts. The gradual increase in mining depth will change the failure mechanism of gas-containing coal and rock mass. In order to further study the failure mechanism of gas-containing coal and rock mass, samples were taken from the gas-containing coal seam in the Wuhushan Coal Mine of the Wuhai Mining area of China. The mechanical parameters of coal samples during the failure process under different gas pressure conditions were measured and analyzed with the SAW-2000 rock mechanics testing machine, the gas-containing coal uniaxial compression device and inflation system. Meanwhile, the failure process and mechanical parameters of coal samples under different gas pressure were simulated by RFPA^2D gas plate numerical simulation software. The results show that with increasing gas pressure in the coal there is decrease in Compressive strength, Elastic modulus, Strain, Peak strength and Bearing capacity and increase in Poisson’s ratio. When the failure state appears in the coal, the cracks are longer and wider, more random cracks are genareted, and the damage degree of the coal is greater. The numerical analysis’ results are in good agreement with experimental results. The research results are applicable to the gas bearing coal with the same or similar gas geological conditions. The tests can be carried out repeatedly and reasonable results can be obtained according to the physical and mechanical parameters of the actual coal seam and the occurrence of gas. On this basis, physical experiments and numerical simulations of triaxial compression can also be carried out to further study the mechanical characteristics of deformation and the failure of gas-containing coal under gas pressure and provide technical support for revealing the mechanism of coal and gas outbursts. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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17 pages, 8921 KiB

Open AccessArticle

A Comparison Study of the Radial and Non-Radial Support Schemes in the Deep Coal Mine Roadways under TBM Excavation by the 3-D Equivalent Continuum Approach

by Feng Ding, Yanan Gao, Feng Gao, Minghui Li, Yiqiang Lu and Fei He

Appl. Sci. 2022, 12(19), 9442; https://doi.org/10.3390/app12199442 - 21 Sep 2022

Viewed by 912

Abstract

In recent years, TBM has been widely applied in deep coal mining to lift the TBM tunneling efficiency and avoid accidents, and more flexible support schemes are expected and required. This research aims to evaluate the performance of the two different support schemes [...] Read more.

In recent years, TBM has been widely applied in deep coal mining to lift the TBM tunneling efficiency and avoid accidents, and more flexible support schemes are expected and required. This research aims to evaluate the performance of the two different support schemes and to study the law of the support effect. A three-dimensional continuum model was established based on the available geological information, and the support effect at the three-dimensional level was studied by changing the support spacing and was assessed in term of the stress, displacement, failure zone, bolt force distributions, and axial deformation behavior of the tunnel. The results show that no matter what the geological condition was, the support of the non-radial support was a little lower than that of the radial support, and with a worse geological condition, the non-radial support had a better performance in the stabilizing tunnel. Additionally, the law of the support effect was found. The interlapping of the support influence domain was the essence of the increase in the support effect. Finally, according to the support law, the appropriate support spacing is found, which is smaller than 50% of the support influence domain length. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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17 pages, 6262 KiB

Open AccessArticle

Investigation into Dynamic Behaviors of High-Temperature Sandstone under Cyclic Impact Loading Using DIC Technology

by Hua Lu, Qiaoli Chen and Xiaotong Ma

Appl. Sci. 2022, 12(18), 9247; https://doi.org/10.3390/app12189247 - 15 Sep 2022

Cited by 2 | Viewed by 1185

Abstract

Coal resources are rich in Ningxia. Long-term mining creates mine goaf, which causes coal to burn spontaneously for a very long time. Unavoidably, the rocks around the coal fire area are affected by high temperatures, which can alter the characteristics of rocks and [...] Read more.

Coal resources are rich in Ningxia. Long-term mining creates mine goaf, which causes coal to burn spontaneously for a very long time. Unavoidably, the rocks around the coal fire area are affected by high temperatures, which can alter the characteristics of rocks and lead to safety accidents. To explore the temperature influence of sandstone in coal fire areas under cyclic impact loading, the sandstone treated under different temperatures is tested by a split Hopkinson pressure bar (SHPB). The mechanical properties of rocks treated at different temperatures are obtained. The composition of rock is determined, and the energy dissipation is calculated. Meantime, the digital image correlation (DIC) method is applied to study the mechanical behaviors of sandstone. The results show that at the first impact, the peak stress of sandstone decreases as the temperature increases. However, there is no obvious trend in the peak strain. Under the SHPB cyclic impact, the sandstone specimen is completely destroyed after two to three times of impact at different temperatures. At 25~1000 °C, the dynamic peak stress of sandstone decreases with the increase in impact times, and brittle failure occurs. When the impact pressure is 0.6 MPa, the incident energy increases with the impact velocity; the dynamic peak stress increases with the transmitted energy. Using the DIC method, it is found that when the temperature is below 800 °C, the dynamic characteristics of rock specimen have a close correlation with the crack initiation point and extreme point. When the temperature exceeds 800 °C, the rock specimen is seriously damaged, the overall strain is small, and the stress transfer efficiency is low. These findings show that temperature significantly affects the mechanical properties and initial damage of the sandstone, and the performance and damage are abrupt at 800 °C. Meanwhile, the DIC technology can effectively characterize the strain evolution of rock materials and explain the formation and propagation process of cracks, which provides a valid means for studying the damage and crack evolution of materials. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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18 pages, 7062 KiB

Open AccessArticle

A Finite Element Model for Investigating Unsteady-State Temperature Distribution and Thermomechanical Behavior of Underground Energy Piles

by Peng Zhao, Xiaozhao Li, Lihua Hu, Yun Wu and Chenyang Zhang

Appl. Sci. 2022, 12(17), 8401; https://doi.org/10.3390/app12178401 - 23 Aug 2022

Viewed by 1273

Abstract

The underground energy geostructure represented by the energy pile is one of the key paths for the cooperative development of underground space and geothermal energy. Because of its advantages of low cost, high efficiency and no extra occupation of underground space, it has [...] Read more.

The underground energy geostructure represented by the energy pile is one of the key paths for the cooperative development of underground space and geothermal energy. Because of its advantages of low cost, high efficiency and no extra occupation of underground space, it has become a feasible alternative to the borehole heat exchanger. The change in the temperature field of the energy pile and its surrounding ground not only affects the geological environment but also influences the thermomechanical performance and the durability of the structure. However, the temporal and spatial unsteady-state temperature distribution of piles and surrounding rock under typical intermittent and unbalanced thermal load conditions is still unclear. In this paper, a finite element model was applied to analyze the unsteady-state temperature distribution, and the thermomechanical behavior of the energy pile group was developed and verified. The temperature field distribution of pile and surrounding rock under typical intermittent working and unbalanced thermal load conditions were determined. Moreover, the thermomechanical behavior characteristics of the energy pile group were investigated. Finally, the influences of pile layout on the thermomechanical behavior of the energy pile group were identified by designing six different scenarios. The results indicate that under typical intermittent operation conditions, the temperature of the energy pile and surrounding ground near the heat exchange pipe varies periodically. For areas with unbalanced cooling and heating loads, long-term operation of energy piles leads to thermal accumulation, and the maximum temperature of energy piles occurs in the first daily cycle. In summer/winter working conditions, the increase/decrease in pile temperature induces axial compression/tensile stress. When the pile group is partially used as the energy pile, the non-energy pile acts as the “anchor pile”, and it generates the added tensile stress. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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21 pages, 16404 KiB

Open AccessArticle

Simulation of the Fracturing Process of Inclusions Embedded in Rock Matrix under Compression

by Chaoyun Yu, Bin Gong, Na Wu, Penglei Xu and Xiankai Bao

Appl. Sci. 2022, 12(16), 8041; https://doi.org/10.3390/app12168041 - 11 Aug 2022

Cited by 6 | Viewed by 1065

Abstract

Typical parallel fractures are often observed in the outcrops of inclusions in the field. To reveal the failure mechanism of inclusions embedded in rock matrix, a series of heterogeneous models are established and tested based on the damage mechanics, statistical strength theory, and [...] Read more.

Typical parallel fractures are often observed in the outcrops of inclusions in the field. To reveal the failure mechanism of inclusions embedded in rock matrix, a series of heterogeneous models are established and tested based on the damage mechanics, statistical strength theory, and continuum mechanics. The results show that, with the spacing between two adjacent fractures decreasing, the stress is firstly transferred from negative to positive, then from positive to negative. Stress transition is profound for the fracture spacing. Meanwhile, three types of fractures, i.e., consecutive fracture, non-consecutive fracture, and debonding fracture, are found, which are consistent with the observed modes in the field. Multiple inclusions are often fractured easier than an isolated inclusion due to the stress disturbance between inclusions and newly generated fractures. Either in single or multiple inclusions, tensile stresses inside the inclusions are the main driving force for fracture initiation and propagation. Besides, although the material heterogeneity has a small effect on the stress variation, it has an evident impact on the fracturing mode of inclusions. The stiffness ratio is critical for the stress transition and failure pattern; the interface debonding occurs earlier than the fracture initiation inside the inclusion when the stiffness ratio is relatively high. Additionally, the inclusions content only affects the sequence of fracture initiation rather than the final fracture spacing pattern. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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20 pages, 3499 KiB

Open AccessArticle

Evaluating the Influence of Fracture Roughness and Tortuosity on Fluid Seepage Based on Fluid Seepage Experiments

by Shuai Wang, Ying Xu, Yanbo Zhang, Qinglei Yu and Ling Wang

Appl. Sci. 2022, 12(15), 7661; https://doi.org/10.3390/app12157661 - 29 Jul 2022

Cited by 3 | Viewed by 1187

Abstract

The roughness and tortuosity of fractures are essential parameters affecting the fluid flow in a jointed rock mass. This paper investigates the influence of fracture roughness and tortuosity on fluid seepage behavior. A rough fracture surface was characterized by means of three-dimensional scanning [...] Read more.

The roughness and tortuosity of fractures are essential parameters affecting the fluid flow in a jointed rock mass. This paper investigates the influence of fracture roughness and tortuosity on fluid seepage behavior. A rough fracture surface was characterized by means of three-dimensional scanning and three-reconstruction technology, and the roughness and tortuosity of rock fractures were calculated. Hydraulic tests were conducted on deformed sandstone fractures with a self-made fracture seepage device, and the variation in the seepage flow was analyzed in rough fractures. The experimental results showed that the seepage flow of fluid decreased non-linearly with the increase in fracture roughness. Under different normal pressures, the friction resistance coefficient and tortuous resistance coefficient decreased with the increase in the Reynolds number. The friction resistance coefficient model and tortuous resistance coefficient model were used to quantitatively analyze the influence of fracture tortuosity and roughness on fluid flow, respectively. A modified model of the frictional resistance coefficient, considering fracture tortuosity and roughness, was established, which clearly expresses the law that with the increase in fracture tortuosity and roughness, the seepage flow of fluid decreases, and the head loss increases. The results of this research can provide a theoretical and experimental basis for studying fluid seepage behavior in deformed sandstone fractures. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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20 pages, 10352 KiB

Open AccessArticle

Monitoring and Analysis of Deformation Evolution Law of Fault Activation Caused by Deep Mining in Shizishan Copper Mine, China

by Yanhui Guo and Luo Luo

Appl. Sci. 2022, 12(14), 6863; https://doi.org/10.3390/app12146863 - 07 Jul 2022

Cited by 4 | Viewed by 1127

Abstract

In order to study the deformation evolution law of fault activation caused by deep mining in Shizishan Copper Mine, China, a monitoring system for fault activation slip is designed and implemented on the basis of the field investigation of footwall fault activation of [...] Read more.

In order to study the deformation evolution law of fault activation caused by deep mining in Shizishan Copper Mine, China, a monitoring system for fault activation slip is designed and implemented on the basis of the field investigation of footwall fault activation of the main orebody in the mining area. The displacement and stress of the fault are monitored by the multipoint displacement meter, bolt stress meter, and borehole stress meter. According to the measured results, the activation deformation laws of fault F2, fault F3, and fault F4 during deep continuous mining are analyzed in detail. The results show that, when the influence range of underground mining spreads to the fault, the increase in the additional tensile stress on the fault plane will reduce the shear strength of the fault and increase the slip of the fault. When the shear stress exceeds the shear strength of the fault plane, the shear failure of the fault plane occurs, the rock mass on both sides of the fault loses stability, and the fault becomes active; when the orebody in the deep sublevel 14 and sublevel 15 were continuously stoped, the development of the mining influence area to fault F2 leads to fault F2’s activation. When stoping the orebody in sublevel 16, fault F3 also activates. With the continuous downward mining of the deep part, the slip amount increases continuously. The fault activation sequence is from fault F2 to fault F3, and then to fault F4. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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20 pages, 5823 KiB

Open AccessArticle

Triaxial Compression Fracture Characteristics and Constitutive Model of Frozen–Thawed Fissured Quasi-Sandstone

by Yi Xie, Jianxi Ren, Tailang Caoxi, Xu Chen and Mengchen Yun

Appl. Sci. 2022, 12(13), 6454; https://doi.org/10.3390/app12136454 - 25 Jun 2022

Viewed by 1567

Abstract

The artificial frozen wall crossing the water-rich sand layer is prone to failure during thawing. To study the loading fracture characteristics and damage evolution of single-fissured sandstone after thawing, quasi-sandstones with prefabricated single fissure at different angles were prepared using the sandstone of [...] Read more.

The artificial frozen wall crossing the water-rich sand layer is prone to failure during thawing. To study the loading fracture characteristics and damage evolution of single-fissured sandstone after thawing, quasi-sandstones with prefabricated single fissure at different angles were prepared using the sandstone of the Luohe Formation as the original rock to conduct freeze–thaw tests with various temperature differences, and triaxial compression tests were performed on the samples. Based on the distribution theory of rock micro-element strength and static elastic modulus, a damage constitutive model of single-fissured quasi-sandstone under freezing–thawing and confining pressure was established. The results show that with the decrease in freezing temperature, the amount of flake spalling on the sample surface increases, and the frost-heaving cracks of quasi-sandstone become more numerous and longer, which makes the single-fissured quasi-sandstone tend to have a more complex tensile–shear hybrid failure than a shear failure. Moreover, with the increase in fissure angle, the absolute value of the freezing temperature required to produce frost-heaving cracks increases. An S-shaped damage evolution curve corresponds to each stage of triaxial compression of single-fissured quasi-sandstone. With the decrease in freezing temperature, the strength of rock after thawing decreases, and the brittleness characteristics strengthen. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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24 pages, 7052 KiB

Open AccessArticle

Study of Rock Crack Extension under Liquid Nitrogen Low-Temperature Fracturing

by Chunyan Bao, Meng Zhou and Yuexiang Cai

Appl. Sci. 2022, 12(11), 5739; https://doi.org/10.3390/app12115739 - 05 Jun 2022

Viewed by 1794

Abstract

Shale gas is a promising new energy source stored in shale. This research aims to study the laws of rock crack initiation and propagation under the low-temperature fracturing of liquid nitrogen, explore the influencing factors of the shale reservoir fracturing effect, and identify [...] Read more.

Shale gas is a promising new energy source stored in shale. This research aims to study the laws of rock crack initiation and propagation under the low-temperature fracturing of liquid nitrogen, explore the influencing factors of the shale reservoir fracturing effect, and identify the method that achieves the best fracturing effect and obtains the highest economic benefits. Herein, a visualized physical experiment of the liquid nitrogen effect is carried out, and the fracture process of a numerical model under cold shock is simulated to analyze the effect of homogeneity on shale crack propagation. The results show that two different crack development modes could be observed in the field test. The first one was the horizontal plane radial crack caused by longitudinal thermal shrinkage, and the other one was the vertical tensile crack caused by circumferential shrinkage. A certain interval length was frequently found between the horizontal cracks. The crack propagation of the specimens with different homogenization degrees was basically distributed in the direction perpendicular to the liquid nitrogen contact surface. When the homogenization degrees were m = 2 and 5, the crack surface was rough and the microfracture was disordered and dotted around the crack tip. When m ≥ 10, the dotted damage around the crack tip did not appear, and the crack propagation was close to the results obtained from using the homogeneous materials. Finally, this work simulates the fracture process of a circular hole plane model under cold shock, analyzes the influences of heat transfer coefficient, in situ stress and other parameters on shale temperature, minimum principal stress distribution, and crack propagation, and discusses the optimal method to improve the heat transfer coefficient. The results show that increasing the heat transfer coefficient can increase the tensile stress value and influence the range of the contact boundary, making the rock more prone to cracking and resulting in greater crack development and a better crack initiation effect. The lateral stress coefficient affects the propagation direction of the cracks, and the propagation depths of low-temperature cracks were found to be deeper in the direction of larger principal stress. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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9 pages, 3051 KiB

Open AccessArticle

Deformation and Control of Super-Large-Diameter Shield in the Upper-Soft and Lower-Hard Ground Crossing the Embankment

by Shuang You and Jianan Sun

Appl. Sci. 2022, 12(9), 4324; https://doi.org/10.3390/app12094324 - 25 Apr 2022

Cited by 1 | Viewed by 1276

Abstract

Compared with the small diameter and the single stratum shield tunnel, the surface subsidence is much greater when the large-diameter shield passes through the upper-soft and lower-hard stratum. In this case, it is particularly important to control the deformation of the embankment when [...] Read more.

Compared with the small diameter and the single stratum shield tunnel, the surface subsidence is much greater when the large-diameter shield passes through the upper-soft and lower-hard stratum. In this case, it is particularly important to control the deformation of the embankment when the large-diameter shield passes underneath the embankment. Taking a tunnel project underneath the north embankment in Zhuhai as an engineering example, this paper investigates the deformation characteristics and safety control measures of a super-large-diameter (i.e., 15.80 m) shield tunnel underneath the embankment under complex stratum conditions in upper-soft and lower-hard strata, using on-site monitoring and three-dimensional numerical simulations. The results of the numerical simulation show that grouting can effectively reduce the settlement of the embankment. Grouting is applied to practical engineering, and the monitoring data are in agreement with the numerical simulation results. The surface subsidence of the embankment gradually increases as the shield tail leaves the monitoring section and finally stabilizes. After the shield machine has passed through the embankment, the horizontal deformation troughs on the embankment’s surface conform to the Gaussian normal distribution. The maximum settlement occurs in the area directly above the central axis of the tunnel. The deformation trough covers an area about four times the diameter of the tunnel on both sides of its center line. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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14 pages, 4349 KiB

Open AccessArticle

Energy Evolution Analysis of Coal Fracture Damage Process Based on Digital Image Processing

by Zhonghu Wu, Liping Li, Yili Lou and Wentao Wang

Appl. Sci. 2022, 12(8), 3944; https://doi.org/10.3390/app12083944 - 13 Apr 2022

Cited by 3 | Viewed by 1355

Abstract

Coal rocks often contain calcite, which has a significant effect on the mechanical properties of coal and the energy evolution during rupture damage. In this study, the meso-scale of rock is considered, and the spatial distribution of the internal structure of coal is [...] Read more.

Coal rocks often contain calcite, which has a significant effect on the mechanical properties of coal and the energy evolution during rupture damage. In this study, the meso-scale of rock is considered, and the spatial distribution of the internal structure of coal is characterized by digital image technology. Uniaxial compression tests were conducted using RFPA on coal rocks containing calcite veins with diverse dip angles. The research results show that the different azimuth angles of the calcite veins change the internal stress distribution of the coal, resulting in higher coal compressive strength at low dip angles (0°, 15° and 30°). Under high dip angles (45°, 60°, 75° and 90°), coal has lower compressive strength. The fracture mode of coal is significantly affected by calcite. At low dip angle, the fracture mode of coal and rock is complex, which are inclined Z-type (0°), V-type (15°) and inverted V-type (30°), respectively. At high dip angle, the fracture mode of coal and rock is single, which is type I failure mode. The destruction process of coal rocks is influenced by calcite veins. Under low dip angle, the internal stress distribution of coal is relatively uniform, the weak cementation between matrix and calcite vein in coal is not easy to be damaged, the stress required for coal failure is large and the input energy, accumulated elastic energy and impact energy index are large. Under high dip angle, the internal stress distribution of coal is uneven, the weak cementitious material between matrix and calcite vein in coal is easy to be damaged and the input energy, accumulated elastic energy and impact energy index are small. Full article

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

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