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ISSN 1000-0585
CN 11-1848/P
Started in 1982
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  • Table of Content
      , Volume 61 Issue 8 Previous Issue    Next Issue
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    Preface
    Experimental Research
    X-ray radiography for visualization of fissure fluid flows during rock failures
    SUN Huan, LIU Xiaoli, WANG Enzhi, ZHANG Jianmin, WANG Sijing, LIU Chi
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 778-791.   DOI: 10.16511/j.cnki.qhdxxb.2021.25.017
    Abstract   HTML   PDF (37321KB) ( 196 )
    Fissure fluids can induce engineering hazards due to interactions between the fluid and the rock. Further research is needed to describe the induced hazards mechanism for fluid and rock interactions by observing fissure fluid flows during rock failures. This study used X-ray imaging to observe fissure fluid flows in rocks during failures. This work studied Karst landslides, slope instabilities in the rock bed mudstone (RBM) area and water bursting into the goaf. The results show that the laminar flows change to transitional or turbulent flows in the rocks during pipe rupture into the fissures. This process shows the fractal characteristics of pipe flows evolving with the fissure flows during Karst rock failures with multilevel stress loadings. Also, the RBM damage parameters increase non-linearly as the rock fails due to the coupled effects of the fluid dynamics and the multilevel stress loadings. This indicates that seepage control and diffusion can reduce and prevent landslides in the RBM area. The mean square fissure water flow rates also show fractional evolution during coal and rock failures, with the flow regime changing from sup-diffusion flow to sub-diffusion flow. The non-linear flow characteristics and anomalous diffusion of fissure water are key reasons for water bursting hazards during coal seam mining.
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    Evolution law of water-rock interaction on the shear behavior of granite fractures
    DOU Zihao, ZHAO Zhihong, GAO Tianyang, LI Jinjin, YANG Qiang
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 792-798.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.027
    Abstract   HTML   PDF (11201KB) ( 182 )
    To study the influence of water-rock interaction on the shear behavior of granite fractures, direct shear tests were conducted on Beishan granite fracture samples that were dry or immersed in water for different durations. The immersion affected the shear properties of granite fractures in terms of decreasing peak strength, shear stiffness and dilation, and increasing peak shear displacement. The three-dimensional morphologies of fracture surfaces before and after shear were recorded using a three-dimensional blue light scanner, and the volume changes of sheared-off asperities were calculated. With increased immersion duration, the volume of the sheared-off asperities increased significantly, and an empirical expression is proposed to describe this relation. Moreover, a nanoindentation test was conducted on the granite surfaces under dry and immersed for 12 months conditions. The maximum indentation depth and plastic deformation of the immersed granite samples were significantly larger than those of the dry samples. The nanohardness and Young's modulus of the granite samples decreased significantly after the long-duration immersions, which indicated that the fracture asperities were damaged more easily after the immersions.
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    Macro-micro experimental study of rock static and dynamic fracture toughness
    MAN Ke, LIU Xiaoli, SONG Zhifei, GUO Zhanfeng, LIU Zongxu, YU Yunhe
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 799-808.   DOI: 10.16511/j.cnki.qhdxxb.2021.26.018
    Abstract   HTML   PDF (15109KB) ( 145 )
    The notched semi-circle bend (NSCB) specimen and fracture toughness test method was used to measure the static and dynamic fracture toughness of Beijing Fangshan granite samples. The results showed the dynamic fracture toughness is generally 1.3~2.6 times the static fracture toughness for medium and high strain rates. The surface morphologies of the damaged rock samples were then characterized using SEM and a laser confocal microscope for three-dimensional reconstructions of the fractures and surface roughnesses. The static and dynamic fracture modes were both mode I crack tensile failures. The difference between the dynamic and static crack growth was that the stress wave in the dynamic tests reflected back and forth at the internal interface of rock sample which induced micro-cracks and accelerated the convergence and penetration of existing and new cracks. The crack propagation had acceleration and deceleration stages with the changes in the rock crack propagation speed consistent with the relative height of the reconstructed three-dimensional rock surface morphology, which also corresponded to the rock surface roughness variations. The key difference between the static and dynamic behavior is that the rate effect (inertia effect) during dynamic loading tends to counteract the structural effects of the physical and geometric properties of the rock material itself.
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    Numerical Simulation
    Numerical simulations of double-shield TBM tunneling for analyzing shield jamming control factors
    HOU Shaokang, LIU Yaoru
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 809-817.   DOI: 10.16511/j.cnki.qhdxxb.2021.26.013
    Abstract   HTML   PDF (7387KB) ( 264 )
    Shield jamming is an important engineering geological problem that restricts safe, efficient construction of double-shield tunnels by tunnel boring machines (TBMs). Many factors influence shield jamming with numerical simulations needed to analyze the influences of these many factors. This study used numerical simulations of the tunneling process of a double shield TBM. The simulations use the creep model based on internal variable thermodynamics theory to simulate the time-dependent deformation of the surrounding rock and the interactions between the surrounding rock and the shield. The simulations were used to analyze the influence of two shield jamming control factors, the TBM advance rate and the overcut, for a highway tunnel. The results show that increasing the advance rate somewhat reduces the contact force and the shield jamming risk. The overcut has a larger effect on the contact force. Careful use of these two shield jamming control measures is more important for tunnel construction with high ground stresses and soft rock conditions. These results provide a reference for shield jamming risk analyses and optimizing the TBM parameters such as the advance rate and the overcut.
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    Eccentric four-dimensional lattice spring model for heterogeneous rock fracturing
    ZHAO Gaofeng, QIAO Lei, ZHANG Yuliang, SUN Jianhua, CHEN Hua
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 818-826.   DOI: 10.16511/j.cnki.qhdxxb.2021.26.014
    Abstract   HTML   PDF (11696KB) ( 258 )
    The time needed to generate ultra-large heterogeneous models limits the application of discrete numerical methods in rock engineering. This paper presents an eccentric four-dimensional spring model (eccentric 4D-LSM, ECC4D) based on a 4D-LSM (four-dimensional lattice spring model) for parameter analyses of the effects of Poisson ratio, the elastic modulus and the ultimate normal deformation. The model feasibility is demonstrated through a study of the elastic parameters and failure parameters. The irregular, eccentric four-dimensional model ECC4D is generated using a random offset of the center of mass that can be used for large-scale calculations. ECC4D characterizes the irregularities of the rock materials while quickly generating an arbitrary geometry model. This method has been used to prepare a dam model with more than 100 million particles on a workstation.
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    Effective permeability of three-dimensional fractured rock with low fracture densities
    HE Chen, YAO Chi, SHAO Yulong, HUANG Fan, ZHOU Chuangbing
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 827-832.   DOI: 10.16511/j.cnki.qhdxxb.2021.25.014
    Abstract   HTML   PDF (4035KB) ( 173 )
    The permeability of fractured rock is an important parameter in many underground projects, but the permeability characteristics are normally not well known and differ significantly from the effective permeability of rocks with high fracture densities that have been widely studied in previous research. This paper presents a numerical method to model the flows in three-dimensional fractured porous media based on the equivalent discrete fracture network model. This method uses two-dimensional triangular elements to simulate the flows in the fractures and the permeable matrix in the rock which predicts the fluid exchange between the fractures and the matrix. The results are used to derive an analytical expression for the conductivity coefficient of the equivalent element. Then, the Monte-Carlo method is used to analyze the permeability of fractured rock with various fracture densities and scales. The results show that the presence of a fracture network in the study area greatly influences the effective permeability and that the effective permeability of fractured rock has a unique scale effect for low fracture densities. After classifying the effective permeabilities with and without fracture networks in the domain, the average effective permeabilities for the two types show regular variations depending on the scale of the domain. This rule can be used to establish a relationship between the effective permeability of various size rock masses.
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    A fully coupled and full 3D finite element model for hydraulic fracturing and its verification with physical experiments
    BAO Jinqing, YANG Chenxu, XU Jianguo, LIU Hongxia, WANG Gaocheng, ZHANG Guangming, CHENG Wei, ZHOU Desheng
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 833-841.   DOI: 10.16511/j.cnki.qhdxxb.2021.26.019
    Abstract   HTML   PDF (7253KB) ( 239 )
    Two sets of equation are proposed to describe the key mechanic issues in hydraulic fracturing including rock deformation, fracture propagation, fluid flow and leak-off in fractures, where the finite element method is taken as the numerical foundation. The fully coupled and full 3-D numerical model for hydraulic fracturing is set up via solving the coupled two sets of equation simultaneously. Comparisons of the numerical simulations from the model with two classical physical experiments are made, and they have excellent agreements on net pressure, fracture widths, fracture lengths, fracture propagation modes, et al. The numerical model is verified by the experiments, and shows that the cubic law in the hydraulic fracturing theory is still applicable even when the fracture widths are at the order of microns.
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    Engineering Application
    Characteristics of excavation disasters and long-term in-situ mechanical behavior of the tunnels in the China Jinping Underground Laboratory
    LI Shaojun, ZHENG Minzong, QIU Shili, YAO Zhibin, XIAO Yaxun, ZHOU Jifang, PAN Pengzhi
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 842-852.   DOI: 10.16511/j.cnki.qhdxxb.2021.26.015
    Abstract   HTML   PDF (17479KB) ( 286 )
    The construction of deep high-stress tunnels can face various problems such as rockbursts, rib spalling, and tunnel collapse. The 2 400 m deep China Jinping Underground Laboratory (CJPL-II) is currently the world's largest buried laboratory. The construction of this group of tunnels included field monitoring and numerical analyses of the mechanical response of the rock around the tunnels, such as the deformation, stresses and microseismic events. The complex geological conditions are analyzed to predict tunnel disaster characteristics and the long-term in-situ mechanical response of the rock. The results show that the surrounding rock mass deformation is larger on the north side walls of laboratory 1# and laboratory 4# with a maximum deformation of 83.7 mm. The maximum rock bolt stress is 530 MPa. The rock mass deformation tended to become stable about three months after completion of the excavation. The excavation damage zone revealed by elastic waves and a borehole camera is generally 0.8~3.5 m. The results also show that the internal fractures in the surrounding rock mass evolve with the excavation with zonal disintegration. The high strength, good integrity rock has a small fracture zone while the low strength, poor integrity rock has a large fracture zone. The results also show that there is more micro-seismic activity in the completed rock mass tunnel and the area around the fault. The intensity of the microseismic activity in each laboratory tunnel during excavation was highest for 8# and decreased to 8#, 7#, 4#, 3#, 5#, 6#, 1#, 2# and 9# as the lowest. After excavation, the microseismic activity in each tunnel gradually decreased. The CASRock software analysis showed the high stresses and large relaxation depth of the southern arch shoulder and sidewall after excavation and unloading that created high-risk areas. The results provide direct support for disaster warning system development, stability assessments, dynamic designs, and long-term safe tunnel operation for safe construction of high-stress, deep tunnels with similar geological conditions.
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    Full-space collaborative control technology and its application for deeply buried roadways
    ZUO Jianping, SUN Yunjiang, WEN Jinhao, WU Genshui, YU Meilu
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 853-862.   DOI: 10.16511/j.cnki.qhdxxb.2021.25.018
    Abstract   HTML   PDF (25470KB) ( 325 )
    Deep roadways use a variety of support methods with the support strength influenced by the ground stresses and the mechanical parameters of the surrounding rock. This paper summarizes the deformation characteristics of deep roadways. The full-space collaborative control method given here uses targeted optimization of the support methods and properties. Full-space collaborative control optimizes the soil rigidity and flexibility, pressure release, rock dynamics, and local reinforcement. The objective is to fully use the self-supporting capacity of the surrounding rock. The prestress fields are compared for a single anchor cable, a traditional truss and a space truss for various preloadings. The maximum prestress of the full space truss is 35%~40% higher than that of the single anchor cable. Additionally, simulations show that the cohesion, the internal friction angle and the elastic modulus all greatly influence the roadway deformation. Field practice shows that this design method can effectively control the deformation of the surrounding rock for roadways buried at various depths.
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    Failure mechanism and stability analysis method of the Xiyu conglomerate slope
    WANG Yujie, SUN Ping, LI Wenxin, ZHANG Qiang
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 863-872.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.028
    Abstract   HTML   PDF (29386KB) ( 135 )
    Because the Xiyu conglomerate slope is commonly encountered in the construction of hydraulic projects in Xinjiang, its specific deformation characteristics and failure mechanism were studied through field investigation and discrete element method (DEM) numerical simulation analysis. A stability analysis method that is compatible with the deformation and failure mechanism was proposed according to force and moment balance analysis. The influence of erosion and crack depths on slope stability was analyzed. Results indicate that the failure process of the Xiyu conglomerate slope generally shows the characteristic of "slope toe erosion-tension crack at crown-staggered collapse." This can be divided into four evolutionary stages:slope toe erosion, local collapse, local collapse aggravated, and vertical tension crack formation. As the erosion or crack depth increases, the dumping safety factor of the slope decreases.
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    Influence of the rock mass structure and lithology on the dynamic response characteristics of steep rock slopes during earthquakes
    SONG Danqing, HUANG Jin, LIU Xiaoli, WANG Enzhi
    Journal of Tsinghua University(Science and Technology). 2021, 61 (8): 873-880.   DOI: 10.16511/j.cnki.qhdxxb.2021.25.020
    Abstract   HTML   PDF (22792KB) ( 162 )
    Numerical models were used to study the influence of the rock mass structure and lithology on the seismic response characteristics of steep rock slopes for homogeneous soft/hard rock slopes and layered soft/hard rock slopes. The dynamic analyses used the finite element method. The predictions gave the dynamic acceleration amplification coefficient (MPGA) of the slopes that characterized the influence of the rock structure and lithology on the wave propagation characteristics and the amplification effect. The results show that the rock structure and the lithology influence the wave propagation characteristics in the slopes with weak interlayer interactions leading to local amplification of the seismic waves in the slopes. The dynamic amplification effect is greater for soft rock slopes than for hard rock slopes. The lithology has more effect on the dynamic response of the slopes than the rock structure. The lithology also more greatly influences the seismic amplification of layered slopes than homogeneous slopes. The MPGA ratios of homogeneous soft rock and homogeneous hard rock slopes are smaller than those of layered soft rock and layered hard rock slopes. The soft and hard rock slopes also show elevation and trend magnification effects. The elevation amplification effect of the layered slopes does not vary linearly as with homogeneous slopes. The weak interlayer interactions impact the slope amplification effect while the dynamic magnification effect of layered slopes with weak interlayer interactions is larger than for homogeneous slopes.
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