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Editor Work2021 Vol. 29, No. 5
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2021, 29(5): 1247-1257.
Abstract
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The mechanical properties of jointed rock mass are of great significance for guiding engineering design and construction. In order to study the effect of the combined joint parameters on mechanical properties of specimens,we conducted compression-shear tests. We used rock-like materials to prefabricate ubiquitous jointed specimens with different joint inclinations and spacings. During the test,we used high-definition camera to record the failure characteristics of ubiquitous jointed specimens. We divided the specimen's failure modes into three types: plane shear failure,quasi-integrity shear failure and oblique shear failure. The results show that the joint spacing can change the failure mode of specimens and the joint inclination determines the failure mode of specimens. Then,we analyzed the peak shear strength of specimens with different joint parameters. The results show that the weaken degree of ubiquitous joint on the shear strength reaches 15.95% ~56.62%. The peak shear strength of jointed specimens gradually increased with the joint spacing's increase,and had the first increase and then decrease trend with the increase of the joint inclination. In addition,we used the acoustic emission(AE)device to monitor the specimens' loading process in real time to analyze the AE characteristics of specimens with different joint parameters. During the compaction phase,the AE maximum impact number for jointed specimens increases as the joint spacing decreases. For the AE energy characteristics of specimens,the peak energy of jointed specimens decreases with the joint spacing's decrease,but the mean energy increases with joint spacing's decrease. As the joint inclination increases,the jointed specimens' peak energy and mean energy both increase first and then decrease.
The mechanical properties of jointed rock mass are of great significance for guiding engineering design and construction. In order to study the effect of the combined joint parameters on mechanical properties of specimens,we conducted compression-shear tests. We used rock-like materials to prefabricate ubiquitous jointed specimens with different joint inclinations and spacings. During the test,we used high-definition camera to record the failure characteristics of ubiquitous jointed specimens. We divided the specimen's failure modes into three types: plane shear failure,quasi-integrity shear failure and oblique shear failure. The results show that the joint spacing can change the failure mode of specimens and the joint inclination determines the failure mode of specimens. Then,we analyzed the peak shear strength of specimens with different joint parameters. The results show that the weaken degree of ubiquitous joint on the shear strength reaches 15.95% ~56.62%. The peak shear strength of jointed specimens gradually increased with the joint spacing's increase,and had the first increase and then decrease trend with the increase of the joint inclination. In addition,we used the acoustic emission(AE)device to monitor the specimens' loading process in real time to analyze the AE characteristics of specimens with different joint parameters. During the compaction phase,the AE maximum impact number for jointed specimens increases as the joint spacing decreases. For the AE energy characteristics of specimens,the peak energy of jointed specimens decreases with the joint spacing's decrease,but the mean energy increases with joint spacing's decrease. As the joint inclination increases,the jointed specimens' peak energy and mean energy both increase first and then decrease.
2021, 29(5): 1258-1266.
Previous research of Brazilian disc test and simulation is mostly focused on either macroscopic failure modes or micro evolution rules. The energy evolution characteristics in the splitting process remains to be discussed in detail. In this paper,Brazilian disc tests are carried out on granite. The fracture processes of specimens are monitored using digital image correlation (DIC) and acoustic emission (AE). Both fracture modes and scales during the test are analyzed. On this basis,PFC2D is employed to simulate the process of Brazilian disc test. The energy characteristics of specimen is illustrated by comparing different energy components. The test and simulation results indicate that: (1) The process of Brazilian disc test is divided into four stages,including fracture compaction (Ⅰ),cracks initiation (Ⅱ),cracks growth (Ⅲ) and post-peaking failure (Ⅳ)stage. (2) Tensile microcrack is the major fracture mode in both stage Ⅱ and Ⅲ. Shear microcrack occurs at later period of stage Ⅲ. Then macrocrack forms in the loading diameter direction,and causes the final failure. (3) There is nearly no dissipated energy in first two stages. All the external work is transformed into releasable strain energy in the specimen. At the end of stage Ⅲ,the strain energy releases rapidly,and the energy of AE reaches to the maximum as the stress approaches to the peak. The releasable strain energy decreases remarkably at stage Ⅳ. The portion of dissipated energy increases significantly due to a large number of new fracture surfaces are formed.
Previous research of Brazilian disc test and simulation is mostly focused on either macroscopic failure modes or micro evolution rules. The energy evolution characteristics in the splitting process remains to be discussed in detail. In this paper,Brazilian disc tests are carried out on granite. The fracture processes of specimens are monitored using digital image correlation (DIC) and acoustic emission (AE). Both fracture modes and scales during the test are analyzed. On this basis,PFC2D is employed to simulate the process of Brazilian disc test. The energy characteristics of specimen is illustrated by comparing different energy components. The test and simulation results indicate that: (1) The process of Brazilian disc test is divided into four stages,including fracture compaction (Ⅰ),cracks initiation (Ⅱ),cracks growth (Ⅲ) and post-peaking failure (Ⅳ)stage. (2) Tensile microcrack is the major fracture mode in both stage Ⅱ and Ⅲ. Shear microcrack occurs at later period of stage Ⅲ. Then macrocrack forms in the loading diameter direction,and causes the final failure. (3) There is nearly no dissipated energy in first two stages. All the external work is transformed into releasable strain energy in the specimen. At the end of stage Ⅲ,the strain energy releases rapidly,and the energy of AE reaches to the maximum as the stress approaches to the peak. The releasable strain energy decreases remarkably at stage Ⅳ. The portion of dissipated energy increases significantly due to a large number of new fracture surfaces are formed.
2021, 29(5): 1267-1275.
There are many layer composite rock masses in tunnel engineering. The weakness of the bedding plane directly determines the stability of the tunnel surrounding rock. In order to reveal the influence of the bedding plane on the deformation of surrounding rock,a tunnel cross-section is established by using AutoCAD software. By importing it into the finite element simulation software ABAQUS,and embedding 0 thickness cohesive elements with different angles as the bedding plane,the tunnel section models with different bedding planes are constructed. Based on the numerical model,the response characteristics of bedding plane angle in the surrounding rock deformation are studied. It is found that the deformed region of the surrounding rock changes from the top to the sidewall during the transition from the horizontal bedding to the vertical bedding. Combined with the acoustic emission simulation data obtained by secondary development,the easy fracture zone of surrounding rock under different bedding plane angles is obtained. According to the regional distribution characteristics,3D laser scanning technology can be used to generate point cloud data for 3D reconstruction of tunnel sections after tunnel construction. In order to avoid the disadvantage of a large amount of point cloud data,the idea of hierarchical resampling is introduced to refine the local point cloud in the deformable zone of the layered surrounding rock. Through the real-time comparative analysis of the point cloud model in the deformation area of the tunnel section,the efficient and accurate monitoring of tunnel surrounding rock can be realized,which can guide the safe and stable operation of tunnel surrounding rock.
There are many layer composite rock masses in tunnel engineering. The weakness of the bedding plane directly determines the stability of the tunnel surrounding rock. In order to reveal the influence of the bedding plane on the deformation of surrounding rock,a tunnel cross-section is established by using AutoCAD software. By importing it into the finite element simulation software ABAQUS,and embedding 0 thickness cohesive elements with different angles as the bedding plane,the tunnel section models with different bedding planes are constructed. Based on the numerical model,the response characteristics of bedding plane angle in the surrounding rock deformation are studied. It is found that the deformed region of the surrounding rock changes from the top to the sidewall during the transition from the horizontal bedding to the vertical bedding. Combined with the acoustic emission simulation data obtained by secondary development,the easy fracture zone of surrounding rock under different bedding plane angles is obtained. According to the regional distribution characteristics,3D laser scanning technology can be used to generate point cloud data for 3D reconstruction of tunnel sections after tunnel construction. In order to avoid the disadvantage of a large amount of point cloud data,the idea of hierarchical resampling is introduced to refine the local point cloud in the deformable zone of the layered surrounding rock. Through the real-time comparative analysis of the point cloud model in the deformation area of the tunnel section,the efficient and accurate monitoring of tunnel surrounding rock can be realized,which can guide the safe and stable operation of tunnel surrounding rock.
2021, 29(5): 1276-1285.
The particle fragility of calcareous sand is an important property that causes its deformation and strength characteristics different from standard sand. Based on the critical state theory,this paper quantitatively reveals the evolution of the critical state line of calcareous sand with particle breakage through a series of experiments. The experiment is carried out in two different stages. In the first stage,the deformation characteristics and particle breakage characteristics of calcareous sand under different confining pressures(60~2000 kPa) are analyzed. In the second stage,the samples with different crushing rates produced from the first stage is reshaped,he second shearing under the confining pressure of 100~300 kPa is performed,then the crushing critical state line is acquired. The test results show that under low confining pressure(<300 kPa) both loose and dense sand show the strain softening and dilatancy characteristics. While under high confining pressure(>1 MPa),significant particle breakage causes continuous shear contraction. There is an obvious confining pressure threshold for particle breakage. For loose sand,there is basically no particle breakage when the confining pressure is less than 300 kPa. In the e-lg p' plane,the intercept ΔeΓ and the slope λc of the crushing critical state line decreases with the increase of Br*,so the particle breakage makes the critical state line move downward and rotate counterclockwise. While in the q-p' plane,the critical state points of calcareous sand fall on the same straight line,indicating that there is a unique critical state stress ratio Mcr and critical friction angle φcr.
The particle fragility of calcareous sand is an important property that causes its deformation and strength characteristics different from standard sand. Based on the critical state theory,this paper quantitatively reveals the evolution of the critical state line of calcareous sand with particle breakage through a series of experiments. The experiment is carried out in two different stages. In the first stage,the deformation characteristics and particle breakage characteristics of calcareous sand under different confining pressures(60~2000 kPa) are analyzed. In the second stage,the samples with different crushing rates produced from the first stage is reshaped,he second shearing under the confining pressure of 100~300 kPa is performed,then the crushing critical state line is acquired. The test results show that under low confining pressure(<300 kPa) both loose and dense sand show the strain softening and dilatancy characteristics. While under high confining pressure(>1 MPa),significant particle breakage causes continuous shear contraction. There is an obvious confining pressure threshold for particle breakage. For loose sand,there is basically no particle breakage when the confining pressure is less than 300 kPa. In the e-lg p' plane,the intercept ΔeΓ and the slope λc of the crushing critical state line decreases with the increase of Br*,so the particle breakage makes the critical state line move downward and rotate counterclockwise. While in the q-p' plane,the critical state points of calcareous sand fall on the same straight line,indicating that there is a unique critical state stress ratio Mcr and critical friction angle φcr.
2021, 29(5): 1286-1294.
Polymer treatment of bentonite contributes to enhance effectively the chemical compatibility of vertical barriers. Reported researches are limited by the impacts of organic contaminant and kind of polymer on the anti-seepage performance of polymer treated bentonites. The swell potential and permeability of sodium carboxymethyl cellulose(CMC)treated bentonites subjected to phenol solutions are evaluated via swell index and modified filtration tests. Fourier transform infrared spectrum analysis is conducted to understand the reasons for the improvement of the chemical compatibility due to characteristic groups on CMC polymer chains. The swell index(SI) of CMC treated bentonite is 1.5 to 2.5 times higher than that of untreated bentonite(CB)for a given phenol concentration. The hydraulic conductivity(k) of CB is reduced by an order of magnitude after CMC treatment. The bentonite treated by CMCⅢ shows the best anti-seepage performance. Molecular weight and degree of substitution of CMC have no significant effect on both SI and k in this study. The main reasons for such improvementare as follows:(1)Polymer hydrogels in CMC treated bentonite is able to block macro-pores between bentonite particles. (2)Characteristic groups(carboxyl group and carboxylic acid group) can adsorb phenol through esterification reaction and hydrogen bondings existed between phenol hydroxyl group and water molecules in hydrogels hinder the migration of phenol in solution,leading to reduce the detrimental effect of phenol on the double layer thickness of montmorillonite.
Polymer treatment of bentonite contributes to enhance effectively the chemical compatibility of vertical barriers. Reported researches are limited by the impacts of organic contaminant and kind of polymer on the anti-seepage performance of polymer treated bentonites. The swell potential and permeability of sodium carboxymethyl cellulose(CMC)treated bentonites subjected to phenol solutions are evaluated via swell index and modified filtration tests. Fourier transform infrared spectrum analysis is conducted to understand the reasons for the improvement of the chemical compatibility due to characteristic groups on CMC polymer chains. The swell index(SI) of CMC treated bentonite is 1.5 to 2.5 times higher than that of untreated bentonite(CB)for a given phenol concentration. The hydraulic conductivity(k) of CB is reduced by an order of magnitude after CMC treatment. The bentonite treated by CMCⅢ shows the best anti-seepage performance. Molecular weight and degree of substitution of CMC have no significant effect on both SI and k in this study. The main reasons for such improvementare as follows:(1)Polymer hydrogels in CMC treated bentonite is able to block macro-pores between bentonite particles. (2)Characteristic groups(carboxyl group and carboxylic acid group) can adsorb phenol through esterification reaction and hydrogen bondings existed between phenol hydroxyl group and water molecules in hydrogels hinder the migration of phenol in solution,leading to reduce the detrimental effect of phenol on the double layer thickness of montmorillonite.
2021, 29(5): 1295-1302.
Thixotropy is a very important and unique property of soft clay in engineering. It is described as an isothermal,reversible,time-dependent process occurring under conditions of constant composition and volume,whereby a material stiffens while at rest and softened or liquefied upon remoulding. In this study,the thixotropic strength recovery of representative deep-sea soft clays collected in different areas in South China Sea is investigated with fall cone method. In addition,the microstructural evolution during thixotropy is captured by scanning electron microscopy,and the related microscopic characteristics are quantitatively analysed. The results present that all the deep-sea soft clays in South China Sea can show thixotropic behaviour. However,the thixotropic potential of different clays is quite distinguished,which might be related to the basic properties. The microstructure of clay during thixotropy changes significantly,and can transfer from dispersive to flocculation. The diameter and orientation characteristics of microstructural unit have some adjustment. The proportions of microstructural unit in small-sized parts decrease while the proportions in larger-sized parts increase with time. However,there is no obvious orientation of the arrangement of microstructural unit during thixotropy.
Thixotropy is a very important and unique property of soft clay in engineering. It is described as an isothermal,reversible,time-dependent process occurring under conditions of constant composition and volume,whereby a material stiffens while at rest and softened or liquefied upon remoulding. In this study,the thixotropic strength recovery of representative deep-sea soft clays collected in different areas in South China Sea is investigated with fall cone method. In addition,the microstructural evolution during thixotropy is captured by scanning electron microscopy,and the related microscopic characteristics are quantitatively analysed. The results present that all the deep-sea soft clays in South China Sea can show thixotropic behaviour. However,the thixotropic potential of different clays is quite distinguished,which might be related to the basic properties. The microstructure of clay during thixotropy changes significantly,and can transfer from dispersive to flocculation. The diameter and orientation characteristics of microstructural unit have some adjustment. The proportions of microstructural unit in small-sized parts decrease while the proportions in larger-sized parts increase with time. However,there is no obvious orientation of the arrangement of microstructural unit during thixotropy.
2021, 29(5): 1303-1311.
We study the influence of clay minerals on bound water characteristics and mechanical properties of soft soils. We first use X-ray diffractometry(XRD)to define the mineralogical composition of the soft soils. We also analyze the soft soils under thermogravimetric analysis(TG) and micro-commercial thermogravimetric analysis(DTG)with a simultaneous thermal analyzer. Based on the above analyses,we learn TG and DTG curve features of the soft soils in Huizhou,Shantou,Yangjiang,Zhanjiang,and Shenzhen. These analyses contribute to determining bound water content and boundaries. The test results are as follows. The clay mineral content of the soft soils positively correlates with the adsorbed water content of the soft soils,among which montmorillonite content has a greater influence on the water absorption of the soft soils. As the TG and DTG curves of the soft soils relate to clay mineral types and content,weight-loss valleys in DTG carves may indicate clay mineral types in the soft soils. In addition,we find it difficult to define the boundary between strongly bound water and weakly bound water solely based on TG and DTG curves.
We study the influence of clay minerals on bound water characteristics and mechanical properties of soft soils. We first use X-ray diffractometry(XRD)to define the mineralogical composition of the soft soils. We also analyze the soft soils under thermogravimetric analysis(TG) and micro-commercial thermogravimetric analysis(DTG)with a simultaneous thermal analyzer. Based on the above analyses,we learn TG and DTG curve features of the soft soils in Huizhou,Shantou,Yangjiang,Zhanjiang,and Shenzhen. These analyses contribute to determining bound water content and boundaries. The test results are as follows. The clay mineral content of the soft soils positively correlates with the adsorbed water content of the soft soils,among which montmorillonite content has a greater influence on the water absorption of the soft soils. As the TG and DTG curves of the soft soils relate to clay mineral types and content,weight-loss valleys in DTG carves may indicate clay mineral types in the soft soils. In addition,we find it difficult to define the boundary between strongly bound water and weakly bound water solely based on TG and DTG curves.
2021, 29(5): 1312-1319.
As a lightweight,highly compressible and elastic material,the rubber reinforced soil has been successfully applied in geotechnical engineering. Many researchers have studied the static and dynamic properties of rubber mixtures at room temperature. However,as the third largest frozen soil country in the world,the research on dynamic characteristics of expansive soil-rubber (ESR) mixture in frozen state is still relatively blank. In this paper,the dynamic response characteristics of expansive soil-rubber (ESR) under freezing condition are studied with low-temperature dynamic triaxial test. The results show that:(1)The hysteresis curve of ESR during freezing is oval,and it becomes flat with the increase of rubber content; (2)The shear stress of ESR is slightly lower than that of expansive soil. When the rubber content is 10%,the shear stress of ESR is the smallest; (3)The shear modulus of ESR decreases with the increase of rubber content and shear strain; (4)During the freezing process,the damping ratio of expansive soil and ESR decreases with the increase of shear strain,and the damping ratio of ESR is larger than that of pure soil.
As a lightweight,highly compressible and elastic material,the rubber reinforced soil has been successfully applied in geotechnical engineering. Many researchers have studied the static and dynamic properties of rubber mixtures at room temperature. However,as the third largest frozen soil country in the world,the research on dynamic characteristics of expansive soil-rubber (ESR) mixture in frozen state is still relatively blank. In this paper,the dynamic response characteristics of expansive soil-rubber (ESR) under freezing condition are studied with low-temperature dynamic triaxial test. The results show that:(1)The hysteresis curve of ESR during freezing is oval,and it becomes flat with the increase of rubber content; (2)The shear stress of ESR is slightly lower than that of expansive soil. When the rubber content is 10%,the shear stress of ESR is the smallest; (3)The shear modulus of ESR decreases with the increase of rubber content and shear strain; (4)During the freezing process,the damping ratio of expansive soil and ESR decreases with the increase of shear strain,and the damping ratio of ESR is larger than that of pure soil.
2021, 29(5): 1320-1330.
This paper studies the evolution of pore structure of Guiyang red clay during the whole process of shearing under conventional triaxial compression conditions, explores and analyzes the changes of fine-microstructure morphology of Guiyang red clay, and reveals the correlation between macroscopic damage morphology and fine-microstructure changes of Guiyang red clay. A fully automatic triaxial instrument, low-field NMR technique and SEM scanning electron microscope are used to conduct the whole process shear test on Guiyang red clay, and to analyze the specimens before and after shear by combining with the Particles(Pores) and Cracks Analysis System(PCAS).The research results show that the T2 spectrum of Guiyang red clay shows three types of peaks, corresponding to large pores, medium pores and small pores. The deformation and damage of Guiyang red clay specimens under the action of load are mainly based on the adjustment of the relative position between internal particles. The pore volume shows an overall decrease. With the intensification of stress, the red clay gradually forms small folds visible to the naked eye, the large scale pores gradually develop into small scale pores, the number of small pores gradually increases, the number of large-medium pores gradually decreases, and the pore direction becomes more and more confused. When the small folds gradually expand and penetrate to form a large slip surface, the pore morphology shows transient directionality. Afterward the compression of the slip surface can reduce the number of small pores and increase the number of large-medium pores. During the whole shearing process, the pore morphology of Guiyang red clay gradually tends to be similar and the pore boundary tends to be complex.
This paper studies the evolution of pore structure of Guiyang red clay during the whole process of shearing under conventional triaxial compression conditions, explores and analyzes the changes of fine-microstructure morphology of Guiyang red clay, and reveals the correlation between macroscopic damage morphology and fine-microstructure changes of Guiyang red clay. A fully automatic triaxial instrument, low-field NMR technique and SEM scanning electron microscope are used to conduct the whole process shear test on Guiyang red clay, and to analyze the specimens before and after shear by combining with the Particles(Pores) and Cracks Analysis System(PCAS).The research results show that the T2 spectrum of Guiyang red clay shows three types of peaks, corresponding to large pores, medium pores and small pores. The deformation and damage of Guiyang red clay specimens under the action of load are mainly based on the adjustment of the relative position between internal particles. The pore volume shows an overall decrease. With the intensification of stress, the red clay gradually forms small folds visible to the naked eye, the large scale pores gradually develop into small scale pores, the number of small pores gradually increases, the number of large-medium pores gradually decreases, and the pore direction becomes more and more confused. When the small folds gradually expand and penetrate to form a large slip surface, the pore morphology shows transient directionality. Afterward the compression of the slip surface can reduce the number of small pores and increase the number of large-medium pores. During the whole shearing process, the pore morphology of Guiyang red clay gradually tends to be similar and the pore boundary tends to be complex.
2021, 29(5): 1331-1341.
In order to explore the relationship between the energy evolution and the strength parameters of coarse-grained soils during shearing, a direct-shear test was conducted to analyze the energy evolution process and strength characteristics of coarse-grained soils. According to the principle of conservation of energy, the Mohr-Coulomb strength criterion was used to derive the balance equation between energy and strength parameters. A theoretical model and an applicable model were established, demonstrated and analyzed, to explore the quantitative correlation between energy and strength parameters of coarse-grained soils. Combined with MATLAB analysis function, the application of the models was discussed. The results have shown that: due to the intense energy evolution process of angular particles, the strength parameters of angular contact particles were larger than that of circular particles, with greater energy of the work done by external force to reach the critical peak stress; under different normal stresses, at the final phase, the work done by external force—F(c, φ) was linearly dependent on strength parameter values, and increased with the increase of the strength parameter values. Furthermore, its partial differential equation converged infinitely to the fixed value—f(cα, φβ). The research results can be used to analyze the shearing condition and energy evolution process of the roadbed filler, and to invert the strength parameter values that meet the engineering needs.
In order to explore the relationship between the energy evolution and the strength parameters of coarse-grained soils during shearing, a direct-shear test was conducted to analyze the energy evolution process and strength characteristics of coarse-grained soils. According to the principle of conservation of energy, the Mohr-Coulomb strength criterion was used to derive the balance equation between energy and strength parameters. A theoretical model and an applicable model were established, demonstrated and analyzed, to explore the quantitative correlation between energy and strength parameters of coarse-grained soils. Combined with MATLAB analysis function, the application of the models was discussed. The results have shown that: due to the intense energy evolution process of angular particles, the strength parameters of angular contact particles were larger than that of circular particles, with greater energy of the work done by external force to reach the critical peak stress; under different normal stresses, at the final phase, the work done by external force—F(c, φ) was linearly dependent on strength parameter values, and increased with the increase of the strength parameter values. Furthermore, its partial differential equation converged infinitely to the fixed value—f(cα, φβ). The research results can be used to analyze the shearing condition and energy evolution process of the roadbed filler, and to invert the strength parameter values that meet the engineering needs.
2021, 29(5): 1342-1353.
The influence of meso-parameters on macro-mechanical response of loess in triaxial test is studied from meso-perspective. Based on the PFC3D software and the results of the indoor triaxial test to determine the value range of the basic mesoscopic parameters of the numerical simulation, particle flow simulation of triaxial samples is performed under the confining pressures 50 kPa, 100 kPa, 150 kPa, and 200 kPa. By changing the influence degree of mesoscopic parameters on the macroscopic mechanical behavior of loess numerical samples and orthogonal test design, the sensitivity analysis of friction coefficient, porosity, particle stiffness ratio kn/ks and particle size distribution in the process of loess triaxial numerical simulation is carried out, and the relationship between soil mesoscopic parameters and macroscopic mechanics is established. The results show that the peak strength and residual strength of the numerical samples under different confining pressures are positively correlated with the friction coefficient, and negatively correlated with the particle stiffness ratio and porosity. The influence of particle size distribution on it is less than 2%. Under low confining pressure(50 kPa), porosity has the greatest impact on the peak shear strength and residual strength of numerical samples, followed by particle stiffness ratio kn/ks, and under high confining pressure(200 kPa), the influence of friction coefficient on the strength of numerical sample is more significant than that of particle stiffness ratio. The meso-parameters that have the greatest impact on the initial linear elastic modulus and strain softening characteristics of the numerical sample are porosity, followed by the stiffness ratio kn/ks, and the particle size distribution is the smallest. The result of orthogonal test under 200 kPa confining pressure is consistent with the result of numerical analysis. By changing the value range of meso-parameters, the influence on the macro-mechanical properties of the loess triaxial test is systematically analyzed, which provides a reference for the future use of discrete element software in loess laboratory test research.
The influence of meso-parameters on macro-mechanical response of loess in triaxial test is studied from meso-perspective. Based on the PFC3D software and the results of the indoor triaxial test to determine the value range of the basic mesoscopic parameters of the numerical simulation, particle flow simulation of triaxial samples is performed under the confining pressures 50 kPa, 100 kPa, 150 kPa, and 200 kPa. By changing the influence degree of mesoscopic parameters on the macroscopic mechanical behavior of loess numerical samples and orthogonal test design, the sensitivity analysis of friction coefficient, porosity, particle stiffness ratio kn/ks and particle size distribution in the process of loess triaxial numerical simulation is carried out, and the relationship between soil mesoscopic parameters and macroscopic mechanics is established. The results show that the peak strength and residual strength of the numerical samples under different confining pressures are positively correlated with the friction coefficient, and negatively correlated with the particle stiffness ratio and porosity. The influence of particle size distribution on it is less than 2%. Under low confining pressure(50 kPa), porosity has the greatest impact on the peak shear strength and residual strength of numerical samples, followed by particle stiffness ratio kn/ks, and under high confining pressure(200 kPa), the influence of friction coefficient on the strength of numerical sample is more significant than that of particle stiffness ratio. The meso-parameters that have the greatest impact on the initial linear elastic modulus and strain softening characteristics of the numerical sample are porosity, followed by the stiffness ratio kn/ks, and the particle size distribution is the smallest. The result of orthogonal test under 200 kPa confining pressure is consistent with the result of numerical analysis. By changing the value range of meso-parameters, the influence on the macro-mechanical properties of the loess triaxial test is systematically analyzed, which provides a reference for the future use of discrete element software in loess laboratory test research.
2021, 29(5): 1354-1365.
Turfy soil is a special kind of humus soil formed by oxidation and partial decomposition of plant residues in the environment of waterlogging. Turfy soil contains a lot of plant fibers, and is with adverse engineering geological characteristics such as high permeability, high pore ratio and high-water content. In order to investigate the pore characteristics of the turfy soil and the percolation law, CT sequence images of turfy soil in Dunhua, Jilin Province are obtained. The optimal threshold of image binarization is determined using the mask volume calculation of Mimics and porosity of turfy soil. The 3D reconstruction model is constructed. Meanwhile, based on the Lattice Boltzmann Method principle, the PALABOS code is adopted to conduct the single-phase seepage simulation of the turfy soil. In addition, the differences of simulated permeability values and the seepage field of the soil under constant pressure are studied. The distribution of flow lines and flow velocity within the pore space are analyzed. The results show that internal pores in turfy soil vary in size. The large pore size and small pore size exhibit different patterns. The distribution of plant fibers and root-like pores are the main reason for the formation of large pore size channels in turfy soil. Therefore, the permeability of turfy soils is mainly affected by the content of plant fibers in the soil and the degree of decomposition. The more content of plant fiber, the lower the decomposition degree is and the higher the permeability of turfy soil is, and vice versa.
Turfy soil is a special kind of humus soil formed by oxidation and partial decomposition of plant residues in the environment of waterlogging. Turfy soil contains a lot of plant fibers, and is with adverse engineering geological characteristics such as high permeability, high pore ratio and high-water content. In order to investigate the pore characteristics of the turfy soil and the percolation law, CT sequence images of turfy soil in Dunhua, Jilin Province are obtained. The optimal threshold of image binarization is determined using the mask volume calculation of Mimics and porosity of turfy soil. The 3D reconstruction model is constructed. Meanwhile, based on the Lattice Boltzmann Method principle, the PALABOS code is adopted to conduct the single-phase seepage simulation of the turfy soil. In addition, the differences of simulated permeability values and the seepage field of the soil under constant pressure are studied. The distribution of flow lines and flow velocity within the pore space are analyzed. The results show that internal pores in turfy soil vary in size. The large pore size and small pore size exhibit different patterns. The distribution of plant fibers and root-like pores are the main reason for the formation of large pore size channels in turfy soil. Therefore, the permeability of turfy soils is mainly affected by the content of plant fibers in the soil and the degree of decomposition. The more content of plant fiber, the lower the decomposition degree is and the higher the permeability of turfy soil is, and vice versa.
2021, 29(5): 1366-1372.
Loess is present in many parts of the world, especially in arid and semi-arid climate zones. The loess in China covers approximately 631 000 km2. Many slopes consisting of Malan loess collapsed or slid during past several decades. Macrostructure characteristics and strength properties of the Malan loess are essentially controlled by intergranular pore distribution, which has gradually become a new focus to study loess basic characteristics. Undisturbed Malan loess samples were collected from excavated profiles on the northern side of Zhaojiaan landslide. Depth of the sample was 5 m below ground surface. Pore distribution and pore deformation behavior of the Malan loess in Zhaojiaan area were analyzed based on consolidation test, scanning electron microscope and ImageJ image analysis software. The samples were humidified by deionized water to predetermined water contents of 9%, 12%, 15%, 18% and 21% and subjected to uni-axial compression experiments. The pore structures of the undisturbed loess samples before and after uni-axial compression experiments were determined using scanning electron microscopy. The Ostu method was used as part of the ImageJ analysis to determine the threshold of binary images. The pore area distribution characteristics and the number of the pore of undisturbed samples were calculated under the different water content condition, which supports loess collapse caused by the large and medium trellis pores to a great extent. A pore destroy mode was proposed that large, medium and small trellis pores destroyed gradually in consolidation or wetting condition. The experiments also show that water must promote the failure of the loess pore structure. Large and medium trellis pores maybe accelerate collapse of the Malan loess when the water content increases to the liquid limit.
Loess is present in many parts of the world, especially in arid and semi-arid climate zones. The loess in China covers approximately 631 000 km2. Many slopes consisting of Malan loess collapsed or slid during past several decades. Macrostructure characteristics and strength properties of the Malan loess are essentially controlled by intergranular pore distribution, which has gradually become a new focus to study loess basic characteristics. Undisturbed Malan loess samples were collected from excavated profiles on the northern side of Zhaojiaan landslide. Depth of the sample was 5 m below ground surface. Pore distribution and pore deformation behavior of the Malan loess in Zhaojiaan area were analyzed based on consolidation test, scanning electron microscope and ImageJ image analysis software. The samples were humidified by deionized water to predetermined water contents of 9%, 12%, 15%, 18% and 21% and subjected to uni-axial compression experiments. The pore structures of the undisturbed loess samples before and after uni-axial compression experiments were determined using scanning electron microscopy. The Ostu method was used as part of the ImageJ analysis to determine the threshold of binary images. The pore area distribution characteristics and the number of the pore of undisturbed samples were calculated under the different water content condition, which supports loess collapse caused by the large and medium trellis pores to a great extent. A pore destroy mode was proposed that large, medium and small trellis pores destroyed gradually in consolidation or wetting condition. The experiments also show that water must promote the failure of the loess pore structure. Large and medium trellis pores maybe accelerate collapse of the Malan loess when the water content increases to the liquid limit.
2021, 29(5): 1373-1386.
Huge hazards, which are often caused by earthquake-induced mountain avalanches, are a serious secondary, and the study of dynamic response characteristics and failure mechanism of the avalanches in certain geological condition is an important issue in engineering geology and geotechnical engineering. In this paper, a large-scale shaking table test was carried out to study the Ganqiuchi granite avalanche relic in Cuihua Mountain, which is typical in Qinling District. The testing results show that the amplification coefficient of the acceleration peak value inside of the model slope presents a significant three-stage trend with the increase of the excitation intensity. The horizontal acceleration response increases monotonously with the increase of elevation, while the vertical acceleration response first increases, then decreases and finally increases with the increase of elevation. The natural frequency curve of the model slope can be divided into three stages, showing a downward trend, indicating that the dynamic characteristics of the model have changed. The damaged model can be divided into two regions: the source area and the accumulation area. The failure sliding characteristics and evolution process of the model slope under seismic excitation are as follows: seismic wave excitation input→tension cracks formed on the trailing edge of the slope→the fracture extends downward and penetrates→unstable slope sliding→accumulating at the foot of the slope. Four stages of mountain failure are inversed: vibration-induced fracturing stage, high-speed start-up stage, impact and deceleration stage and accumulating stage. The experimental results are quite consist with field observation and can help to explain the failure mechanism of granite slope, and provide the basic data and scientific support for disaster reduction and the development and protection of Qinling Mountain landscapes and geological heritages.
Huge hazards, which are often caused by earthquake-induced mountain avalanches, are a serious secondary, and the study of dynamic response characteristics and failure mechanism of the avalanches in certain geological condition is an important issue in engineering geology and geotechnical engineering. In this paper, a large-scale shaking table test was carried out to study the Ganqiuchi granite avalanche relic in Cuihua Mountain, which is typical in Qinling District. The testing results show that the amplification coefficient of the acceleration peak value inside of the model slope presents a significant three-stage trend with the increase of the excitation intensity. The horizontal acceleration response increases monotonously with the increase of elevation, while the vertical acceleration response first increases, then decreases and finally increases with the increase of elevation. The natural frequency curve of the model slope can be divided into three stages, showing a downward trend, indicating that the dynamic characteristics of the model have changed. The damaged model can be divided into two regions: the source area and the accumulation area. The failure sliding characteristics and evolution process of the model slope under seismic excitation are as follows: seismic wave excitation input→tension cracks formed on the trailing edge of the slope→the fracture extends downward and penetrates→unstable slope sliding→accumulating at the foot of the slope. Four stages of mountain failure are inversed: vibration-induced fracturing stage, high-speed start-up stage, impact and deceleration stage and accumulating stage. The experimental results are quite consist with field observation and can help to explain the failure mechanism of granite slope, and provide the basic data and scientific support for disaster reduction and the development and protection of Qinling Mountain landscapes and geological heritages.
2021, 29(5): 1387-1400.
In order to research the failure of rock mass induced by earthquakes, we took the rock avalanche at Heyezhai, Sichuan, China as the research object. Firstly, we simulated the failure under self-weight by DDA and RocFall codes. The results show RocFall and DDA could coincide with each other well. Then we simulated the failure of the single rock under seismic loading with the influence of density, height, size and peak acceleration of horizontal simple harmonic waves. Finally, we simulated the failure of the rock with a set of parallel joints. In the simulation, we monitored some important parameters such as trajectories, horizontal displacement, velocity and the maximum bounce height at the slope foot. The simulation results show the follows. (1)The natural slope angle of rock zone 2 is at 65°~75°, the rockfall accelerated on the slope surface and then ejected at the height of about 132 m. The main motion modes are rolling, collision and bouncing. (2)The breakthrough time of weak structural plane decreases with increasing of the density, volume and peak acceleration of seismic waves. (3)The peak velocity increases with increasing of slope height, volume and peak acceleration of seismic waves. (4)The horizontal displacement increases with increasing of the density, slope height and peak acceleration of seismic waves. (5)The maximum jumping height of rockfall at the foot of slope is positively correlated with various factors. We continued to study the failure of rock mass with internal joints under seismic loading: (1)The horizontal displacement range of rockfall is 191.5~243.8 m. (2)The maximum bounce height at the slope foot was 13.8 m.
In order to research the failure of rock mass induced by earthquakes, we took the rock avalanche at Heyezhai, Sichuan, China as the research object. Firstly, we simulated the failure under self-weight by DDA and RocFall codes. The results show RocFall and DDA could coincide with each other well. Then we simulated the failure of the single rock under seismic loading with the influence of density, height, size and peak acceleration of horizontal simple harmonic waves. Finally, we simulated the failure of the rock with a set of parallel joints. In the simulation, we monitored some important parameters such as trajectories, horizontal displacement, velocity and the maximum bounce height at the slope foot. The simulation results show the follows. (1)The natural slope angle of rock zone 2 is at 65°~75°, the rockfall accelerated on the slope surface and then ejected at the height of about 132 m. The main motion modes are rolling, collision and bouncing. (2)The breakthrough time of weak structural plane decreases with increasing of the density, volume and peak acceleration of seismic waves. (3)The peak velocity increases with increasing of slope height, volume and peak acceleration of seismic waves. (4)The horizontal displacement increases with increasing of the density, slope height and peak acceleration of seismic waves. (5)The maximum jumping height of rockfall at the foot of slope is positively correlated with various factors. We continued to study the failure of rock mass with internal joints under seismic loading: (1)The horizontal displacement range of rockfall is 191.5~243.8 m. (2)The maximum bounce height at the slope foot was 13.8 m.
2021, 29(5): 1401-1415.
Loess caves and landslides are micro-geomorphic landscapes under the unique erosion of the Loess Plateau. They reflect the chain process of rapid geomorphic evolution. They have the characteristics of wide distribution and high development density,and can seriously threaten the safety of human settlements in Northwest China. This paper takes Laolang gully small watershed as the research object,adopts field investigation,GIS spatial analysis,UAV mapping and numerical simulation. This paper carries out the research on the spatial-temporal distribution characteristics and risk of loess micro geomorphic disaster chain. The results show that the numbers of loess caves,landslides and shallow gullies developed in Laolanggou study area are 134,38 and 81 respectively. The density of loess caves is about 159/km2,accounting for 1.88% of the total area of the study area. The loess caves are mostly located in the TWI high concave terrain area,and are arranged in a linear distribution,which is closely related to the development of shallow gullies. The results of five nuclear density estimates from 2001 to 2021 show that the high density centers are located in the west slope,with an area of about 5.91×104 m2. Its spatial agglomeration degree of long axis,short axis,area,elevation and perimeter is the highest. The evolution model of loess micro geomorphic disaster chain can be summarized as primary geomorphic stage,early erosion stage,accelerated erosion stage,erosion breakthrough stage and then local destruction stage. The cave environment aggravates the degree of water infiltration,which is more likely to cause landslides. The simulation results show that the potential landslide movement can cause serious damage to the terrace buildings,and the affected area is about 2.02×104 m2,the landslide movement process is 150 s,the average accumulation thickness is about 9.2 m,and the maximum movement distance is about 651 m. This study is a beneficial exploration and attempt to reveal the development law of loess caves and its disaster chain effect,and provides a reference for urban disaster prevention and reduction in the Loess Plateau.
Loess caves and landslides are micro-geomorphic landscapes under the unique erosion of the Loess Plateau. They reflect the chain process of rapid geomorphic evolution. They have the characteristics of wide distribution and high development density,and can seriously threaten the safety of human settlements in Northwest China. This paper takes Laolang gully small watershed as the research object,adopts field investigation,GIS spatial analysis,UAV mapping and numerical simulation. This paper carries out the research on the spatial-temporal distribution characteristics and risk of loess micro geomorphic disaster chain. The results show that the numbers of loess caves,landslides and shallow gullies developed in Laolanggou study area are 134,38 and 81 respectively. The density of loess caves is about 159/km2,accounting for 1.88% of the total area of the study area. The loess caves are mostly located in the TWI high concave terrain area,and are arranged in a linear distribution,which is closely related to the development of shallow gullies. The results of five nuclear density estimates from 2001 to 2021 show that the high density centers are located in the west slope,with an area of about 5.91×104 m2. Its spatial agglomeration degree of long axis,short axis,area,elevation and perimeter is the highest. The evolution model of loess micro geomorphic disaster chain can be summarized as primary geomorphic stage,early erosion stage,accelerated erosion stage,erosion breakthrough stage and then local destruction stage. The cave environment aggravates the degree of water infiltration,which is more likely to cause landslides. The simulation results show that the potential landslide movement can cause serious damage to the terrace buildings,and the affected area is about 2.02×104 m2,the landslide movement process is 150 s,the average accumulation thickness is about 9.2 m,and the maximum movement distance is about 651 m. This study is a beneficial exploration and attempt to reveal the development law of loess caves and its disaster chain effect,and provides a reference for urban disaster prevention and reduction in the Loess Plateau.
2021, 29(5): 1416-1426.
The Hydro-Fluctuation Belt of the Three Gorges Reservoir is a sensitive area for the stability of the reservoir bank slope. The geological disasters occurred many times in the reservoir area were related to the deterioration of the rock mass. With the passage of time,the deterioration of the rock mass in the Hydro-Fluctuation Belt is increasing,and new landslides continue to emerge. Based on field investigation and office analysis,We have summarized the types of rock mass deterioration and the evolution mode of new landslide hazards in the Hydro-Fluctuation Belt from Zigui to Badong in the Three Gorges Reservoir Area. The results have shown that the lithology of the study area is mainly carbonate rock and clastic rock. The degradation types of carbonate rocks mainly include dissolution(subsurface erosion),fracture manifestation,expansion and mechanical erosion. The degradation types of clastic rocks mainly include loosening(spalling),erosion(abrasion),structural plane disintegration and block fracture,and soft-hard interphase erosion,Among them,the loose/flaking type is the most developed. Based on this binding surface development characteristics,slope structure,lithology and structural characteristics and the boundary,obtained the model of new landslide hazard point formed by rock mass degradation and evolution in different Hydro-Fluctuation Belt. Carbonate bank slopes are mainly characterized by base crushing,base hollowing and dumping,and forward slip. The clastic rock bank slopes are mainly dominated by soft-hard interphase collapse(collapse),apparent dip wedge sliding,forward sliding and reverse dumping. The research results can provide technical support for geological disasters monitoring in the Three Gorges Reservoir area.
The Hydro-Fluctuation Belt of the Three Gorges Reservoir is a sensitive area for the stability of the reservoir bank slope. The geological disasters occurred many times in the reservoir area were related to the deterioration of the rock mass. With the passage of time,the deterioration of the rock mass in the Hydro-Fluctuation Belt is increasing,and new landslides continue to emerge. Based on field investigation and office analysis,We have summarized the types of rock mass deterioration and the evolution mode of new landslide hazards in the Hydro-Fluctuation Belt from Zigui to Badong in the Three Gorges Reservoir Area. The results have shown that the lithology of the study area is mainly carbonate rock and clastic rock. The degradation types of carbonate rocks mainly include dissolution(subsurface erosion),fracture manifestation,expansion and mechanical erosion. The degradation types of clastic rocks mainly include loosening(spalling),erosion(abrasion),structural plane disintegration and block fracture,and soft-hard interphase erosion,Among them,the loose/flaking type is the most developed. Based on this binding surface development characteristics,slope structure,lithology and structural characteristics and the boundary,obtained the model of new landslide hazard point formed by rock mass degradation and evolution in different Hydro-Fluctuation Belt. Carbonate bank slopes are mainly characterized by base crushing,base hollowing and dumping,and forward slip. The clastic rock bank slopes are mainly dominated by soft-hard interphase collapse(collapse),apparent dip wedge sliding,forward sliding and reverse dumping. The research results can provide technical support for geological disasters monitoring in the Three Gorges Reservoir area.
2021, 29(5): 1427-1436.
Colluvial landslide is an important type of geological hazard during the operation of the Three Gorges Reservoir, and its deformation evolution often lag behind the change of rainfall and reservoir level, show the time lag effect, which brings a great challenge to landslide hazard prediction and disaster alert release. In this paper, firstly we construct a landslide weighted displacement vector calculation model by adopting set pair analysis method and analytic hierarchy process method, then carry out the qualitative analysis of the relationship between landslide weighted displacement evolution and reservoir water level, then look for the number of translation steps when the correlation between landslide weighted displacement and reservoir water level change rate reaches the maximum, finally calculate the time that landslide deformation lags behind the reservoir water level change. Taking Shuping landslide which is a typical colluvial landslide in Three Gorges Reservoir Area as an example, based on the analysis of the landslide deformation evolution law, the monitoring data of surface displacement and reservoir water level decreasing rate during flood and rain periods in 2012, 2013, and 2014 were respectively selected to carry out research on landslide deformation lag time. The results show that when the decreasing rate of reservoir water level is less than or equal to 0.43 m·d-1, the deformation lag time of Shuping landslide is greater than or equal to 5 d; When the decreasing rate of reservoir water level is between 0.43 m·d-1 and 0.7 m·d-1, the deformation lag time of Shuping landslide is between 2 d and 5 d; When the decreasing rate of reservoir water level is greater than or equal to 0.7 m·d-1, the deformation lag time of Shuping landslide is less than or equal to 2 d. Therefore, the decreasing rate of reservoir water level increases, the lag time of landslide deformation decreases. By analyzing the lag time of monitoring points at different spatial locations of the landslide, it is found that the closer the landslide front edge, the shorter the landslide deformation lag time. When the decreasing rate of reservoir water level is between 0.43 m·d-1 and 0.7 m·d-1, the deformation lag time at the front edge of Shuping landslide is between 2.4 d and 5.4 d, the deformation lag time at the middle part of Shuping landslide is between 3.4 d and 5.6 d, and the deformation lag time interval between the front and middle parts of Shuping landslide is from 0.2 d to 1.4 d. The research results can provide reference for the monitoring and early warning of Shuping landslide, and have certain reference significance for the monitoring and early warning of partly submerged landslides in major water conservancy projects.
Colluvial landslide is an important type of geological hazard during the operation of the Three Gorges Reservoir, and its deformation evolution often lag behind the change of rainfall and reservoir level, show the time lag effect, which brings a great challenge to landslide hazard prediction and disaster alert release. In this paper, firstly we construct a landslide weighted displacement vector calculation model by adopting set pair analysis method and analytic hierarchy process method, then carry out the qualitative analysis of the relationship between landslide weighted displacement evolution and reservoir water level, then look for the number of translation steps when the correlation between landslide weighted displacement and reservoir water level change rate reaches the maximum, finally calculate the time that landslide deformation lags behind the reservoir water level change. Taking Shuping landslide which is a typical colluvial landslide in Three Gorges Reservoir Area as an example, based on the analysis of the landslide deformation evolution law, the monitoring data of surface displacement and reservoir water level decreasing rate during flood and rain periods in 2012, 2013, and 2014 were respectively selected to carry out research on landslide deformation lag time. The results show that when the decreasing rate of reservoir water level is less than or equal to 0.43 m·d-1, the deformation lag time of Shuping landslide is greater than or equal to 5 d; When the decreasing rate of reservoir water level is between 0.43 m·d-1 and 0.7 m·d-1, the deformation lag time of Shuping landslide is between 2 d and 5 d; When the decreasing rate of reservoir water level is greater than or equal to 0.7 m·d-1, the deformation lag time of Shuping landslide is less than or equal to 2 d. Therefore, the decreasing rate of reservoir water level increases, the lag time of landslide deformation decreases. By analyzing the lag time of monitoring points at different spatial locations of the landslide, it is found that the closer the landslide front edge, the shorter the landslide deformation lag time. When the decreasing rate of reservoir water level is between 0.43 m·d-1 and 0.7 m·d-1, the deformation lag time at the front edge of Shuping landslide is between 2.4 d and 5.4 d, the deformation lag time at the middle part of Shuping landslide is between 3.4 d and 5.6 d, and the deformation lag time interval between the front and middle parts of Shuping landslide is from 0.2 d to 1.4 d. The research results can provide reference for the monitoring and early warning of Shuping landslide, and have certain reference significance for the monitoring and early warning of partly submerged landslides in major water conservancy projects.
2021, 29(5): 1437-1451.
The existence of tensional structural planes in sub-horizontal rock slope is very important for the formation and evolution of the sub-horizontal translational landslides. After the occurrence of sub-horizontal translational landslides, the water-filled tensile structural planes can be widened to form a tension trough. We find that the distribution directions of the tension troughs of medium size, medium thickness or above sub-horizontal translational landslidesare almost perpendicular to the tectonic belt at the basin edge. On the bases of others' research results and the analysis of river system of Sichuan Basin inversion of neotectonic stress field, we find that the tension trough strike directionsare close to the principle stress direction of neotectonic stress field. We believe that the formation and development of these sub-horizontal translational landslides are closely related to the stress field of neotectonics. The influences of tectonic stress field on the formation and evolution of sub-horizontal translational landslides are mainly manifested in the transformation of rock mass structure, the control of permeability of structural plane and the influence of hydrological condition of slope. The change of principal stress direction makes the structural plane with compressive(shear) property close to parallel transform into tensile property, resulting in tensile crack extension and expansion of scale. The structural planes are connected with each other along the strike and crosses the layer along the dip. The permeability of the structural plane with the three dimensional stress state is improved remarkably. The permeability, scale, connectivity and opening of the structural plane are simultaneously expanded. The characteristics of vertical and horizontal migration are one of the main factors for the occurrence of sub-horizontal translational landslides and provide the formation and evolution conditions for the occurrence of landslide. Due to the existence of cross-layer tensile structural planes, groundwater is able to migrate vertically to the depth of the slope and have physical and chemical interactions with the deep soft layers, thus providing conditions for the formation of larger scale and thickness of sub-horizontal translational landslides.
The existence of tensional structural planes in sub-horizontal rock slope is very important for the formation and evolution of the sub-horizontal translational landslides. After the occurrence of sub-horizontal translational landslides, the water-filled tensile structural planes can be widened to form a tension trough. We find that the distribution directions of the tension troughs of medium size, medium thickness or above sub-horizontal translational landslidesare almost perpendicular to the tectonic belt at the basin edge. On the bases of others' research results and the analysis of river system of Sichuan Basin inversion of neotectonic stress field, we find that the tension trough strike directionsare close to the principle stress direction of neotectonic stress field. We believe that the formation and development of these sub-horizontal translational landslides are closely related to the stress field of neotectonics. The influences of tectonic stress field on the formation and evolution of sub-horizontal translational landslides are mainly manifested in the transformation of rock mass structure, the control of permeability of structural plane and the influence of hydrological condition of slope. The change of principal stress direction makes the structural plane with compressive(shear) property close to parallel transform into tensile property, resulting in tensile crack extension and expansion of scale. The structural planes are connected with each other along the strike and crosses the layer along the dip. The permeability of the structural plane with the three dimensional stress state is improved remarkably. The permeability, scale, connectivity and opening of the structural plane are simultaneously expanded. The characteristics of vertical and horizontal migration are one of the main factors for the occurrence of sub-horizontal translational landslides and provide the formation and evolution conditions for the occurrence of landslide. Due to the existence of cross-layer tensile structural planes, groundwater is able to migrate vertically to the depth of the slope and have physical and chemical interactions with the deep soft layers, thus providing conditions for the formation of larger scale and thickness of sub-horizontal translational landslides.
2021, 29(5): 1452-1459.
The microscopic characteristics such as the shape and roughness of coral sand can affect the macro-mechanical properties of sand. Image processing technology is used to analyze the aspect ratio,area,roundness and fractal dimension of coral sand. 3D measuring microscope technology is used to measure and calculate the surface roughness of the sand grains,the interface expansion area ratio,and the arithmetic mean curvature of the peak apex. The test results show that the aspect ratio of most coral sand particles is between 1.2~1.5,and the aspect ratio presents a negative skew distribution. The roundness of sand particles is mainly concentrated in the range of 0.75~0.9,which shows a negative skew distribution. The area of sand particles is mainly between 0.015~0.035 cm2,and the area of the particles has a positive skew distribution. The fractal dimension D of sand particles is basically between 1.04~1.10,indicating that sand particles have strong self-similarity. The roughness analysis results show that the coral sand particles have poor surface flatness,with many erosion pits distributed,and sharp bulges locally. The sand surface roughness Sa is 20.078 μm. The interface expansion ratio Sdr is 0.167,and there are several inclined surfaces distributed on the surface of the sand particles. The arithmetic mean curvature Spc of the peak apex is 216.641 mm-1,which shows that the contact between sand particles is sharp. The above research results can provide references for the study of the macro and micro mechanical properties of coral sand.
The microscopic characteristics such as the shape and roughness of coral sand can affect the macro-mechanical properties of sand. Image processing technology is used to analyze the aspect ratio,area,roundness and fractal dimension of coral sand. 3D measuring microscope technology is used to measure and calculate the surface roughness of the sand grains,the interface expansion area ratio,and the arithmetic mean curvature of the peak apex. The test results show that the aspect ratio of most coral sand particles is between 1.2~1.5,and the aspect ratio presents a negative skew distribution. The roundness of sand particles is mainly concentrated in the range of 0.75~0.9,which shows a negative skew distribution. The area of sand particles is mainly between 0.015~0.035 cm2,and the area of the particles has a positive skew distribution. The fractal dimension D of sand particles is basically between 1.04~1.10,indicating that sand particles have strong self-similarity. The roughness analysis results show that the coral sand particles have poor surface flatness,with many erosion pits distributed,and sharp bulges locally. The sand surface roughness Sa is 20.078 μm. The interface expansion ratio Sdr is 0.167,and there are several inclined surfaces distributed on the surface of the sand particles. The arithmetic mean curvature Spc of the peak apex is 216.641 mm-1,which shows that the contact between sand particles is sharp. The above research results can provide references for the study of the macro and micro mechanical properties of coral sand.
2021, 29(5): 1460-1468.
To analyze the influence of sampling size and sampling spacing on the evaluation results of structural surface roughness,this paper takes the structural surface on the left bank of the high steep slope of the Sequ Special Bridge in the section from Chengdu to Linzi on the Sichuan-Tibet line as the research object. The 3D coordinates and surface texture data of the slope are obtained with the nap-of-the-object. The 3D reconstruction of the slope and the point cloud processing are realized indoors. 37 structural surface information are artificially extracted from the point cloud model and the structural surface mesh reconstruction is performed with MATLAB. The variations of roughness in different sizes and intervals are described based on the Z2S method. The analysis results show that the scale effect and interval effect of structural surface roughness are obvious; the increase in the sampling interval can reduce the effective sampling size value. Thus,a smaller sampling interval should be selected when conducting roughness research,and the sampling size should be greater than or equal to the true effective sampling size value. In addition,this paper is a case of the application of the nap-of-the-object in the field of structural surface data collection and is a useful supplement to the research on structural surface roughness characteristics.
To analyze the influence of sampling size and sampling spacing on the evaluation results of structural surface roughness,this paper takes the structural surface on the left bank of the high steep slope of the Sequ Special Bridge in the section from Chengdu to Linzi on the Sichuan-Tibet line as the research object. The 3D coordinates and surface texture data of the slope are obtained with the nap-of-the-object. The 3D reconstruction of the slope and the point cloud processing are realized indoors. 37 structural surface information are artificially extracted from the point cloud model and the structural surface mesh reconstruction is performed with MATLAB. The variations of roughness in different sizes and intervals are described based on the Z2S method. The analysis results show that the scale effect and interval effect of structural surface roughness are obvious; the increase in the sampling interval can reduce the effective sampling size value. Thus,a smaller sampling interval should be selected when conducting roughness research,and the sampling size should be greater than or equal to the true effective sampling size value. In addition,this paper is a case of the application of the nap-of-the-object in the field of structural surface data collection and is a useful supplement to the research on structural surface roughness characteristics.
2021, 29(5): 1469-1476.
There are numerous mineral resources on the seafloor,and its prospect is broad. Compared with land mining,seabed mining is more threatened by overlying water bodies. In order to ensure the safe and economic operation of subsea mining projects,it is of great significance to find out the characteristics of the deformation,movement and destruction of the surrounding rocks in the mined-out area,and to determine the critical thickness of the waterproof pillars left at the top of the mining area. In this paper,the San-shan-dao Gold Mine Xinli Mining Area is selected as the research subject. A geo-mechanical experiment is carried out to reproduce the process of undersea filling mining and clarify the deformation and failure process characteristics of overlying strata. The test results show that the critical mining height of Xinli mining area is - 85 m,and the critical thickness of the reserved isolation pillar at the top is 50 m. When the critical value is exceeded,the goaf and the top aquifer suffer through failure,and the instability mode is fault activation. The research results can provide a theoretical basis for the selection of reserved pillar height in Xinli mining area,and also provide a reference for the mining safety of mines under similar geological conditions.
There are numerous mineral resources on the seafloor,and its prospect is broad. Compared with land mining,seabed mining is more threatened by overlying water bodies. In order to ensure the safe and economic operation of subsea mining projects,it is of great significance to find out the characteristics of the deformation,movement and destruction of the surrounding rocks in the mined-out area,and to determine the critical thickness of the waterproof pillars left at the top of the mining area. In this paper,the San-shan-dao Gold Mine Xinli Mining Area is selected as the research subject. A geo-mechanical experiment is carried out to reproduce the process of undersea filling mining and clarify the deformation and failure process characteristics of overlying strata. The test results show that the critical mining height of Xinli mining area is - 85 m,and the critical thickness of the reserved isolation pillar at the top is 50 m. When the critical value is exceeded,the goaf and the top aquifer suffer through failure,and the instability mode is fault activation. The research results can provide a theoretical basis for the selection of reserved pillar height in Xinli mining area,and also provide a reference for the mining safety of mines under similar geological conditions.
2021, 29(5): 1477-1487.
Chinese economic integration promotes the rapid development of urban transportation networks. Many underwater shield tunnels were constructed in recent years. Since the 21st century,the completion of a series of long distance and large diameter cross-river(or cross-sea) tunnels has marked the great progress of construction technologies for underwater tunnels. In order to promote the development of tunneling technology in complex ground and improve the construction efficiency of cross-river (or cross-sea)tunnels,the present study takes the recently constructed and representative underwater tunnels as research subjects and analyzes tunnel geological environment,shield tunneling technology and construction project management. Firstly,engineering difficulties and technical challenges for long distance shield tunnels under high water pressure are presented. These tunnels include the Nanjing Yangtze River Tunnel,Jinan Yellow River Tunnel,Nanjing Metro Line 10 Crossing River Tunnel,Sutong GIL Yangtze River Crossing Cable Tunnel and Xiamen Metro Line 2 Subsea Tunnel. Then,construction technologies for underwater tunnels in complex geological conditions are summarized. The conditions include abrasive sandy cobble ground,cohesive silty clay ground,high water pressure and permeable ground,underwater gas-rich ground and undersea dense boulder group ground. Finally,development trends of cross-river (or cross-sea)tunnels are prospected. They include the complexity of engineering geology,the diversification of shield equipment and the intelligentization of tunnel construction. The present study provides theoretical basis and technical support for the survey,design and construction of underwater shield tunnels under complex geological conditions in the future.
Chinese economic integration promotes the rapid development of urban transportation networks. Many underwater shield tunnels were constructed in recent years. Since the 21st century,the completion of a series of long distance and large diameter cross-river(or cross-sea) tunnels has marked the great progress of construction technologies for underwater tunnels. In order to promote the development of tunneling technology in complex ground and improve the construction efficiency of cross-river (or cross-sea)tunnels,the present study takes the recently constructed and representative underwater tunnels as research subjects and analyzes tunnel geological environment,shield tunneling technology and construction project management. Firstly,engineering difficulties and technical challenges for long distance shield tunnels under high water pressure are presented. These tunnels include the Nanjing Yangtze River Tunnel,Jinan Yellow River Tunnel,Nanjing Metro Line 10 Crossing River Tunnel,Sutong GIL Yangtze River Crossing Cable Tunnel and Xiamen Metro Line 2 Subsea Tunnel. Then,construction technologies for underwater tunnels in complex geological conditions are summarized. The conditions include abrasive sandy cobble ground,cohesive silty clay ground,high water pressure and permeable ground,underwater gas-rich ground and undersea dense boulder group ground. Finally,development trends of cross-river (or cross-sea)tunnels are prospected. They include the complexity of engineering geology,the diversification of shield equipment and the intelligentization of tunnel construction. The present study provides theoretical basis and technical support for the survey,design and construction of underwater shield tunnels under complex geological conditions in the future.
2021, 29(5): 1488-1496.
In order to evaluate the adaptability and application potential of CO2 flooding in the extra-low permeability reservoir, Changqing oilfield carried out a pilot project of CO2 enhanced oil recovery and storage and obtained satisfactory results. Given the surrounding gas source conditions, reservoir conditions, field road and CO2 trial transportation, the Chang 8 reservoir in Huang 3 district was determined as the pilot test block for CO2 enhanced oil recovery and storage engineering. The reservoir is characterized by a large scale, abundant surrounding gas sources, and good field road conditions. In addition, the reservoir condition can be able to reach the condition of miscible flooding. However, it is an ultra-low permeability reservoir and in the early development stage, showing a steady decline in the production efficiency by water flooding. The field test result of CO2 flooding pilot shows that the daily oil production and daily gas production increases significantly, while the water content decreases after CO2 flooding, indicating that CO2 flooding has a good prospect for further promotion in Changqing oilfield. Compared to water flooding, the injection pressure is independent with the injection volume and without significant increase, indicating that the test reservoir has a good capacity for CO2 injection and storage. However, the change of injection pressure is relatively lower than that of water flooding, optimization has to be applied to the parameters of gas injection mode, injection rate and pressure for the further application of CO2 flooding. In addition, in order to improve the effect of CO2 flooding and facilitate the overall evaluation of the pilot project, it is suggested that the scale of the pilot project can be further expanded in the future.
In order to evaluate the adaptability and application potential of CO2 flooding in the extra-low permeability reservoir, Changqing oilfield carried out a pilot project of CO2 enhanced oil recovery and storage and obtained satisfactory results. Given the surrounding gas source conditions, reservoir conditions, field road and CO2 trial transportation, the Chang 8 reservoir in Huang 3 district was determined as the pilot test block for CO2 enhanced oil recovery and storage engineering. The reservoir is characterized by a large scale, abundant surrounding gas sources, and good field road conditions. In addition, the reservoir condition can be able to reach the condition of miscible flooding. However, it is an ultra-low permeability reservoir and in the early development stage, showing a steady decline in the production efficiency by water flooding. The field test result of CO2 flooding pilot shows that the daily oil production and daily gas production increases significantly, while the water content decreases after CO2 flooding, indicating that CO2 flooding has a good prospect for further promotion in Changqing oilfield. Compared to water flooding, the injection pressure is independent with the injection volume and without significant increase, indicating that the test reservoir has a good capacity for CO2 injection and storage. However, the change of injection pressure is relatively lower than that of water flooding, optimization has to be applied to the parameters of gas injection mode, injection rate and pressure for the further application of CO2 flooding. In addition, in order to improve the effect of CO2 flooding and facilitate the overall evaluation of the pilot project, it is suggested that the scale of the pilot project can be further expanded in the future.
2021, 29(5): 1497-1506.
Seasonal coarse-grained frozen soil is widely distributed along the Sichuan-Tibet railway, and the stability of this type of soil slope is prominent. It is necessary to discuss the natural evolution characteristics and laws of this type of slope. This paper carries out the experimental study under the conditions of single factor and multi-factor combination and examines the variation law of coarse-grained soil slope under the conditions of rainfall, sunlight, freeze-thaw cycles. The results show that for a specific coarse-grained soil slope with steep initial slope angle, under the action of natural rainfall and sunlight, the slope can develop to a natural stable slope. Under repeated freeze-thaw action, the freezing and thawing of slope soil can lead to residual deformation. The frost deformation is about 0.09~0.1 times of the freezing depth, the residual deformation is about 0.052~0.062 times of the freezing depth, and the soil moisture of the slope surface increases by 2% ~3%. After repeated freezing and thawing, the soil structure becomes loose, the voids increase, and the density decreases. The loose soil is easy to erosion under the rainfall condition, and the slope degenerates backward. The natural evolution characteristics of coarse-grained soil slope in seasonal frozen soil area follow the following process: influence of rainfall and sunlight→slope tends to be stable→freeze-thaw action→soil of slope surface becomes loose→rainfall scouring→ slope degenerates backward→freezing and thawing of deeper soil. This process is repeated with seasonal changes, and the slope degenerates year by year.
Seasonal coarse-grained frozen soil is widely distributed along the Sichuan-Tibet railway, and the stability of this type of soil slope is prominent. It is necessary to discuss the natural evolution characteristics and laws of this type of slope. This paper carries out the experimental study under the conditions of single factor and multi-factor combination and examines the variation law of coarse-grained soil slope under the conditions of rainfall, sunlight, freeze-thaw cycles. The results show that for a specific coarse-grained soil slope with steep initial slope angle, under the action of natural rainfall and sunlight, the slope can develop to a natural stable slope. Under repeated freeze-thaw action, the freezing and thawing of slope soil can lead to residual deformation. The frost deformation is about 0.09~0.1 times of the freezing depth, the residual deformation is about 0.052~0.062 times of the freezing depth, and the soil moisture of the slope surface increases by 2% ~3%. After repeated freezing and thawing, the soil structure becomes loose, the voids increase, and the density decreases. The loose soil is easy to erosion under the rainfall condition, and the slope degenerates backward. The natural evolution characteristics of coarse-grained soil slope in seasonal frozen soil area follow the following process: influence of rainfall and sunlight→slope tends to be stable→freeze-thaw action→soil of slope surface becomes loose→rainfall scouring→ slope degenerates backward→freezing and thawing of deeper soil. This process is repeated with seasonal changes, and the slope degenerates year by year.
2021, 29(5): 1507-1514.
In recent years,there have been a number of medium-large landslides in Ningzhen area. It is found that the structure of this kind of gentle slope is special: The lower part is coarse-grained soil with good permeability,while the shallow stratum is clay soil with poor permeability. And it is easy to form temporaryly confined water with high head,which causes large-scale intermittent creep under the action of its buoyancy. According to the structural characteristics of the gentle slope in Ningzhen area,this paper constructs a new hydrogeological model,then analyses the different part's stress characteristics under extreme heavy rainfall. And then this paper derives its stability coefficient formula by Janbu method. Finally,this paper calculates and compares the stability coefficient of the Paoma Mountain's landslide under different confined water heads. The results show that the confined water has a big difference in different part of gentle slope,which essentially explains the cause of sliding failure and the inaccuracy of the stability coefficient obtained by the traditional limit equilibrium method. This recognition provides a theoretical basis for the stability evaluation and landslide control of the Piedmont gentle slope in Ningzhen area.
In recent years,there have been a number of medium-large landslides in Ningzhen area. It is found that the structure of this kind of gentle slope is special: The lower part is coarse-grained soil with good permeability,while the shallow stratum is clay soil with poor permeability. And it is easy to form temporaryly confined water with high head,which causes large-scale intermittent creep under the action of its buoyancy. According to the structural characteristics of the gentle slope in Ningzhen area,this paper constructs a new hydrogeological model,then analyses the different part's stress characteristics under extreme heavy rainfall. And then this paper derives its stability coefficient formula by Janbu method. Finally,this paper calculates and compares the stability coefficient of the Paoma Mountain's landslide under different confined water heads. The results show that the confined water has a big difference in different part of gentle slope,which essentially explains the cause of sliding failure and the inaccuracy of the stability coefficient obtained by the traditional limit equilibrium method. This recognition provides a theoretical basis for the stability evaluation and landslide control of the Piedmont gentle slope in Ningzhen area.
2021, 29(5): 1515-1524.
In order to study the seepage characteristics of horizontal well in low permeability formation,a self-designed model test device is used to monitor the seepage regular pattern of different aquifer thicknesses and different pump positions by embedding micro pore pressure sensor. The thickness of different aquifers was 60,79,94,113,and 123cm,respectively. The locations of the pump are at the ramp section,the entrance of the horizontal section,and the horizontal section. The test results show that the seepage regular pattern of different aquifer thicknesses is that with the increase of aquifer thickness,the pore water pressure and unit discharge increase,but there is a limit value. According to this experiment,it can be concluded that there are constraints among aquifer thickness,formation permeability and horizontal well completion structure. When the thickness of aquifer exceeds 207cm,under the same test conditions,the unit flow tends to be stable,indicating the advantages of horizontal well in low permeability and thin layer mining. When the pump is located at different positions in the horizontal well,the pore water pressure distribution in the aquifer is similar. The position of the pump mainly affects the radius of influence of the falling curve and the corresponding flow. According to the slope building section,the entrance of the horizontal section and the range of influence of precipitation in the horizontal section,the flow increases. Therefore,it is beneficial to increase production capacity to place pump in horizontal section in low permeability formation.
In order to study the seepage characteristics of horizontal well in low permeability formation,a self-designed model test device is used to monitor the seepage regular pattern of different aquifer thicknesses and different pump positions by embedding micro pore pressure sensor. The thickness of different aquifers was 60,79,94,113,and 123cm,respectively. The locations of the pump are at the ramp section,the entrance of the horizontal section,and the horizontal section. The test results show that the seepage regular pattern of different aquifer thicknesses is that with the increase of aquifer thickness,the pore water pressure and unit discharge increase,but there is a limit value. According to this experiment,it can be concluded that there are constraints among aquifer thickness,formation permeability and horizontal well completion structure. When the thickness of aquifer exceeds 207cm,under the same test conditions,the unit flow tends to be stable,indicating the advantages of horizontal well in low permeability and thin layer mining. When the pump is located at different positions in the horizontal well,the pore water pressure distribution in the aquifer is similar. The position of the pump mainly affects the radius of influence of the falling curve and the corresponding flow. According to the slope building section,the entrance of the horizontal section and the range of influence of precipitation in the horizontal section,the flow increases. Therefore,it is beneficial to increase production capacity to place pump in horizontal section in low permeability formation.
2021, 29(5): 1525-1544.
As an interdisciplinary subject of planetary geology and planetary engineering,the subject of planetary geological engineering directly supports the construction of planetary research stations and bases,the development of planetary resources and the future migration of humans. Thus,the related research is imminent. Compared with the earth,planetary rock and soil are quite different in material,structure and environment,which determine that the engineering characteristics of planetary rock and soil are very different. In-situ testing of planetary geological engineering is the most direct way to accurately obtain the engineering parameters of planetary rock and soil. The article reviewed the geological engineering in-situ testing methods during lunar and martian exploration missions,analyzed the differences of method and apparatus. These testing methods are classificated into penetration test,bucket test,drilling test,geophysical exploration and photogrammetry. The in-situ test results of lunar physical and mechanical characteristics were summarized as: particle size distribution,density,void ratio,shear resistance,compressibility and bearing capacity. We pointed out the difference in the physical and mechanical properties between lunar soil and earth soil. In the future,based on the ground low-gravity simulation test platform and the physical and mechanical constitutive theory research,we should develop miniaturized and lightweight in-situ test instruments with high level automation and intelligence,and obtain more accurate engineering data for subsequent construction of lunar scientific research stations and bases.
As an interdisciplinary subject of planetary geology and planetary engineering,the subject of planetary geological engineering directly supports the construction of planetary research stations and bases,the development of planetary resources and the future migration of humans. Thus,the related research is imminent. Compared with the earth,planetary rock and soil are quite different in material,structure and environment,which determine that the engineering characteristics of planetary rock and soil are very different. In-situ testing of planetary geological engineering is the most direct way to accurately obtain the engineering parameters of planetary rock and soil. The article reviewed the geological engineering in-situ testing methods during lunar and martian exploration missions,analyzed the differences of method and apparatus. These testing methods are classificated into penetration test,bucket test,drilling test,geophysical exploration and photogrammetry. The in-situ test results of lunar physical and mechanical characteristics were summarized as: particle size distribution,density,void ratio,shear resistance,compressibility and bearing capacity. We pointed out the difference in the physical and mechanical properties between lunar soil and earth soil. In the future,based on the ground low-gravity simulation test platform and the physical and mechanical constitutive theory research,we should develop miniaturized and lightweight in-situ test instruments with high level automation and intelligence,and obtain more accurate engineering data for subsequent construction of lunar scientific research stations and bases.
2021, 29(5): 1545-1554.
The two-receiver sonar method for cavity detection beneath piles is an effective detection method in the special environment of mud during the construction of bored holes. The Instantaneous Phase Difference Intensity(IPDI)can be applied to extract weak reflection signal from the top and bottom of a cavity,hence discovering the location of the cavity. However,during the process,how the specific valuing method of some critical parameters influence the detecting results remains unknown. The uncertainty may lead to loss in accuracy. The purposes of this paper are to find out(1)how these critical parameters,such as sampling rate,the number of ensemble members and standard deviation,can influence the results of cavity detecting and (2)the suitable way to value these parameters. First of all,we establish a numerical mode for simulating cavity detection. Secondly,we explore the optimization of valuing the critical parameters through lots of tests of simulated numerical signals. At last,we apply the optimization results to the IPDI analysis of a real-field signal to verify its effectiveness. The gray prediction model is applied to deal with the end effect appearing in the IPDI image. The result shows that a sampling rate of 0.5~1MHz,accepting 0.1 as standard deviation and 300~600 as the number of ensemble members can improve the accuracy of cavity's location and size's detecting results. In addition,grey prediction model can help release the end effect of IPDI image. A solution to address signal's zero drift phenomenon is promoted. By subtracting the trend item from the reconstructed signal,the hidden reflection signal from the bottom of the cavity is discovered. The calculated depth of the cavity's bottom is identical to what it demonstrates in borehole logs. Compared with the original detection results,the optimization method is proved effective in elevating the accuracy of two-receiver sonar method for cavity detection.
The two-receiver sonar method for cavity detection beneath piles is an effective detection method in the special environment of mud during the construction of bored holes. The Instantaneous Phase Difference Intensity(IPDI)can be applied to extract weak reflection signal from the top and bottom of a cavity,hence discovering the location of the cavity. However,during the process,how the specific valuing method of some critical parameters influence the detecting results remains unknown. The uncertainty may lead to loss in accuracy. The purposes of this paper are to find out(1)how these critical parameters,such as sampling rate,the number of ensemble members and standard deviation,can influence the results of cavity detecting and (2)the suitable way to value these parameters. First of all,we establish a numerical mode for simulating cavity detection. Secondly,we explore the optimization of valuing the critical parameters through lots of tests of simulated numerical signals. At last,we apply the optimization results to the IPDI analysis of a real-field signal to verify its effectiveness. The gray prediction model is applied to deal with the end effect appearing in the IPDI image. The result shows that a sampling rate of 0.5~1MHz,accepting 0.1 as standard deviation and 300~600 as the number of ensemble members can improve the accuracy of cavity's location and size's detecting results. In addition,grey prediction model can help release the end effect of IPDI image. A solution to address signal's zero drift phenomenon is promoted. By subtracting the trend item from the reconstructed signal,the hidden reflection signal from the bottom of the cavity is discovered. The calculated depth of the cavity's bottom is identical to what it demonstrates in borehole logs. Compared with the original detection results,the optimization method is proved effective in elevating the accuracy of two-receiver sonar method for cavity detection.
2021, 29(5): 1555-1566.
This paper provides the theoretical basis and technical support for related large-diameter slurry shield engineering in silty clay formations. It analyzes the ground deformation monitoring and construction parameter data of Jinan Yellow River Tunnel project. It presents the relationship between the characteristics of surface deformation and the construction parameters. It puts forward the key control construction parameters of the ultra-large diameter slurry shield tunneling passing through important structures. The research results show that under different construction parameters there are significant differences in the characteristics of ground deformation. The amount of grouting has the greatest influence on surface deformation. The cutterhead thrust,mud pressure,grouting pressure and tunneling speed are the smallest. And the influence of the grouting amount on the surface deformation varies with the buried depth of the tunnel. It has significant impact on the surface deformation when the buried depth is less than the tunnel diameter. In addition,when the muddy water pressure is about 0.1MPa bigger than the soil pressure and the grouting pressure is about 0.3MPa bigger than the soil pressure,the ground deformation of the structure area is acceptable,and the geological adaptability of the shield is significantly optimized.
This paper provides the theoretical basis and technical support for related large-diameter slurry shield engineering in silty clay formations. It analyzes the ground deformation monitoring and construction parameter data of Jinan Yellow River Tunnel project. It presents the relationship between the characteristics of surface deformation and the construction parameters. It puts forward the key control construction parameters of the ultra-large diameter slurry shield tunneling passing through important structures. The research results show that under different construction parameters there are significant differences in the characteristics of ground deformation. The amount of grouting has the greatest influence on surface deformation. The cutterhead thrust,mud pressure,grouting pressure and tunneling speed are the smallest. And the influence of the grouting amount on the surface deformation varies with the buried depth of the tunnel. It has significant impact on the surface deformation when the buried depth is less than the tunnel diameter. In addition,when the muddy water pressure is about 0.1MPa bigger than the soil pressure and the grouting pressure is about 0.3MPa bigger than the soil pressure,the ground deformation of the structure area is acceptable,and the geological adaptability of the shield is significantly optimized.
2021, 29(5): 1567-1576.
Sand liquefaction is one of main reasons leading to severe earthquake disasters. In this study,liquefaction resistance of sand reinforced by natural fiber under cyclic loading is discussed. A series of cyclic triaxial tests is carried out on reinforced sand specimens with different fiber contents under undrained conditions. The influence of cyclic shear strain amplitude and fiber content on liquefaction resistance of saturated sand is studied. Furthermore,a two-dimensional finite element numerical model for simulating finished cyclic triaxial test is established. The parameter calibration of reinforced sand with different fiber contents is performed. The results show that:(1)Increasing the amplitude of cyclic shear strain can promote the accumulation of excess pore pressure,which can speed up the reduction of slope of hysteresis loops and mean effective stress. (2)The presence of fiber can mitigate the accumulation of excess pore pressure,and liquefaction resistance of reinforced sand is significantly improved with increase of fiber content. (3)Calibrated constitutive model parameters can be reliably used to simulate liquefaction response of fiber reinforced sand. The results provide a valuable reference for liquefaction resistance of saturated sand and the numerical modeling of fiber reinforced sand.
Sand liquefaction is one of main reasons leading to severe earthquake disasters. In this study,liquefaction resistance of sand reinforced by natural fiber under cyclic loading is discussed. A series of cyclic triaxial tests is carried out on reinforced sand specimens with different fiber contents under undrained conditions. The influence of cyclic shear strain amplitude and fiber content on liquefaction resistance of saturated sand is studied. Furthermore,a two-dimensional finite element numerical model for simulating finished cyclic triaxial test is established. The parameter calibration of reinforced sand with different fiber contents is performed. The results show that:(1)Increasing the amplitude of cyclic shear strain can promote the accumulation of excess pore pressure,which can speed up the reduction of slope of hysteresis loops and mean effective stress. (2)The presence of fiber can mitigate the accumulation of excess pore pressure,and liquefaction resistance of reinforced sand is significantly improved with increase of fiber content. (3)Calibrated constitutive model parameters can be reliably used to simulate liquefaction response of fiber reinforced sand. The results provide a valuable reference for liquefaction resistance of saturated sand and the numerical modeling of fiber reinforced sand.
EXPERIMENTAL STUDY ON DIFFERENTIAL SETTLEMENT CONTROL PER￣FORMANCE OF PRESTRESSED REINFORCED CUSHION
2021, 29(5): 1577-1586.
Differential settlement is one of the main factors that cause road damage and traffic accidents. Reinforced soil technology is widely used as an economic control measure. We design a comparative model test between reinforced cushion and prestressed reinforced cushion and study the deformation law and the differential settlement control performance of the prestressed reinforced cushion. The comparative tests revealed the excellent deformation control ability of prestressed reinforced cushion under differential settlement. The results show that prestressed reinforced cushion can effectively reduce differential settlement, especially the serious differential settlement near the interface of soft and hard roadbed. In addition, prestressed reinforced cushion has better energy dissipation capacity. And stress superposition occurs when the additional stress is transferred from the cushion to the roadbed, and increasing trend will appear. The paper can provide reference and basis for the performance research and engineering application of prestressed reinforced structures under differential settlement.
Differential settlement is one of the main factors that cause road damage and traffic accidents. Reinforced soil technology is widely used as an economic control measure. We design a comparative model test between reinforced cushion and prestressed reinforced cushion and study the deformation law and the differential settlement control performance of the prestressed reinforced cushion. The comparative tests revealed the excellent deformation control ability of prestressed reinforced cushion under differential settlement. The results show that prestressed reinforced cushion can effectively reduce differential settlement, especially the serious differential settlement near the interface of soft and hard roadbed. In addition, prestressed reinforced cushion has better energy dissipation capacity. And stress superposition occurs when the additional stress is transferred from the cushion to the roadbed, and increasing trend will appear. The paper can provide reference and basis for the performance research and engineering application of prestressed reinforced structures under differential settlement.
2021, 29(5): 1587-1598.
Shield tunneling can inevitably pass through areas with dense building structures. Especially when the building structure that passes through is constructed for a long time, the foundation is weak, and the stratum deformation exceeds a certain limit, the building foundation is prone to uneven settlement and superstructure the additional deformation. This paper aims to clarify the influencing factors of the formation deformation of the large-diameter muddy water shield tunnel through the complex environment, and to better grasp the law of the formation deformation. It is based on the Wuhan Rail Transit Line 8 Huangpu Road Station-Xujiapeng Station River Crossing Tunnel Project. It uses the element software Plaxis 3D and establishes a three-dimensional finite element model to simulate the construction process. It studies the thickness of the overburden, the support pressure of the excavation surface, the soil loss of the shield shell, and the sensitivity of the shield tail grouting pressure to the surface settlement law. The numerical simulation results are compared and analyzed with the measured values on site. It is found that the finite element calculation results are in good agreement with the measured results, which verifies the validity of the numerical model. The research in this paper can provide method guidance for the subsequent selection of parameters of large-diameter mud-water balance shield.
Shield tunneling can inevitably pass through areas with dense building structures. Especially when the building structure that passes through is constructed for a long time, the foundation is weak, and the stratum deformation exceeds a certain limit, the building foundation is prone to uneven settlement and superstructure the additional deformation. This paper aims to clarify the influencing factors of the formation deformation of the large-diameter muddy water shield tunnel through the complex environment, and to better grasp the law of the formation deformation. It is based on the Wuhan Rail Transit Line 8 Huangpu Road Station-Xujiapeng Station River Crossing Tunnel Project. It uses the element software Plaxis 3D and establishes a three-dimensional finite element model to simulate the construction process. It studies the thickness of the overburden, the support pressure of the excavation surface, the soil loss of the shield shell, and the sensitivity of the shield tail grouting pressure to the surface settlement law. The numerical simulation results are compared and analyzed with the measured values on site. It is found that the finite element calculation results are in good agreement with the measured results, which verifies the validity of the numerical model. The research in this paper can provide method guidance for the subsequent selection of parameters of large-diameter mud-water balance shield.
2021, 29(5): 1599-1610.
A three-dimensional numerical model for wave-induced seabed response around mono-pile foundation is established in the OpenFOAM platform. The numerical model is governed by the Reynolds-Averaged Navier-Stokes equations to simulate the nonlinear motion of the waves. The governing equation of the seabed model is the Biot equation to study the liquefaction law of the seabed around mono-pile. The previous numerical analysis results and experimental results are compared and analyzed to verify the accuracy and effectiveness of the three-dimensional wave model and seabed model established in this paper. In this paper,the seabed response around mono-pile is analyzed and evaluated. The results show that the pore water pressure field and effective stress field of the seabed around mono-pile change greatly in the water depth direction and less in the horizontal direction. Under this condition,the maximum liquefaction depth of seabed can reach 10 m,and the seabed perpendicular to the wave movement direction is more prone to liquefaction. The numerical model in this paper can effectively predict the liquefaction depth and liquefaction range of the actual engineering seabed.
A three-dimensional numerical model for wave-induced seabed response around mono-pile foundation is established in the OpenFOAM platform. The numerical model is governed by the Reynolds-Averaged Navier-Stokes equations to simulate the nonlinear motion of the waves. The governing equation of the seabed model is the Biot equation to study the liquefaction law of the seabed around mono-pile. The previous numerical analysis results and experimental results are compared and analyzed to verify the accuracy and effectiveness of the three-dimensional wave model and seabed model established in this paper. In this paper,the seabed response around mono-pile is analyzed and evaluated. The results show that the pore water pressure field and effective stress field of the seabed around mono-pile change greatly in the water depth direction and less in the horizontal direction. Under this condition,the maximum liquefaction depth of seabed can reach 10 m,and the seabed perpendicular to the wave movement direction is more prone to liquefaction. The numerical model in this paper can effectively predict the liquefaction depth and liquefaction range of the actual engineering seabed.
2021, 29(5): 1611-1620.
In pipe jacking construction, thixotropic mud plays the role of lubrication, drag reduction and supporting formation. The mud is an important material to ensure the safe and rapid pipe jacking construction. Its resistance reduction performance is an important factor affecting the pipe jacking construction. This paper is based on the comprehensive pipe gallery project of Lucheng Town, Tongzhou City, the vice center of Beijing City. It takes bentonite, CMC and soda as the raw materials of thixotropic slurry, carries out the orthogonal test of different material ratio, and optimizes the optimal ratio of thixotropic slurry. The drag reduction effect of thixotropic slurry is investigated by scale model test. In addition, the microstructure and drag reduction mechanism of thixotropic mud are studied by SEM. The results show that: the content of thixotropic slurry raw materials has a great influence on the performance of the mud. 10% bentonite, 0.2% CMC, 0.5% soda ash and 89.3% water are the optimal proportion of slurry raw materials. Under this ratio, the slurry has good fluidity and thixotropy, small water loss, dense "filter cake" and optimal comprehensive performance. In addition, the friction coefficient between the test block and sand can be effectively reduced by 40% with the thixotropic slurry, and the drag reduction effect is remarkable. The thixotropic slurry has a lamellar structure, and its main mineral component montmorillonite has micro characteristics such as lattice substitution and cation exchange, which makes the thixotropic slurry show thixotropy macroscopically and plays the role of drag reduction. Finally, the pipe and surrounding soil are fully isolated in the form of mud sleeve, so as to realize the drag reduction function.
In pipe jacking construction, thixotropic mud plays the role of lubrication, drag reduction and supporting formation. The mud is an important material to ensure the safe and rapid pipe jacking construction. Its resistance reduction performance is an important factor affecting the pipe jacking construction. This paper is based on the comprehensive pipe gallery project of Lucheng Town, Tongzhou City, the vice center of Beijing City. It takes bentonite, CMC and soda as the raw materials of thixotropic slurry, carries out the orthogonal test of different material ratio, and optimizes the optimal ratio of thixotropic slurry. The drag reduction effect of thixotropic slurry is investigated by scale model test. In addition, the microstructure and drag reduction mechanism of thixotropic mud are studied by SEM. The results show that: the content of thixotropic slurry raw materials has a great influence on the performance of the mud. 10% bentonite, 0.2% CMC, 0.5% soda ash and 89.3% water are the optimal proportion of slurry raw materials. Under this ratio, the slurry has good fluidity and thixotropy, small water loss, dense "filter cake" and optimal comprehensive performance. In addition, the friction coefficient between the test block and sand can be effectively reduced by 40% with the thixotropic slurry, and the drag reduction effect is remarkable. The thixotropic slurry has a lamellar structure, and its main mineral component montmorillonite has micro characteristics such as lattice substitution and cation exchange, which makes the thixotropic slurry show thixotropy macroscopically and plays the role of drag reduction. Finally, the pipe and surrounding soil are fully isolated in the form of mud sleeve, so as to realize the drag reduction function.
2021, 29(5): 1621-1631.
This paper introduces the time series prediction principle,the basic model and the prediction procedure in detail. It uses the measured data of foundation pit displacement and crack deformation during the construction of the foundation pit of Xintian Yangtze river bridge. It establishes the Autoregressive Integrated Moving Average Model(ARMA)using the time series method. On this basis,the deformation trend of the foundation pit and the crack of the dangerous rock in the construction process are predicted. The model prediction data are compared with the international monitoring data. The results show that the ARMA model can solve the problem of data instability by using difference algorithms,and has higher accuracy in the short-time prediction period. With the increase of the prediction period,the prediction accuracy of ARMA model decreases. The reason is that with the increase of the prediction period,the historical data on which the prediction relies are decreases. Therefore,when establishing the ARMA model,new data should be added regularly to avoid the reduction of prediction accuracy caused by too long prediction period. The research results of this paper can be a reference for the monitoring research of similar projects in the future.
This paper introduces the time series prediction principle,the basic model and the prediction procedure in detail. It uses the measured data of foundation pit displacement and crack deformation during the construction of the foundation pit of Xintian Yangtze river bridge. It establishes the Autoregressive Integrated Moving Average Model(ARMA)using the time series method. On this basis,the deformation trend of the foundation pit and the crack of the dangerous rock in the construction process are predicted. The model prediction data are compared with the international monitoring data. The results show that the ARMA model can solve the problem of data instability by using difference algorithms,and has higher accuracy in the short-time prediction period. With the increase of the prediction period,the prediction accuracy of ARMA model decreases. The reason is that with the increase of the prediction period,the historical data on which the prediction relies are decreases. Therefore,when establishing the ARMA model,new data should be added regularly to avoid the reduction of prediction accuracy caused by too long prediction period. The research results of this paper can be a reference for the monitoring research of similar projects in the future.
2021, 29(5): 1632-1639.
Due to the limitation of site conditions,the construction of Xi'an's underground utility tunnel needs to cross the ground fractures at different angles,which can be affected by the action of ground fractures. A three-dimensional finite element model of an underground utility tunnel crossing an active ground fracture is established. When the underground utility tunnel crossed the ground fracture orthogonally,6 different settlement conditions of 5 cm,10 cm,15 cm,20 cm,25 cm and 30 cm,and 3 crossing angles of orthogonal,oblique angle of 60°and oblique angle of 30°are set up. The research results show that the deformation and axial stress of the underground utility tunnel increase with the increase of ground fracture activity. The horizontal displacement,vertical displacement and axial stress of the underground utility tunnel are concentrated in a certain range on both sides of the ground fracture. The peak value of axial stress on the upper surface of the underground utility tunnel is greater than that on the lower surface. The smaller the intersection angle between the underground utility tunnel and the ground fracture,the smaller the horizontal displacement of the underground utility tunnel,the larger the influence range of the vertical displacement,and the greater the axial stress of the underground utility tunnel. The actual project has to avoid the underground utility tunnel from crossing the ground fractures with strong activity at a small angle without joints. The research results have certain reference value for the analysis of the interaction mechanism between the soil body and the underground utility tunnel,as well as for disaster prevention and mitigation.
Due to the limitation of site conditions,the construction of Xi'an's underground utility tunnel needs to cross the ground fractures at different angles,which can be affected by the action of ground fractures. A three-dimensional finite element model of an underground utility tunnel crossing an active ground fracture is established. When the underground utility tunnel crossed the ground fracture orthogonally,6 different settlement conditions of 5 cm,10 cm,15 cm,20 cm,25 cm and 30 cm,and 3 crossing angles of orthogonal,oblique angle of 60°and oblique angle of 30°are set up. The research results show that the deformation and axial stress of the underground utility tunnel increase with the increase of ground fracture activity. The horizontal displacement,vertical displacement and axial stress of the underground utility tunnel are concentrated in a certain range on both sides of the ground fracture. The peak value of axial stress on the upper surface of the underground utility tunnel is greater than that on the lower surface. The smaller the intersection angle between the underground utility tunnel and the ground fracture,the smaller the horizontal displacement of the underground utility tunnel,the larger the influence range of the vertical displacement,and the greater the axial stress of the underground utility tunnel. The actual project has to avoid the underground utility tunnel from crossing the ground fractures with strong activity at a small angle without joints. The research results have certain reference value for the analysis of the interaction mechanism between the soil body and the underground utility tunnel,as well as for disaster prevention and mitigation.
2021, 29(5): 1640-1646.
In this study, a dry bentonite powder was selected as a sealing material to backfill the joint area between the compacted bentonite-sand blocks. The swelling property of the block system after backfilling was studied considering the initial dry density of blocks as the basic variable. During the swelling test, the swelling stresses at radial and axial direction were monitored in a constant volume condition. The test results show that the swelling stress develops preferentially to the joint area, and the radial swelling stress develops faster than that on the axial direction, which contributes to the sealing of the joint. The final swelling stress in the joint area of the block system increases with the initial dry density of the block, the time to reach the maximum swelling stress shortens, and the healing speed of the joint area improves under the same sealing condition. The swelling deformation of block develops preferentially to the joint area and fills the pores in the joint area resulting in the phenomenon of "stress compensation" that the final axial stresses of blocks decrease. The lower the dry density of the block, the more stress compensation is needed. According to the axial stress curve of the block, the development process of axial stress of block can be divided into three stages: rapid-growth stage, steady-development stage and stress-adjustment stage. The rapid development stage is mainly controlled by the initial dry density of the block, while the steady growth stage is more affected by water migration, side wall friction and the sealing rate of joint. Compared with the intact block, the stress distribution of block with backfilled joint tends to be isotropic, which improves the homogenization degree of the whole barrier system.
In this study, a dry bentonite powder was selected as a sealing material to backfill the joint area between the compacted bentonite-sand blocks. The swelling property of the block system after backfilling was studied considering the initial dry density of blocks as the basic variable. During the swelling test, the swelling stresses at radial and axial direction were monitored in a constant volume condition. The test results show that the swelling stress develops preferentially to the joint area, and the radial swelling stress develops faster than that on the axial direction, which contributes to the sealing of the joint. The final swelling stress in the joint area of the block system increases with the initial dry density of the block, the time to reach the maximum swelling stress shortens, and the healing speed of the joint area improves under the same sealing condition. The swelling deformation of block develops preferentially to the joint area and fills the pores in the joint area resulting in the phenomenon of "stress compensation" that the final axial stresses of blocks decrease. The lower the dry density of the block, the more stress compensation is needed. According to the axial stress curve of the block, the development process of axial stress of block can be divided into three stages: rapid-growth stage, steady-development stage and stress-adjustment stage. The rapid development stage is mainly controlled by the initial dry density of the block, while the steady growth stage is more affected by water migration, side wall friction and the sealing rate of joint. Compared with the intact block, the stress distribution of block with backfilled joint tends to be isotropic, which improves the homogenization degree of the whole barrier system.
2021, 29(5): 1647-1656.
During the construction of large rectangular cross-section pipe corridor in anhydrous sand layer, due to the poor flow plasticity of sand, the bad phenomena such as "blocking", "gushing" and "cake forming" are easy to appear on the face of the pipe, which can affect the construction progress. This paper is based on the construction of rectangular pipe jacking with large section in Changhe West Road(Zhaoshan Street-Luyang Street), Tongzhou District, Beijing. It carries out the experimental study on the improvement of muck of front face. It uses different ratios of sodium base bentonite, polyacrylamide(PAM) and water as additives of sandy soil improvement. It uses viscosity test and filtration test of different ratios of modifying agent(bentonite added polyacrylamide) performance. It further uses the modified waste residue of the slump test, direct shear test and compression test, and evaluates the constraints of waste residue. The results show that the effect of adding polyacrylamide to bentonite on anhydrous sand is remarkable. According to the performance test of the improver, four good modifiers of bentonite:PAM:water: 60:2:1000, 70:1.5:1000, 70:2:1000 and 80:1.5:1000 are screened. The results show that the optimal ratio of modifier bentonite:PAM:water is 70:2:1000, and the optimal injection ratio of modifier is 15%. By adding the modifier, the slump of the modified soil is reduced by 30 mm, the cohesion is increased by 9.3 kPa, the internal friction angle is reduced by 6.5°, and the compression coefficient is increased by 0.090 MPa-1. At this time, the anhydrous sand soil is close to the plastic-flow state, which effectively reduces the side wall resistance of large section rectangular pipe and improves the jacking efficiency.
During the construction of large rectangular cross-section pipe corridor in anhydrous sand layer, due to the poor flow plasticity of sand, the bad phenomena such as "blocking", "gushing" and "cake forming" are easy to appear on the face of the pipe, which can affect the construction progress. This paper is based on the construction of rectangular pipe jacking with large section in Changhe West Road(Zhaoshan Street-Luyang Street), Tongzhou District, Beijing. It carries out the experimental study on the improvement of muck of front face. It uses different ratios of sodium base bentonite, polyacrylamide(PAM) and water as additives of sandy soil improvement. It uses viscosity test and filtration test of different ratios of modifying agent(bentonite added polyacrylamide) performance. It further uses the modified waste residue of the slump test, direct shear test and compression test, and evaluates the constraints of waste residue. The results show that the effect of adding polyacrylamide to bentonite on anhydrous sand is remarkable. According to the performance test of the improver, four good modifiers of bentonite:PAM:water: 60:2:1000, 70:1.5:1000, 70:2:1000 and 80:1.5:1000 are screened. The results show that the optimal ratio of modifier bentonite:PAM:water is 70:2:1000, and the optimal injection ratio of modifier is 15%. By adding the modifier, the slump of the modified soil is reduced by 30 mm, the cohesion is increased by 9.3 kPa, the internal friction angle is reduced by 6.5°, and the compression coefficient is increased by 0.090 MPa-1. At this time, the anhydrous sand soil is close to the plastic-flow state, which effectively reduces the side wall resistance of large section rectangular pipe and improves the jacking efficiency.
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