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Editor Work2019 Vol. 27, No. 5
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2019, 27(5): 933-945.
Abstract
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The pre-existing discontinuous such as fractures have significant influences on the mechanical properties of the rock mass. Understanding the fracture initiation, propagation, interaction, and coalescence mechanisms are critical for characterizing mechanical behaviors of engineering rock mass and evaluating their engineering performance. In this study, a discrete element method is developed on the basis of continuum mechanics models. It is applied and a series of fracture models are established. In these models, the loading conditions and its relationship with the fracture occurrences are considered during numerical simulations. The evolution of fracture propagation and rock failure mechanisms of different fracture models are studied and the fracture propagation and its influence on rock failure characteristics are analyzed. The results show that:(1)The locations of fracture initiation, final coalescence patterns, and damage distributions are influenced by the confining pressures of the models, fracture occurrence, and the angle between fracture orientations and the maximum principal compressive stress. (2)When the weak surface of the fracture is obliquely intersected with the orientation of the maximum principal compressive stress, fracture initiation is more prone to occur at the end of the fracture. These new cracks dominate the coalescence pattern of the rock mass. Conversely, when the weak surface of the fracture is consistent with the orientation of the maximum principal compressive stress, the weak surface of the fracture only passively affects the initiation and propagation of the newly induced fractures and no further propagation or damage are observed during the whole loading process. (3)The increase of the number of pre-existing fractures and the confining pressure of the model could significantly increase the number of internal shear fractures and the degree of failure once the model is failed. The damaged area inside the model shows a slip-line shaped distribution and mainly distributes around the rupture surface, while the damage of the non-ruptured surface area shows a strip-shaped cross distribution. (4)When the weak surface of the fracture is oblique with the orientation of the maximum principal compressive stress, the weaken degree of the strength of the rock model is higher than that at the opposing situation, while the residual strength of the fractured model is the exact opposite.
The pre-existing discontinuous such as fractures have significant influences on the mechanical properties of the rock mass. Understanding the fracture initiation, propagation, interaction, and coalescence mechanisms are critical for characterizing mechanical behaviors of engineering rock mass and evaluating their engineering performance. In this study, a discrete element method is developed on the basis of continuum mechanics models. It is applied and a series of fracture models are established. In these models, the loading conditions and its relationship with the fracture occurrences are considered during numerical simulations. The evolution of fracture propagation and rock failure mechanisms of different fracture models are studied and the fracture propagation and its influence on rock failure characteristics are analyzed. The results show that:(1)The locations of fracture initiation, final coalescence patterns, and damage distributions are influenced by the confining pressures of the models, fracture occurrence, and the angle between fracture orientations and the maximum principal compressive stress. (2)When the weak surface of the fracture is obliquely intersected with the orientation of the maximum principal compressive stress, fracture initiation is more prone to occur at the end of the fracture. These new cracks dominate the coalescence pattern of the rock mass. Conversely, when the weak surface of the fracture is consistent with the orientation of the maximum principal compressive stress, the weak surface of the fracture only passively affects the initiation and propagation of the newly induced fractures and no further propagation or damage are observed during the whole loading process. (3)The increase of the number of pre-existing fractures and the confining pressure of the model could significantly increase the number of internal shear fractures and the degree of failure once the model is failed. The damaged area inside the model shows a slip-line shaped distribution and mainly distributes around the rupture surface, while the damage of the non-ruptured surface area shows a strip-shaped cross distribution. (4)When the weak surface of the fracture is oblique with the orientation of the maximum principal compressive stress, the weaken degree of the strength of the rock model is higher than that at the opposing situation, while the residual strength of the fractured model is the exact opposite.
2019, 27(5): 946-954.
In recent years, earthquakes occur frequently in China. Slope instability induced by strong earthquakes are the most common secondary disasters, which makes the number of earthquake landslides in China ranks first in the world. For the dynamic response of rock slopes under strong earthquakes, this paper adopts iron powder, barite powder, quartz sand, gypsum and water as the similar materials and carries out the shaking table test of large homogeneous rock slope. The ground motion response characteristics of the homogeneous slope model under different frequencies and amplitudes of seismic wave input are analyzed in detail. It is found that when the frequency is low, the horizontal acceleration amplification factor of the monitoring point along the horizontal distance of the slope surface increases monotonously. The horizontal acceleration amplification factor at the shoulder reaches the maximum value. When the frequency further increases or exceeds the natural vibration frequency of the model, the slope model no longer exhibits a typical amplification phenomenon. Under the same amplitude and different frequency loading conditions, the variation of the natural frequency of the model is not obvious overall, and the change of the input acceleration amplitude has a more significant effect on the natural vibration frequency. The low frequency component is not obvious to the model damage. The high frequency and the natural vibration frequency are more obvious on the damage of the homogeneous slope, resulting in a significant decrease in the natural frequency of the model. The research of this problem has certain guiding significance for the study of ground motion response and deformation failure mechanism of rock slope under strong earthquakes.
In recent years, earthquakes occur frequently in China. Slope instability induced by strong earthquakes are the most common secondary disasters, which makes the number of earthquake landslides in China ranks first in the world. For the dynamic response of rock slopes under strong earthquakes, this paper adopts iron powder, barite powder, quartz sand, gypsum and water as the similar materials and carries out the shaking table test of large homogeneous rock slope. The ground motion response characteristics of the homogeneous slope model under different frequencies and amplitudes of seismic wave input are analyzed in detail. It is found that when the frequency is low, the horizontal acceleration amplification factor of the monitoring point along the horizontal distance of the slope surface increases monotonously. The horizontal acceleration amplification factor at the shoulder reaches the maximum value. When the frequency further increases or exceeds the natural vibration frequency of the model, the slope model no longer exhibits a typical amplification phenomenon. Under the same amplitude and different frequency loading conditions, the variation of the natural frequency of the model is not obvious overall, and the change of the input acceleration amplitude has a more significant effect on the natural vibration frequency. The low frequency component is not obvious to the model damage. The high frequency and the natural vibration frequency are more obvious on the damage of the homogeneous slope, resulting in a significant decrease in the natural frequency of the model. The research of this problem has certain guiding significance for the study of ground motion response and deformation failure mechanism of rock slope under strong earthquakes.
2019, 27(5): 955-965.
The study on seepage-damage-stress coupling of underground engineering rock mass is of great significance for the stability analysis of surrounding rock. This paper presents an improved model of seepage-damage-stress coupling based on elastic-plastic mechanics, seepage mechanics and damage theory. In this model, we fully consider the heterogeneity of rock mass, the dynamic weakening process of rock mass mechanical parameters, the post-peak characteristics of plastic yield of surrounding rock and the abrupt change of permeability coefficient in the process of damage. This model can better reflect the yield damage and progressive failure process of the surrounding rock, which is obtained from the analysis of numerical simulation results with the multi-physics coupling simulation software. In order to verify its validity, this model is used in the analysis of the progressive failure process of surrounding rock at different depths. The result is in good agreement with the results from the engineering practice.
The study on seepage-damage-stress coupling of underground engineering rock mass is of great significance for the stability analysis of surrounding rock. This paper presents an improved model of seepage-damage-stress coupling based on elastic-plastic mechanics, seepage mechanics and damage theory. In this model, we fully consider the heterogeneity of rock mass, the dynamic weakening process of rock mass mechanical parameters, the post-peak characteristics of plastic yield of surrounding rock and the abrupt change of permeability coefficient in the process of damage. This model can better reflect the yield damage and progressive failure process of the surrounding rock, which is obtained from the analysis of numerical simulation results with the multi-physics coupling simulation software. In order to verify its validity, this model is used in the analysis of the progressive failure process of surrounding rock at different depths. The result is in good agreement with the results from the engineering practice.
2019, 27(5): 966-972.
Under the long-term of water-rock interaction, the red-bed soft rock shows the characteristics of poor diagenesis, easy weathering and softening, which brings challenges to the soft rock engineering design and construction. Firstly, in order to analyze the mechanical properties of soft rock, uniaxial and triaxial compression tests are carried out on the samples of mudstone taken from the red-bed soft rock area of central Yunnan in the natural state and saturated state. Then the microstructure of soft rock in different water-bearing states is studied by X-ray diffraction. Finally, based on the experimental results, the softening mechanism is discussed. The results show that the mechanical properties of soft rock are obviously deteriorated under the action of hydro-rock, and the compressive strength of rock peak value is significantly reduced, the failure degree of rock mass increases. When the soft rock clay minerals absorb water, the microstructure changes from compact granular structure to loose and porous irregular flake structure. Clay mineral particles such as illite react with water, resulting in uneven stress and a large number of micropores, which causes the destruction of the internal structure of soft rock. The water-softening mechanism of red-bed soft rock includes the dissolution and secondary action of soft rock minerals, the water-absorbing expansion and disintegration of clay minerals, and the failure of intergranular bonding caused by the interaction between soft rock and water.
Under the long-term of water-rock interaction, the red-bed soft rock shows the characteristics of poor diagenesis, easy weathering and softening, which brings challenges to the soft rock engineering design and construction. Firstly, in order to analyze the mechanical properties of soft rock, uniaxial and triaxial compression tests are carried out on the samples of mudstone taken from the red-bed soft rock area of central Yunnan in the natural state and saturated state. Then the microstructure of soft rock in different water-bearing states is studied by X-ray diffraction. Finally, based on the experimental results, the softening mechanism is discussed. The results show that the mechanical properties of soft rock are obviously deteriorated under the action of hydro-rock, and the compressive strength of rock peak value is significantly reduced, the failure degree of rock mass increases. When the soft rock clay minerals absorb water, the microstructure changes from compact granular structure to loose and porous irregular flake structure. Clay mineral particles such as illite react with water, resulting in uneven stress and a large number of micropores, which causes the destruction of the internal structure of soft rock. The water-softening mechanism of red-bed soft rock includes the dissolution and secondary action of soft rock minerals, the water-absorbing expansion and disintegration of clay minerals, and the failure of intergranular bonding caused by the interaction between soft rock and water.
2019, 27(5): 973-979.
The mechanical property of shale gas reservoir is an important factor affecting the stability of shale wellbore. According to the results of shale uniaxial and triaxial compression tests under different confining pressures, energy evolution laws, damage evolution laws and brittle property are analyzed. The results show that before yielding stage, the total absorption energy of shales almost transforms into the elastic strain energy. During the period between yielding stage and peak strength stage, the ratio of the elastic strain energy to the total absorption energy gradually decreases, while the dissipation energy gradually increases. After the peak strength stage, the elastic strain energy sharply decreases and the dissipation energy rapidly increases. The dissipation of energy can lead to the damage of the rock. The damage evolution laws determined from energy perspective conform to the "S-shaped" curve. The brittleness index of shale becomes weaker with the increase of confining pressure. The study of improving the reservoir fracturing effect and evaluating the wellbore stability has important practical significance and application value.
The mechanical property of shale gas reservoir is an important factor affecting the stability of shale wellbore. According to the results of shale uniaxial and triaxial compression tests under different confining pressures, energy evolution laws, damage evolution laws and brittle property are analyzed. The results show that before yielding stage, the total absorption energy of shales almost transforms into the elastic strain energy. During the period between yielding stage and peak strength stage, the ratio of the elastic strain energy to the total absorption energy gradually decreases, while the dissipation energy gradually increases. After the peak strength stage, the elastic strain energy sharply decreases and the dissipation energy rapidly increases. The dissipation of energy can lead to the damage of the rock. The damage evolution laws determined from energy perspective conform to the "S-shaped" curve. The brittleness index of shale becomes weaker with the increase of confining pressure. The study of improving the reservoir fracturing effect and evaluating the wellbore stability has important practical significance and application value.
2019, 27(5): 980-988.
Due to the long-term geological and structural forces, there are a large number of weak fracture zones in the underground rock mass. Tunnel excavation through this area is prone to large deformations and even accidents such as landslides. This paper uses the method of TGP advanced geological prediction combining with the investigation of the surrounding rock of the face. It carries out the detailed geological survey of the undisturbed rock mass in the tunnel face and the front. It obtains the Hoek-Brown parameters of the rock surrounding the tunnel. The paper uses the FLAC3D numerical simulation method to analyze the stability of surrounding rock in the crushing zone under the given supporting conditions. It compares the numerical results with the field monitoring data to verify the correctness of this method. The results show that the numerical simulation results are in good agreement with the actual monitoring results. We can use this method to guide the stability prediction of unexcavated tunnels to ensure the construction safety of the tunneling process.
Due to the long-term geological and structural forces, there are a large number of weak fracture zones in the underground rock mass. Tunnel excavation through this area is prone to large deformations and even accidents such as landslides. This paper uses the method of TGP advanced geological prediction combining with the investigation of the surrounding rock of the face. It carries out the detailed geological survey of the undisturbed rock mass in the tunnel face and the front. It obtains the Hoek-Brown parameters of the rock surrounding the tunnel. The paper uses the FLAC3D numerical simulation method to analyze the stability of surrounding rock in the crushing zone under the given supporting conditions. It compares the numerical results with the field monitoring data to verify the correctness of this method. The results show that the numerical simulation results are in good agreement with the actual monitoring results. We can use this method to guide the stability prediction of unexcavated tunnels to ensure the construction safety of the tunneling process.
2019, 27(5): 989-999.
The rock mass in the unloading disturbance zone near the excavation face of project is affected by the structural plane and tensile stress. Deformation and failure of the rock mass are characterized by the loading combining tension and shear. However, previous studies have focused on the tensile failure and compression-shear composite failure of intact rock or rock mass. The deformation and failure behavior of rock mass under tensile and shear stress conditions was less involved. This paper aims to study the mechanical behavior of jointed rock mass under combined tension and shear condition. A series of numerical simulation studies on coplanar discontinuous jointed rock mass are carried out using particle discrete element method. It assumes that the micro mechanical parameters of the intergranular contact obey the Weibull distribution to characterize the heterogeneity of the rock mass. The influence of the heterogeneity, homogeneity, normal tensile stress and joint connectivity rate on the tensile strength and failure mode of the jointed rock mass is discussed. The research results indicate that the heterogeneous jointed rock mass is primarily destroyed along the stepped fracture surface under the condition of combined tension and shear stress. The shear stress-horizontal displacement curve can be divided into the linear deformation stage, the nonlinear deformation stage, the peak and the post-peak stage. With the increase of homogeneity degree, the shear strength of jointed rock mass increases gradually and the lifting amplitude decreases gradually, which tends to be the same as that of homogeneous rock mass and the degree of breakage of the fracture surface is reduced. The micro-cracks in the rock mass are concentrated from the diffuse distribution to the fracture surface. Peak shear strength of jointed rock mass and the magnitude of the normal tensile stress are nonlinearly and negatively correlated. With the increase of the normal tensile stress, the failure mode of the jointed rock mass is transformed from tension-shear mixed type to tension type, and the roughness of the fracture surface is increased to varying degrees. The shear strength of jointed rock mass decreases significantly with the increase of joint connectivity rate.
The rock mass in the unloading disturbance zone near the excavation face of project is affected by the structural plane and tensile stress. Deformation and failure of the rock mass are characterized by the loading combining tension and shear. However, previous studies have focused on the tensile failure and compression-shear composite failure of intact rock or rock mass. The deformation and failure behavior of rock mass under tensile and shear stress conditions was less involved. This paper aims to study the mechanical behavior of jointed rock mass under combined tension and shear condition. A series of numerical simulation studies on coplanar discontinuous jointed rock mass are carried out using particle discrete element method. It assumes that the micro mechanical parameters of the intergranular contact obey the Weibull distribution to characterize the heterogeneity of the rock mass. The influence of the heterogeneity, homogeneity, normal tensile stress and joint connectivity rate on the tensile strength and failure mode of the jointed rock mass is discussed. The research results indicate that the heterogeneous jointed rock mass is primarily destroyed along the stepped fracture surface under the condition of combined tension and shear stress. The shear stress-horizontal displacement curve can be divided into the linear deformation stage, the nonlinear deformation stage, the peak and the post-peak stage. With the increase of homogeneity degree, the shear strength of jointed rock mass increases gradually and the lifting amplitude decreases gradually, which tends to be the same as that of homogeneous rock mass and the degree of breakage of the fracture surface is reduced. The micro-cracks in the rock mass are concentrated from the diffuse distribution to the fracture surface. Peak shear strength of jointed rock mass and the magnitude of the normal tensile stress are nonlinearly and negatively correlated. With the increase of the normal tensile stress, the failure mode of the jointed rock mass is transformed from tension-shear mixed type to tension type, and the roughness of the fracture surface is increased to varying degrees. The shear strength of jointed rock mass decreases significantly with the increase of joint connectivity rate.
2019, 27(5): 1000-1009.
The topographic measurement of high-steep to upright slopes of complex terrain and the establishment of three-dimensional numerical models have always been puzzling geologists. In recent years, the unmanned aerial vehicle(UAV)has been widely used in geological surveys due to its small size, maneuverability, and the ability to acquire high-resolution images. This paper focuses on a new measuring and modeling method for high-steep/upright slopes with integration of UAV and many software. Firstly, on the basis of low-altitude UAV tilt photography, the Agisoft Photoscan 3D real scene modeling software and reverse engineering-based Geomagic Studio's powerful point cloud data processing function, we generate the topographic maps of high-steep/upright slope with complex terrain quickly, combine with the topographic mapping function of CASS. Subsequently, the high-steep/upright slope closed CAD surface models of complex terrain are reconstructed utilizing the CAD surface modeling function of Geomagic Studio. Then we take the advantages of the powerful geometric processing and meshing ability of Hypermesh to mesh the CAD surface models, before establishing a fine three-dimensional(3D)numerical model for high-steep/upright slopes of complex terrain. Finally, the 3D numerical model is converted into a file recognizable by FLAC3D for calculation and analysis. In order to illustrate this method in details, we take the Feitian Waterfall Scenic Spot in Shenxianju Scenic Spot in Zhejiang Province as an example. The results indicate that the use of UAV enables fast, efficient and accurate mapping and 3D modeling of high-steep/upright slopes in complex terrain. The method has the advantages of being simple and practical, fast and convenient, and strong practicality.
The topographic measurement of high-steep to upright slopes of complex terrain and the establishment of three-dimensional numerical models have always been puzzling geologists. In recent years, the unmanned aerial vehicle(UAV)has been widely used in geological surveys due to its small size, maneuverability, and the ability to acquire high-resolution images. This paper focuses on a new measuring and modeling method for high-steep/upright slopes with integration of UAV and many software. Firstly, on the basis of low-altitude UAV tilt photography, the Agisoft Photoscan 3D real scene modeling software and reverse engineering-based Geomagic Studio's powerful point cloud data processing function, we generate the topographic maps of high-steep/upright slope with complex terrain quickly, combine with the topographic mapping function of CASS. Subsequently, the high-steep/upright slope closed CAD surface models of complex terrain are reconstructed utilizing the CAD surface modeling function of Geomagic Studio. Then we take the advantages of the powerful geometric processing and meshing ability of Hypermesh to mesh the CAD surface models, before establishing a fine three-dimensional(3D)numerical model for high-steep/upright slopes of complex terrain. Finally, the 3D numerical model is converted into a file recognizable by FLAC3D for calculation and analysis. In order to illustrate this method in details, we take the Feitian Waterfall Scenic Spot in Shenxianju Scenic Spot in Zhejiang Province as an example. The results indicate that the use of UAV enables fast, efficient and accurate mapping and 3D modeling of high-steep/upright slopes in complex terrain. The method has the advantages of being simple and practical, fast and convenient, and strong practicality.
2019, 27(5): 1010-1018.
The effect of Cu2+ on the variation of physical-mechanical properties of Guilin red clay is studied. XRD、XRF and mercury intrusion tests are carried out to investigate the variation law of main mineral components and the change trend of micro-pore structure in Cu2+ contaminated red clay. The results show that the main mineral components in red clay are kaolinite, quartz and goethite. Cu2+ pollutant has a significant impact on the content of these three main mineral components. With the increase of Cu2+ concentration, the content of kaolinite and goethite gradually decrease, while the content of quartz gradually increases. The change rate of content is kaolinite>goethite>quartz. When the Cu2+ concentration is 2%, the content loss rate of kaolinite and goethite are as high as 10.69% and 5.38%, respectively. The pore distribution curve of red clay is bimodal. The double peaks are distributed between 0.01~0.1 μm and 1~10 μm respectively. The micro-pores between 0.01~0.1 μm account for the absolute advantage. With the increase of Cu2+ concentration, the "double peak" move to the right gradually, and the pores become larger, the "peak width" widens gradually, and the pores become more numerous. A series of tests are conducted to observe the variation law of the deformation strength characteristics of red clay contaminated by Cu2+. They include unconfined compressive strength test, direct shear test and compression consolidation test. The results show that Cu2+ contamination has a significant effect on the deformation strength properties of red clay. With the increase of Cu2+ concentration, the unconfined compressive strength, shear strength and its index(C, φ) of the red clay decrease gradually, the initial void ratio e0 and the compression coefficient α increase gradually. When the Cu2+ concentration increases from 0 to 2%, the unconfined compressive stress-strain curve of soil changes from typical strain softening type to weak strain hardening one, the peak value of unconfined compressive strength decreases by 76.91% and the average loss rate of shear strength reaches 69.36%.
The effect of Cu2+ on the variation of physical-mechanical properties of Guilin red clay is studied. XRD、XRF and mercury intrusion tests are carried out to investigate the variation law of main mineral components and the change trend of micro-pore structure in Cu2+ contaminated red clay. The results show that the main mineral components in red clay are kaolinite, quartz and goethite. Cu2+ pollutant has a significant impact on the content of these three main mineral components. With the increase of Cu2+ concentration, the content of kaolinite and goethite gradually decrease, while the content of quartz gradually increases. The change rate of content is kaolinite>goethite>quartz. When the Cu2+ concentration is 2%, the content loss rate of kaolinite and goethite are as high as 10.69% and 5.38%, respectively. The pore distribution curve of red clay is bimodal. The double peaks are distributed between 0.01~0.1 μm and 1~10 μm respectively. The micro-pores between 0.01~0.1 μm account for the absolute advantage. With the increase of Cu2+ concentration, the "double peak" move to the right gradually, and the pores become larger, the "peak width" widens gradually, and the pores become more numerous. A series of tests are conducted to observe the variation law of the deformation strength characteristics of red clay contaminated by Cu2+. They include unconfined compressive strength test, direct shear test and compression consolidation test. The results show that Cu2+ contamination has a significant effect on the deformation strength properties of red clay. With the increase of Cu2+ concentration, the unconfined compressive strength, shear strength and its index(C, φ) of the red clay decrease gradually, the initial void ratio e0 and the compression coefficient α increase gradually. When the Cu2+ concentration increases from 0 to 2%, the unconfined compressive stress-strain curve of soil changes from typical strain softening type to weak strain hardening one, the peak value of unconfined compressive strength decreases by 76.91% and the average loss rate of shear strength reaches 69.36%.
2019, 27(5): 1019-1026.
Soil water characteristic curve(SWCC) is a basic physical-mechanical relation of unsaturated soils and changes the physical index of water content to the mechanical index of forces among soil particles. Soil water characteristic curve is controlled by soil structures. To investigate the effect of soil structure on the SWCC, the compacted loess samples are made with three different moisture contents in the study. They are less than the optimum 8%, at the optimum 17%, and more than the optimum 19%. Mercury intrusion poroimetry(MIP) and scanning electronic method(SEM)as well as filter paper method are respectively used to explore the pore size distribution(PSD), the soil water characteristics(SWCC) and the microstructure of the compacted loess at difference water contents. It can be observed from the SWCCs and PSds that compacted loess at different moisture contents have a significant impact on the microstructure and soil water characteristics. In low suction zone, the SWCCs of three compacted loess are significantly different. The SWCC of dry of optimum is the steepest. It tends to be the same in the high suction region. The PSD varies greatly in the macro-pores and is almost the same in the micro-pores, which is similar to the SWCC. By comparing the SWCC and PSD, it is found that the pore distribution of loess compacted with different moisture content is tightly related to SWCC. The larger the pore size density, the steeper slope of SWCC. SEM imagines also show the typical characters of the three compacted soils as follows:(1)the specimen with the less optimum moisture content is dominated with large inter-particle pores; (2)the specimen with more than the optimum moisture content is dominated with smaller inter-particle pores; and (3)the specimen at the optimum has a wide range of pore distribution.
Soil water characteristic curve(SWCC) is a basic physical-mechanical relation of unsaturated soils and changes the physical index of water content to the mechanical index of forces among soil particles. Soil water characteristic curve is controlled by soil structures. To investigate the effect of soil structure on the SWCC, the compacted loess samples are made with three different moisture contents in the study. They are less than the optimum 8%, at the optimum 17%, and more than the optimum 19%. Mercury intrusion poroimetry(MIP) and scanning electronic method(SEM)as well as filter paper method are respectively used to explore the pore size distribution(PSD), the soil water characteristics(SWCC) and the microstructure of the compacted loess at difference water contents. It can be observed from the SWCCs and PSds that compacted loess at different moisture contents have a significant impact on the microstructure and soil water characteristics. In low suction zone, the SWCCs of three compacted loess are significantly different. The SWCC of dry of optimum is the steepest. It tends to be the same in the high suction region. The PSD varies greatly in the macro-pores and is almost the same in the micro-pores, which is similar to the SWCC. By comparing the SWCC and PSD, it is found that the pore distribution of loess compacted with different moisture content is tightly related to SWCC. The larger the pore size density, the steeper slope of SWCC. SEM imagines also show the typical characters of the three compacted soils as follows:(1)the specimen with the less optimum moisture content is dominated with large inter-particle pores; (2)the specimen with more than the optimum moisture content is dominated with smaller inter-particle pores; and (3)the specimen at the optimum has a wide range of pore distribution.
2019, 27(5): 1027-1034.
The gas permeability values of in-situ and undisturbed Malan loess samples at different areas are measured with improved air permeameter. The reliability of the in-situ gas permeability test is verified by comparing the results of in-suit and indoor gas permeability. The effects of particle size, pore ratio and water content on the gas permeability are compared and analyzed with a method of combining the two kinds of gas permeability. The effects of collapsible cracks(fractures) and suffosion caves on the gas permeability of loess are analyzed with in-situ test. The results show that the indoor and the in-situ gas permeability are basically consistent, the in-situ gas permeability test results are reliable. The in-situ gas permeability test removes the impact of transport disturbances and can be more accurate and reflect the real situation of loess. The indoor gas permeability test is only suitable for the uniform loess sample without obvious cracks. As the coarse particles of the Malan loess increase the gas permeability increases, otherwise the gas permeability decreases. As the loess depth increases, the void ratio decreases, the water content increases, and the gas permeability decreases. As the loess near the collapsible cracks(fractures) and suffosion caves, the gas permeability increases obviously. As the loess far from the collapsible cracks(fractures) and suffusion caves the gas permeability is tend to an average value. The in-situ gas permeability measurement process is fast and convenient. This method has broad application prospects in evaluating the permeability and structural defects in the loess.
The gas permeability values of in-situ and undisturbed Malan loess samples at different areas are measured with improved air permeameter. The reliability of the in-situ gas permeability test is verified by comparing the results of in-suit and indoor gas permeability. The effects of particle size, pore ratio and water content on the gas permeability are compared and analyzed with a method of combining the two kinds of gas permeability. The effects of collapsible cracks(fractures) and suffosion caves on the gas permeability of loess are analyzed with in-situ test. The results show that the indoor and the in-situ gas permeability are basically consistent, the in-situ gas permeability test results are reliable. The in-situ gas permeability test removes the impact of transport disturbances and can be more accurate and reflect the real situation of loess. The indoor gas permeability test is only suitable for the uniform loess sample without obvious cracks. As the coarse particles of the Malan loess increase the gas permeability increases, otherwise the gas permeability decreases. As the loess depth increases, the void ratio decreases, the water content increases, and the gas permeability decreases. As the loess near the collapsible cracks(fractures) and suffosion caves, the gas permeability increases obviously. As the loess far from the collapsible cracks(fractures) and suffusion caves the gas permeability is tend to an average value. The in-situ gas permeability measurement process is fast and convenient. This method has broad application prospects in evaluating the permeability and structural defects in the loess.
2019, 27(5): 1035-1040.
This paper investigates the shear strength characteristics of Guilin remolded red clay. It examines the influence of dry density and moisture content on shear strength of saturated and unsaturated remolded red clay with triaxial test. Experimental results show that under the conditions of optimal moisture content and full saturation, the cohesive force and dry density of the remolded red clay have the concave quadratic polynomial fitting curves. The cohesive force and dry density of unsaturated soil have the convex quadratic polynomial fitting curve. The cohesive forces of the saturated and un-saturated clay at the vicinity of dry density of 1.41 g·cm-3 have the biggest difference. The internal friction angle and the dry density of the remolded red clay have the quadratic polynomial fitting curve of concave shape. The angle of internal friction and the dry density of unsaturated soil have the convex quadratic polynomial fitting curve. Near the dry density of 1.35g·cm-3, the difference between the internal frictional angles of the saturated and unsaturated remolded red clay has the biggest difference. he water content of unsaturated remolded red clay has significant influence on shear strength parameters.
This paper investigates the shear strength characteristics of Guilin remolded red clay. It examines the influence of dry density and moisture content on shear strength of saturated and unsaturated remolded red clay with triaxial test. Experimental results show that under the conditions of optimal moisture content and full saturation, the cohesive force and dry density of the remolded red clay have the concave quadratic polynomial fitting curves. The cohesive force and dry density of unsaturated soil have the convex quadratic polynomial fitting curve. The cohesive forces of the saturated and un-saturated clay at the vicinity of dry density of 1.41 g·cm-3 have the biggest difference. The internal friction angle and the dry density of the remolded red clay have the quadratic polynomial fitting curve of concave shape. The angle of internal friction and the dry density of unsaturated soil have the convex quadratic polynomial fitting curve. Near the dry density of 1.35g·cm-3, the difference between the internal frictional angles of the saturated and unsaturated remolded red clay has the biggest difference. he water content of unsaturated remolded red clay has significant influence on shear strength parameters.
2019, 27(5): 1041-1047.
The loess contains a large amount of Na+, Ca2+, Cl-, SO42-, etc. When the water enters loess, it dissolves these ions in the water and carries mineral ions away. It affects the structure and strength of loess. In order to study the influence of mineral ions' concentration on the strength of loess, we used the TFB-1 type triaxial apparatus of stress and strain controlled for unsaturated soil and carried out the consolidation and undrained(CU)test with remolded sample from Heifangtai in Gansu Province. We did CU test with the samples of loess reconstituted with different concentration's solution of sodium sulfate under the confining pressures of 100 kPa, 200 kPa and 300 kPa, respectively. We discussed the effect of sodium sulfate's concentration on strength of loess. The test result shows that under the same confining pressure, the peak shear strength and residual shear strength of loess decrease first and then increase with the increase of sodium sulfate's concentration. Under the same dry density, with the increase of sodium sulfate's concentration, the total internal friction angle, effective internal friction angle and total cohesion of loess decreases first and then increases, while the effective cohesion of loess increases monotonously. The increase of sodium sulfate's concentration reduces thickness of clay's double electric layer, which enhances the force between clay particles. So the aggregate of clay particles increases, which affects the strength of the loess.
The loess contains a large amount of Na+, Ca2+, Cl-, SO42-, etc. When the water enters loess, it dissolves these ions in the water and carries mineral ions away. It affects the structure and strength of loess. In order to study the influence of mineral ions' concentration on the strength of loess, we used the TFB-1 type triaxial apparatus of stress and strain controlled for unsaturated soil and carried out the consolidation and undrained(CU)test with remolded sample from Heifangtai in Gansu Province. We did CU test with the samples of loess reconstituted with different concentration's solution of sodium sulfate under the confining pressures of 100 kPa, 200 kPa and 300 kPa, respectively. We discussed the effect of sodium sulfate's concentration on strength of loess. The test result shows that under the same confining pressure, the peak shear strength and residual shear strength of loess decrease first and then increase with the increase of sodium sulfate's concentration. Under the same dry density, with the increase of sodium sulfate's concentration, the total internal friction angle, effective internal friction angle and total cohesion of loess decreases first and then increases, while the effective cohesion of loess increases monotonously. The increase of sodium sulfate's concentration reduces thickness of clay's double electric layer, which enhances the force between clay particles. So the aggregate of clay particles increases, which affects the strength of the loess.
2019, 27(5): 1048-1055.
This paper studies the creep characteristics of saturated sand and its influencing factors. It carries out the high pressure unidirectional compression tests of different density,different grading methods and different loading methods. The test results show that the creep of sand mainly undergoes three stages:instantaneous deformation,rapid deformation and steady-state creep deformation; the compressive modulus increases with the increase of density. The weighted average particle size of samples prepared by similar grading method is 42.4%smaller than that by equivalent substitution method,and the compression modulus is 8%~18% higher. Different loading methods have a certain influence on the final deformation of the sample. After cyclic loading,the compression modulus of samples decreased by 4.3%~10.1%,and the final strain variable increased by 4.6%~6.0%. The statistical analysis of the test data shows that the power function can well fit the stress,deformation and time relationship of saturated sand soil,and finally the creep model is suitable for this sand soil. The research results can be applied to the design and evaluation of sand foundation.
This paper studies the creep characteristics of saturated sand and its influencing factors. It carries out the high pressure unidirectional compression tests of different density,different grading methods and different loading methods. The test results show that the creep of sand mainly undergoes three stages:instantaneous deformation,rapid deformation and steady-state creep deformation; the compressive modulus increases with the increase of density. The weighted average particle size of samples prepared by similar grading method is 42.4%smaller than that by equivalent substitution method,and the compression modulus is 8%~18% higher. Different loading methods have a certain influence on the final deformation of the sample. After cyclic loading,the compression modulus of samples decreased by 4.3%~10.1%,and the final strain variable increased by 4.6%~6.0%. The statistical analysis of the test data shows that the power function can well fit the stress,deformation and time relationship of saturated sand soil,and finally the creep model is suitable for this sand soil. The research results can be applied to the design and evaluation of sand foundation.
2019, 27(5): 1056-1062.
It is agreed that the simplified Bishop method is "strictly", easy, accurate for the use of the slip circle in the stability analysis of slopes. Some scholars studied its stringency. but There is no corresponding calculation of landslide residual sliding force. So, it is difficult for the comprehensive control of landslide and slope stability. In this paper, the two analysis models of residual sliding force are obtained using simplified Bishop Method. They are the model Ⅰ for Ti ≥ 0 and the model Ⅱ for Ti < 0. They consider the weight of the slope body, the external force, the horizontal seismic force, and the pore water pressure of sliding surface. They use the laws of force polygon. Sliding thrusts of the slice i about the two models above are deduced respectively. Then, using the calculated sliding thrust of the slice i-1 to calculate the sliding thrust of slice i. The calculation of landslide residual sliding force of slice i can easily be deduced from working the way downward. The results of the analytic method are compared with the results of the transfer coefficient calculation in residual sliding force. It is concluded that the values of solved residual sliding force based on the analytic method is higher than those based on the transfer coefficient calculation in general. The difference of the two landslide residual sliding forces above decreases with the increase of safety factor. Moreover, this method is applicable to landslide and slope stability and has higher safety factor. The present work provides a theoretical basis for the control of landslides of sliding circular.
It is agreed that the simplified Bishop method is "strictly", easy, accurate for the use of the slip circle in the stability analysis of slopes. Some scholars studied its stringency. but There is no corresponding calculation of landslide residual sliding force. So, it is difficult for the comprehensive control of landslide and slope stability. In this paper, the two analysis models of residual sliding force are obtained using simplified Bishop Method. They are the model Ⅰ for Ti ≥ 0 and the model Ⅱ for Ti < 0. They consider the weight of the slope body, the external force, the horizontal seismic force, and the pore water pressure of sliding surface. They use the laws of force polygon. Sliding thrusts of the slice i about the two models above are deduced respectively. Then, using the calculated sliding thrust of the slice i-1 to calculate the sliding thrust of slice i. The calculation of landslide residual sliding force of slice i can easily be deduced from working the way downward. The results of the analytic method are compared with the results of the transfer coefficient calculation in residual sliding force. It is concluded that the values of solved residual sliding force based on the analytic method is higher than those based on the transfer coefficient calculation in general. The difference of the two landslide residual sliding forces above decreases with the increase of safety factor. Moreover, this method is applicable to landslide and slope stability and has higher safety factor. The present work provides a theoretical basis for the control of landslides of sliding circular.
2019, 27(5): 1063-1069.
Stiffened deep mixed column(SDCM)with short core pile is commonly used in application. However, people have limit understanding regarding the failure mode and mechanism of such column-supported embankment over soft clay, leading to a challenging to reasonably evaluate the stability of such column-supported embankment in design. This paper conducts a scaled-down 1 g model test of short-cored stiffened deep cement mixed columns-supported embankment. The progressive failure modes of columns and global failure characteristics of embankment are detected according to comprehensive analyses of the change of conductive path pasted in core piles, vertical stresses on pile and surrounding soil, displacement of embankment combined with PIV technology. The results show that the embankment load is mostly carried by the core pile under embankment crest. With the increase of surcharge, the loading shear action of column reduces, but the core pile still has certain loading shear action after the embankment failure. In the process of embankment failure, the column under the embankment crest fails by compression and bulging failure occurs below the core pile section. The columns under the embankment slope fails by bending. The slip surface did not completely pass through the failure positions of columns.
Stiffened deep mixed column(SDCM)with short core pile is commonly used in application. However, people have limit understanding regarding the failure mode and mechanism of such column-supported embankment over soft clay, leading to a challenging to reasonably evaluate the stability of such column-supported embankment in design. This paper conducts a scaled-down 1 g model test of short-cored stiffened deep cement mixed columns-supported embankment. The progressive failure modes of columns and global failure characteristics of embankment are detected according to comprehensive analyses of the change of conductive path pasted in core piles, vertical stresses on pile and surrounding soil, displacement of embankment combined with PIV technology. The results show that the embankment load is mostly carried by the core pile under embankment crest. With the increase of surcharge, the loading shear action of column reduces, but the core pile still has certain loading shear action after the embankment failure. In the process of embankment failure, the column under the embankment crest fails by compression and bulging failure occurs below the core pile section. The columns under the embankment slope fails by bending. The slip surface did not completely pass through the failure positions of columns.
2019, 27(5): 1070-1077.
Thermal environment control is a powerful guarantee for the safe operation of utility tunnel. A precise estimation for soil thermal conductivity is just the critical basis for the thermal load of environment control system in underground space. Soft soils are very common in the development of underground space, especially in highly urbanized areas. Most available studies on soil thermal conductivity focus on the range of low and intermediate water content, but there is rarely a model suitable for unsaturated soft soils with high water content. Based on mesoscopic simulation, a numerical model is proposed in the present study for unsaturated soft soils with high water content. This model can include traditional factors like water content and dry density. It also can reveal the influence of mineralogy and particle size. Finally, the numerical model is validated by the experimental data of 20 soil samples in Su-Tong GIL utility tunnel project. The mineralogy composition and particle size distribution are measured in advance from X-ray diffraction analysis and laser particle size analyzer, from which the model parameters are derived and then the effective thermal conductivity can be reached by mesoscopic simulation of heat conduction. Results show that the values of most the modelled thermal conductivity are located within the±20%ranges of experimental data, which verifies the reliability of the numerical model in the present study. In the same time, it indicates that this numerical model has a potential application in predicting the thermal conductivity of unsaturated soft soils. Besides, this model can give a direct exhibition of local heat flux in soils, which lays a foundation for a better understanding on the mechanism of soil thermal behavior. The results in the present study can provide a new strategy and/or approach to estimate soils thermal conductivity.
Thermal environment control is a powerful guarantee for the safe operation of utility tunnel. A precise estimation for soil thermal conductivity is just the critical basis for the thermal load of environment control system in underground space. Soft soils are very common in the development of underground space, especially in highly urbanized areas. Most available studies on soil thermal conductivity focus on the range of low and intermediate water content, but there is rarely a model suitable for unsaturated soft soils with high water content. Based on mesoscopic simulation, a numerical model is proposed in the present study for unsaturated soft soils with high water content. This model can include traditional factors like water content and dry density. It also can reveal the influence of mineralogy and particle size. Finally, the numerical model is validated by the experimental data of 20 soil samples in Su-Tong GIL utility tunnel project. The mineralogy composition and particle size distribution are measured in advance from X-ray diffraction analysis and laser particle size analyzer, from which the model parameters are derived and then the effective thermal conductivity can be reached by mesoscopic simulation of heat conduction. Results show that the values of most the modelled thermal conductivity are located within the±20%ranges of experimental data, which verifies the reliability of the numerical model in the present study. In the same time, it indicates that this numerical model has a potential application in predicting the thermal conductivity of unsaturated soft soils. Besides, this model can give a direct exhibition of local heat flux in soils, which lays a foundation for a better understanding on the mechanism of soil thermal behavior. The results in the present study can provide a new strategy and/or approach to estimate soils thermal conductivity.
2019, 27(5): 1078-1084.
Slope failure is often a large-deformation process of soils consisting of initiation, run-out and deposition stages. The deformation process determines the landslide consequence. Conventional slope stability analysis approaches such as Limit Equilibrium Method and Finite Element Method may face difficulties in modeling the whole process of slope failure, especially the post-failure process with large deformation of soils. Slope failure is controlled by uncertainties, and one important uncertainty is the spatial variability of soil properties. This study aims to investigate the evolution of slope failure modes at large deformation in spatially variable soils using Random Limit Equilibrium and Material Point Methods. Different types of failure modes are identified from different spatial distributions of undrained shear strength. Results reveal the spatial distribution of soil properties plays a significant role in the evolution of slope failure mode. It is of importance to characterize the in-situ soil properties for slope risk assessment and mitigation.
Slope failure is often a large-deformation process of soils consisting of initiation, run-out and deposition stages. The deformation process determines the landslide consequence. Conventional slope stability analysis approaches such as Limit Equilibrium Method and Finite Element Method may face difficulties in modeling the whole process of slope failure, especially the post-failure process with large deformation of soils. Slope failure is controlled by uncertainties, and one important uncertainty is the spatial variability of soil properties. This study aims to investigate the evolution of slope failure modes at large deformation in spatially variable soils using Random Limit Equilibrium and Material Point Methods. Different types of failure modes are identified from different spatial distributions of undrained shear strength. Results reveal the spatial distribution of soil properties plays a significant role in the evolution of slope failure mode. It is of importance to characterize the in-situ soil properties for slope risk assessment and mitigation.
2019, 27(5): 1085-1092.
The morphology of clay particles not only reflects the mineral composition of clay, but also is one of the important factors affecting its physical and mechanical properties. In order to study the effect of material composition on the micro-macro properties of clay, we use the discrete element method to simulate the triaxial compression of soft clay samples with different particle morphology. Firstly, based on the SEM image, the orientation angle and concave convexity of clay particles in natural state are counted. The sphericity and concave-convexity are introduced as the characteristic parameters of particle morphology. Secondly, we construct a clay particle model based on the structural features of clay minerals that we count at the first step. Finally, with triaxial test discrete element simulation method, we analyze the effect of soft clay particle morphology on its macroscopic mechanical and microscopic properties. The results show that the flaky particle sample has higher initial modulus than the spherical particle sample, and the shear strength is larger. The flaky particle sample even tends to be horizontally distributed with the loading. We divide the loading processes into two stages as the clay particle model showing different characteristics in each stage. At the initial stage of loading, the sphericity of particles has a significant effect on the initial elastic modulus, and the initial elastic modulus decreases with the increase of sphericity. At the later stage of loading, the effect of concave and convex degree of particles on shear strength index is gradually obvious, and the internal friction angle and cohesion of samples decrease with the increase of concave and convex degree. Microscopically, the shape of particles also has a great influence on the displacement and rotation of particles.
The morphology of clay particles not only reflects the mineral composition of clay, but also is one of the important factors affecting its physical and mechanical properties. In order to study the effect of material composition on the micro-macro properties of clay, we use the discrete element method to simulate the triaxial compression of soft clay samples with different particle morphology. Firstly, based on the SEM image, the orientation angle and concave convexity of clay particles in natural state are counted. The sphericity and concave-convexity are introduced as the characteristic parameters of particle morphology. Secondly, we construct a clay particle model based on the structural features of clay minerals that we count at the first step. Finally, with triaxial test discrete element simulation method, we analyze the effect of soft clay particle morphology on its macroscopic mechanical and microscopic properties. The results show that the flaky particle sample has higher initial modulus than the spherical particle sample, and the shear strength is larger. The flaky particle sample even tends to be horizontally distributed with the loading. We divide the loading processes into two stages as the clay particle model showing different characteristics in each stage. At the initial stage of loading, the sphericity of particles has a significant effect on the initial elastic modulus, and the initial elastic modulus decreases with the increase of sphericity. At the later stage of loading, the effect of concave and convex degree of particles on shear strength index is gradually obvious, and the internal friction angle and cohesion of samples decrease with the increase of concave and convex degree. Microscopically, the shape of particles also has a great influence on the displacement and rotation of particles.
2019, 27(5): 1093-1100.
The analysis of frozen soil temperature field plays an important role in the research on frozen soil and engineering construction in permafrost regions. The latent heat caused by the phase change between ice and water increases the complexity of the analysis. In order to solve this problem, the volumetric heat capacity due to phase change and thermal conductivity is obtained through back calculation based on the linear heat source model and the basic theory of heat transfer in frozen soil. In this paper, a series of laboratory tests on frozen soil are conducted for the same initial moisture content. Based on the method of Actively Heated Fiber Optics(AHFO), the FBG corundum tube sensor is used as a heat source and temperature sensor to detect thermal response characteristics of frozen soil under different initial temperature. The results show that under the experimental conditions, the influence radius of the FBG corundum tube sensor is less than 5 cm. The temperature increment measured by the FBG corundum tube sensor has a linear relationship with the logarithm of time. With the increase of initial temperature, the measured thermal conductivity of frozen soil has a linear relationship with temperature. The phase change heat capacity tends to be stable when the initial temperature is lower than -6℃, and increases gradually with the increase of temperature from -6℃ to 0℃. When the initial temperature is higher than -5℃, the phase change heat capacity is even greater than the volume heat capacity of the frozen soil itself. These results provide references for the modification of the AHFO-based monitoring method of ice content in frozen soil.
The analysis of frozen soil temperature field plays an important role in the research on frozen soil and engineering construction in permafrost regions. The latent heat caused by the phase change between ice and water increases the complexity of the analysis. In order to solve this problem, the volumetric heat capacity due to phase change and thermal conductivity is obtained through back calculation based on the linear heat source model and the basic theory of heat transfer in frozen soil. In this paper, a series of laboratory tests on frozen soil are conducted for the same initial moisture content. Based on the method of Actively Heated Fiber Optics(AHFO), the FBG corundum tube sensor is used as a heat source and temperature sensor to detect thermal response characteristics of frozen soil under different initial temperature. The results show that under the experimental conditions, the influence radius of the FBG corundum tube sensor is less than 5 cm. The temperature increment measured by the FBG corundum tube sensor has a linear relationship with the logarithm of time. With the increase of initial temperature, the measured thermal conductivity of frozen soil has a linear relationship with temperature. The phase change heat capacity tends to be stable when the initial temperature is lower than -6℃, and increases gradually with the increase of temperature from -6℃ to 0℃. When the initial temperature is higher than -5℃, the phase change heat capacity is even greater than the volume heat capacity of the frozen soil itself. These results provide references for the modification of the AHFO-based monitoring method of ice content in frozen soil.
2019, 27(5): 1101-1109.
Exploring the mechanical behavior of fault gouge is the basis of studying the engineering geology effects of fault zone. Taking the gouges of Shendaogou Fault in Yan'an as an object, the fabric characteristics of three color fault gouges are studied with particle analysis and X-ray diffraction(XRD). In addition, the mechanical behavior of remolded fault gouges under different water contents are analyzed with ring shear test. The results show that the gradation curve with single peak of fault gouge particle is significantly better than that with double-peaks. Quartz, mica and feldspar are the main non-clay minerals, while illite and kaolinite are the main clay minerals. The color difference of fault gouge is brought by the relative content of hematite and chloriter. The strain softening characteristics of fault gouge are mainly affected by water content and coarse particle content. The strain softening is first strengthened and then weakened with the increase of water content. When the water content is less than the plastic limit, the strain softening characteristics weaken with the increase of coarse particle content. Internal friction angle(φ) is the main mechanical parameter affecting strain softening characteristics. Peak internal friction angle and residual internal friction angle are negatively correlated with water content. The variation of φ is influenced by the mineral composition, which indicates that φ increases with the increase of non-clay mineral under 5% and 10%water content.
Exploring the mechanical behavior of fault gouge is the basis of studying the engineering geology effects of fault zone. Taking the gouges of Shendaogou Fault in Yan'an as an object, the fabric characteristics of three color fault gouges are studied with particle analysis and X-ray diffraction(XRD). In addition, the mechanical behavior of remolded fault gouges under different water contents are analyzed with ring shear test. The results show that the gradation curve with single peak of fault gouge particle is significantly better than that with double-peaks. Quartz, mica and feldspar are the main non-clay minerals, while illite and kaolinite are the main clay minerals. The color difference of fault gouge is brought by the relative content of hematite and chloriter. The strain softening characteristics of fault gouge are mainly affected by water content and coarse particle content. The strain softening is first strengthened and then weakened with the increase of water content. When the water content is less than the plastic limit, the strain softening characteristics weaken with the increase of coarse particle content. Internal friction angle(φ) is the main mechanical parameter affecting strain softening characteristics. Peak internal friction angle and residual internal friction angle are negatively correlated with water content. The variation of φ is influenced by the mineral composition, which indicates that φ increases with the increase of non-clay mineral under 5% and 10%water content.
2019, 27(5): 1110-1115.
This paper investigates the influence of clay contents on the shear strength characteristics of sand clay mixture by ring shear test. A series of consolidated drained ring shear tests are conducted under the axial loads of 75, 150, 300, and 600 kPa. The relationship between shear stress and rotation angle is obtained. According to the peak shear stress obtained from experimental results and the Mohr-Coulomb failure criterion, the cohesion and friction angle are obtained. The cohesion value is located between 5 kPa and 18 kPa, and it goes to the minimum value when the clay content is around 50%. The internal friction angle decreases with the content of clay, and the decreasing trend is more significant when the clay content exceeds 50%than that one lower than 50%.
This paper investigates the influence of clay contents on the shear strength characteristics of sand clay mixture by ring shear test. A series of consolidated drained ring shear tests are conducted under the axial loads of 75, 150, 300, and 600 kPa. The relationship between shear stress and rotation angle is obtained. According to the peak shear stress obtained from experimental results and the Mohr-Coulomb failure criterion, the cohesion and friction angle are obtained. The cohesion value is located between 5 kPa and 18 kPa, and it goes to the minimum value when the clay content is around 50%. The internal friction angle decreases with the content of clay, and the decreasing trend is more significant when the clay content exceeds 50%than that one lower than 50%.
2019, 27(5): 1116-1121.
This paper investigates the influence of subway induced vibration on a proposed residential office building. It performed filed measurements lasting 24 hours at this building site. Applying a high-precision instrument TROMINO, we recorded the vibration data of acceleration and velocity on three directions simultaneously. For analyzing the environmental vibration characteristics at different periods in a day, we used the one-third octave band spectrum method to analyze the test data. The results show that the peak of environmental vibration during subway operating time is greater than that without subway. The ground vibration acceleration beyond subway operating time is in the range of 10-5~10-1 gal, and has no obvious dominant frequency. The acceleration amplitude of ground vibration in subway operation hours is between 10-4-100 gal. Under the superposition of various factors in the evening peak period, the vibration acceleration of the proposed site reaches the maximum value of 0.8 gal. The dominant frequency of ground vibration induced by the subway lies in the frequency range of 40~50 Hz. In the low frequency range(2~8 Hz), the peak ground acceleration is 0.030 gal, and the maximum value of velocity reaches 26.715 μm·s-1 in the medium-high frequency range(8~250 Hz). One-third octave band spectra of testing vibrations locates below the standard curves specified in the ISO 2631 totally, which indicates that the design requirements are satisfied.
This paper investigates the influence of subway induced vibration on a proposed residential office building. It performed filed measurements lasting 24 hours at this building site. Applying a high-precision instrument TROMINO, we recorded the vibration data of acceleration and velocity on three directions simultaneously. For analyzing the environmental vibration characteristics at different periods in a day, we used the one-third octave band spectrum method to analyze the test data. The results show that the peak of environmental vibration during subway operating time is greater than that without subway. The ground vibration acceleration beyond subway operating time is in the range of 10-5~10-1 gal, and has no obvious dominant frequency. The acceleration amplitude of ground vibration in subway operation hours is between 10-4-100 gal. Under the superposition of various factors in the evening peak period, the vibration acceleration of the proposed site reaches the maximum value of 0.8 gal. The dominant frequency of ground vibration induced by the subway lies in the frequency range of 40~50 Hz. In the low frequency range(2~8 Hz), the peak ground acceleration is 0.030 gal, and the maximum value of velocity reaches 26.715 μm·s-1 in the medium-high frequency range(8~250 Hz). One-third octave band spectra of testing vibrations locates below the standard curves specified in the ISO 2631 totally, which indicates that the design requirements are satisfied.
PROBABILITY OF COSEISMIC LANDSLIDES: A NEW GENERATION OF EARTHQUAKE-TRIGGERED LANDSLIDE HAZARD MODEL
2019, 27(5): 1122-1130.
The probability of the occurrence of coseismic landslides is basically blank. In this study, the Bayesian Probability Method and the Machine Model are used to carry out the real probability of coseismic landslides of China. The first generation of coseismic landslide hazard probability map of China is produced on the basis of nine earthquake cases. They include 1999 Chi-chi, Taiwan, 2005 Kashmir, 2008 Wenchuan, 2010 Yushu, 2013 Lushan, 2013 Minxian, 2014 Ludian, 2015 Nepal, and 2017 Jiuzhaigou earthquakes. Seven of the nine earthquakes occurred in China. The 2005 Kashmir and the 2015 Nepal quakes occurred in China's neighboring areas, which can better control the accuracy of the model. All these earthquake events have detailed and complete coseismic landslide inventories. They include 306 435 landslide polygons. Considering the real earthquake landslide occurrence area, the difference of landslide size, the ratio of landslide to non-slip sample ratio, a total of 5 117 000 samples are selected. A total of 13 factors are selected. They are absolute elevation, relative elevation, slope angle, slope aspect, slope curvature, slope position, topographic humidity index, land cover, vegetation coverage percentage, fault distance, stratum, average annual precipitation, and peak ground acceleration. The Bayesian probability method is combined with the machine learning model to establish a multi-factor impact model for the probability of earthquake-triggered landslide. Then the weights of each continuous factor and the weight of each class of the classification factor are obtained. The model is applied in China considering the peak ground acceleration as the triggering factor of landslides and considering the real probability of earthquake landslides in China under different peak ground accelerations(0.1~1 g, one result per 0.1 g, a total of 10 results). In addition, combined with Seismic Ground Motion Parameters Zonation Map of China, the corresponding true probability of earthquake-triggered landslides of China is generated.
The probability of the occurrence of coseismic landslides is basically blank. In this study, the Bayesian Probability Method and the Machine Model are used to carry out the real probability of coseismic landslides of China. The first generation of coseismic landslide hazard probability map of China is produced on the basis of nine earthquake cases. They include 1999 Chi-chi, Taiwan, 2005 Kashmir, 2008 Wenchuan, 2010 Yushu, 2013 Lushan, 2013 Minxian, 2014 Ludian, 2015 Nepal, and 2017 Jiuzhaigou earthquakes. Seven of the nine earthquakes occurred in China. The 2005 Kashmir and the 2015 Nepal quakes occurred in China's neighboring areas, which can better control the accuracy of the model. All these earthquake events have detailed and complete coseismic landslide inventories. They include 306 435 landslide polygons. Considering the real earthquake landslide occurrence area, the difference of landslide size, the ratio of landslide to non-slip sample ratio, a total of 5 117 000 samples are selected. A total of 13 factors are selected. They are absolute elevation, relative elevation, slope angle, slope aspect, slope curvature, slope position, topographic humidity index, land cover, vegetation coverage percentage, fault distance, stratum, average annual precipitation, and peak ground acceleration. The Bayesian probability method is combined with the machine learning model to establish a multi-factor impact model for the probability of earthquake-triggered landslide. Then the weights of each continuous factor and the weight of each class of the classification factor are obtained. The model is applied in China considering the peak ground acceleration as the triggering factor of landslides and considering the real probability of earthquake landslides in China under different peak ground accelerations(0.1~1 g, one result per 0.1 g, a total of 10 results). In addition, combined with Seismic Ground Motion Parameters Zonation Map of China, the corresponding true probability of earthquake-triggered landslides of China is generated.
2019, 27(5): 1131-1145.
Water-induced landslide refers to the slope instability events directly caused by water-related factors(including rainfall, fluctuation of reservoir level, surface runoff, groundwater activities and others). Southwest China is a high-incidence area of water-induced landslides, especially for reservoir landslides. The water-induced landslides directly threaten the safety of human life, property and infrastructure, and may cause far-reaching secondary disasters. It is extremely urgent to improve the level of monitoring, early warning, comprehensive control and emergency disposal of water-induced landslides. Water-induced landslide can be induced in loose accumulated formation, broken rock mass, soft rock and rock slope with soft rock interlayer or weak structural plane. Long-term evolution of geological environment, hydrological conditions and human disturbance play key roles in the development of water-induced landslide. Under uninterrupted alternation of various adverse factors, the slope gradually produces deformation and local failure with the decrease of stability and tends to the limit instability state. Finally, the short-term change of hydrological conditions makes a large-scale slope instability. The dynamic process of landslide after slope instability is very complex, especially the large-scale landslide in high elevation, which can produce strong impact disturbance fragmentation and erosion with scraping along the movement course, resulting in the change of landslide movement pattern and the increase of accumulation volume. The presence of water can intensify the erosion with scraping along the movement course and lead to the change of movement pattern to fluidization, resulting in a longer movement distance and a wider disaster-causing scope. Water-induced landslide is a complex and systematic problem. The deformation and failure processes of landslides under different geological structures and hydrodynamic conditions are quite different. Long-distance, non-contact early identification and monitoring technology, early prediction and warning method for landslide based on artificial intelligence, large data and self-learning are the important development directions in the future. The prevention and control of hydrodynamic landslide is involved with many factors such as engineering construction, economy, livelihood, and society, which needs a comprehensive application of engineering measures and non-engineering measures. In the future construction of water conservancy and hydropower engineering, much attention should be paid to the hazards of reservoir landslides, and the reconstruction facilities should be apart from the affected areas of reservoir landslides as far as possible.
Water-induced landslide refers to the slope instability events directly caused by water-related factors(including rainfall, fluctuation of reservoir level, surface runoff, groundwater activities and others). Southwest China is a high-incidence area of water-induced landslides, especially for reservoir landslides. The water-induced landslides directly threaten the safety of human life, property and infrastructure, and may cause far-reaching secondary disasters. It is extremely urgent to improve the level of monitoring, early warning, comprehensive control and emergency disposal of water-induced landslides. Water-induced landslide can be induced in loose accumulated formation, broken rock mass, soft rock and rock slope with soft rock interlayer or weak structural plane. Long-term evolution of geological environment, hydrological conditions and human disturbance play key roles in the development of water-induced landslide. Under uninterrupted alternation of various adverse factors, the slope gradually produces deformation and local failure with the decrease of stability and tends to the limit instability state. Finally, the short-term change of hydrological conditions makes a large-scale slope instability. The dynamic process of landslide after slope instability is very complex, especially the large-scale landslide in high elevation, which can produce strong impact disturbance fragmentation and erosion with scraping along the movement course, resulting in the change of landslide movement pattern and the increase of accumulation volume. The presence of water can intensify the erosion with scraping along the movement course and lead to the change of movement pattern to fluidization, resulting in a longer movement distance and a wider disaster-causing scope. Water-induced landslide is a complex and systematic problem. The deformation and failure processes of landslides under different geological structures and hydrodynamic conditions are quite different. Long-distance, non-contact early identification and monitoring technology, early prediction and warning method for landslide based on artificial intelligence, large data and self-learning are the important development directions in the future. The prevention and control of hydrodynamic landslide is involved with many factors such as engineering construction, economy, livelihood, and society, which needs a comprehensive application of engineering measures and non-engineering measures. In the future construction of water conservancy and hydropower engineering, much attention should be paid to the hazards of reservoir landslides, and the reconstruction facilities should be apart from the affected areas of reservoir landslides as far as possible.
2019, 27(5): 1146-1152.
Jiacha-Langxian section of the Yarlung Zangbo suture zone is located in the southeastern part of the Qinghai-Tibet Plateau. The terrain is highly undulated and the geological disasters are densely distributed. This paper is mainly based on 10 m precision image data acquired by airborne radar, satellite remote sensing data, and high precision drone aerial data to identify the geological hazards of collapse and landslide, and to study its main control factors. We identified 41 collapses and 92 landslides, and used statistical methods to analyze the correlation between main controlling factors and hazards. The thickness identification of the identified collapse and landslides establishes a functional relationship between the area and the volume of the hazards. The estimation of the scale of the collapse and landslide on the base of known area is realized. This paper clarifies the spatial distribution of geological hazards in the area, and studies the main controlling factors of geological hazards of collapse and landslides in the area. The results show that the landslide is mainly developed on the south bank of the Yarlung Zangbo River and the middle and lower part of the slope of the north bank, while the collapse mainly occurs in the middle and upper part of the slope of the north bank. From the analysis of stratum lithology, geological structure, rock mass structure, climatic conditions and human engineering, the collapse is mainly concentrated in conglomerate and granite areas, while landslides are often developed in phyllite. The collapse in the study area is controlled by the slope, the slope direction and the elevation. The slope is the main controlling factor. Landslides are mainly controlled by faults. Slopes have a certain influence on the development of landslides. Elevation and slope direction have little effect on landslides. The landslide is mainly traction type, and the directions of most landslides are generally perpendicular to the direction of the fracture. The collapse is mainly based on the slip type. By analyzing the structural plane of the rock mass, the development of the structural plane can be analyzed, and the genetic mechanism is analyzed.
Jiacha-Langxian section of the Yarlung Zangbo suture zone is located in the southeastern part of the Qinghai-Tibet Plateau. The terrain is highly undulated and the geological disasters are densely distributed. This paper is mainly based on 10 m precision image data acquired by airborne radar, satellite remote sensing data, and high precision drone aerial data to identify the geological hazards of collapse and landslide, and to study its main control factors. We identified 41 collapses and 92 landslides, and used statistical methods to analyze the correlation between main controlling factors and hazards. The thickness identification of the identified collapse and landslides establishes a functional relationship between the area and the volume of the hazards. The estimation of the scale of the collapse and landslide on the base of known area is realized. This paper clarifies the spatial distribution of geological hazards in the area, and studies the main controlling factors of geological hazards of collapse and landslides in the area. The results show that the landslide is mainly developed on the south bank of the Yarlung Zangbo River and the middle and lower part of the slope of the north bank, while the collapse mainly occurs in the middle and upper part of the slope of the north bank. From the analysis of stratum lithology, geological structure, rock mass structure, climatic conditions and human engineering, the collapse is mainly concentrated in conglomerate and granite areas, while landslides are often developed in phyllite. The collapse in the study area is controlled by the slope, the slope direction and the elevation. The slope is the main controlling factor. Landslides are mainly controlled by faults. Slopes have a certain influence on the development of landslides. Elevation and slope direction have little effect on landslides. The landslide is mainly traction type, and the directions of most landslides are generally perpendicular to the direction of the fracture. The collapse is mainly based on the slip type. By analyzing the structural plane of the rock mass, the development of the structural plane can be analyzed, and the genetic mechanism is analyzed.
2019, 27(5): 1153-1163.
The research area of this paper is located from Diexi Township to Shidaguan Township, Maoxian County, Sichuan Province. Ten impact factors are selected according to the field data and the basic conditions of the study area. They are the slope, slope direction, section curvature, elevation, undulation, stratum, distance from the river, distance from the fault, land type and vegetation coverage. Using GIS technology as the operating platform, three methods are used to evaluate the sensitivity of landslide hazard in the study area. They are the deterministic coefficient+analytic hierarchy process(CF-AHP), the deterministic coefficient+logistic regression method(CF-LR) and the deterministic coefficient+multi-layer perceptron method of neural network(CF-MLP), The landslide hazard in this area is divided into four categories:extremely low, low, medium and high sensitive areas. The effect of the model is tested by receiver operating characteristic curve(ROC). The under-line area(AUC) of the ROC curves of the CF-AHP, CF-LR and CF-MLP combined models are 0.850, 0.884 and 0.867, respectively. The CF-LR combination model works best. In the CF-LR combination model, the areas of high, medium, low and extremely low sensitive area account for 11.3%, 25.1%, 22.5% and 41.1% of the total area of the study area, respectively. The results show that the high sensitive area is mainly located around the main water system and fault-developed area. The calculated sensitivity zoning results are close to the actual situation in field. Thus, the results are valuable for risk assessment of geological disasters.
The research area of this paper is located from Diexi Township to Shidaguan Township, Maoxian County, Sichuan Province. Ten impact factors are selected according to the field data and the basic conditions of the study area. They are the slope, slope direction, section curvature, elevation, undulation, stratum, distance from the river, distance from the fault, land type and vegetation coverage. Using GIS technology as the operating platform, three methods are used to evaluate the sensitivity of landslide hazard in the study area. They are the deterministic coefficient+analytic hierarchy process(CF-AHP), the deterministic coefficient+logistic regression method(CF-LR) and the deterministic coefficient+multi-layer perceptron method of neural network(CF-MLP), The landslide hazard in this area is divided into four categories:extremely low, low, medium and high sensitive areas. The effect of the model is tested by receiver operating characteristic curve(ROC). The under-line area(AUC) of the ROC curves of the CF-AHP, CF-LR and CF-MLP combined models are 0.850, 0.884 and 0.867, respectively. The CF-LR combination model works best. In the CF-LR combination model, the areas of high, medium, low and extremely low sensitive area account for 11.3%, 25.1%, 22.5% and 41.1% of the total area of the study area, respectively. The results show that the high sensitive area is mainly located around the main water system and fault-developed area. The calculated sensitivity zoning results are close to the actual situation in field. Thus, the results are valuable for risk assessment of geological disasters.
2019, 27(5): 1172-1178.
Landslide hazards occur frequently in China, but the prediction and forecast of landslide deformation have always been a problem. Therefore, the deformation and failure of landslides caused major property damage and loss of human lives each year. Based on the monitoring data for more than ten years, we studied and analyzed the deformation characteristics of the Bazimen landslide. The main reasons of the deformation are the decline of reservoir water level and the rainfall. Moreover the cumulative displacement curve has the "step type" deformation characteristic. When we remove or reduce the external factors, the cumulative displacement-time curve becomes smooth. According to this characteristic, this paper selects the monitoring data of the annual step of the deformation curve(June-August), and uses the cumulative displacement as the objective function. Based on the one-variable linear regression model, the landslide monitoring data of the Bazimen landslide from 2004 to 2017 are analyzed. The results show that the one-variable linear regression model can simulate the deformation process of the "step segment" of the Bazimen landslide well. The cumulative displacement with time in this deformation phase is linear, and the slope of the line is basically the same. According to this linear relationship, we can predict the cumulative displacement of the landslide. The results show that prediction error is less than ±5 mm comparing with the actual monitoring data, and the relative error is below 1%. The accuracy can meet the requirements of landslide monitoring and early warning. It can be used for the prevention of the landslide work and provides reference.
Landslide hazards occur frequently in China, but the prediction and forecast of landslide deformation have always been a problem. Therefore, the deformation and failure of landslides caused major property damage and loss of human lives each year. Based on the monitoring data for more than ten years, we studied and analyzed the deformation characteristics of the Bazimen landslide. The main reasons of the deformation are the decline of reservoir water level and the rainfall. Moreover the cumulative displacement curve has the "step type" deformation characteristic. When we remove or reduce the external factors, the cumulative displacement-time curve becomes smooth. According to this characteristic, this paper selects the monitoring data of the annual step of the deformation curve(June-August), and uses the cumulative displacement as the objective function. Based on the one-variable linear regression model, the landslide monitoring data of the Bazimen landslide from 2004 to 2017 are analyzed. The results show that the one-variable linear regression model can simulate the deformation process of the "step segment" of the Bazimen landslide well. The cumulative displacement with time in this deformation phase is linear, and the slope of the line is basically the same. According to this linear relationship, we can predict the cumulative displacement of the landslide. The results show that prediction error is less than ±5 mm comparing with the actual monitoring data, and the relative error is below 1%. The accuracy can meet the requirements of landslide monitoring and early warning. It can be used for the prevention of the landslide work and provides reference.
2019, 27(5): 1179-1187.
Due to the unique above-ground and underground binary hydrological structure in the karst area, the karst water system has a special circulation evolution. So the water can not only carry the surface loss of the soil, but also the underground loss, resulting in large soil loss and karst rock desertification in the area. The phenomenon is outstanding and the governance is difficult. This paper takes the demonstration area of rocky desertification control in Huanjiang Gorge Plateau of Guizhou Province as the research area. Through field investigation, water sample analysis and numerical simulation method, the karst water system's recharge, runoff, drainage characteristics, water chemistry characteristics and water balance are analyzed and simulated to calculate the groundwater level and underground dark river flow changes. The equilibrium amount of karst water in this area is obtained. The results basically reflect the changes of actual groundwater system discharge and groundwater flow characteristics, and preliminarily clarify the cyclic evolution characteristics of the karst water system in this area. They provide a theoretical basis for the study and prevention of soil surface loss and underground loss in karst rocky desertification areas.
Due to the unique above-ground and underground binary hydrological structure in the karst area, the karst water system has a special circulation evolution. So the water can not only carry the surface loss of the soil, but also the underground loss, resulting in large soil loss and karst rock desertification in the area. The phenomenon is outstanding and the governance is difficult. This paper takes the demonstration area of rocky desertification control in Huanjiang Gorge Plateau of Guizhou Province as the research area. Through field investigation, water sample analysis and numerical simulation method, the karst water system's recharge, runoff, drainage characteristics, water chemistry characteristics and water balance are analyzed and simulated to calculate the groundwater level and underground dark river flow changes. The equilibrium amount of karst water in this area is obtained. The results basically reflect the changes of actual groundwater system discharge and groundwater flow characteristics, and preliminarily clarify the cyclic evolution characteristics of the karst water system in this area. They provide a theoretical basis for the study and prevention of soil surface loss and underground loss in karst rocky desertification areas.
2019, 27(5): 1188-1198.
In the current classification of rock and soil, there is a fragmentation problem to cause lots of fuzzy zones among various rock or soil types. The general classification of rock and soil should be based on the basic material compositions, ie, soild, liquid and gas. Generally, the combination scheme of special components, grain diameter, porosity and degree of saturation can be adopted. In this paper, special components include orgnic matter and special minerals including layered minerals represented by montmorillonite, soluble salts and carbonate minerals. The general classification index has to be applicable to various kinds of rock and soil. But some parameters only suitable for a special rock or soil can be used as the secondary classification index. Grain sizes and special components are adopted to classify particles of rock & soil, porosity to measure the absolute compactness among particles and degree of saturation to measure moisture content. A rock or soil stratum is defined as the one with the same geological age, the same genetic type and the same special components, grain sizes, compactness and degree of saturation. The standard sequence of rock & soil strata is defined as the complete sequence of rock and soil strata for a special geological age and a genetic type in a region. It is an important foundation of large database of rock and soil strata to establish the standard sequence of rock & soil strata for all the geological ages and genetic types in a region, nation or the whole world.
In the current classification of rock and soil, there is a fragmentation problem to cause lots of fuzzy zones among various rock or soil types. The general classification of rock and soil should be based on the basic material compositions, ie, soild, liquid and gas. Generally, the combination scheme of special components, grain diameter, porosity and degree of saturation can be adopted. In this paper, special components include orgnic matter and special minerals including layered minerals represented by montmorillonite, soluble salts and carbonate minerals. The general classification index has to be applicable to various kinds of rock and soil. But some parameters only suitable for a special rock or soil can be used as the secondary classification index. Grain sizes and special components are adopted to classify particles of rock & soil, porosity to measure the absolute compactness among particles and degree of saturation to measure moisture content. A rock or soil stratum is defined as the one with the same geological age, the same genetic type and the same special components, grain sizes, compactness and degree of saturation. The standard sequence of rock & soil strata is defined as the complete sequence of rock and soil strata for a special geological age and a genetic type in a region. It is an important foundation of large database of rock and soil strata to establish the standard sequence of rock & soil strata for all the geological ages and genetic types in a region, nation or the whole world.
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