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Editor Work2020 Vol. 28, No. 3
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2020, 28(3): 431-440.
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This paper studies the effect and fluid-solid coupling mechanism of hydraulic fracturing in rock samples with different compactness. It prepares two kinds of remolded samples with different compactness. The acoustic data and void ratio of two kinds of samples are obtained by the acoustic wave & true density test system. Combined with the hydraulic fracturing monitoring system under triaxial loading, the hydraulic fracturing tests of samples of different fluid rates under certain formation pressure are simulated. Analyzing the acoustic data obtained from the acoustic test shows that the density of the samples is different and the wave velocity is positively correlated with the compactness of the samples. There is lower compactness with higher porosity. In the hydraulic fracturing test, the crack propagation direction of the compact specimen is parallel to the maximum principal stress. The variation trend of the hydraulic pressure curve is as follows. At low compactness, the time to reach the peak pressure is shorter than the back part of the peak, while the high density sample is opposite. The fracture pressure of low compactness specimen is lower than the peak value of hydraulic pressure curve in hydraulic fracturing, and that of high compactness sample is the same value. Combined with above analysis and non-confining pressure test of high compactness specimen, it can be concluded that the change of low flow rate has little effect on the crack propagation direction of the specimen with higher density and it's seepage effect is weak. The stress concentration effect is obvious in the samples without confining pressure and the samples with low compactness, which indicates that the stress concentration phenomenon in the hydraulic fracturing test is obvious in the samples with low density, that is the seepage flow rate is larger and the seepage effect is stronger. The seepage of hydraulic fracturing is positively correlated with the flow rate of injection and the low density sample has remarkable performance.
This paper studies the effect and fluid-solid coupling mechanism of hydraulic fracturing in rock samples with different compactness. It prepares two kinds of remolded samples with different compactness. The acoustic data and void ratio of two kinds of samples are obtained by the acoustic wave & true density test system. Combined with the hydraulic fracturing monitoring system under triaxial loading, the hydraulic fracturing tests of samples of different fluid rates under certain formation pressure are simulated. Analyzing the acoustic data obtained from the acoustic test shows that the density of the samples is different and the wave velocity is positively correlated with the compactness of the samples. There is lower compactness with higher porosity. In the hydraulic fracturing test, the crack propagation direction of the compact specimen is parallel to the maximum principal stress. The variation trend of the hydraulic pressure curve is as follows. At low compactness, the time to reach the peak pressure is shorter than the back part of the peak, while the high density sample is opposite. The fracture pressure of low compactness specimen is lower than the peak value of hydraulic pressure curve in hydraulic fracturing, and that of high compactness sample is the same value. Combined with above analysis and non-confining pressure test of high compactness specimen, it can be concluded that the change of low flow rate has little effect on the crack propagation direction of the specimen with higher density and it's seepage effect is weak. The stress concentration effect is obvious in the samples without confining pressure and the samples with low compactness, which indicates that the stress concentration phenomenon in the hydraulic fracturing test is obvious in the samples with low density, that is the seepage flow rate is larger and the seepage effect is stronger. The seepage of hydraulic fracturing is positively correlated with the flow rate of injection and the low density sample has remarkable performance.
2020, 28(3): 441-449.
Crack closure stress(σcc), crack initiation stress(σci) and crack damage stress(σcd) are the key stress thresholds in rock compression failure process. The determination of the stress thresholds is of great significance for the analysis of the macroscopic mechanical properties of rocks and the evolution of microscopic cracks. Based on strain measurement method, this paper examines the methods of determining stress threshold of siliceous siltstone under uniaxial compression. The stress and strain curves of siliceous siltstone were analyzed by different methods. It determines the reliability of stress thresholds obtained under different methods, and selects an optimized method to obtain the stress thresholds of siliceous siltstone samples. The results show that the accuracy of obtaining the crack closure stress and crack initiation stress by the crack volumetric strain method is easily affected by the Poisson's ratio. The moving point regression method is susceptible to subjective factors. The lateral strain response method can reduce the subjectivity of human beings, but the accuracy of the solution does not explain the physical meaning. The crack volumetric strain method based on the moving point regression technique mitigates the interference of the rock Poisson's ratio on the stress thresholds, and the physical meaning of the obtained results is relatively clear.
Crack closure stress(σcc), crack initiation stress(σci) and crack damage stress(σcd) are the key stress thresholds in rock compression failure process. The determination of the stress thresholds is of great significance for the analysis of the macroscopic mechanical properties of rocks and the evolution of microscopic cracks. Based on strain measurement method, this paper examines the methods of determining stress threshold of siliceous siltstone under uniaxial compression. The stress and strain curves of siliceous siltstone were analyzed by different methods. It determines the reliability of stress thresholds obtained under different methods, and selects an optimized method to obtain the stress thresholds of siliceous siltstone samples. The results show that the accuracy of obtaining the crack closure stress and crack initiation stress by the crack volumetric strain method is easily affected by the Poisson's ratio. The moving point regression method is susceptible to subjective factors. The lateral strain response method can reduce the subjectivity of human beings, but the accuracy of the solution does not explain the physical meaning. The crack volumetric strain method based on the moving point regression technique mitigates the interference of the rock Poisson's ratio on the stress thresholds, and the physical meaning of the obtained results is relatively clear.
2020, 28(3): 450-458.
This paper is based on a case history of a landslide occurred at the soil-rock interface. By mixing different ratios of medium sand, silicon powder and cement, rock specimens with different shapes of surface and compressive strengths are constructed to model the rocks of different degrees of roughness and weathering. The strength behavior of gravel-rock interface is investigated through laboratory large-scale simple shear tests. The results show that the shear strength of gravel-rock interface varies significantly with the shape of rock surface. The larger the surface undulation, the higher the shear strength, and the closer the shear strength is to that of the gravel. The shear strength of zigzagged interface with h=20 mm is greater than those with h=10 mm and h=0. The magnitudes of the shear strengths of gravel-rock interfaces with the same undulating height are as follows: waved interface>staged interface ≈ zigzagged interface>singly-bulged interface. The degrees of damage due to shearing are different for rocks with different degrees of weathering, which are modeled using rock specimens with different compressive strengths. The shear strength of interface changes with the degree of damage. The degree of weathering of rock is an important influencing factor on the shear strength of gravel-rock interface. A shape parameter and a damage parameter are used to characterize the effects of the shape of rock surface and the degree of weathering on the gravel-rock interface, respectively. By using these two parameters, an equation is proposed to estimate the shear strength of gravel-rock interface. It provides a theoretical basis for the stability analysis of soil-rock slope.
This paper is based on a case history of a landslide occurred at the soil-rock interface. By mixing different ratios of medium sand, silicon powder and cement, rock specimens with different shapes of surface and compressive strengths are constructed to model the rocks of different degrees of roughness and weathering. The strength behavior of gravel-rock interface is investigated through laboratory large-scale simple shear tests. The results show that the shear strength of gravel-rock interface varies significantly with the shape of rock surface. The larger the surface undulation, the higher the shear strength, and the closer the shear strength is to that of the gravel. The shear strength of zigzagged interface with h=20 mm is greater than those with h=10 mm and h=0. The magnitudes of the shear strengths of gravel-rock interfaces with the same undulating height are as follows: waved interface>staged interface ≈ zigzagged interface>singly-bulged interface. The degrees of damage due to shearing are different for rocks with different degrees of weathering, which are modeled using rock specimens with different compressive strengths. The shear strength of interface changes with the degree of damage. The degree of weathering of rock is an important influencing factor on the shear strength of gravel-rock interface. A shape parameter and a damage parameter are used to characterize the effects of the shape of rock surface and the degree of weathering on the gravel-rock interface, respectively. By using these two parameters, an equation is proposed to estimate the shear strength of gravel-rock interface. It provides a theoretical basis for the stability analysis of soil-rock slope.
2020, 28(3): 459-470.
The existing study of 3D passive earth pressure on the rigid retaining walls is mainly based on translation mode(T mode) of the retaining wall,while the study of 3D passive earth pressure during retaining wall rotation around the top mode(RT mode) still remains insufficient. Therefore,this paper systematically studies 3D passive earth pressures and the properties of 3D spatial slip surface in RT mode with numerical method. For the non-cohesive soil,the width and depth ratios of retaining wall,the values of soil internal friction angle and the soil-to-wall interface friction ratios have effects on the 3D passive earth pressure coefficients and the slip surface of soil behind retaining walls. Meanwhile there is a quantitative comparison between RT mode and T mode. The results show that: both values of soil internal friction angle and soil-to-wall interface friction ratios have significant influence on the 3D passive earth pressure coefficients and the spatial slip surface behind retaining wall in RT mode,and there is a connection between them. The 3D passive earth pressure coefficients and the spatial slip surface have significant difference between T mode and RT mode. The 3D passive earth pressure coefficients and the spatial slip surface in RT mode are smaller than those in T mode. The results can provide a reference for further study of 3D passive earth pressure and related engineering design in RT mode.
The existing study of 3D passive earth pressure on the rigid retaining walls is mainly based on translation mode(T mode) of the retaining wall,while the study of 3D passive earth pressure during retaining wall rotation around the top mode(RT mode) still remains insufficient. Therefore,this paper systematically studies 3D passive earth pressures and the properties of 3D spatial slip surface in RT mode with numerical method. For the non-cohesive soil,the width and depth ratios of retaining wall,the values of soil internal friction angle and the soil-to-wall interface friction ratios have effects on the 3D passive earth pressure coefficients and the slip surface of soil behind retaining walls. Meanwhile there is a quantitative comparison between RT mode and T mode. The results show that: both values of soil internal friction angle and soil-to-wall interface friction ratios have significant influence on the 3D passive earth pressure coefficients and the spatial slip surface behind retaining wall in RT mode,and there is a connection between them. The 3D passive earth pressure coefficients and the spatial slip surface have significant difference between T mode and RT mode. The 3D passive earth pressure coefficients and the spatial slip surface in RT mode are smaller than those in T mode. The results can provide a reference for further study of 3D passive earth pressure and related engineering design in RT mode.
2020, 28(3): 471-481.
This paper investigates the slope protection effect by vegetation and its mechanical role in increasing soil strength. It chooses six types of combinations of herbs and shrubs with their growing period of 1 a as the tested species. Built on these, the undisturbed root-soil composite systems of six types of vegetation combinations received direct shear tests to evaluate their contribution in increasing soil strength and to analysis their roots distribution characteristics. The results exhibit that for the six types of vegetation combinations, most of their roots distribute in a range of 0 to 30 cm beneath the ground surface, with their roots distribution type exhibiting a linear or exponential declining trend. The mass ratio of herb roots to shrub roots exhibit an initial decreasing and then increasing trend with soil depth increasing, which can be described with a quadratic function. In the depth of 0 to 30 cm beneath the ground surface, the cohesion of the root-soil composite systems of six types of vegetation combinations increases by 2.47% ~70.96%, comparing with that of soil without roots. The result indicates the roots of the six types of vegetation combinations exerts a significant role in increasing soil cohesion. Of these vegetation combinations, the vegetation combination of Medicago sativa L.+ Elymus nutans Griseb.+ Puccinellia distanx(L.)+ Poa pratensis L.+ Caragana korshinskii Kom exerts a dominant role in increasing soil cohesion compared to other vegetation combinations. The results of this study have practical guiding significance and theoretical research value for the ecological restoration and vegetation slope protection project in Xining Basin and its surrounding areas.
This paper investigates the slope protection effect by vegetation and its mechanical role in increasing soil strength. It chooses six types of combinations of herbs and shrubs with their growing period of 1 a as the tested species. Built on these, the undisturbed root-soil composite systems of six types of vegetation combinations received direct shear tests to evaluate their contribution in increasing soil strength and to analysis their roots distribution characteristics. The results exhibit that for the six types of vegetation combinations, most of their roots distribute in a range of 0 to 30 cm beneath the ground surface, with their roots distribution type exhibiting a linear or exponential declining trend. The mass ratio of herb roots to shrub roots exhibit an initial decreasing and then increasing trend with soil depth increasing, which can be described with a quadratic function. In the depth of 0 to 30 cm beneath the ground surface, the cohesion of the root-soil composite systems of six types of vegetation combinations increases by 2.47% ~70.96%, comparing with that of soil without roots. The result indicates the roots of the six types of vegetation combinations exerts a significant role in increasing soil cohesion. Of these vegetation combinations, the vegetation combination of Medicago sativa L.+ Elymus nutans Griseb.+ Puccinellia distanx(L.)+ Poa pratensis L.+ Caragana korshinskii Kom exerts a dominant role in increasing soil cohesion compared to other vegetation combinations. The results of this study have practical guiding significance and theoretical research value for the ecological restoration and vegetation slope protection project in Xining Basin and its surrounding areas.
2020, 28(3): 482-489.
Dispersive soil has the characteristics of low erosion resistance and easy to disperse in water. It is easy to cause accidents such as roadbed instability and dam piping. Due to the lack of the study of dispersed soils in China, there is no unified standard yet, and different identification tests often produce different results. At the same time, the different determination experiments have different grades. The dispersive degree of soil cannot be precisely distinguished. This paper takes Qian'an soil as the study samples, measures soil properties and mineral composition. It divides the crumb test, the double hydrometer test, the pinhole test and the exchangeable sodium ion test into four grades. They are high dispersive, dispersive, transitional dispersive, and non-dispersive. It gives a different score for each dispersion level. According to the weight value and score, the paper determines the dispersive degree of soil sample. This method of evaluating soil dispersion grade can be used as a reference for comprehensive judgment of dispersion grade of dispersed soil mass.
Dispersive soil has the characteristics of low erosion resistance and easy to disperse in water. It is easy to cause accidents such as roadbed instability and dam piping. Due to the lack of the study of dispersed soils in China, there is no unified standard yet, and different identification tests often produce different results. At the same time, the different determination experiments have different grades. The dispersive degree of soil cannot be precisely distinguished. This paper takes Qian'an soil as the study samples, measures soil properties and mineral composition. It divides the crumb test, the double hydrometer test, the pinhole test and the exchangeable sodium ion test into four grades. They are high dispersive, dispersive, transitional dispersive, and non-dispersive. It gives a different score for each dispersion level. According to the weight value and score, the paper determines the dispersive degree of soil sample. This method of evaluating soil dispersion grade can be used as a reference for comprehensive judgment of dispersion grade of dispersed soil mass.
2020, 28(3): 490-499.
This paper investigates the effect of particle morphology on the shear strength of sand. We prepared five sets of spherical quartz sand and laboratory standard sand with different particle sizes. Based on the high-performance DEM analysis software MatDEM, we established a three-dimensional direct shear test simulator through deeper development. We compared test data with simulated data. The results are summarized as follows. (1)The natural repose angle of quartz sand has a good positive correlation with its shear strength. (2)Spherical quartz sand can reach the peak strength faster than the laboratory standard sand, but its value is relatively low. Standard sand has a certain cohesion. (3)In the direct shearing process, the displacement of the particles in the middle of the shear box is different from that of the surrounding particles, forming a distinct shear band. The displacement of the particles in the shear band of the spherical quartz sand sample is smaller. (4)The coordination number and shear stress have a good positive correlation. The results of this paper show that under the same conditions, the shear strength of quartz sand with irregular particle morphology is significantly greater than that of spherical quartz sand.
This paper investigates the effect of particle morphology on the shear strength of sand. We prepared five sets of spherical quartz sand and laboratory standard sand with different particle sizes. Based on the high-performance DEM analysis software MatDEM, we established a three-dimensional direct shear test simulator through deeper development. We compared test data with simulated data. The results are summarized as follows. (1)The natural repose angle of quartz sand has a good positive correlation with its shear strength. (2)Spherical quartz sand can reach the peak strength faster than the laboratory standard sand, but its value is relatively low. Standard sand has a certain cohesion. (3)In the direct shearing process, the displacement of the particles in the middle of the shear box is different from that of the surrounding particles, forming a distinct shear band. The displacement of the particles in the shear band of the spherical quartz sand sample is smaller. (4)The coordination number and shear stress have a good positive correlation. The results of this paper show that under the same conditions, the shear strength of quartz sand with irregular particle morphology is significantly greater than that of spherical quartz sand.
2020, 28(3): 500-509.
The red clay in northern China is also known as the Sanzhima red clay. It is discontinuously distributed under laying loess at the Loess Plateau, where is a typical slide-prone stratum. We can usually observe the exposed red clay in the valleys. The mechanical properties of the red clay can decrease with the increase of its water content. Accordingly, its deformation can increase and the sliding can occur. It is believed that all these behavior of the red clay are closely related to its creeping characteristics. Most of the recent related studies focus on the red clay in southern China. However, the engineering properties of the red clay in the south and north of China are quite different. We cannot treat them as the same. We take the red clay in Lantian County at Shaanxi Province as an example. There is widely distributed red clay. The landslides related to loess and red clay soil are also intensively developed along the loess tableland there. We study the long-term strength and deformation characteristics of the red clay taken from the section at the Lantian County. Based on the theory of unsaturated soil mechanics, the triaxial creeping test under different confining pressures and different matric suctions are conducted. The unsaturated triaxial test apparatus, FSR-60, is used. The pressure sensor is updated to improve the accuracy and long-term stability. According to the range of the volumatric water content(from 11.4%to 22.3%) of the studied red clay, the suction is controlled as 0, 100 kPa and 200 kPa respectively in the test. The step-loading method is used. The deformation less than 0.01 mm within 1 day is taken as the criterion for stability. During the process of adding load step by step, the lasting time for each step is about 1 to 2 weeks. The test and analysis results show the follows: (1)The red clay in northern China is characterized with obvious creeping property. The changes of stress with strain under different confining pressures are similar, which obey the law of power function or logarithm function. (2)When the value of deviatoric stress is less than the failure load, the instantaneous strain is first observed, and then the deformation decreased until stable. When the value of deviatoric stress is close to the failure load, creeping deformation occurs, then the deformation accelerates until the tested sample failure. When the value of deviatoric stress is bigger than the failure load, the tested sample is quickly destroyed during the loading process. (3)When the confining pressure and load are constant, the creep deformation increases with the decrease of suction, and the lasting time for stability increases with the decrease of suction. The creep probability increase and yield strength decrease under small suction. (4)The long-term strength for the saturated red clay(with the suction of 0) is about 50%lower than that of red clay with a suction of 200 kPa, indicating that the increase of water content can increase the creep deformation of the red clay and led to the occurrence of landslides. This study can provide a basis for the study on the mechanism of landslide related to red clay at the loess plateau.
The red clay in northern China is also known as the Sanzhima red clay. It is discontinuously distributed under laying loess at the Loess Plateau, where is a typical slide-prone stratum. We can usually observe the exposed red clay in the valleys. The mechanical properties of the red clay can decrease with the increase of its water content. Accordingly, its deformation can increase and the sliding can occur. It is believed that all these behavior of the red clay are closely related to its creeping characteristics. Most of the recent related studies focus on the red clay in southern China. However, the engineering properties of the red clay in the south and north of China are quite different. We cannot treat them as the same. We take the red clay in Lantian County at Shaanxi Province as an example. There is widely distributed red clay. The landslides related to loess and red clay soil are also intensively developed along the loess tableland there. We study the long-term strength and deformation characteristics of the red clay taken from the section at the Lantian County. Based on the theory of unsaturated soil mechanics, the triaxial creeping test under different confining pressures and different matric suctions are conducted. The unsaturated triaxial test apparatus, FSR-60, is used. The pressure sensor is updated to improve the accuracy and long-term stability. According to the range of the volumatric water content(from 11.4%to 22.3%) of the studied red clay, the suction is controlled as 0, 100 kPa and 200 kPa respectively in the test. The step-loading method is used. The deformation less than 0.01 mm within 1 day is taken as the criterion for stability. During the process of adding load step by step, the lasting time for each step is about 1 to 2 weeks. The test and analysis results show the follows: (1)The red clay in northern China is characterized with obvious creeping property. The changes of stress with strain under different confining pressures are similar, which obey the law of power function or logarithm function. (2)When the value of deviatoric stress is less than the failure load, the instantaneous strain is first observed, and then the deformation decreased until stable. When the value of deviatoric stress is close to the failure load, creeping deformation occurs, then the deformation accelerates until the tested sample failure. When the value of deviatoric stress is bigger than the failure load, the tested sample is quickly destroyed during the loading process. (3)When the confining pressure and load are constant, the creep deformation increases with the decrease of suction, and the lasting time for stability increases with the decrease of suction. The creep probability increase and yield strength decrease under small suction. (4)The long-term strength for the saturated red clay(with the suction of 0) is about 50%lower than that of red clay with a suction of 200 kPa, indicating that the increase of water content can increase the creep deformation of the red clay and led to the occurrence of landslides. This study can provide a basis for the study on the mechanism of landslide related to red clay at the loess plateau.
2020, 28(3): 510-519.
The thermophysical parameters of stratum must be clearly defined in the freezing method. This paper aims to ascertain the thermophysical parameters of water-rich gravel stratum and the interactions between the parameters and the influencing factors. It examines the effect of salt content on the initial freezing temperature of gravel samples and the interactions among freezing temperature, specific heat capacity and thermal conductivity of the samples based on their initial parameters measured by a self-made test device. The experimental results show that the increasing salt content leads to continuous decrease of the vapor pressure of water in the samples, which requires lower temperature and more energy for the samples to freeze. Specially, with the increase of salt content, the initial freezing temperature of the samples 1, 2 and 3 decreased from -0.46 to -1.15 ℃. With the decrease of freezing temperature, the ice content in the samples increased while the unfrozen water content decreased. Because the specific heat capacity of ice is half of that of water, the specific heat capacity of the samples decreased continuously from 1.60 J·(g·℃)-1 to 1.06 J·(g·℃)-1. Because the thermal conductivity of ice is much larger than that of water, the thermal conductivity of the samples increased from 1.71 W·(m·K)-1 to 2.13 W·(m·K)-1. As the ice content and unfrozen water content in the samples changed with temperature, the liquid-solid phase transformation of the water caused the variation in physical and mechanical properties of the samples.
The thermophysical parameters of stratum must be clearly defined in the freezing method. This paper aims to ascertain the thermophysical parameters of water-rich gravel stratum and the interactions between the parameters and the influencing factors. It examines the effect of salt content on the initial freezing temperature of gravel samples and the interactions among freezing temperature, specific heat capacity and thermal conductivity of the samples based on their initial parameters measured by a self-made test device. The experimental results show that the increasing salt content leads to continuous decrease of the vapor pressure of water in the samples, which requires lower temperature and more energy for the samples to freeze. Specially, with the increase of salt content, the initial freezing temperature of the samples 1, 2 and 3 decreased from -0.46 to -1.15 ℃. With the decrease of freezing temperature, the ice content in the samples increased while the unfrozen water content decreased. Because the specific heat capacity of ice is half of that of water, the specific heat capacity of the samples decreased continuously from 1.60 J·(g·℃)-1 to 1.06 J·(g·℃)-1. Because the thermal conductivity of ice is much larger than that of water, the thermal conductivity of the samples increased from 1.71 W·(m·K)-1 to 2.13 W·(m·K)-1. As the ice content and unfrozen water content in the samples changed with temperature, the liquid-solid phase transformation of the water caused the variation in physical and mechanical properties of the samples.
2020, 28(3): 520-529.
This paper examines the kinematical characteristics of the collapse-debris flow in red beds. It presents an evaluation method about spatial prediction of the collapse-debris flow in red beds. It takes the Kangsu collapse in red beds as an example. It uses the aerial photography of the unmanned aerial vehicle and the field geological survey, and combines them with the engineering geological elements of the collapse study area. It analyses the formation mechanism and the failure types of the collapse. The research shows that the collapse is the crack-type collapse. The collapse is mainly controlled by the combination of lithology of dangerous rock mass and structural plane of slope body. The hazard mode is given as follows: Firstly, rocks have the different weathering resistance under the conditions of wind and other external forces. Secondly, under the action of freezing and thawing, the rock mass structure is destroyed. Thirdly, when the unloading fracture zone is expanded further, the bottom of dangerous rock mass can be in an empty state. At last, under the action of gravity, the collapse occurs unsteadily and impacts to the slope surface to form debris flow. And the DAN-W model is used to simulate the motion process of the collapse-debris flow. The calculation results show that the duration of the collapse-debris flow movement is about 50 s, the average thickness about accumulation body reaches 2 m and the maximum speed is 11.5 m·s-1. And the longest impact distance is 315 m. The calculated results are in good agreement with the actual situation. It proves that DAN-W model can be used to analyze the dynamic hazard effects of the collapse-debris flow in red beds area. The paper providesareference for the analysis of the formation characteristics and movement effects of similar potential collapse-debris flow hazards in red beds.
This paper examines the kinematical characteristics of the collapse-debris flow in red beds. It presents an evaluation method about spatial prediction of the collapse-debris flow in red beds. It takes the Kangsu collapse in red beds as an example. It uses the aerial photography of the unmanned aerial vehicle and the field geological survey, and combines them with the engineering geological elements of the collapse study area. It analyses the formation mechanism and the failure types of the collapse. The research shows that the collapse is the crack-type collapse. The collapse is mainly controlled by the combination of lithology of dangerous rock mass and structural plane of slope body. The hazard mode is given as follows: Firstly, rocks have the different weathering resistance under the conditions of wind and other external forces. Secondly, under the action of freezing and thawing, the rock mass structure is destroyed. Thirdly, when the unloading fracture zone is expanded further, the bottom of dangerous rock mass can be in an empty state. At last, under the action of gravity, the collapse occurs unsteadily and impacts to the slope surface to form debris flow. And the DAN-W model is used to simulate the motion process of the collapse-debris flow. The calculation results show that the duration of the collapse-debris flow movement is about 50 s, the average thickness about accumulation body reaches 2 m and the maximum speed is 11.5 m·s-1. And the longest impact distance is 315 m. The calculated results are in good agreement with the actual situation. It proves that DAN-W model can be used to analyze the dynamic hazard effects of the collapse-debris flow in red beds area. The paper providesareference for the analysis of the formation characteristics and movement effects of similar potential collapse-debris flow hazards in red beds.
2020, 28(3): 530-540.
The threats of earthquakes to humans are not only the casualties and property losses, but also the hidden danger of geological hazards. To assess the risk of landslides in the area after earthquakes, a quick and reliable model for the landslide risk assessment is necessary. Based on the study area along the "Chuanzhusi-Jiuzhaigou" road, we proposes a comprehensive analysis model for the landslide hazard in the post-earthquake area. It can be called the DInSAR-BP model. The model results show that the area of the high-risk landsides in Jiuzhaigou after the earthquake is about 2602.35 km2 and 3.4 times larger than that before the earthquake. And these areas are mainly distributed in the Jiuzhaigou scenic area and the slope of the first 70 km along the "Chuanzhusi-Jiuzhaigou" road. These results match the survey results after the earthquake. The Multivariate nonlinear regression can consider the impact from the earthquake to the risk of landslides, which improves the accuracy of post-earthquake risk assessment results by 13.9%. And it proves that the model has good applicability in the study area.
The threats of earthquakes to humans are not only the casualties and property losses, but also the hidden danger of geological hazards. To assess the risk of landslides in the area after earthquakes, a quick and reliable model for the landslide risk assessment is necessary. Based on the study area along the "Chuanzhusi-Jiuzhaigou" road, we proposes a comprehensive analysis model for the landslide hazard in the post-earthquake area. It can be called the DInSAR-BP model. The model results show that the area of the high-risk landsides in Jiuzhaigou after the earthquake is about 2602.35 km2 and 3.4 times larger than that before the earthquake. And these areas are mainly distributed in the Jiuzhaigou scenic area and the slope of the first 70 km along the "Chuanzhusi-Jiuzhaigou" road. These results match the survey results after the earthquake. The Multivariate nonlinear regression can consider the impact from the earthquake to the risk of landslides, which improves the accuracy of post-earthquake risk assessment results by 13.9%. And it proves that the model has good applicability in the study area.
2020, 28(3): 541-556.
A large scale of mountain rock avalanche occurred at about 20:40 on July 23, 2019 in Jichang town, Shuicheng county, Guizhou Province. The rock mass moved down and scraped the original loose deposits and finally formed deposits of 191.2×104 m3. It destroyed parts of Pingdi Village, resulted in the death of 43 persons and missing of 9 persons. Based on site investigation, unmanned aerial vehicle(UAV)photography and other technologies, this paper provides a detailed description on the characteristics and a comprehensive analysis on the dynamic process and failure mechanism of the long runout landslide. And there is still a risk that the accumulation of the landslide can be translated into debris flow during rainfall. The results indicate that the special geometry of the source area, the weathered and shattered basalt mass and favorable combination of discontinuities are the internal cause factors for sliding. The rainfall infiltrating and its softening effect, and the road construction cut-slope are the external cause factors for the landslide. There is no obvious signal before the occurrence of Jichang landslide. The masses have a long runout distance of 1300 m. When it occurred, it demonstrated the characteristics of confidentiality and strong paroxysm. It caused great danger in large range. There can be a big attribution if such case can be studied further. It can provide the significant experience on dealing with this kind of masses failure and how to provide advice on early warning and remediation project on similar cases in southwestern mountain areas of China.
A large scale of mountain rock avalanche occurred at about 20:40 on July 23, 2019 in Jichang town, Shuicheng county, Guizhou Province. The rock mass moved down and scraped the original loose deposits and finally formed deposits of 191.2×104 m3. It destroyed parts of Pingdi Village, resulted in the death of 43 persons and missing of 9 persons. Based on site investigation, unmanned aerial vehicle(UAV)photography and other technologies, this paper provides a detailed description on the characteristics and a comprehensive analysis on the dynamic process and failure mechanism of the long runout landslide. And there is still a risk that the accumulation of the landslide can be translated into debris flow during rainfall. The results indicate that the special geometry of the source area, the weathered and shattered basalt mass and favorable combination of discontinuities are the internal cause factors for sliding. The rainfall infiltrating and its softening effect, and the road construction cut-slope are the external cause factors for the landslide. There is no obvious signal before the occurrence of Jichang landslide. The masses have a long runout distance of 1300 m. When it occurred, it demonstrated the characteristics of confidentiality and strong paroxysm. It caused great danger in large range. There can be a big attribution if such case can be studied further. It can provide the significant experience on dealing with this kind of masses failure and how to provide advice on early warning and remediation project on similar cases in southwestern mountain areas of China.
2020, 28(3): 557-564.
The stability analysis of unstable rock mass in steep slope has always been the focus of researchers' interests. The traditional stability evaluation methods are based on qualitative analysis. The numerical simulation can accurately establish complex 3D geological model rapidly and analyze the stability of unstable rock mass quantitatively, with the support of reverse engineering modeling method. Based on the geological survey results, the paper scans the potential geological hazard points of Shenlong Waterfall Slope in Shenxianju, and gives a model reconstruction of the scanned data through the Geomagic Studio software. It uses Hypermesh to process the surface model, simultaneously calculates the obtained grid data and carries out the finite difference numerical simulation calculation with FLAC3D. According to the results, the maximum tensile stress of the unstable rock mass is close to its tensile strength. It is likely that rockslide-rockfall can occur. In addition, the paper uses the FISH language in FLAC3D to calculate the failure probability of the unstable rock mass based on the proportion of the number of plastic failure elements in discontinuities. The results show that the probability of failure is more than 20%. The results correspond well with the results of the geological survey and numerical analysis. The paper also forecasts the failure of the unstable rock mass, and proposes the preventive and control measures. The paper can provide the theoretical basis and have certain reference value for the disaster prevention works of the local tourism administrative department.
The stability analysis of unstable rock mass in steep slope has always been the focus of researchers' interests. The traditional stability evaluation methods are based on qualitative analysis. The numerical simulation can accurately establish complex 3D geological model rapidly and analyze the stability of unstable rock mass quantitatively, with the support of reverse engineering modeling method. Based on the geological survey results, the paper scans the potential geological hazard points of Shenlong Waterfall Slope in Shenxianju, and gives a model reconstruction of the scanned data through the Geomagic Studio software. It uses Hypermesh to process the surface model, simultaneously calculates the obtained grid data and carries out the finite difference numerical simulation calculation with FLAC3D. According to the results, the maximum tensile stress of the unstable rock mass is close to its tensile strength. It is likely that rockslide-rockfall can occur. In addition, the paper uses the FISH language in FLAC3D to calculate the failure probability of the unstable rock mass based on the proportion of the number of plastic failure elements in discontinuities. The results show that the probability of failure is more than 20%. The results correspond well with the results of the geological survey and numerical analysis. The paper also forecasts the failure of the unstable rock mass, and proposes the preventive and control measures. The paper can provide the theoretical basis and have certain reference value for the disaster prevention works of the local tourism administrative department.
2020, 28(3): 565-573.
In order to slow down the development of cracks in the rock roof of No. 3 cavern in Longyou Grottoes, this paper adopts the method of the investigation of factors and the mechanical calculation and analysis of the cracks development mechanism. The paper analyzes the variation law of the rock roof by monitoring date. The results show that:(1)The factors causing cracks of the rock roof are complicated, and mainly are due to the low fracture toughness of the surrounding rock of mainly sandstone. (2)The cracks in the unsupported area are intensified developed and the uneven settlement of roof increases gradually. According to the deformation law, this paper designs a supporting scheme and visualizes the supporting structure by BIM(Building Information Modeling). This paper further proposes an intelligent monitoring program for the supported caverns. The support schemes and intelligent monitoring program can play an important role in the long-term protection of the rock caverns.
In order to slow down the development of cracks in the rock roof of No. 3 cavern in Longyou Grottoes, this paper adopts the method of the investigation of factors and the mechanical calculation and analysis of the cracks development mechanism. The paper analyzes the variation law of the rock roof by monitoring date. The results show that:(1)The factors causing cracks of the rock roof are complicated, and mainly are due to the low fracture toughness of the surrounding rock of mainly sandstone. (2)The cracks in the unsupported area are intensified developed and the uneven settlement of roof increases gradually. According to the deformation law, this paper designs a supporting scheme and visualizes the supporting structure by BIM(Building Information Modeling). This paper further proposes an intelligent monitoring program for the supported caverns. The support schemes and intelligent monitoring program can play an important role in the long-term protection of the rock caverns.
2020, 28(3): 574-583.
Reinforced soil embankments are one of the common structures for rockfall protection in engineering. It is usually used in mountainous areas where giant rocks fall frequently or where rocks roll down at high speed. It can stop rockfall with high impact energy effectively. However, the design methods of the reinforced soil embankments for rockfall protection are not perfect as(1)the large and irreversible deformations induced, (2)the complex elastic-plastic stress-strain relationship of reinforced soil, and (3)the interaction between different components. This paper presents an overview of the state of art and the design methods in the field of the rockfall protection reinforced soil embankments. The impact response of protection reinforced soil embankments is elaborated. This paper discusses their design methods, from the aspects of block's trajectory control and stability of embankments. The comparisons and shortcomings of the design methods are also discussed. It points out that the rotational kinetic energy of the block can't be ignored in the study. It also shows that further investigation is needed in the aspects of maximum impact force calculation method and analysis method of impact response.
Reinforced soil embankments are one of the common structures for rockfall protection in engineering. It is usually used in mountainous areas where giant rocks fall frequently or where rocks roll down at high speed. It can stop rockfall with high impact energy effectively. However, the design methods of the reinforced soil embankments for rockfall protection are not perfect as(1)the large and irreversible deformations induced, (2)the complex elastic-plastic stress-strain relationship of reinforced soil, and (3)the interaction between different components. This paper presents an overview of the state of art and the design methods in the field of the rockfall protection reinforced soil embankments. The impact response of protection reinforced soil embankments is elaborated. This paper discusses their design methods, from the aspects of block's trajectory control and stability of embankments. The comparisons and shortcomings of the design methods are also discussed. It points out that the rotational kinetic energy of the block can't be ignored in the study. It also shows that further investigation is needed in the aspects of maximum impact force calculation method and analysis method of impact response.
2020, 28(3): 584-596.
The uncertainties are great challenges for seismic design of slope engineering. This paper reviews the recent researches on random dynamic analysis methods of slope, and discusses the application prospect of those methods in engineering seismic design. Firstly, this paper reviews previous researches on dynamic stability of slopes and analyses the uncertainty problems involved in slope engineering. Secondly, it summarizes the processes of slope random dynamic analysis. It introduces a framework of slope random dynamic analysis based on probability density evolution. Then, it introduces the research on the uncertainty of ground motions, where the recorded waves of a typical station are used to explain the existence of the ground motion uncertainty, and a random ground motion model based on the site characteristics is discussed. On this basis, the paper identifies the specific contents of random dynamic analysis of slope. They include(1)random dynamic response analysis of slope, (2)dynamic reliability analysis of slope, (3)seismic fragility analysis of slope, (4)dynamic instability and risk assessment of slope, (5)optimal design of seismic performance of slope. At last, the paper briefly introduces some advances of slope random dynamic analysis method, analyses the achievements and prospects. The research of slope seismic is extended from the deterministic analysis to the uncertainty analysis. The paper provides a theoretical reference for the aseismic design of slope engineering. However, it still needs to be further improved in the aspects of random ground motion simulation, multi random factors coupling, and support structure optimization design.
The uncertainties are great challenges for seismic design of slope engineering. This paper reviews the recent researches on random dynamic analysis methods of slope, and discusses the application prospect of those methods in engineering seismic design. Firstly, this paper reviews previous researches on dynamic stability of slopes and analyses the uncertainty problems involved in slope engineering. Secondly, it summarizes the processes of slope random dynamic analysis. It introduces a framework of slope random dynamic analysis based on probability density evolution. Then, it introduces the research on the uncertainty of ground motions, where the recorded waves of a typical station are used to explain the existence of the ground motion uncertainty, and a random ground motion model based on the site characteristics is discussed. On this basis, the paper identifies the specific contents of random dynamic analysis of slope. They include(1)random dynamic response analysis of slope, (2)dynamic reliability analysis of slope, (3)seismic fragility analysis of slope, (4)dynamic instability and risk assessment of slope, (5)optimal design of seismic performance of slope. At last, the paper briefly introduces some advances of slope random dynamic analysis method, analyses the achievements and prospects. The research of slope seismic is extended from the deterministic analysis to the uncertainty analysis. The paper provides a theoretical reference for the aseismic design of slope engineering. However, it still needs to be further improved in the aspects of random ground motion simulation, multi random factors coupling, and support structure optimization design.
2020, 28(3): 597-609.
The excavation and filling works is frequently conducted in geotechnical engineering activities in collapsible loess area,which can easily lead to a series of geo-environmental problems such as land subsidence and vegetation damage. This paper examines the land subsidence and vegetation restoration characteristics of the mountain excavation and city construction project for Yan'an New District. Firstly,the basic characteristics of the project are obtained using visual interpretation and DEM extraction method from the remote sensing images. Then,the ground settlement of the study area is detected based on SBAS techniques. Finally,the NDVI time series data are extracted by GF-1 remote sensing images. The conclusions can be found as follows. (1)The urban construction projects of Yan'an New District are two years later after the large-scale land creation projects. The filling area is distributed along the valley of the original terrain. The maximum thickness of the excavation and filling square is more than 90m. (2)The main settlement area of Yan'an New District is the filling area. The maximum subsidence rate is 45mm ·a-1 from 2017 to 2018. There is no obvious land subsidence in the excavation area where the main construction project is located. The man-made loess slopes are mostly in a stable state. (3)The average NDVI of Yan'an New District increases 230% from 2013 to 2015, and increases 50% from 2015 to 2018 with apparent vegetation restoration. The planning and construction of the mountain excavation and city construction project in Yan'an New District has reasonably avoided land subsidence and alleviated the deterioration of ecological environment. The results can provide scientific references for the similar projects.
The excavation and filling works is frequently conducted in geotechnical engineering activities in collapsible loess area,which can easily lead to a series of geo-environmental problems such as land subsidence and vegetation damage. This paper examines the land subsidence and vegetation restoration characteristics of the mountain excavation and city construction project for Yan'an New District. Firstly,the basic characteristics of the project are obtained using visual interpretation and DEM extraction method from the remote sensing images. Then,the ground settlement of the study area is detected based on SBAS techniques. Finally,the NDVI time series data are extracted by GF-1 remote sensing images. The conclusions can be found as follows. (1)The urban construction projects of Yan'an New District are two years later after the large-scale land creation projects. The filling area is distributed along the valley of the original terrain. The maximum thickness of the excavation and filling square is more than 90m. (2)The main settlement area of Yan'an New District is the filling area. The maximum subsidence rate is 45mm ·a-1 from 2017 to 2018. There is no obvious land subsidence in the excavation area where the main construction project is located. The man-made loess slopes are mostly in a stable state. (3)The average NDVI of Yan'an New District increases 230% from 2013 to 2015, and increases 50% from 2015 to 2018 with apparent vegetation restoration. The planning and construction of the mountain excavation and city construction project in Yan'an New District has reasonably avoided land subsidence and alleviated the deterioration of ecological environment. The results can provide scientific references for the similar projects.
2020, 28(3): 610-618.
Considering the desiccation cracks in the earthen sites,the crack initiation conditions,propagation characteristics and unstable propagation conditions of the desiccation cracks are discussed based on the section of Moon Bay Wall in Sanxingdui. The research results help reveal the cracking law of soil sites and have guiding significance for the engineering practice in earthen sites protection. The drying test is carried out for the soil of Moon Bay Wall. The sample water loss process is recorded by electronic balance. The cracking and expansion process of the crack is obtained with image analysis technology. The displacement field and strain field of the cracking process are obtained by digital image correlation technology. The results show that the surface defects of the sample reduce the tensile stress required for cracking. The smaller the distance between the defect and the boundary,the smaller the critical defect dimension of the crack initiation. Therefore,the crack is more likely to be generated near the boundary of the sample. According to the theory of fracture mechanics,considering the influence of water content on the soil properties,the crack unstable propagation need to meet the condition that the increase rate of stress intensity factor to water content should be greater than the fracture toughness. Theoretically,the calculation method for critical length of crack unstable propagation is derived,and the method is initially verified with measured data. As the water content decreases from 45.6%to 29.1%,the ratio between the critical length of the instability propagation to the width of the sample is reduced from 0.109 to 0.024. The lower the water content,the easier the crack enters the instability stage,and a small defect in the crack can trigger a rapid crack expansion. When protecting the surface of soil sites,the work should focus on repairing large-scale defects distributed in the boundary area of soil layers,should regularly monitor the surface of the soil sites. Once cracking is found,the cracks and their nearby defects should be timely repaired to prevent crack instability expansion.
Considering the desiccation cracks in the earthen sites,the crack initiation conditions,propagation characteristics and unstable propagation conditions of the desiccation cracks are discussed based on the section of Moon Bay Wall in Sanxingdui. The research results help reveal the cracking law of soil sites and have guiding significance for the engineering practice in earthen sites protection. The drying test is carried out for the soil of Moon Bay Wall. The sample water loss process is recorded by electronic balance. The cracking and expansion process of the crack is obtained with image analysis technology. The displacement field and strain field of the cracking process are obtained by digital image correlation technology. The results show that the surface defects of the sample reduce the tensile stress required for cracking. The smaller the distance between the defect and the boundary,the smaller the critical defect dimension of the crack initiation. Therefore,the crack is more likely to be generated near the boundary of the sample. According to the theory of fracture mechanics,considering the influence of water content on the soil properties,the crack unstable propagation need to meet the condition that the increase rate of stress intensity factor to water content should be greater than the fracture toughness. Theoretically,the calculation method for critical length of crack unstable propagation is derived,and the method is initially verified with measured data. As the water content decreases from 45.6%to 29.1%,the ratio between the critical length of the instability propagation to the width of the sample is reduced from 0.109 to 0.024. The lower the water content,the easier the crack enters the instability stage,and a small defect in the crack can trigger a rapid crack expansion. When protecting the surface of soil sites,the work should focus on repairing large-scale defects distributed in the boundary area of soil layers,should regularly monitor the surface of the soil sites. Once cracking is found,the cracks and their nearby defects should be timely repaired to prevent crack instability expansion.
2020, 28(3): 619-625.
The stability of reservoir slope is affected by various factors so that it is still difficult to accurately evaluate their stability. This paper overcomes the two existing problems:(1)the eigenvalues of cloud model are not unified and (2)the expected rather than actual membership degrees are used as the traditional normal cloud model method is actually applied. It proposes an improved normal cloud model. A program for model application is developed on MATLAB software platform. Twelve factors that mainly affect the reservoir slope stability are selected as the evaluation indexes. The comprehensive cloud model of each stability level in regard to every evaluation index is generated from the cloud generator. Meanwhile,the weight of every evaluation index are obtained by the analytic hierarchy process on the basis of improved entropy weight. The maximum average membership degree is determined as the stability grade of a slope. The proposed method is employed in the stability evaluation of 34 engineering examples of reservoir slopes. And the evaluation results are compared with the ones by the CSMR method recommended by the current national code as well as the actual state of these slopes. It shows that the accuracy of this method is higher than that of CSMR method,and it has good reliability and practicability. The method can also be referred when evaluating the stability of other rock slopes such as road,railway or building slopes.
The stability of reservoir slope is affected by various factors so that it is still difficult to accurately evaluate their stability. This paper overcomes the two existing problems:(1)the eigenvalues of cloud model are not unified and (2)the expected rather than actual membership degrees are used as the traditional normal cloud model method is actually applied. It proposes an improved normal cloud model. A program for model application is developed on MATLAB software platform. Twelve factors that mainly affect the reservoir slope stability are selected as the evaluation indexes. The comprehensive cloud model of each stability level in regard to every evaluation index is generated from the cloud generator. Meanwhile,the weight of every evaluation index are obtained by the analytic hierarchy process on the basis of improved entropy weight. The maximum average membership degree is determined as the stability grade of a slope. The proposed method is employed in the stability evaluation of 34 engineering examples of reservoir slopes. And the evaluation results are compared with the ones by the CSMR method recommended by the current national code as well as the actual state of these slopes. It shows that the accuracy of this method is higher than that of CSMR method,and it has good reliability and practicability. The method can also be referred when evaluating the stability of other rock slopes such as road,railway or building slopes.
2020, 28(3): 626-638.
Rock slopes with soft interlayer are common in nature and engineering projects. They are usually the characteristic of poor stability and low safety level. Landslides that induced by the instability and destruction of rock slopes with soft interlayer have caused a large number of casualties and heavy economic losses around the world. Therefore,the stability of rock slopes containing soft interlayer is one of important research topic in engineering geology and geotechnical engineering. This paper is based on the collection and review of the domestic and foreign literatures related to the stability of rock slopes with soft interlayer. It summarizes the advances of some important aspects on the research current situations and progress of rock slope stability in the aspect of gravity load conditions,excavation conditions,heavy rain and reservoir storage conditions,and earthquake conditions. The findings are as follows. (1)The presence of a soft interlayer usually plays an adverse role in slope stability. The stability is poorer for rock slopes with soft interlayer than for homogeneous rock slopes and layered rock slopes without soft interlayer. (2)The stability and deformation failure mechanism of rock slopes containing soft interlayer are linked to the water content,shear strength,dip angles,thickness,the number of layers,intervals of soft interlayer and the slope angles. (3)Slope failure can easily occur along soft interlayer after the completion of slope excavation without support. When excavation with supporting in time,the stability of slopes can be effectively controlled. (4)Lamination effect caused by blasting changes the contact state between soft interlayer and surrounding rock and decreases the cohesion and friction of contact interfaces,thus decreases the stability of rock slopes. (5)Heavy rainfall and reservoir impoundment are both against the stability of rock slopes with soft interlayer. (6)The dynamic responses,deformation and failure process of rock slopes with soft interlayer under earthquakes are related to the property of soft interlayer(thickness,dip angle and water content),the characteristics of seismic wave(wave types,vibration intensity and frequency of earthquake waves,loading direction) and slope structure(bedding and counter-tilt rock slopes with soft interlayer). (7)The amplification or weakening effect of soft interlayer on the horizontal dynamic responses of rock slopes under earthquake and the role of thick soft interlayer in energy dissipation and shock absorption during shakings are not unified. Subsequently,on this basis,the advantages and disadvantages of research methods are analyzed in detail. Furthermore,the main problems existing in current research are also discussed. Finally,on account of research status in this field,some important research emphases and directions in the future are proposed. The contents are listed as follows: the progressive failure process and stability of rock slopes with multi-soft interlayer; the effect mechanism of individual factors(including excavation unloading,blasting,earthquake,heavy rain,the change of reservoir water levels,underground water) on the stability of rock slopes with soft interlayer; the stability of rock slopes with multi-soft interlayer under multi-factor coupling participation; and the reinforcement mechanism of retaining structural system for rock slopes with multi-soft interlayer.
Rock slopes with soft interlayer are common in nature and engineering projects. They are usually the characteristic of poor stability and low safety level. Landslides that induced by the instability and destruction of rock slopes with soft interlayer have caused a large number of casualties and heavy economic losses around the world. Therefore,the stability of rock slopes containing soft interlayer is one of important research topic in engineering geology and geotechnical engineering. This paper is based on the collection and review of the domestic and foreign literatures related to the stability of rock slopes with soft interlayer. It summarizes the advances of some important aspects on the research current situations and progress of rock slope stability in the aspect of gravity load conditions,excavation conditions,heavy rain and reservoir storage conditions,and earthquake conditions. The findings are as follows. (1)The presence of a soft interlayer usually plays an adverse role in slope stability. The stability is poorer for rock slopes with soft interlayer than for homogeneous rock slopes and layered rock slopes without soft interlayer. (2)The stability and deformation failure mechanism of rock slopes containing soft interlayer are linked to the water content,shear strength,dip angles,thickness,the number of layers,intervals of soft interlayer and the slope angles. (3)Slope failure can easily occur along soft interlayer after the completion of slope excavation without support. When excavation with supporting in time,the stability of slopes can be effectively controlled. (4)Lamination effect caused by blasting changes the contact state between soft interlayer and surrounding rock and decreases the cohesion and friction of contact interfaces,thus decreases the stability of rock slopes. (5)Heavy rainfall and reservoir impoundment are both against the stability of rock slopes with soft interlayer. (6)The dynamic responses,deformation and failure process of rock slopes with soft interlayer under earthquakes are related to the property of soft interlayer(thickness,dip angle and water content),the characteristics of seismic wave(wave types,vibration intensity and frequency of earthquake waves,loading direction) and slope structure(bedding and counter-tilt rock slopes with soft interlayer). (7)The amplification or weakening effect of soft interlayer on the horizontal dynamic responses of rock slopes under earthquake and the role of thick soft interlayer in energy dissipation and shock absorption during shakings are not unified. Subsequently,on this basis,the advantages and disadvantages of research methods are analyzed in detail. Furthermore,the main problems existing in current research are also discussed. Finally,on account of research status in this field,some important research emphases and directions in the future are proposed. The contents are listed as follows: the progressive failure process and stability of rock slopes with multi-soft interlayer; the effect mechanism of individual factors(including excavation unloading,blasting,earthquake,heavy rain,the change of reservoir water levels,underground water) on the stability of rock slopes with soft interlayer; the stability of rock slopes with multi-soft interlayer under multi-factor coupling participation; and the reinforcement mechanism of retaining structural system for rock slopes with multi-soft interlayer.
2020, 28(3): 639-649.
The suction caisson is a new type of foundation having advantages such as easy construction,reuse and low cost. It has been successfully applied to offshore platforms and offshore wind turbines. Uplift capacity is the key factor in design of suction caissons used to fix the floating offshore platforms in deep water. Moreover,offshore wind farms should be updated with increasing the diameter of the wind turbine blades and the tower height,thus the existing foundation is not available to meet new design requirements. From the perspective of sustainable development,pullout and reuse of suction caissons are gradually attracting more attention. This paper presents an overview of the researches on uplift capacity and pullout characteristics of suction caissons,and also discusses the influencing factors of pullout capacity in the aspects of loading rate and soil properties. Investigations from the literature show that pumping water into suction caissons can cause seepage failure in sand,splitting failure in clay,and at this time the caissons cannot continue to be pulled out. It is recommended to be extracted by pullout loading or vibrating.
The suction caisson is a new type of foundation having advantages such as easy construction,reuse and low cost. It has been successfully applied to offshore platforms and offshore wind turbines. Uplift capacity is the key factor in design of suction caissons used to fix the floating offshore platforms in deep water. Moreover,offshore wind farms should be updated with increasing the diameter of the wind turbine blades and the tower height,thus the existing foundation is not available to meet new design requirements. From the perspective of sustainable development,pullout and reuse of suction caissons are gradually attracting more attention. This paper presents an overview of the researches on uplift capacity and pullout characteristics of suction caissons,and also discusses the influencing factors of pullout capacity in the aspects of loading rate and soil properties. Investigations from the literature show that pumping water into suction caissons can cause seepage failure in sand,splitting failure in clay,and at this time the caissons cannot continue to be pulled out. It is recommended to be extracted by pullout loading or vibrating.
2020, 28(3): 650-657.
The stability of single and double side slopes is studied with the strength reduction method and the finite element analysis software ABAQUS. Firstly,the feasibility of the method is validated with typical examples. Then the stability of single and double side soil slopes with different slope parameters,including slope angle and height,soil cohesive strength and friction angle,is analyzed by the model simulation. The influencing characteristics and laws of the slope parameters are discussed on the factor of safety for the two kinds of slopes. The occurrence and expansion of soil plastic strain zone are also displayed. The following results are found. (1)The variation of the factor with single slope parameter,slope angle,slope height or soil cohesive strength,soil friction angle is basically consistent and approximately linear. The influence laws on the stability of single and double side slopes are the same. (2)The stability of the double side slope is lower than that of the single side slope. But when the geometric and physical parameters of the slope are within a certain range,the two tend to be consistent. (3)It does not appear at the same time between the inflection point of displacement curve at the top of the slope and the plastic zone of the slope. There are differences between the different criteria for evaluating the stability of the soil slope. It is suggested that double side slope or even more complex slope analysis model should be used for slope stability evaluation in practical engineering,and different critical state criteria of slope stability should be used synthetically.
The stability of single and double side slopes is studied with the strength reduction method and the finite element analysis software ABAQUS. Firstly,the feasibility of the method is validated with typical examples. Then the stability of single and double side soil slopes with different slope parameters,including slope angle and height,soil cohesive strength and friction angle,is analyzed by the model simulation. The influencing characteristics and laws of the slope parameters are discussed on the factor of safety for the two kinds of slopes. The occurrence and expansion of soil plastic strain zone are also displayed. The following results are found. (1)The variation of the factor with single slope parameter,slope angle,slope height or soil cohesive strength,soil friction angle is basically consistent and approximately linear. The influence laws on the stability of single and double side slopes are the same. (2)The stability of the double side slope is lower than that of the single side slope. But when the geometric and physical parameters of the slope are within a certain range,the two tend to be consistent. (3)It does not appear at the same time between the inflection point of displacement curve at the top of the slope and the plastic zone of the slope. There are differences between the different criteria for evaluating the stability of the soil slope. It is suggested that double side slope or even more complex slope analysis model should be used for slope stability evaluation in practical engineering,and different critical state criteria of slope stability should be used synthetically.
2020, 28(3): 658-666.
The reinforcement of expansive soil slope has always been a difficult problem in railway(highway) construction. Conventional rigid structure strictly restricts the deformation of expansive soil,and easily lead to structural failure under excessive force. Instead,the flexible structure also may cause the slope to be damaged by greater deformation. Both of them can not achieve the expectation effect when they are used separately. This paper presents the geogrid-steel tube expansive soil retaining wall. It can coordinate the stress and deformation of the structure reasonably and reinforce the expansive soil slope. This paper uses the similar material model in large scale for the reinforcement test. Expansive soil is used as retaining wall filler. Geogrids and longitudinal steel tubes are used to enhance the integrity and stiffness of the wall. The reinforcement effects of the new and conventional structures are compared under simulated rainfall conditions. Furthermore,the reinforcement mechanism of geogrid-steel tube expansive soil retaining wall is studied,by means of monitoring the horizontal displacement of the retaining wall,the earth pressure on the back of the wall and the deformation of the steel tube. The results show that the deformation of geogrid-steel tube retaining wall is significantly reduced and more coordinated than that of conventional geogrid retaining wall. The bearing capacity of geogrid-steel tube retaining wall is significantly improved. The steel tubes penetrate the wall longitudinally and are rooted in the foundation,effectively improve the stress of the retaining wall. The research results can provide theoretical reference for the selection of reinforcement structure and its structural design of expansive soil slope.
The reinforcement of expansive soil slope has always been a difficult problem in railway(highway) construction. Conventional rigid structure strictly restricts the deformation of expansive soil,and easily lead to structural failure under excessive force. Instead,the flexible structure also may cause the slope to be damaged by greater deformation. Both of them can not achieve the expectation effect when they are used separately. This paper presents the geogrid-steel tube expansive soil retaining wall. It can coordinate the stress and deformation of the structure reasonably and reinforce the expansive soil slope. This paper uses the similar material model in large scale for the reinforcement test. Expansive soil is used as retaining wall filler. Geogrids and longitudinal steel tubes are used to enhance the integrity and stiffness of the wall. The reinforcement effects of the new and conventional structures are compared under simulated rainfall conditions. Furthermore,the reinforcement mechanism of geogrid-steel tube expansive soil retaining wall is studied,by means of monitoring the horizontal displacement of the retaining wall,the earth pressure on the back of the wall and the deformation of the steel tube. The results show that the deformation of geogrid-steel tube retaining wall is significantly reduced and more coordinated than that of conventional geogrid retaining wall. The bearing capacity of geogrid-steel tube retaining wall is significantly improved. The steel tubes penetrate the wall longitudinally and are rooted in the foundation,effectively improve the stress of the retaining wall. The research results can provide theoretical reference for the selection of reinforcement structure and its structural design of expansive soil slope.
2020, 28(3): 667-676.
Rockburst disasters during excavation of hard rock tunnels are closely related to the stress state of tunnel surrounding rock. Pressure relief blasting can reduce the stress concentration of surrounding rock mass,which is a direct and effective rockburst prevention and control measure. This paper takes the pressure relief blasting of the rock burst section of the Mountain Mi-Cang Tunnel of Bashan Expressway as an example and carries out the numerical simulation analysis of tunnel step-by-step excavation considering high geostress and rock heterogeneity conditions using GDEM-BlockDyna software. It then takes the pressure relief blasting as the only variable and carries out the numerical simulation comparison analysis of two sets of schemes(whether or not pressure relief blasting is carried out). It uses the microseismic monitoring data and tests the effect of pressure relief blasting on the surrounding rock of the tunnel. Under the condition of surrounding rock pressure relief blasting,the stress concentration of the surrounding rock and the degree of damage are reduced,and the rockburst risk is suppressed. Comparing the two sets of simulation results,the damage is concentrated on the vault and the arch bottom. The damage radius ratio(pressure relief/non-pressure relief) is 3.4 times at vault and 1.47 times at arch bottom. The damage volume ratio(pressure relief/non-pressure relief) is 5.36 times. But the maximum damage value of the area is reduced from 1((no pressure relief) to 0.6 pressure relief). The peak stress of the surrounding rock is reduced by about 5 MPa. The stress gradient is reduced by 0.8 MPa/m. These results indicate that the surrounding rock is still self-stability and the risk of rockburst is reduced. The relevant results can provide a theoretical basis for the implementation of pressure relief blasting and reasonable design of pressure relief blasting in the surrounding rock section of tunnel high ground stress.
Rockburst disasters during excavation of hard rock tunnels are closely related to the stress state of tunnel surrounding rock. Pressure relief blasting can reduce the stress concentration of surrounding rock mass,which is a direct and effective rockburst prevention and control measure. This paper takes the pressure relief blasting of the rock burst section of the Mountain Mi-Cang Tunnel of Bashan Expressway as an example and carries out the numerical simulation analysis of tunnel step-by-step excavation considering high geostress and rock heterogeneity conditions using GDEM-BlockDyna software. It then takes the pressure relief blasting as the only variable and carries out the numerical simulation comparison analysis of two sets of schemes(whether or not pressure relief blasting is carried out). It uses the microseismic monitoring data and tests the effect of pressure relief blasting on the surrounding rock of the tunnel. Under the condition of surrounding rock pressure relief blasting,the stress concentration of the surrounding rock and the degree of damage are reduced,and the rockburst risk is suppressed. Comparing the two sets of simulation results,the damage is concentrated on the vault and the arch bottom. The damage radius ratio(pressure relief/non-pressure relief) is 3.4 times at vault and 1.47 times at arch bottom. The damage volume ratio(pressure relief/non-pressure relief) is 5.36 times. But the maximum damage value of the area is reduced from 1((no pressure relief) to 0.6 pressure relief). The peak stress of the surrounding rock is reduced by about 5 MPa. The stress gradient is reduced by 0.8 MPa/m. These results indicate that the surrounding rock is still self-stability and the risk of rockburst is reduced. The relevant results can provide a theoretical basis for the implementation of pressure relief blasting and reasonable design of pressure relief blasting in the surrounding rock section of tunnel high ground stress.
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