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Editor Work2021 Vol. 29, No. 3
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2021, 29(3): 575-582.
Abstract
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Since the impoundment of the Three Gorges Reservoir in 2003, different types of landslides have taken place in the main stream and tributaries of Yangtze River. Based on the structure of reservoir bank slope, taking Qianjiangping landslide and Shuping landslide as examples, this paper attempts to analyze the deformation and failure mechanism of slopes with different structures under the condition of reservoir water level fluctuation through the field investigation, long-term observation and simple model tests. It aims to provide scientific insight for the prevention and control of landslide disasters in the operation process of reservoir area. The results show that the dip-structure slope is easy to lose its stability and cause rapid landsliding in the process of water level rising, while the loose structure slope is easy to produce obvious deformation in the process of water level drawn-down due to the influence of seepage force. The deformation rate is positively correlated with the speed of water level drawn-down.
Since the impoundment of the Three Gorges Reservoir in 2003, different types of landslides have taken place in the main stream and tributaries of Yangtze River. Based on the structure of reservoir bank slope, taking Qianjiangping landslide and Shuping landslide as examples, this paper attempts to analyze the deformation and failure mechanism of slopes with different structures under the condition of reservoir water level fluctuation through the field investigation, long-term observation and simple model tests. It aims to provide scientific insight for the prevention and control of landslide disasters in the operation process of reservoir area. The results show that the dip-structure slope is easy to lose its stability and cause rapid landsliding in the process of water level rising, while the loose structure slope is easy to produce obvious deformation in the process of water level drawn-down due to the influence of seepage force. The deformation rate is positively correlated with the speed of water level drawn-down.
2021, 29(3): 583-592.
The Jiweishan extra-large landslide is a unique large-scale rock slope instability mode in the karst mountain area of southwest China, and it has received great attention from engineering geology and other fields. A series of researches were carried out on the cause and mechanism of landslide, the starting and movement process of landslide, the distribution of deposits and secondary disasters, etc. Many results were obtained. For the structural plane and sliding plane of the landslide, it is considered that the vertical structural plane is composed of two groups, and the horizontal sliding plane is on the weak interlayer. This paper uses field geological survey, sample test and comprehensive analysis. It investigates the characteristics of karst and fissure development, the distribution of weak interlayer and its underlying nodular limestone, and the conditions of recharge and discharge of karst groundwater in landslide area. A large number of nodular limestone blocks are found in the landslide deposit. The relationship between karst and the vertical structural plane and horizontal sliding plane of the landslide are discussed. The main conclusions are as follows. (1)There are 3 groups of vertical structural planes. They are formed by 3 groups of structural fissures after long-term corrosion and erosion. (2)Part of the sliding plane passes through the weak interlayer, enters or passes through the nodular limestone, and the sliding plane is mainly controlled by bedding karst, weak cementation between rock layers, and weak interlayer. (3)The karst groundwater plays an expanding role on the vertical structural plane and the horizontal sliding plane in the process of landslide disaster. It has no hydrodynamic effect in the starting period of landslide. This study is of practical significance to the analysis and judgment of the genesis of Jiweishan landslide.
The Jiweishan extra-large landslide is a unique large-scale rock slope instability mode in the karst mountain area of southwest China, and it has received great attention from engineering geology and other fields. A series of researches were carried out on the cause and mechanism of landslide, the starting and movement process of landslide, the distribution of deposits and secondary disasters, etc. Many results were obtained. For the structural plane and sliding plane of the landslide, it is considered that the vertical structural plane is composed of two groups, and the horizontal sliding plane is on the weak interlayer. This paper uses field geological survey, sample test and comprehensive analysis. It investigates the characteristics of karst and fissure development, the distribution of weak interlayer and its underlying nodular limestone, and the conditions of recharge and discharge of karst groundwater in landslide area. A large number of nodular limestone blocks are found in the landslide deposit. The relationship between karst and the vertical structural plane and horizontal sliding plane of the landslide are discussed. The main conclusions are as follows. (1)There are 3 groups of vertical structural planes. They are formed by 3 groups of structural fissures after long-term corrosion and erosion. (2)Part of the sliding plane passes through the weak interlayer, enters or passes through the nodular limestone, and the sliding plane is mainly controlled by bedding karst, weak cementation between rock layers, and weak interlayer. (3)The karst groundwater plays an expanding role on the vertical structural plane and the horizontal sliding plane in the process of landslide disaster. It has no hydrodynamic effect in the starting period of landslide. This study is of practical significance to the analysis and judgment of the genesis of Jiweishan landslide.
2021, 29(3): 593-601.
The mechanical properties and stability of shallow rock mass in Three Gorges Reservoir area are significantly affected by climate and environment conditions. This paper examine the deterioration effect of cyclic temperature and humidity climatic environment due to diurnal variation on the weathering damage and mechanical properties of silty mudstone. The standard rock samples processed from silty mudstone are subjected to different numbers of high-low temperature and humidity cycles. Then the mechanical properties under uniaxial compression are tested to analyze the stress-strain curve, strength, deformation, fracture properties and energy characteristics. The results show that, with the number of temperature and humidity cycles increasing from 0 to 5, the uniaxial compression strength, elastic modulus, deformation modulus, total energy, elastic energy and its proportion of silty mudstone are linearly and negatively correlated with the number of temperature and humidity cycles. The values are decline by 49.21%, 53.11%, 57.78%, 42.29%, 58.7% and 32.45%, respectively. The dissipated energy and its proportion are positively correlated with the cyclic number, and their growths are 80.86% and 215.27%, respectively. On the mesoscopic scale, the bonding force between silty mudstone particles after cyclic temperature and humidity treatments is weakened, leading to the decrease of strength, the increase of brittleness and the reduction of viscosity between particles. The amplitude of variation is positively correlated with the number of temperature and humidity cycles. Meanwhile, the reduction amplitude of bonding force between particles is not uniformly distributed in the rock mass, but shows a certain gradient variation from surface to interior, and the closer to the surface, the greater the reduction amplitude. These changes make it is easier to produce tensile airfoil cracks or oblique tensile-shear mixed airfoil cracks due to transverse expansion of silty mudstone specimen under uniaxial compression. The specimen is even more incomplete. On the macroscopic scale, the silty mudstone shows lower brittleness and higher plasticity after cyclic temperature and humidity treatment.
The mechanical properties and stability of shallow rock mass in Three Gorges Reservoir area are significantly affected by climate and environment conditions. This paper examine the deterioration effect of cyclic temperature and humidity climatic environment due to diurnal variation on the weathering damage and mechanical properties of silty mudstone. The standard rock samples processed from silty mudstone are subjected to different numbers of high-low temperature and humidity cycles. Then the mechanical properties under uniaxial compression are tested to analyze the stress-strain curve, strength, deformation, fracture properties and energy characteristics. The results show that, with the number of temperature and humidity cycles increasing from 0 to 5, the uniaxial compression strength, elastic modulus, deformation modulus, total energy, elastic energy and its proportion of silty mudstone are linearly and negatively correlated with the number of temperature and humidity cycles. The values are decline by 49.21%, 53.11%, 57.78%, 42.29%, 58.7% and 32.45%, respectively. The dissipated energy and its proportion are positively correlated with the cyclic number, and their growths are 80.86% and 215.27%, respectively. On the mesoscopic scale, the bonding force between silty mudstone particles after cyclic temperature and humidity treatments is weakened, leading to the decrease of strength, the increase of brittleness and the reduction of viscosity between particles. The amplitude of variation is positively correlated with the number of temperature and humidity cycles. Meanwhile, the reduction amplitude of bonding force between particles is not uniformly distributed in the rock mass, but shows a certain gradient variation from surface to interior, and the closer to the surface, the greater the reduction amplitude. These changes make it is easier to produce tensile airfoil cracks or oblique tensile-shear mixed airfoil cracks due to transverse expansion of silty mudstone specimen under uniaxial compression. The specimen is even more incomplete. On the macroscopic scale, the silty mudstone shows lower brittleness and higher plasticity after cyclic temperature and humidity treatment.
2021, 29(3): 602-616.
In the Three Gorges Reservoir area, there are many anti-dip slopes with soft-hard interbedded structures in the Gongjiafang-Dulong section of Yangtze river, and the phenomena of flexural toppling are developed. This paper explores the mechanism of flexural toppling, as well as the effect of slope morphology, structure, and mechanical parameters of rock mass on the toppling. Based on the centrifugal model test and discrete element numerical simulation, this paper analyzes the toppling process of soft-hard interbedded toppling slopes and the failure mechanism under external disturbance. The single factor analysis method and orthogonal design method are combined to study the influence factors of toppling. And the sensitivity of each factor is obtained by range analysis. The random fractures are preset in the rock strata to get the initiation and extension laws of the fracture surfaces. The results show that the centrifugal model test and the discrete element simulation can well reproduce the toppling process, and the coincidence between them is good. The toppling process can be divided into three stages: initial deformation, steady deformation, and unstable failure. After the rock bending accumulates to a certain extent, the arc fracture surfaces form rapidly. The slope can form a multi-level fracture from bottom to top. Under the external disturbance, the toppling part can gradually form progressive backward collapse along different fracture surfaces from the outside to the inside. The stability of the anti-dip slope is mainly affected by the slope shape and structure. The sensitivity of influence factors is as follows: slope angle>dip angle>slope height>slope shape>layer thickness ratio>layer thickness. The larger the slope angle, dip angle and slope height are, the more convex the slope shape is, the smaller the thickness difference between soft and hard strata is, and the easier the slope is to be damaged. The slope damage scale is mainly controlled by the dip and slope angle. The research results have guiding significance for the reinforcement and treatment of such slopes.
In the Three Gorges Reservoir area, there are many anti-dip slopes with soft-hard interbedded structures in the Gongjiafang-Dulong section of Yangtze river, and the phenomena of flexural toppling are developed. This paper explores the mechanism of flexural toppling, as well as the effect of slope morphology, structure, and mechanical parameters of rock mass on the toppling. Based on the centrifugal model test and discrete element numerical simulation, this paper analyzes the toppling process of soft-hard interbedded toppling slopes and the failure mechanism under external disturbance. The single factor analysis method and orthogonal design method are combined to study the influence factors of toppling. And the sensitivity of each factor is obtained by range analysis. The random fractures are preset in the rock strata to get the initiation and extension laws of the fracture surfaces. The results show that the centrifugal model test and the discrete element simulation can well reproduce the toppling process, and the coincidence between them is good. The toppling process can be divided into three stages: initial deformation, steady deformation, and unstable failure. After the rock bending accumulates to a certain extent, the arc fracture surfaces form rapidly. The slope can form a multi-level fracture from bottom to top. Under the external disturbance, the toppling part can gradually form progressive backward collapse along different fracture surfaces from the outside to the inside. The stability of the anti-dip slope is mainly affected by the slope shape and structure. The sensitivity of influence factors is as follows: slope angle>dip angle>slope height>slope shape>layer thickness ratio>layer thickness. The larger the slope angle, dip angle and slope height are, the more convex the slope shape is, the smaller the thickness difference between soft and hard strata is, and the easier the slope is to be damaged. The slope damage scale is mainly controlled by the dip and slope angle. The research results have guiding significance for the reinforcement and treatment of such slopes.
2021, 29(3): 617-627.
Geological disasters occur frequently in the Three Gorges Reservoir Area(TGRA). Colluvial landslide is the most common type of landslides with the largest total amount. Information of 145 colluvial landslides on the reservoir bank in TGRA are systematically collected. This paper selects landslide topography, geological lithology, and slope structure as the controlling factors, and precipitation and reservoir water fluctuation as the main inducing factors. It characterizes the distribution and deformation and failure response of the landslides under these factors. The mechanisms of landsliding are analyzed. The main conclusions are as follows. (1)Under the dominance of regional tectonics and bedrock stratigraphic lithology, the occurrence frequency and scale of landslides have obvious spatial differences along Yangtze river. (2)Lithology combinations of sandstone and mudstone intercalated with shale and coal seams(SC), and marl and shale intercalated with mudstone(MSM) are the most harmful to landslide stability. The existences of soft rock,"soft-hard" interbedded dual structure, and water-rock(soil) interaction are the main influencing factors that dominate the development of landslides. (3)Most of the landslides are wade into river, locate on the slopes of 10°~30°, and have the elevation of the front edge mainly concentrated at 100~175 m above sea level. These slopes are seriously affected by the fluctuation of reservoir water. The river sides and slope structures have no obvious effect on the colluvial landslide development. (4)The landslides are mainly caused by the joint influences of rainfall and reservoir water drew-down. The joint influences can result in the slip-cracking deformation at the front of sliding body, which triggers retrogressive landslides. There are obvious differences in the pattern and extent of the impact of rainfall and reservoir water fluctuations on landslides. The research results are expected to provide scientific guidance for the overall targeted landslide prevention and control in the TGRA.
Geological disasters occur frequently in the Three Gorges Reservoir Area(TGRA). Colluvial landslide is the most common type of landslides with the largest total amount. Information of 145 colluvial landslides on the reservoir bank in TGRA are systematically collected. This paper selects landslide topography, geological lithology, and slope structure as the controlling factors, and precipitation and reservoir water fluctuation as the main inducing factors. It characterizes the distribution and deformation and failure response of the landslides under these factors. The mechanisms of landsliding are analyzed. The main conclusions are as follows. (1)Under the dominance of regional tectonics and bedrock stratigraphic lithology, the occurrence frequency and scale of landslides have obvious spatial differences along Yangtze river. (2)Lithology combinations of sandstone and mudstone intercalated with shale and coal seams(SC), and marl and shale intercalated with mudstone(MSM) are the most harmful to landslide stability. The existences of soft rock,"soft-hard" interbedded dual structure, and water-rock(soil) interaction are the main influencing factors that dominate the development of landslides. (3)Most of the landslides are wade into river, locate on the slopes of 10°~30°, and have the elevation of the front edge mainly concentrated at 100~175 m above sea level. These slopes are seriously affected by the fluctuation of reservoir water. The river sides and slope structures have no obvious effect on the colluvial landslide development. (4)The landslides are mainly caused by the joint influences of rainfall and reservoir water drew-down. The joint influences can result in the slip-cracking deformation at the front of sliding body, which triggers retrogressive landslides. There are obvious differences in the pattern and extent of the impact of rainfall and reservoir water fluctuations on landslides. The research results are expected to provide scientific guidance for the overall targeted landslide prevention and control in the TGRA.
2021, 29(3): 628-639.
The development degree of macro cracks in rock mass has an important influence on the stability of karst bank slope in reservoir area. This paper aims to effectively detect and identify the geometric shape and distribution characteristics of macro cracks in rock mass. It adopts and simplifies the model of vertical cracks in rock mass in Wuxia typical karst bank slope of Three Gorges Reservoir area. It is based on the non-destructive GPR geophysical technology and uses theoretical analysis and numerical simulation. It studies the variation law of time distance curve and amplitude characteristics of cracks in radar echo profile when the width, length and inclination of vertical cracks in rock mass change. The results show that the radar echo response at the top and bottom of the fracture is hyperbolic, which can be used as a sign of fracture identification. The width of fracture is closely related to the maximum amplitude and energy mass of radar echo. The angle of fracture can affect the shape of radar echo energy mass. The field test results show that the ground penetrating radar with 900MHz antenna frequency can be used to detect the distribution of vertical macro cracks with centimeter width within the depth of about 1.5 meters inside the karst bank slope, and can be used as a monitoring means to investigate the macro crack propagation inside the rock mass of the bank slope in the reservoir water area.
The development degree of macro cracks in rock mass has an important influence on the stability of karst bank slope in reservoir area. This paper aims to effectively detect and identify the geometric shape and distribution characteristics of macro cracks in rock mass. It adopts and simplifies the model of vertical cracks in rock mass in Wuxia typical karst bank slope of Three Gorges Reservoir area. It is based on the non-destructive GPR geophysical technology and uses theoretical analysis and numerical simulation. It studies the variation law of time distance curve and amplitude characteristics of cracks in radar echo profile when the width, length and inclination of vertical cracks in rock mass change. The results show that the radar echo response at the top and bottom of the fracture is hyperbolic, which can be used as a sign of fracture identification. The width of fracture is closely related to the maximum amplitude and energy mass of radar echo. The angle of fracture can affect the shape of radar echo energy mass. The field test results show that the ground penetrating radar with 900MHz antenna frequency can be used to detect the distribution of vertical macro cracks with centimeter width within the depth of about 1.5 meters inside the karst bank slope, and can be used as a monitoring means to investigate the macro crack propagation inside the rock mass of the bank slope in the reservoir water area.
2021, 29(3): 640-646.
Anti-slide pile is one of the common engineering measures in landslide treatment. In order to improve the reinforcement effect and optimize the location of anti-slide piles, we take a typical reservoir bank landslide as an example, analyze the stress level of the landslide, and put forward a method for determining the location of the anti-slide pile reinforcement. The method is based on the stress level distribution characteristics of the sliding zone. The research results show that:(1)The distribution of high stress level area in landslide body is in good agreement with that of large displacement area and plastic zone. According to the distribution law of stress level, the failure type of landslide can be determined more accurately. (2)When the anti-slide pile is arranged in the area with high stress level in the sliding zone, it can effectively improve the plastic zone in the slope, improve the safety factor of the landslide, and achieve better reinforcement effect. (3)Compared with the existing method of determining the optimal reinforcement position of anti-slide pile based on deformation field distribution and plastic zone distribution, the proposed method based on the distribution characteristics of the stress level of sliding zone can accurately and quantitatively determine the optimal reinforcement position of the landslide, so as to give full play to anti-slide pile reinforcement. We hope that the relevant analysis methods and research results can provide a good reference for the optimization of the anti-slide pile reinforcement position.
Anti-slide pile is one of the common engineering measures in landslide treatment. In order to improve the reinforcement effect and optimize the location of anti-slide piles, we take a typical reservoir bank landslide as an example, analyze the stress level of the landslide, and put forward a method for determining the location of the anti-slide pile reinforcement. The method is based on the stress level distribution characteristics of the sliding zone. The research results show that:(1)The distribution of high stress level area in landslide body is in good agreement with that of large displacement area and plastic zone. According to the distribution law of stress level, the failure type of landslide can be determined more accurately. (2)When the anti-slide pile is arranged in the area with high stress level in the sliding zone, it can effectively improve the plastic zone in the slope, improve the safety factor of the landslide, and achieve better reinforcement effect. (3)Compared with the existing method of determining the optimal reinforcement position of anti-slide pile based on deformation field distribution and plastic zone distribution, the proposed method based on the distribution characteristics of the stress level of sliding zone can accurately and quantitatively determine the optimal reinforcement position of the landslide, so as to give full play to anti-slide pile reinforcement. We hope that the relevant analysis methods and research results can provide a good reference for the optimization of the anti-slide pile reinforcement position.
2021, 29(3): 647-656.
With the long-term operation of the Three Gorges Reservoir, owing to the fluctuation of water level and wave action, the bank collapse in the Three Gorges Reservoir area becomes more and more evident. Bank collapse not only causes soil loss, but also induces the landslide occurrence. Daping landslide is one of the landslides in Three Gorges Reservoir area. Its bank collapse development is serious. This paper combines the surface deformation with GPS displacement data of Daping landslide. It analyzes the characteristics of landslide deformation and the impact of bank collapse on landslide deformation. On this basis, it uses the model change method to realize the numerical simulation of the influence of the front bank collapse on the stability of the landslide, and further explores the response relationship of the landslide deformation to the front bank collapse. The results show the follows. The deformation of Daping landslide is mainly affected by the fluctuation of the reservoir water level. The bank collapse of the front edge is relatively developed. The surface displacement of GPS monitoring point on the side of the bank collapse has evident changes. Evidently, the bank collapse has a greater impact on the surface deformation of Daping landslide. The ABAQUS finite element calculation results show that the bank collapse has a significant impact on the landslide deformation, and the impact magnitude is mainly related to the amount of bank collapse. The landslide deformation caused by bank collapse is mainly concentrated in the front part of the landslide and gradually expands to the rear part. The model change method in ABAQUS can well simulate the influence of bank collapse on the landslide stability.
With the long-term operation of the Three Gorges Reservoir, owing to the fluctuation of water level and wave action, the bank collapse in the Three Gorges Reservoir area becomes more and more evident. Bank collapse not only causes soil loss, but also induces the landslide occurrence. Daping landslide is one of the landslides in Three Gorges Reservoir area. Its bank collapse development is serious. This paper combines the surface deformation with GPS displacement data of Daping landslide. It analyzes the characteristics of landslide deformation and the impact of bank collapse on landslide deformation. On this basis, it uses the model change method to realize the numerical simulation of the influence of the front bank collapse on the stability of the landslide, and further explores the response relationship of the landslide deformation to the front bank collapse. The results show the follows. The deformation of Daping landslide is mainly affected by the fluctuation of the reservoir water level. The bank collapse of the front edge is relatively developed. The surface displacement of GPS monitoring point on the side of the bank collapse has evident changes. Evidently, the bank collapse has a greater impact on the surface deformation of Daping landslide. The ABAQUS finite element calculation results show that the bank collapse has a significant impact on the landslide deformation, and the impact magnitude is mainly related to the amount of bank collapse. The landslide deformation caused by bank collapse is mainly concentrated in the front part of the landslide and gradually expands to the rear part. The model change method in ABAQUS can well simulate the influence of bank collapse on the landslide stability.
2021, 29(3): 657-667.
This paper take the Jiaodingfeng Ⅱ landslide in Wushan County of the Three Gorges Reservoir area as an example and combines the reservoir water level changes and geological environment conditions data. It analyzes the deformation characteristics,formation mechanism and development trend of the landslide in detail. It uses field survey and mapping,UAV,engineering geological drilling,surface and deep displacement monitoring and other techniques. The volume of the landslide is about 250×104 m3. It is a large monoclinal neogenic bedrock landslide. The height difference between the front and rear edges of the landslide is about 380 m. The leading-edge shear outlet is high and steep and is located below the reservoir water level. The deep monitoring data show that there are two sliding zone in the water-level-fluctuation zone. The two zones are located between 12.0~17.0 m and 25.0~30.0 m from the slope. At present,the landslide is in the creep deformation stage. Under the influence of landform,stratigraphic structure,deterioration of the water-fluctuation zone and water,the vertical displacement continues to increase. The front deterioration zone has a high possibility of sliding failure and instability,and there is a risk of landslide surge disaster chain. It is suggested to carry out continuous observation and research on the deterioration system of the fluctuation zone in the reservoir area to improve the ability of identification and early warning of new landslide disasters in the reservoir area.
This paper take the Jiaodingfeng Ⅱ landslide in Wushan County of the Three Gorges Reservoir area as an example and combines the reservoir water level changes and geological environment conditions data. It analyzes the deformation characteristics,formation mechanism and development trend of the landslide in detail. It uses field survey and mapping,UAV,engineering geological drilling,surface and deep displacement monitoring and other techniques. The volume of the landslide is about 250×104 m3. It is a large monoclinal neogenic bedrock landslide. The height difference between the front and rear edges of the landslide is about 380 m. The leading-edge shear outlet is high and steep and is located below the reservoir water level. The deep monitoring data show that there are two sliding zone in the water-level-fluctuation zone. The two zones are located between 12.0~17.0 m and 25.0~30.0 m from the slope. At present,the landslide is in the creep deformation stage. Under the influence of landform,stratigraphic structure,deterioration of the water-fluctuation zone and water,the vertical displacement continues to increase. The front deterioration zone has a high possibility of sliding failure and instability,and there is a risk of landslide surge disaster chain. It is suggested to carry out continuous observation and research on the deterioration system of the fluctuation zone in the reservoir area to improve the ability of identification and early warning of new landslide disasters in the reservoir area.
2021, 29(3): 668-679.
The periodic water level regulation between 145~175 m above sea level in Three Gorges Reservoir area promotes the deterioration damage and stability attenuation of the limestone bank slope in the reservoir area. This paper is based on the example of "sliding-bending" failure of bedding limestone bank slope in Wuxia section and the model of elastic-plastic plate warping. It introduces the concept of rock mass deterioration and deduces the equilibrium equation of critical flexural section. Compared with the traditional static calculation method,the proposed method has wider applicability and can consider the dynamic evolution process of bank slope with deterioration time t. The proposed method is more in line with the engineering practice. The calculation results show that the critical deflection section lcr gradually decreases with deterioration,and the stability of bank slope becomes worse. The sliding thrust F gradually increases and bending stiffness K gradually decreases. These two factors lead to the rapid decrease of the critical flexural segment lcr. Since the rock mass deterioration GSI(t) is an exponential deterioration form,lcr presents that decreases rapidly and gradually slows down to a certain value,which can be defined as the"key section" of the bank slope treatment. With the increase of dip angle α,lcr and relative plate length ratio RPL decrease rapidly. With the increase of hi,lcr and RPL gradually increase. The influence of rock dip α on lcr and RPL is obviously greater than the rock thickness hi. Moreover,the rapid descent angle α of bank slope lcr is about 40°~60°,which can be used as a geological criterion for similarly bedding bank slopes that are prone to sliding-bending instability. The critical balance equation deduced in this paper considering the deterioration of the rock mass is calculated and discussed through the Qingshi 6# bank slope of the Three Gorges Reservoir. It can provide references for the damage evolution calculation of other similar bedding slopes. The difference is to determine the GSI(t)function of the specific slope damage deterioration.
The periodic water level regulation between 145~175 m above sea level in Three Gorges Reservoir area promotes the deterioration damage and stability attenuation of the limestone bank slope in the reservoir area. This paper is based on the example of "sliding-bending" failure of bedding limestone bank slope in Wuxia section and the model of elastic-plastic plate warping. It introduces the concept of rock mass deterioration and deduces the equilibrium equation of critical flexural section. Compared with the traditional static calculation method,the proposed method has wider applicability and can consider the dynamic evolution process of bank slope with deterioration time t. The proposed method is more in line with the engineering practice. The calculation results show that the critical deflection section lcr gradually decreases with deterioration,and the stability of bank slope becomes worse. The sliding thrust F gradually increases and bending stiffness K gradually decreases. These two factors lead to the rapid decrease of the critical flexural segment lcr. Since the rock mass deterioration GSI(t) is an exponential deterioration form,lcr presents that decreases rapidly and gradually slows down to a certain value,which can be defined as the"key section" of the bank slope treatment. With the increase of dip angle α,lcr and relative plate length ratio RPL decrease rapidly. With the increase of hi,lcr and RPL gradually increase. The influence of rock dip α on lcr and RPL is obviously greater than the rock thickness hi. Moreover,the rapid descent angle α of bank slope lcr is about 40°~60°,which can be used as a geological criterion for similarly bedding bank slopes that are prone to sliding-bending instability. The critical balance equation deduced in this paper considering the deterioration of the rock mass is calculated and discussed through the Qingshi 6# bank slope of the Three Gorges Reservoir. It can provide references for the damage evolution calculation of other similar bedding slopes. The difference is to determine the GSI(t)function of the specific slope damage deterioration.
2021, 29(3): 680-692.
This paper further summarizes the deformation characteristics and mechanism of geological hazards in Three Gorges Reservoir Area. It bases on the professional monitoring data of 189 geological hazards. It deeply studies the displacement-time curve of geological hazards. The results show the following findings. (1)From 2017 to 2019,the deformation situation of the geological hazards generally tended to be gentle. The deformation mainly occurred from May to September,and rainfall became the main inducing factor. According to the annual deformation and the macroscopic deformation of geological hazards,it can be divided into four categories: non-deformation or micro-deformation,slow deformation,relatively obvious deformation,obvious deformation. (2)According to the cumulative displacement-time curve characteristics of GNSS,the deformation characteristics of geological hazards can be divided into three basic types: Oscillatory type,linear type and step-like type. The displacement of step-like landslides is the main deformation form of geological hazards with obvious deformation and relatively obvious deformation,which is the focus of geological hazards. (3)Through the analysis of deformation characteristics and trigger factors,eight kinds of step-like type deformation characteristics of geological hazards during the operation period of the Three Gorges Reservoir are summarized. The deformation modes under the action of different trigger factors are put forward. The differences of the cumulative displacement-time curve in four aspects,such as the morphological characteristics of the deformation of step-like,the height,the occurrence time and the repetition regularity are concluded. The paper can provide a basis for deformation analysis and early warning of geological disasters in the Three Gorges Reservoir area.
This paper further summarizes the deformation characteristics and mechanism of geological hazards in Three Gorges Reservoir Area. It bases on the professional monitoring data of 189 geological hazards. It deeply studies the displacement-time curve of geological hazards. The results show the following findings. (1)From 2017 to 2019,the deformation situation of the geological hazards generally tended to be gentle. The deformation mainly occurred from May to September,and rainfall became the main inducing factor. According to the annual deformation and the macroscopic deformation of geological hazards,it can be divided into four categories: non-deformation or micro-deformation,slow deformation,relatively obvious deformation,obvious deformation. (2)According to the cumulative displacement-time curve characteristics of GNSS,the deformation characteristics of geological hazards can be divided into three basic types: Oscillatory type,linear type and step-like type. The displacement of step-like landslides is the main deformation form of geological hazards with obvious deformation and relatively obvious deformation,which is the focus of geological hazards. (3)Through the analysis of deformation characteristics and trigger factors,eight kinds of step-like type deformation characteristics of geological hazards during the operation period of the Three Gorges Reservoir are summarized. The deformation modes under the action of different trigger factors are put forward. The differences of the cumulative displacement-time curve in four aspects,such as the morphological characteristics of the deformation of step-like,the height,the occurrence time and the repetition regularity are concluded. The paper can provide a basis for deformation analysis and early warning of geological disasters in the Three Gorges Reservoir area.
2021, 29(3): 693-701.
To study the damage evolution characteristics of Jurassic soft rock at different reservoir water depth,the MTS triaxial compression and acoustic emission test of Jurassic argillaceous siltstone in Three Gorges Reservoir area is carried out under different water pressure states. The damage evolution equation considering initial damage caused by water is established based on acoustic emission ringing count. The mechanical deterioration characteristics,damage evolution stage and damage evolution characteristic value of Jurassic soft rock under water pressure are analyzed. The results show that,with the increase of water pressure(from 0MPa to 1MPa),the crack initial stress,damage stress,peak stress,residual stress and elastic modulus of soft rock all show a decreasing trend and decrease by 74.2%,66.9%,62.4%,43.4% and 51.9% respectively. The damage evolution process of Jurassic soft rock can be divided into damage formation stage,damage stability development stage,damage failure stage and post-damage failure stage. The damage evolution stage and crack development stage are basically synchronous. The damage evolution characteristic value can quantitatively reflect the influence of water pressure on Jurassic soft rock deterioration degree. These parameters have a good mathematical correlation with water pressure. The initial damage variable and crack initiation damage variable increase with the increase of water pressure. The crack initiation damage increment and the crack initiation damage strain decrease with the increase of water pressure. In the process of increasing water pressure,the values of initial damage variables and crack initiation damage variables are approaching gradually,the crack initiation damage increment and the crack initiation damage strain are gradually approaching to 0. These result show that the reservoir water pressure can cause different damage degrees to the Jurassic soft rock and can accelerate the crack development and failure process of the rock.
To study the damage evolution characteristics of Jurassic soft rock at different reservoir water depth,the MTS triaxial compression and acoustic emission test of Jurassic argillaceous siltstone in Three Gorges Reservoir area is carried out under different water pressure states. The damage evolution equation considering initial damage caused by water is established based on acoustic emission ringing count. The mechanical deterioration characteristics,damage evolution stage and damage evolution characteristic value of Jurassic soft rock under water pressure are analyzed. The results show that,with the increase of water pressure(from 0MPa to 1MPa),the crack initial stress,damage stress,peak stress,residual stress and elastic modulus of soft rock all show a decreasing trend and decrease by 74.2%,66.9%,62.4%,43.4% and 51.9% respectively. The damage evolution process of Jurassic soft rock can be divided into damage formation stage,damage stability development stage,damage failure stage and post-damage failure stage. The damage evolution stage and crack development stage are basically synchronous. The damage evolution characteristic value can quantitatively reflect the influence of water pressure on Jurassic soft rock deterioration degree. These parameters have a good mathematical correlation with water pressure. The initial damage variable and crack initiation damage variable increase with the increase of water pressure. The crack initiation damage increment and the crack initiation damage strain decrease with the increase of water pressure. In the process of increasing water pressure,the values of initial damage variables and crack initiation damage variables are approaching gradually,the crack initiation damage increment and the crack initiation damage strain are gradually approaching to 0. These result show that the reservoir water pressure can cause different damage degrees to the Jurassic soft rock and can accelerate the crack development and failure process of the rock.
2021, 29(3): 702-710.
Rock mass in the natural state generally contain micro-cracks. As a result,mechanism for the initiation,propagation,development and penetration of cracks in rock mass has always been a hot research topic in rock mechanics. In addition,the spatial variability characteristics of physical and mechanical parameters of rock mass can also affect the strength of rock samples as well as the failure process. In this paper,a new random peridynamics method(RPD) is proposed to simulate the crack propagation and the development process of cracked rock mass under uniaxial compression. This method combines the use and advantages of the peridynamics method in simulating crack development and the random field method in characterizing the spatial variation of material parameters. The presented numerical framework can produce simulation results of well agreement with the real rock mass state. MATLAB is adopted to verify the accuracy of the inclined prefabricated crack modeling. The results show that the proposed method can simulate the crack development process well. In addition,it is proved that the prefabricated crack inclination has great influence on the subsequent crack development. Furthermore,the evolution velocity can be greatly underestimated if the spatial variability characteristic of rock mass material parameters is neglected. Therefore,it is of great importance to determine the reasonable values of random field parameters,especially the coefficient of variation and the horizontal fluctuation range,for determining the strength and damage of rock mass under uniaxial compression.
Rock mass in the natural state generally contain micro-cracks. As a result,mechanism for the initiation,propagation,development and penetration of cracks in rock mass has always been a hot research topic in rock mechanics. In addition,the spatial variability characteristics of physical and mechanical parameters of rock mass can also affect the strength of rock samples as well as the failure process. In this paper,a new random peridynamics method(RPD) is proposed to simulate the crack propagation and the development process of cracked rock mass under uniaxial compression. This method combines the use and advantages of the peridynamics method in simulating crack development and the random field method in characterizing the spatial variation of material parameters. The presented numerical framework can produce simulation results of well agreement with the real rock mass state. MATLAB is adopted to verify the accuracy of the inclined prefabricated crack modeling. The results show that the proposed method can simulate the crack development process well. In addition,it is proved that the prefabricated crack inclination has great influence on the subsequent crack development. Furthermore,the evolution velocity can be greatly underestimated if the spatial variability characteristic of rock mass material parameters is neglected. Therefore,it is of great importance to determine the reasonable values of random field parameters,especially the coefficient of variation and the horizontal fluctuation range,for determining the strength and damage of rock mass under uniaxial compression.
2021, 29(3): 711-723.
The purpose of this paper is to determine the relationship between macro and micro mechanical parameters of particle flow in unsaturated purple soil. We use three-dimensional particle flow software(PFC3D),combine the control variable method and Mohr-Coulomb failure criterion,and carry out numerical simulations of triaxial consolidation and undrained tests of unsaturated purple soils under different water contents(8%、10%、12%、14%、16%、18%) and confining pressure conditions(100kPa、200kPa、300kPa、400kPa). We compare with the indoor lab test results. The results show that the cohesion of remolded purple soil first increases and then decreases with the increase of water content,and there is a critical water content of 12%. The internal friction angle is negatively correlated with water content. The macroscopic strength parameter cohesion of the purple soil is linearly positively correlated with the microscopic parameter tangential bond strength. The value of the internal friction angle is determined by the particle tangential bonding strength and the friction coefficient. The quantitative relationship between the water content of the purple soil and the shear bond strength and friction coefficient is established. The changes of cohesion and internal friction angle of purple soil under different water content are simulated by changing the shear bond strength and friction coefficient. The correctness of the relationship is verified by comparing the stress-strain curve of numerical simulation with that of laboratory test. The movement of triaxial micro particles and the distribution characteristics of indirect contact force of particles in purple soil can be observed by using the slicing tool built in PFC3D. It provides a reference for further study on shear strength characteristics and stress-strain characteristics of purple soil.
The purpose of this paper is to determine the relationship between macro and micro mechanical parameters of particle flow in unsaturated purple soil. We use three-dimensional particle flow software(PFC3D),combine the control variable method and Mohr-Coulomb failure criterion,and carry out numerical simulations of triaxial consolidation and undrained tests of unsaturated purple soils under different water contents(8%、10%、12%、14%、16%、18%) and confining pressure conditions(100kPa、200kPa、300kPa、400kPa). We compare with the indoor lab test results. The results show that the cohesion of remolded purple soil first increases and then decreases with the increase of water content,and there is a critical water content of 12%. The internal friction angle is negatively correlated with water content. The macroscopic strength parameter cohesion of the purple soil is linearly positively correlated with the microscopic parameter tangential bond strength. The value of the internal friction angle is determined by the particle tangential bonding strength and the friction coefficient. The quantitative relationship between the water content of the purple soil and the shear bond strength and friction coefficient is established. The changes of cohesion and internal friction angle of purple soil under different water content are simulated by changing the shear bond strength and friction coefficient. The correctness of the relationship is verified by comparing the stress-strain curve of numerical simulation with that of laboratory test. The movement of triaxial micro particles and the distribution characteristics of indirect contact force of particles in purple soil can be observed by using the slicing tool built in PFC3D. It provides a reference for further study on shear strength characteristics and stress-strain characteristics of purple soil.
2021, 29(3): 724-733.
Since the construction of the Three Gorges Project,the land use types in the Reservoir Area have changed greatly. In order to get better coordination between landslide development and land use change since the impoundment of the Three Gorges Reservoir,this paper studies the land use change of the Yangtze River mainstream from Zigui to Badong,where is the head area of the Three Gorges Reservoir,based on image data in the year of 1987,2000,and 2010. The Landslide Area Modulus Ratio(Rsi) is used for analyzing and valuating the relationship between landslide development and land use change. The research result shows that:(1)From 1987 to 2010,the vegetation coverage rate in the study area decreased continuously,while the residential building area and cultivated land area increased continuously. (2)The Rsi value increases in vegetation area,cultivated land and residential building area in turn,increasing with the more intensive human activities,which indicates that landslide development is positively correlated with human activity intensity; (3)According to Rsi value,the study area is divided into three sub areas: high,medium and low area,and landslides are mainly distributed in high and medium areas,which is in good agreement with the actual situation. Therefore,it is necessary to consider the change of land use when carrying out landslide risk assessment in the reservoir area. This research is expected to provide theoretical support for land use planning,disaster prevention and mitigation in the Three Gorges Reservoir Area.
Since the construction of the Three Gorges Project,the land use types in the Reservoir Area have changed greatly. In order to get better coordination between landslide development and land use change since the impoundment of the Three Gorges Reservoir,this paper studies the land use change of the Yangtze River mainstream from Zigui to Badong,where is the head area of the Three Gorges Reservoir,based on image data in the year of 1987,2000,and 2010. The Landslide Area Modulus Ratio(Rsi) is used for analyzing and valuating the relationship between landslide development and land use change. The research result shows that:(1)From 1987 to 2010,the vegetation coverage rate in the study area decreased continuously,while the residential building area and cultivated land area increased continuously. (2)The Rsi value increases in vegetation area,cultivated land and residential building area in turn,increasing with the more intensive human activities,which indicates that landslide development is positively correlated with human activity intensity; (3)According to Rsi value,the study area is divided into three sub areas: high,medium and low area,and landslides are mainly distributed in high and medium areas,which is in good agreement with the actual situation. Therefore,it is necessary to consider the change of land use when carrying out landslide risk assessment in the reservoir area. This research is expected to provide theoretical support for land use planning,disaster prevention and mitigation in the Three Gorges Reservoir Area.
2021, 29(3): 734-741.
Yanguan landslide was on the right bank of Xiangxi River in Guizhou Town,Zigui County and suffered from overall damage on October 30,2017. It became one of few failed landslides in the Three Gorges reservoir area in recent years. According to the site emergency investigation,the landslide went through the process of early warning to complete destruction in less than three days. In this paper,we use module SEEP/W in the finite element geotechnical simulation software Geo-Studio to simulate the seepage field of Yanguan landslide under the boundary conditions of actual rainfall infiltration and reservoir water level value. Next,we calculate the change of pore water pressure value with time. Then,we apply the results to another module Slope/W to calculate the curve of safe of factor. We propose the mode of landslide instability and failure under the condition of pore water pressure change rate combining with the principle of effective stress and take Yanguan landslide as an example to analyze the influence of pore pressure on landslide failure. Then,we verify the cause of failure that the pore water pressure soars after the soil inside the landslide is saturated under the action of combined effect,rainfall and high reservoir water level.
Yanguan landslide was on the right bank of Xiangxi River in Guizhou Town,Zigui County and suffered from overall damage on October 30,2017. It became one of few failed landslides in the Three Gorges reservoir area in recent years. According to the site emergency investigation,the landslide went through the process of early warning to complete destruction in less than three days. In this paper,we use module SEEP/W in the finite element geotechnical simulation software Geo-Studio to simulate the seepage field of Yanguan landslide under the boundary conditions of actual rainfall infiltration and reservoir water level value. Next,we calculate the change of pore water pressure value with time. Then,we apply the results to another module Slope/W to calculate the curve of safe of factor. We propose the mode of landslide instability and failure under the condition of pore water pressure change rate combining with the principle of effective stress and take Yanguan landslide as an example to analyze the influence of pore pressure on landslide failure. Then,we verify the cause of failure that the pore water pressure soars after the soil inside the landslide is saturated under the action of combined effect,rainfall and high reservoir water level.
2021, 29(3): 742-750.
The Baijiabao landslide is a representative of landslides with hysteretic"step-type" deformation. According to qualitative analysis, the reservoir water level decline is the main influencing factor of Baijiabao landslide deformation, and its impact degree is greater than rainfall. In order to clarify the response degree of Baijiabao landslide deformation to reservoir water level fluctuation and rainfall, this paper divides it into five stages according to the annual fluctuation of reservoir water level. The Pearson correlation coefficient method is used to quantitatively calculate the correlation on deformation of Baijiabao landslide between water level and rainfall. The calculation results show that the deformation of each monitoring point and the reservoir water level and rainfall are the most relevant in the comprehensive calculation from May to June. The absolute correlation coefficient between deformation and reservoir water level is about 0.75, and is greater than the maximum correlation coefficient between deformation and rainfall(about 0.45). In order to completely eliminate the interaction between reservoir water level and rainfall, the Pearson correlation coefficient is used to calculate the net correlation coefficient between Baijiabao landslide deformation and reservoir water level and rainfall. Finally, the net correlation coefficients are used to compare with the daily deformation displacement of the GPS automatic monitoring point in 2017-2018 to verify. The results show that the response of the deformation of the Baijiabao landslide to the reservoir water level is greater than to the rainfall. The response degree is the highest in the rapid decline of the reservoir water level, and it is consistent with the monitoring results of the newly built GPS automatic monitoring points, the results of qualitative analysis and Pearson correlation coefficient analysis. This study can provide a scientific basis for the quantitative analysis of the factors affecting landslide deformation.
The Baijiabao landslide is a representative of landslides with hysteretic"step-type" deformation. According to qualitative analysis, the reservoir water level decline is the main influencing factor of Baijiabao landslide deformation, and its impact degree is greater than rainfall. In order to clarify the response degree of Baijiabao landslide deformation to reservoir water level fluctuation and rainfall, this paper divides it into five stages according to the annual fluctuation of reservoir water level. The Pearson correlation coefficient method is used to quantitatively calculate the correlation on deformation of Baijiabao landslide between water level and rainfall. The calculation results show that the deformation of each monitoring point and the reservoir water level and rainfall are the most relevant in the comprehensive calculation from May to June. The absolute correlation coefficient between deformation and reservoir water level is about 0.75, and is greater than the maximum correlation coefficient between deformation and rainfall(about 0.45). In order to completely eliminate the interaction between reservoir water level and rainfall, the Pearson correlation coefficient is used to calculate the net correlation coefficient between Baijiabao landslide deformation and reservoir water level and rainfall. Finally, the net correlation coefficients are used to compare with the daily deformation displacement of the GPS automatic monitoring point in 2017-2018 to verify. The results show that the response of the deformation of the Baijiabao landslide to the reservoir water level is greater than to the rainfall. The response degree is the highest in the rapid decline of the reservoir water level, and it is consistent with the monitoring results of the newly built GPS automatic monitoring points, the results of qualitative analysis and Pearson correlation coefficient analysis. This study can provide a scientific basis for the quantitative analysis of the factors affecting landslide deformation.
2021, 29(3): 751-763.
Fault has significant impact on the hydraulic fracturing operations in shale gas reservoir, and even induces deep seismic events and near-surface environment issues. This article adopts the multi-physics coupling method and considered the coupling progress of seepage and stress. The appearance、extend and evolution of hydraulic damage area in the fault and closed caprock during hydraulic fracturing in shale reservoirs are simulated and analyzed. The mechanism of fluid migration and diffusion along high permeability channels is analyzed at last. The main conclusions can be drawn as follows: (1)Fault changes the shape of reservoir hydraulic damage area and expands the space of the hydraulic fracturing damage area. High water-injection pressure extends reservoir hydraulic damage area to the closed cap rock and bottom plate. The hydraulic damage area is obviously affected by the fault and developed along the fault rapidly. High water-injection pressure leads to a sharp increase in the height of fault hydraulic damage area, resulting in changes in reservoir sealing integrity. (2)Under the conditions of internal high-risk geological structures and high water-injection pressures in shale reservoirs, hydraulic fracturing operations produce micro-seismic events induced by rock rupture and local reactivation of fractures, but it is difficult to cause destructive seismic events. Most of seismic events are possibly induced by regional hydraulic coupling damage during reactivation of fault or large fractures. (3)The penetration of the hydraulic damage zone into the fault induces fluid migration along the fault zone. Considering the higher permeability in fault damage zone, the well-connected pathway of the hydraulic damage zone and the upper high-permeability rock formation can speed up the escape of fluid and increase the risk of fracturing fluid contaminating the upper formation. This leads to a decrease in fracturing efficiency and reduces the economic value of shale gas development.
Fault has significant impact on the hydraulic fracturing operations in shale gas reservoir, and even induces deep seismic events and near-surface environment issues. This article adopts the multi-physics coupling method and considered the coupling progress of seepage and stress. The appearance、extend and evolution of hydraulic damage area in the fault and closed caprock during hydraulic fracturing in shale reservoirs are simulated and analyzed. The mechanism of fluid migration and diffusion along high permeability channels is analyzed at last. The main conclusions can be drawn as follows: (1)Fault changes the shape of reservoir hydraulic damage area and expands the space of the hydraulic fracturing damage area. High water-injection pressure extends reservoir hydraulic damage area to the closed cap rock and bottom plate. The hydraulic damage area is obviously affected by the fault and developed along the fault rapidly. High water-injection pressure leads to a sharp increase in the height of fault hydraulic damage area, resulting in changes in reservoir sealing integrity. (2)Under the conditions of internal high-risk geological structures and high water-injection pressures in shale reservoirs, hydraulic fracturing operations produce micro-seismic events induced by rock rupture and local reactivation of fractures, but it is difficult to cause destructive seismic events. Most of seismic events are possibly induced by regional hydraulic coupling damage during reactivation of fault or large fractures. (3)The penetration of the hydraulic damage zone into the fault induces fluid migration along the fault zone. Considering the higher permeability in fault damage zone, the well-connected pathway of the hydraulic damage zone and the upper high-permeability rock formation can speed up the escape of fluid and increase the risk of fracturing fluid contaminating the upper formation. This leads to a decrease in fracturing efficiency and reduces the economic value of shale gas development.
2021, 29(3): 764-776.
The Zhouchangping large deep creeping ancient landslide is located on the left bank of the Minjiang River in Maoxian County, Sichuan Province. It had a large-scale rapid reactivation and dammed the Minjiang River in 1982. Based on field investigation and drilling analysis, the landslide is currently in deep-buried creeping deformation. Rainfall has an important influence on the deformation rate of the landslide. The moisture content of sliding zone soil and the sliding rate of Zhouchangping landslide have a great influence on its mechanical strength characteristics and the occurrence and development of the landslide. In order to study the influence of moisture content and sliding rate on the mechanical strength, this paper carried out ring shear tests of sliding zone soil with different moisture contents(8%, 15%, 25%) and different shear rates(0.1 mm·min-1, 5 mm·min-1, 100 mm·min-1). The results show that the shear strength of sliding zone soil decreases with the increase of moisture content, and the reduction is greater under high moisture content conditions. The strain softening phenomenon of the specimen becomes more obvious with the increase of shear rate, and its peak strength and residual strength generally increase first and then decrease. The internal friction angle of the strength parameter, linearly fitted by the peak strength and the residual strength, first increases and then decreases. Research suggests that in the case of rainfall conditions, high moisture content significantly reduces the shear strength of the sliding zone soil, which is likely to cause the Zhouchangping landslide to periodically creep. The accelerated sliding rate can again reduce the shear strength of the sliding zone soil, which can cause the landslide to reactive and block the Minjiang River as a whole again.
The Zhouchangping large deep creeping ancient landslide is located on the left bank of the Minjiang River in Maoxian County, Sichuan Province. It had a large-scale rapid reactivation and dammed the Minjiang River in 1982. Based on field investigation and drilling analysis, the landslide is currently in deep-buried creeping deformation. Rainfall has an important influence on the deformation rate of the landslide. The moisture content of sliding zone soil and the sliding rate of Zhouchangping landslide have a great influence on its mechanical strength characteristics and the occurrence and development of the landslide. In order to study the influence of moisture content and sliding rate on the mechanical strength, this paper carried out ring shear tests of sliding zone soil with different moisture contents(8%, 15%, 25%) and different shear rates(0.1 mm·min-1, 5 mm·min-1, 100 mm·min-1). The results show that the shear strength of sliding zone soil decreases with the increase of moisture content, and the reduction is greater under high moisture content conditions. The strain softening phenomenon of the specimen becomes more obvious with the increase of shear rate, and its peak strength and residual strength generally increase first and then decrease. The internal friction angle of the strength parameter, linearly fitted by the peak strength and the residual strength, first increases and then decreases. Research suggests that in the case of rainfall conditions, high moisture content significantly reduces the shear strength of the sliding zone soil, which is likely to cause the Zhouchangping landslide to periodically creep. The accelerated sliding rate can again reduce the shear strength of the sliding zone soil, which can cause the landslide to reactive and block the Minjiang River as a whole again.
2021, 29(3): 777-785.
Expansive soil is a kind of clay soil that expands rapidly after being immersed in water and shrinks significantly after losing water. The expansive soil slopes exposed to the atmosphere are in the process of continuous dry and wet cycles and are extremely unstable under the conditions of rainfall. Based on saturated-unsaturated seepage theory, the unsaturated seepage process and humidification process of the expansive soil slope under rainfall are simulated, and a related FORTRAN language program is developed. The effects of matrix suction changes, seepage softening, and moistening expansion during the seepage process are considered. The effects of strength attenuation, seepage softening, and moistening expansion on the overall stability of the expansive soil slope are analyzed. The results show that under the action of rainfall, the failure mode of the expansive soil slope after multiple dry-wet cycles is shallow collapse, and the failure surface is located in the weathered area, which is quite different from the traction landslide of the homogeneous expansive soil slope. After considering the moistening expansion and softening effects, the maximum displacement of the slope can be increased by an order of magnitude, and the safety factor can be decreased significantly. The research results well explain the special tractive and shallow properties of typical expansive soil landslides.
Expansive soil is a kind of clay soil that expands rapidly after being immersed in water and shrinks significantly after losing water. The expansive soil slopes exposed to the atmosphere are in the process of continuous dry and wet cycles and are extremely unstable under the conditions of rainfall. Based on saturated-unsaturated seepage theory, the unsaturated seepage process and humidification process of the expansive soil slope under rainfall are simulated, and a related FORTRAN language program is developed. The effects of matrix suction changes, seepage softening, and moistening expansion during the seepage process are considered. The effects of strength attenuation, seepage softening, and moistening expansion on the overall stability of the expansive soil slope are analyzed. The results show that under the action of rainfall, the failure mode of the expansive soil slope after multiple dry-wet cycles is shallow collapse, and the failure surface is located in the weathered area, which is quite different from the traction landslide of the homogeneous expansive soil slope. After considering the moistening expansion and softening effects, the maximum displacement of the slope can be increased by an order of magnitude, and the safety factor can be decreased significantly. The research results well explain the special tractive and shallow properties of typical expansive soil landslides.
2021, 29(3): 786-797.
The initiation mechanisms of post-fire debris flow can be divided into two categories: surface runoff initiated(SRI) and shallow landslides initiated(SLI). To date, most researches focus on SRI post-fire debris flow, which normally occurs within the first two years after forest fire, causing serious surface erosion and civil destructions. However, there are relatively few researches on the SLI post-fire debris flow after a long time of the occurrence of forest fire, which restricts practitioner's cognition of the starting conditions and initiation mechanism of SLI post-fire debris flow. Compared with SRI post-fire debris flow, SLI debris flow is mainly attributed to the root-soil strength degradation. Hence, quantitative determination of the long-term decline of root and soil strength by supplying new experimental data is a necessity to understand the initiation mechanism of SLI post-fire debris flow. To achieve the objective, the current research was carried out in a source area of SLI post-fire debris flow with a mixed conifer forest dominated by masson pine in Muli County, Liangshan Prefecture, Sichuan province. Root quantity statistics, root ultimate tensile strength test, triaxial test on root-soil mixtures, and slope stability analysis were conducted from the samples of three different timeslots after forest fire. The results indicate that the root quantity and ultimate tensile force decrease with the time after fire. In addition, root-soil mixture experimental triaxial results revealed that cohesion can drop dramatically 9 years after forest fire. Accordingly, factor of safety of local slope is calculated to be less than 1. The main reason for the occurrence of shallow landslide after fire is the decrease of shear strength of root-soil mixture caused by tree mortality and subsequent decay of tree root networks. The research results provide scientific data to support the risk assessment of SLI post-fire debris flow.
The initiation mechanisms of post-fire debris flow can be divided into two categories: surface runoff initiated(SRI) and shallow landslides initiated(SLI). To date, most researches focus on SRI post-fire debris flow, which normally occurs within the first two years after forest fire, causing serious surface erosion and civil destructions. However, there are relatively few researches on the SLI post-fire debris flow after a long time of the occurrence of forest fire, which restricts practitioner's cognition of the starting conditions and initiation mechanism of SLI post-fire debris flow. Compared with SRI post-fire debris flow, SLI debris flow is mainly attributed to the root-soil strength degradation. Hence, quantitative determination of the long-term decline of root and soil strength by supplying new experimental data is a necessity to understand the initiation mechanism of SLI post-fire debris flow. To achieve the objective, the current research was carried out in a source area of SLI post-fire debris flow with a mixed conifer forest dominated by masson pine in Muli County, Liangshan Prefecture, Sichuan province. Root quantity statistics, root ultimate tensile strength test, triaxial test on root-soil mixtures, and slope stability analysis were conducted from the samples of three different timeslots after forest fire. The results indicate that the root quantity and ultimate tensile force decrease with the time after fire. In addition, root-soil mixture experimental triaxial results revealed that cohesion can drop dramatically 9 years after forest fire. Accordingly, factor of safety of local slope is calculated to be less than 1. The main reason for the occurrence of shallow landslide after fire is the decrease of shear strength of root-soil mixture caused by tree mortality and subsequent decay of tree root networks. The research results provide scientific data to support the risk assessment of SLI post-fire debris flow.
2021, 29(3): 798-806.
Large and complex landslides are often rich in groundwater due to the hydraulic recharge of fault structures. Regional hydrogeological conditions mainly control the water supply of fault structure, and have the characteristics of stable water quality and quantity, which has a great and long-term influence on the stability of landslide. At present, there are few studies on the effect of fault structure on the water supply of landslide, and there is no systematic study on the type and formation conditions of fault structure on the water supply of landslide. This paper studies the typical example of water supply landslide in fault structure, according to the distribution location of the fault structure, the mechanical properties, the way to recharge the groundwater of the landslide and the hydraulic connection. Fault structure causes on the water supply types of the landslide are divided into three types: boundary supply type, sliding bed supply type and compound type. Four mechanisms of groundwater action on landslide are analyzed, including increasing pore water pressure in the sliding zone, lifting of confined water in the sliding bed, sliding zone clay minerals swell with water, and sliding zone chemical dissolution. The research results have important guiding value for revealing the law of groundwater migration, quantitatively evaluating the effect of groundwater on landslide, and reasonably determining the design of interception and drainage of groundwater.
Large and complex landslides are often rich in groundwater due to the hydraulic recharge of fault structures. Regional hydrogeological conditions mainly control the water supply of fault structure, and have the characteristics of stable water quality and quantity, which has a great and long-term influence on the stability of landslide. At present, there are few studies on the effect of fault structure on the water supply of landslide, and there is no systematic study on the type and formation conditions of fault structure on the water supply of landslide. This paper studies the typical example of water supply landslide in fault structure, according to the distribution location of the fault structure, the mechanical properties, the way to recharge the groundwater of the landslide and the hydraulic connection. Fault structure causes on the water supply types of the landslide are divided into three types: boundary supply type, sliding bed supply type and compound type. Four mechanisms of groundwater action on landslide are analyzed, including increasing pore water pressure in the sliding zone, lifting of confined water in the sliding bed, sliding zone clay minerals swell with water, and sliding zone chemical dissolution. The research results have important guiding value for revealing the law of groundwater migration, quantitatively evaluating the effect of groundwater on landslide, and reasonably determining the design of interception and drainage of groundwater.
2021, 29(3): 807-816.
On July 16, 2018, a large debris flow occurred in the Yunmeng Mountain region of Huairou district, Beijing. The event seriously destroyed two downstream highways of G111 and S310, forced an emergent transfer of 506 people in adjacent 4 towns. This paper interprets the high-resolution satellite images of pre-and post-event and in-situ investigation, and studies the geomorphologic and geologic characteristics of two typical debris flow gullies and the sources of solid materials, analyzes the rainfall condition of debris flow initiation, calculates the major parameters such as bulk density, flow velocity and total amount of the primary process, and deduces a quantitative relationship between the initiation of debris flow and the rainfall intensity on the basis of stability analysis of gully blocks. The results reveal that "7·16" event belongs to dilute debris flow triggered by the short-time heavy rainfall. Under continuous rainfall for 2 hours, the critical rainfall intensity that could trigger debris flow is no less than 8~15 mm·h-1. The solid materials of debris flow are mainly sourced from the gully-bed erosion and the lateral erosion. It suggests that debris flow of same type and scale can be very likely to take place under similar rainfall conditions. The study can provide a useful reference for future hazard prevention and mitigation.
On July 16, 2018, a large debris flow occurred in the Yunmeng Mountain region of Huairou district, Beijing. The event seriously destroyed two downstream highways of G111 and S310, forced an emergent transfer of 506 people in adjacent 4 towns. This paper interprets the high-resolution satellite images of pre-and post-event and in-situ investigation, and studies the geomorphologic and geologic characteristics of two typical debris flow gullies and the sources of solid materials, analyzes the rainfall condition of debris flow initiation, calculates the major parameters such as bulk density, flow velocity and total amount of the primary process, and deduces a quantitative relationship between the initiation of debris flow and the rainfall intensity on the basis of stability analysis of gully blocks. The results reveal that "7·16" event belongs to dilute debris flow triggered by the short-time heavy rainfall. Under continuous rainfall for 2 hours, the critical rainfall intensity that could trigger debris flow is no less than 8~15 mm·h-1. The solid materials of debris flow are mainly sourced from the gully-bed erosion and the lateral erosion. It suggests that debris flow of same type and scale can be very likely to take place under similar rainfall conditions. The study can provide a useful reference for future hazard prevention and mitigation.
2021, 29(3): 817-824.
The 3.0 km long highway from Zhangmu to Youyiqiao was destroyed by the mountain torrents and mud-rock flow in the upstream of Zhangmu town on July 5, 2016 after the "4.25" Nepal earthquake. In order to restore the highway as soon as possible, we conducted an in-depth research on the deformation history of Youyiqiao landslide group based on the monitoring data of deep displacement. It is concluded that the deformation rate of the landslide is closely related to the rainfall. The Youyiqiao landslide group is basically stable in the dry season. Its deformation starts with the increase of rainfall from June to July and intensifies in August. After entering September, the deformation converges rapidly with the decrease of rainfall. The earthquake, mountain torrent and mud-rock flow were the main inducing factors for the reactivation of landslides, and continuous rainfall intensified the activities of landslide group. Besides, in highway renovation, low frequency catastrophes would continuously threaten the highway with large-scale and ultra-deep landslides failed to be controlled permanently in a short term. So, we suggest that landslide control and highway traffic recovery should be our top priority currently, while in the long run, we should make further studies from policy-making in a macrocosmic sense relating to route selection along valleys and river banks.
The 3.0 km long highway from Zhangmu to Youyiqiao was destroyed by the mountain torrents and mud-rock flow in the upstream of Zhangmu town on July 5, 2016 after the "4.25" Nepal earthquake. In order to restore the highway as soon as possible, we conducted an in-depth research on the deformation history of Youyiqiao landslide group based on the monitoring data of deep displacement. It is concluded that the deformation rate of the landslide is closely related to the rainfall. The Youyiqiao landslide group is basically stable in the dry season. Its deformation starts with the increase of rainfall from June to July and intensifies in August. After entering September, the deformation converges rapidly with the decrease of rainfall. The earthquake, mountain torrent and mud-rock flow were the main inducing factors for the reactivation of landslides, and continuous rainfall intensified the activities of landslide group. Besides, in highway renovation, low frequency catastrophes would continuously threaten the highway with large-scale and ultra-deep landslides failed to be controlled permanently in a short term. So, we suggest that landslide control and highway traffic recovery should be our top priority currently, while in the long run, we should make further studies from policy-making in a macrocosmic sense relating to route selection along valleys and river banks.
2021, 29(3): 825-833.
The apparent friction coefficient(or the test factor in this paper),is expressed as the ratio(H/L) of the maximum vertical height(H)to the maximum horizontal landslide displacement(L). It is an important and straightforward variable for characterizing landslide mobility. The authors statistically analyze the distribution characteristics of influencing factors(landslide volume V,slope angle θ,maximum vertical movement distance of landslide H) and the test factor(H/L)in landslides in southeastern Tibet and landslides triggered by earthquakes. The authors further analyze the influence characteristics of each influencing factor on this test factor,and simulate the temporal and spatial evolution process of the landslide movement using a numerical simulation method based on the continuum mechanics method. The results indicate that θ is a significant influencing factor of the H/L and mainly concentrated in 20°~40°. The H/L value shows a decreasing trend with the decrease of θ and H as well as the increase of V. The H/L value of the southeast Tibet landslide is generally greater than that of seismic landslide. Seismic landslides have significant remote characteristics,and seismic action can weaken the correlation between θ,V,H and H/L. The movement speed of the landslide near the foot of the slope often reaches its peak,and the foot of the slope has a significant blocking effect on the movement of the landslide. The research results have reference significance for the study of landslide movement characteristics and disaster prevention and mitigation.
The apparent friction coefficient(or the test factor in this paper),is expressed as the ratio(H/L) of the maximum vertical height(H)to the maximum horizontal landslide displacement(L). It is an important and straightforward variable for characterizing landslide mobility. The authors statistically analyze the distribution characteristics of influencing factors(landslide volume V,slope angle θ,maximum vertical movement distance of landslide H) and the test factor(H/L)in landslides in southeastern Tibet and landslides triggered by earthquakes. The authors further analyze the influence characteristics of each influencing factor on this test factor,and simulate the temporal and spatial evolution process of the landslide movement using a numerical simulation method based on the continuum mechanics method. The results indicate that θ is a significant influencing factor of the H/L and mainly concentrated in 20°~40°. The H/L value shows a decreasing trend with the decrease of θ and H as well as the increase of V. The H/L value of the southeast Tibet landslide is generally greater than that of seismic landslide. Seismic landslides have significant remote characteristics,and seismic action can weaken the correlation between θ,V,H and H/L. The movement speed of the landslide near the foot of the slope often reaches its peak,and the foot of the slope has a significant blocking effect on the movement of the landslide. The research results have reference significance for the study of landslide movement characteristics and disaster prevention and mitigation.
2021, 29(3): 834-842.
Capillary action is one of significant sources of matric suction in the unsaturated soil. So we propose a new method based on two-dimensional Discontinuous Deformation Analysis(DDA)to simulate the distribution of capillary water and to calculate soil water characteristic curve(SWCC)under different saturation. Firstly,the radius of capillary water meniscus is calculated with iterative algorithm method,and circle center trajectory intersection method is used to determine capillary water distribution in this algorithm. Then,the matric suction,calculated with Young-Laplace equation,is taken into account in the control equation of the original DDA method with the surface tension. Moreover,in order to verify the accuracy of the algorithm,an ideal loess structure model is established. In this model,the soil particles used are mainly coarse particles with high content of loess. The particle size distribution and shape accord with the actual values. And then,the model is used to simulate the distribution of capillary water and to calculate soil water characteristic curve(SWCC). The results show that simulated SWCC contains boundary effect stage,transition stage and residual stage. The distribution of capillary water is simulated at each stage. In addition,the result comparison between the numerical simulation and the experimental data shows the simulation results agree well with experimental data,which suggest that this method is available for simulating capillary action in unsaturated soil.
Capillary action is one of significant sources of matric suction in the unsaturated soil. So we propose a new method based on two-dimensional Discontinuous Deformation Analysis(DDA)to simulate the distribution of capillary water and to calculate soil water characteristic curve(SWCC)under different saturation. Firstly,the radius of capillary water meniscus is calculated with iterative algorithm method,and circle center trajectory intersection method is used to determine capillary water distribution in this algorithm. Then,the matric suction,calculated with Young-Laplace equation,is taken into account in the control equation of the original DDA method with the surface tension. Moreover,in order to verify the accuracy of the algorithm,an ideal loess structure model is established. In this model,the soil particles used are mainly coarse particles with high content of loess. The particle size distribution and shape accord with the actual values. And then,the model is used to simulate the distribution of capillary water and to calculate soil water characteristic curve(SWCC). The results show that simulated SWCC contains boundary effect stage,transition stage and residual stage. The distribution of capillary water is simulated at each stage. In addition,the result comparison between the numerical simulation and the experimental data shows the simulation results agree well with experimental data,which suggest that this method is available for simulating capillary action in unsaturated soil.
2021, 29(3): 843-850.
The red bed soft rocks in central Yunnan are widely distributed and have significant rheological properties. In order to study the creep properties of red bed mudstones under saturated-dehydration cycle conditions,Longchuan River valley mudstones are used as research objects. YZJL-300 rock shear rheometer performs 30,50,and 100 shear creep tests on saturated-dehydration cycles. A constitutive model of mudstone creep is proposed by analyzing the shear creep characteristics of mudstone. The test results show that:(1)the limit instantaneous creep rate gradually increases 0.273 mm·h-1,0.365 mm·h-1,and 0.452 mm·h-1 as the number of saturated-dehydrated cycles increases and the long-term strength gradually decreases; (2)under the same shear load,the greater the number of cycles,the lower the viscoelastic modulus of mudstone; the same number of cycles,the larger the shear load,the smaller the viscoelastic modulus of mudstone. The function of the number of cycles and time is E(n,t). A non-linear viscoplastic body is introduced to establish the shear creep constitutive model(LCJN) of the inverted siphon mudstone of the Longchuan River and a general global optimization method to identify parameters of LCJN model,and verify the accuracy and applicability of the model.
The red bed soft rocks in central Yunnan are widely distributed and have significant rheological properties. In order to study the creep properties of red bed mudstones under saturated-dehydration cycle conditions,Longchuan River valley mudstones are used as research objects. YZJL-300 rock shear rheometer performs 30,50,and 100 shear creep tests on saturated-dehydration cycles. A constitutive model of mudstone creep is proposed by analyzing the shear creep characteristics of mudstone. The test results show that:(1)the limit instantaneous creep rate gradually increases 0.273 mm·h-1,0.365 mm·h-1,and 0.452 mm·h-1 as the number of saturated-dehydrated cycles increases and the long-term strength gradually decreases; (2)under the same shear load,the greater the number of cycles,the lower the viscoelastic modulus of mudstone; the same number of cycles,the larger the shear load,the smaller the viscoelastic modulus of mudstone. The function of the number of cycles and time is E(n,t). A non-linear viscoplastic body is introduced to establish the shear creep constitutive model(LCJN) of the inverted siphon mudstone of the Longchuan River and a general global optimization method to identify parameters of LCJN model,and verify the accuracy and applicability of the model.
2021, 29(3): 851-861.
Expansive soil is a kind of poor engineering geology soil because it has the property of water expansion and water shrinkage. In this paper,cement,steel slag powder and NaOH are used to improve the expansive soil and carry out the freeze-thaw cycle test. Then,the physical and mechanical properties of the improved expansive soil and the unmodified expansive soil after different freeze-thaw cycles are tested,such as volume,free expansion rate and unconfined compressive strength. At the same time,the micro electron microscope scanning analysis is carried out,in order to study the change rule of physical and mechanical properties of expansive soil improved by steel slag cement under the action of freeze-thaw cycle. The test results show that: Under the action of freeze-thaw cycle,the volume change rate of improved expansive soil is significantly lower than that of unmodified expansive soil. With the increase of curing age,the volume expansion of the improved expansive soil becomes smaller under the action of freeze-thaw cycles. With the increase of freeze-thaw cycles,the volume change rate tends to be stable,and the free expansion rate decreases with the increase of freeze-thaw cycles. Under the action of freeze-thaw cycle,the no-load expansion rate of ES-SSP-C sample is low. Under the action of freeze-thaw cycle,the improved expansive soil has better anti expansion performance than the unmodified expansive soil,and the effect of steel slag powder cement modified expansive soil(ES-SSP-C-SH)with NaOH active activator is the most obvious.
Expansive soil is a kind of poor engineering geology soil because it has the property of water expansion and water shrinkage. In this paper,cement,steel slag powder and NaOH are used to improve the expansive soil and carry out the freeze-thaw cycle test. Then,the physical and mechanical properties of the improved expansive soil and the unmodified expansive soil after different freeze-thaw cycles are tested,such as volume,free expansion rate and unconfined compressive strength. At the same time,the micro electron microscope scanning analysis is carried out,in order to study the change rule of physical and mechanical properties of expansive soil improved by steel slag cement under the action of freeze-thaw cycle. The test results show that: Under the action of freeze-thaw cycle,the volume change rate of improved expansive soil is significantly lower than that of unmodified expansive soil. With the increase of curing age,the volume expansion of the improved expansive soil becomes smaller under the action of freeze-thaw cycles. With the increase of freeze-thaw cycles,the volume change rate tends to be stable,and the free expansion rate decreases with the increase of freeze-thaw cycles. Under the action of freeze-thaw cycle,the no-load expansion rate of ES-SSP-C sample is low. Under the action of freeze-thaw cycle,the improved expansive soil has better anti expansion performance than the unmodified expansive soil,and the effect of steel slag powder cement modified expansive soil(ES-SSP-C-SH)with NaOH active activator is the most obvious.
2021, 29(3): 862-870.
Prediction of in-situ stress for deep-buried and extra-long tunnel area is always a difficulty encountered by many engineers,while the comprehensive engineering geological analysis presents a unique advantage in evaluating geostress regime of complex engineering projects. Therefore,exemplified by a typical deep-buried and extra-long highway tunnel in northeast Yunnan region,this solution is illustrated and validated. Firstly,based on the tectonic stress field zoning,stress orientations for the targeted area is obtained by analyzing Anderson's theory of faulting mechanics,focal mechanism and statistics of in-situ stress measurement. Secondly,the strength of rock masses for the targeted area is estimated by using Hoek-Brown criterion. The modified Sheorey model then is utilized to estimate the stress magnitudes. The results show that modern tectonic stress plays dominant role in the stress field. The dominant azimuth of maximum horizontal principal stress shows N20°~60°W. The stress orientation for the engineering area maintains relatively stable state. Prediction of stress values indicates that the maximum and minimum horizontal principal stress values respectively score respectively at the values of 11.2~20.5 MPa and 6.6~12.2 MPa,at a burial depth of 500 m approximately. The values reach up to 25.9~28.2 MPa and 15.4~17.1 MPa with a burial depth of nearly 1000 m. It is also pointed that,under such high stress,rock bust or brittle failure of hard rocks can occur in the situation of over 500 meters' buried depth,while large deformation nearly occur in the surrounding rocks masses. The predicted results can be well validated by in-situ stress measurements. The proposed method outperforms traditional ones in predicting stress state of linear engineering projects,and shows a promising application prospect.
Prediction of in-situ stress for deep-buried and extra-long tunnel area is always a difficulty encountered by many engineers,while the comprehensive engineering geological analysis presents a unique advantage in evaluating geostress regime of complex engineering projects. Therefore,exemplified by a typical deep-buried and extra-long highway tunnel in northeast Yunnan region,this solution is illustrated and validated. Firstly,based on the tectonic stress field zoning,stress orientations for the targeted area is obtained by analyzing Anderson's theory of faulting mechanics,focal mechanism and statistics of in-situ stress measurement. Secondly,the strength of rock masses for the targeted area is estimated by using Hoek-Brown criterion. The modified Sheorey model then is utilized to estimate the stress magnitudes. The results show that modern tectonic stress plays dominant role in the stress field. The dominant azimuth of maximum horizontal principal stress shows N20°~60°W. The stress orientation for the engineering area maintains relatively stable state. Prediction of stress values indicates that the maximum and minimum horizontal principal stress values respectively score respectively at the values of 11.2~20.5 MPa and 6.6~12.2 MPa,at a burial depth of 500 m approximately. The values reach up to 25.9~28.2 MPa and 15.4~17.1 MPa with a burial depth of nearly 1000 m. It is also pointed that,under such high stress,rock bust or brittle failure of hard rocks can occur in the situation of over 500 meters' buried depth,while large deformation nearly occur in the surrounding rocks masses. The predicted results can be well validated by in-situ stress measurements. The proposed method outperforms traditional ones in predicting stress state of linear engineering projects,and shows a promising application prospect.
2021, 29(3): 871-878.
The microstructure controls the physical and mechanical properties of loess and further affects engineering stability. In this paper,we quantitatively investigated the three-dimensional particle structure characteristics of loess from Jingyang,Shaanxi province through X-ray computed tomography(CT)experiment and Avizo software. We established the numerical simulation model of loess based on the results of triaxial shear test,and analyzed the influences of particle size,shape and arrangement on the shear behaviors of loess by using PFC software. The results show that the equivalent diameter and sphericity distribution of loess particles obey the Weibull and Gamma probability distribution function,respectively. The loess sample has a transversely isotropic arrangement,and the maximum Feret diameters of most particles are nearly parallel to bedding direction. Particle size distribution and particle sphericity mainly affect strength characteristics,and have little effect on the failure mode of loess sample. As particle size distribution becomes more uniform,the shear strength of sample increases,and strain hardening phenomenon becomes more obvious. The shear strength of samples with elliptic particles reduces with decreasing particle sphericity. Particle arrangement has a noticeable impact on the shear behaviors of sample. The sample shows obvious stress drop and develops brittle shear failure when the maximum Feret diameters of most particles are parallel to the axial loading direction. The sample has lower shear strength and develops plastic shearing band when the maximum Feret diameters of most particles are perpendicular to the axial loading direction.
The microstructure controls the physical and mechanical properties of loess and further affects engineering stability. In this paper,we quantitatively investigated the three-dimensional particle structure characteristics of loess from Jingyang,Shaanxi province through X-ray computed tomography(CT)experiment and Avizo software. We established the numerical simulation model of loess based on the results of triaxial shear test,and analyzed the influences of particle size,shape and arrangement on the shear behaviors of loess by using PFC software. The results show that the equivalent diameter and sphericity distribution of loess particles obey the Weibull and Gamma probability distribution function,respectively. The loess sample has a transversely isotropic arrangement,and the maximum Feret diameters of most particles are nearly parallel to bedding direction. Particle size distribution and particle sphericity mainly affect strength characteristics,and have little effect on the failure mode of loess sample. As particle size distribution becomes more uniform,the shear strength of sample increases,and strain hardening phenomenon becomes more obvious. The shear strength of samples with elliptic particles reduces with decreasing particle sphericity. Particle arrangement has a noticeable impact on the shear behaviors of sample. The sample shows obvious stress drop and develops brittle shear failure when the maximum Feret diameters of most particles are parallel to the axial loading direction. The sample has lower shear strength and develops plastic shearing band when the maximum Feret diameters of most particles are perpendicular to the axial loading direction.
2021, 29(3): 879-890.
Loess is widely deposited on the Hipparion red clay and forms weak interfaces,which is a typical slippery layer in northwest China. To investigate the influences of interface contact angle on shear strength characteristics of interfaces between loess and Hipparion red clay,we performed a series of direct shear experiments using a home-developed shear equipment. The results indicate that the failure modes of interfaces between loess and Hipparion red clay can be divided into sliding,sliding and shear-off,and cutoff. The interface contact angle affects the failure mode of interfaces. When the interface contact angle of interface is larger,the failure mode tends to be cutoff. When the interface contact angle of interface is smaller,the failure mode tends to be sliding. The shear stress-displacement curves of interfaces show that the shear failure characteristics of interfaces are brittle obviously. As the interface contact angle of interface increases,the shear stiffness and shear failure displacement of interfaces increase,and the "dropping" phenomenon of shear stress-shear displacement curves becomes more obvious. The dilatancy effect of interfaces is obvious in the shear process. The peak dilatancy angle of interfaces decreases first and then increases with the increasing interface contact angles,which reflects the different shear failure modes of interfaces. The shear strength of interfaces varies nonlinearly with the normal stresses and interface contact angles. The peak strength of interfaces increases first and then decreases with the increasing interface contact angles,and the variation range of peak strength increases with the increasing normal stresses. The residual strength of interfaces increases with the increasing interface contact angles,and the variation range of peak strength increases with the increasing normal stresses.
Loess is widely deposited on the Hipparion red clay and forms weak interfaces,which is a typical slippery layer in northwest China. To investigate the influences of interface contact angle on shear strength characteristics of interfaces between loess and Hipparion red clay,we performed a series of direct shear experiments using a home-developed shear equipment. The results indicate that the failure modes of interfaces between loess and Hipparion red clay can be divided into sliding,sliding and shear-off,and cutoff. The interface contact angle affects the failure mode of interfaces. When the interface contact angle of interface is larger,the failure mode tends to be cutoff. When the interface contact angle of interface is smaller,the failure mode tends to be sliding. The shear stress-displacement curves of interfaces show that the shear failure characteristics of interfaces are brittle obviously. As the interface contact angle of interface increases,the shear stiffness and shear failure displacement of interfaces increase,and the "dropping" phenomenon of shear stress-shear displacement curves becomes more obvious. The dilatancy effect of interfaces is obvious in the shear process. The peak dilatancy angle of interfaces decreases first and then increases with the increasing interface contact angles,which reflects the different shear failure modes of interfaces. The shear strength of interfaces varies nonlinearly with the normal stresses and interface contact angles. The peak strength of interfaces increases first and then decreases with the increasing interface contact angles,and the variation range of peak strength increases with the increasing normal stresses. The residual strength of interfaces increases with the increasing interface contact angles,and the variation range of peak strength increases with the increasing normal stresses.
2021, 29(3): 891-900.
The ultra-low permeability and porosity of shale reservoir dominants gas storage and transport capabilities. Recent research shows that the acid solution would dissolve some specified minerals in shale matrix,improve the porosity together with the permeability,therefore and enhance the gas flow. The hydrochloric acid,hydrofluoric acid and mixed acid are applied to immerse the shale matrix with the SEM together with MATLAB applied to identity the porosity variation. Also the SEM and XRD are combined to measure the element changes before and post the immersion. The reaction mechanism and degree between shale samples and different acid solutions are analyzed. The results and discussions show the initial porosity of the shale matrix is 10.68% while enhanced to 1.21,1.3,2.61 and 2.99 times of the initial value after immersion. After soaking for one hour,a quantity of quartz and clay are remained on the surface of shale matrix after hydrofluoric acid immersion; and the content of clay minerals decreases obviously and lots of quartz remain when treated with mixed acid,which indicates that the clay minerals are more likely to be dissolved. After 24 hours of acidification,the mixed acid reacts violently with quartz and clay minerals because of its strongest acid. The porosity network is well developed after acidification with mixed acid prompting the gas reservoir performance.
The ultra-low permeability and porosity of shale reservoir dominants gas storage and transport capabilities. Recent research shows that the acid solution would dissolve some specified minerals in shale matrix,improve the porosity together with the permeability,therefore and enhance the gas flow. The hydrochloric acid,hydrofluoric acid and mixed acid are applied to immerse the shale matrix with the SEM together with MATLAB applied to identity the porosity variation. Also the SEM and XRD are combined to measure the element changes before and post the immersion. The reaction mechanism and degree between shale samples and different acid solutions are analyzed. The results and discussions show the initial porosity of the shale matrix is 10.68% while enhanced to 1.21,1.3,2.61 and 2.99 times of the initial value after immersion. After soaking for one hour,a quantity of quartz and clay are remained on the surface of shale matrix after hydrofluoric acid immersion; and the content of clay minerals decreases obviously and lots of quartz remain when treated with mixed acid,which indicates that the clay minerals are more likely to be dissolved. After 24 hours of acidification,the mixed acid reacts violently with quartz and clay minerals because of its strongest acid. The porosity network is well developed after acidification with mixed acid prompting the gas reservoir performance.
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