2017 Vol. 39 No. 11
2017, 39(11): 1948-1948.
Abstract：
The structural safety performance of Dashixia concrete face gravel dam (CFGD), an extremely high CFGD in Xinjiang, is studied based on the large-scale triaxial experiments on dam materials and the detailed finite element method-based numerical analyses. Compacted gravel materials are found to exhibit high deformation modulus, small magnitude of creep and small magnitude of shaking-induced residual deformation. The maximum settlement of Dashixia CFGD during operation is evaluated to be about 0.64% of the dam height, while the earthquake-induced permanent settlement at the crest is about 0.40% of the dam height. The maximum settlement and tension of the peripheral joints do not exceed the maximum measured ones in the investigated existing cases. The shear displacement, on the other hand, is larger than all the measured ones in these cases. However, the advancement in sealing structures and materials achieved in China can already satisfy the need of CFGDs as high as 250 m. Therefore, it can be concluded that the difficulty in deformation control is relatively less compared with that of rockfill dams with similar height. What deserves special attention, on the other hand, is the high static and dynamic stresses in concrete slabs, both along the axial and slope directions. The reinforcements near some peripheral joints and the compressive vertical joints are necessary.
The structural safety performance of Dashixia concrete face gravel dam (CFGD), an extremely high CFGD in Xinjiang, is studied based on the large-scale triaxial experiments on dam materials and the detailed finite element method-based numerical analyses. Compacted gravel materials are found to exhibit high deformation modulus, small magnitude of creep and small magnitude of shaking-induced residual deformation. The maximum settlement of Dashixia CFGD during operation is evaluated to be about 0.64% of the dam height, while the earthquake-induced permanent settlement at the crest is about 0.40% of the dam height. The maximum settlement and tension of the peripheral joints do not exceed the maximum measured ones in the investigated existing cases. The shear displacement, on the other hand, is larger than all the measured ones in these cases. However, the advancement in sealing structures and materials achieved in China can already satisfy the need of CFGDs as high as 250 m. Therefore, it can be concluded that the difficulty in deformation control is relatively less compared with that of rockfill dams with similar height. What deserves special attention, on the other hand, is the high static and dynamic stresses in concrete slabs, both along the axial and slope directions. The reinforcements near some peripheral joints and the compressive vertical joints are necessary.
Abstract：
In order to study the relationship between the accumulative plastic strain growth and the energy dissipation, and to further classify the pattern of the accumulative plastic strain growth, dynamic consolidated-undrained triaxial shear tests are conducted on tailing silts with different void ratios under different cyclic stress ratios. The energy dissipation in soil is divided into plastic strain and viscous accumulative energy dissipations. According to the relationships, failure modes of the accumulative plastic strain growth are reclassified, and the development of accumulative plastic strain is interpreted based on the improved energy dissipation mechanism. The research highlights that as the cyclic stress ratio keeps increasing, the rate of dissipation of viscous accumulative energy will exceed that of the plastic strain accumulative energy,and the development mode of accumulative plastic strain will turn from the stability to the failure. Furthermore, the failure modes are reasonably divided into four categories: stable type, stable damage type, damage type and collapse. Essentially, the plastic strain accumulative energy will dissipate due to the rearrangement among relatively large particles such as sand and silt grains. Similarly, the viscous accumulative energy will dissipate because of the relative slippage among clay particles and colloid particles under the relaxation of weak bound water in the double electrical layers. This energy dissipation mechanism is consistent with the proposed failure pattern of the accumulative plastic strain growth. The research results may provide a basis for further model researches on the accumulative plastic strain growth.
In order to study the relationship between the accumulative plastic strain growth and the energy dissipation, and to further classify the pattern of the accumulative plastic strain growth, dynamic consolidated-undrained triaxial shear tests are conducted on tailing silts with different void ratios under different cyclic stress ratios. The energy dissipation in soil is divided into plastic strain and viscous accumulative energy dissipations. According to the relationships, failure modes of the accumulative plastic strain growth are reclassified, and the development of accumulative plastic strain is interpreted based on the improved energy dissipation mechanism. The research highlights that as the cyclic stress ratio keeps increasing, the rate of dissipation of viscous accumulative energy will exceed that of the plastic strain accumulative energy,and the development mode of accumulative plastic strain will turn from the stability to the failure. Furthermore, the failure modes are reasonably divided into four categories: stable type, stable damage type, damage type and collapse. Essentially, the plastic strain accumulative energy will dissipate due to the rearrangement among relatively large particles such as sand and silt grains. Similarly, the viscous accumulative energy will dissipate because of the relative slippage among clay particles and colloid particles under the relaxation of weak bound water in the double electrical layers. This energy dissipation mechanism is consistent with the proposed failure pattern of the accumulative plastic strain growth. The research results may provide a basis for further model researches on the accumulative plastic strain growth.
Abstract：
The principal purpose of the present work is to investigate the effects of hydrochemistry on the structural strength of Zhanjiang formation clay. Several hydrogeological data and chemical composition of groundwater by drilling are collected and analyzed to evaluate the chemical environment of groundwater in Zhanjiang area. The water solution in soils is extracted by using the centrifugal filter method to measure its chemical composition. Furthermore, changes of microstructure before and after removing the cementing material are studied using the scanning electron microscopy and energy spectrum analysis. The results show that the water solution in Zhanjiang formation clay has chemical compositions and relative content similar to the groundwater with the chemical types of Cl-Na, showing acidic and iron-rich characteristics. The high content of iron can provide cementing materials, and free iron oxides further produce a strong bond between soil particles in the acidic environment. According to the results, an effect mechanism of hydrochemistry on the structural strength of Zhanjiang formation clay is proposed. The strong structural characteristic is caused by an open flocculation structure and the bond between the particles.
The principal purpose of the present work is to investigate the effects of hydrochemistry on the structural strength of Zhanjiang formation clay. Several hydrogeological data and chemical composition of groundwater by drilling are collected and analyzed to evaluate the chemical environment of groundwater in Zhanjiang area. The water solution in soils is extracted by using the centrifugal filter method to measure its chemical composition. Furthermore, changes of microstructure before and after removing the cementing material are studied using the scanning electron microscopy and energy spectrum analysis. The results show that the water solution in Zhanjiang formation clay has chemical compositions and relative content similar to the groundwater with the chemical types of Cl-Na, showing acidic and iron-rich characteristics. The high content of iron can provide cementing materials, and free iron oxides further produce a strong bond between soil particles in the acidic environment. According to the results, an effect mechanism of hydrochemistry on the structural strength of Zhanjiang formation clay is proposed. The strong structural characteristic is caused by an open flocculation structure and the bond between the particles.
Abstract：
In order to study the effect of discharge load rate and pore water pressure on the mechanical properties of sandstone, the triaxial unloading tests with different unloading rates (0.005, 0.02, 0.05 and 0.1 MPa/s) and different osmotic pressures (0, 0.3, 0.6, 0.9 and 1.2 MPa) are designed and perfomed. The results show that: (1) At the loading stage, with the increase of the pore water pressure, the gradient of stress-strain curve of rock decreaseds gradually; (2) At the stage of unloading, with the increase of the unloading rate, the soft number of confining pressure is smaller, the confining pressure of rock failure is smaller, the rock strength is relatively high, but the degree of fragmentation is more serious. Moreover, under the same unloading rate, the higher the pore water pressure is, the more obvious the rock lateral expansion phenomenon is, and the rock is much easier to fail; (3) At the stage of unloading, the deformation modulus of rock shows a slow and steep trend, and the smaller the unloading rate is, the higher the pore water pressure is, the larger the deformation modulus deterioration amplitude is; (4) During the process of unloading, with the increase of the unloading rate, the features of rock brittle failure are more obvious. With the increase of pore water pressure, the number of axial tensile cracks and tracking secondary cracks near failure is larger. The stress concentration caused by the pore water pressure in specimen cracks or crack tips is the main reason that leads to the deformation failure. The related research results can provide good references for the analysis of the discharging mechanical properties of the hydrous rock mass.
In order to study the effect of discharge load rate and pore water pressure on the mechanical properties of sandstone, the triaxial unloading tests with different unloading rates (0.005, 0.02, 0.05 and 0.1 MPa/s) and different osmotic pressures (0, 0.3, 0.6, 0.9 and 1.2 MPa) are designed and perfomed. The results show that: (1) At the loading stage, with the increase of the pore water pressure, the gradient of stress-strain curve of rock decreaseds gradually; (2) At the stage of unloading, with the increase of the unloading rate, the soft number of confining pressure is smaller, the confining pressure of rock failure is smaller, the rock strength is relatively high, but the degree of fragmentation is more serious. Moreover, under the same unloading rate, the higher the pore water pressure is, the more obvious the rock lateral expansion phenomenon is, and the rock is much easier to fail; (3) At the stage of unloading, the deformation modulus of rock shows a slow and steep trend, and the smaller the unloading rate is, the higher the pore water pressure is, the larger the deformation modulus deterioration amplitude is; (4) During the process of unloading, with the increase of the unloading rate, the features of rock brittle failure are more obvious. With the increase of pore water pressure, the number of axial tensile cracks and tracking secondary cracks near failure is larger. The stress concentration caused by the pore water pressure in specimen cracks or crack tips is the main reason that leads to the deformation failure. The related research results can provide good references for the analysis of the discharging mechanical properties of the hydrous rock mass.
Abstract：
A summary about the analytical solutions in the literatures for the seepage fields of underwater tunnels is given, whose advantages and disadvantages are pointed out. Based on the governing equation for the steady-state seepage, the analytical solutions for the seepage fields of underwater tunnels are derived rigorously using the conformal mapping method. The water inflow and water pressure distribution for the grouted lined underwater tunnel with arbitrary burial depth can be obtained according to the new solutions. The analytical solutions can be degenerated to two limit cases including the unlined underwater tunnel and the underwater pipeline. A numerical model is established using the software Comsol to validate the newly derived solutions. The effects of the burial depth of tunnel, the permeability of lining and the thickness of lining on the water inflow, the pore water pressure around the lining and the distribution of the total water head are investigated. It is found that the maximum water pressure appears at the top of the lining when the tunnel is buried shallowly, which is opposite for the case with a large burial depth. The average pore pressure and the non-uniform degree of distribution of the pore water pressure around the lining decrease first then increase as the burial depth increases gradually. And for a given working condition, there is a burial depth or thickness of lining, which can make the pore water pressure around the lining distributed uniformly.
A summary about the analytical solutions in the literatures for the seepage fields of underwater tunnels is given, whose advantages and disadvantages are pointed out. Based on the governing equation for the steady-state seepage, the analytical solutions for the seepage fields of underwater tunnels are derived rigorously using the conformal mapping method. The water inflow and water pressure distribution for the grouted lined underwater tunnel with arbitrary burial depth can be obtained according to the new solutions. The analytical solutions can be degenerated to two limit cases including the unlined underwater tunnel and the underwater pipeline. A numerical model is established using the software Comsol to validate the newly derived solutions. The effects of the burial depth of tunnel, the permeability of lining and the thickness of lining on the water inflow, the pore water pressure around the lining and the distribution of the total water head are investigated. It is found that the maximum water pressure appears at the top of the lining when the tunnel is buried shallowly, which is opposite for the case with a large burial depth. The average pore pressure and the non-uniform degree of distribution of the pore water pressure around the lining decrease first then increase as the burial depth increases gradually. And for a given working condition, there is a burial depth or thickness of lining, which can make the pore water pressure around the lining distributed uniformly.
Abstract：
The mechanism of water and salt migration in the freezing process has long been a hot spot in the study of frozen soil. The water and salt transport processes and the deformation of soil are studied through the unidirectional freezing experiments. The results show that the deformation of soil with the initial salt content decreases significantly compared with that of desalinized soil, and the salt has a strong impact on the accumulation of water in the frozen fringe, and it can mitigate the frost heave of soil in the freezing process. Supplied with different concentrations of NaCl solution, the deformations of the samples are consistent in the early stage. With the accumulation of salt at the freezing front, the driving force of moisture migration is not so sufficient during the subsequent period when the frost heave decreases significantly. Based on the properties of the solution, a model is established to describe the water and salt migration and deformation in the freezing process, where the influence of salt on the freezing temperature and unfrozen water content are considered. The calculated results illustrate that the proposed model can reflect the laws of temperature, water, salt and deformation in the freezing process well, and it will provide a theoretical basis for understanding the deformation mechanism of saline freezing soil.
The mechanism of water and salt migration in the freezing process has long been a hot spot in the study of frozen soil. The water and salt transport processes and the deformation of soil are studied through the unidirectional freezing experiments. The results show that the deformation of soil with the initial salt content decreases significantly compared with that of desalinized soil, and the salt has a strong impact on the accumulation of water in the frozen fringe, and it can mitigate the frost heave of soil in the freezing process. Supplied with different concentrations of NaCl solution, the deformations of the samples are consistent in the early stage. With the accumulation of salt at the freezing front, the driving force of moisture migration is not so sufficient during the subsequent period when the frost heave decreases significantly. Based on the properties of the solution, a model is established to describe the water and salt migration and deformation in the freezing process, where the influence of salt on the freezing temperature and unfrozen water content are considered. The calculated results illustrate that the proposed model can reflect the laws of temperature, water, salt and deformation in the freezing process well, and it will provide a theoretical basis for understanding the deformation mechanism of saline freezing soil.
Abstract：
The characteristics of filter cake formation on the excavation face of permeable sand strata are the critical factors for the growth and decrease law of the soil and water pressure in the strata as well as the stability of excavation face. Then the requirement of these analyses is to establish the quantitative relationship between the pore characteristics and filter cake forming characteristics. The quantitative indexes of pore characteristics and filter cake forming characteristics are raised by carrying out the slurry filtration tests in the strata with different pore characteristics. Based on the quantitative relationship between the both characteristics, the filter cake formation laws influenced by the pore characteristics are studied. The results show that the two-parameters model for the filtration curve can describe the variation characteristics of seepage discharge. Th model parameters own clear physical significances to quantify filter cake forming characteristics such as curvature characteristics of seepage discharge, formation time and forming stages of filter cake. Based on the relationship of power function between parameters a, b and the pore volume v, the quantitative relationship of pore characteristics and the filter cake forming characteristics is established. The research results can determine the reasonable slurry proportion and construction parameters and may provide experimental and theoretical support for guaranteeing the stability of the excavation face for slurry shield tunneling projects excavated in highly permeable sand soils.
The characteristics of filter cake formation on the excavation face of permeable sand strata are the critical factors for the growth and decrease law of the soil and water pressure in the strata as well as the stability of excavation face. Then the requirement of these analyses is to establish the quantitative relationship between the pore characteristics and filter cake forming characteristics. The quantitative indexes of pore characteristics and filter cake forming characteristics are raised by carrying out the slurry filtration tests in the strata with different pore characteristics. Based on the quantitative relationship between the both characteristics, the filter cake formation laws influenced by the pore characteristics are studied. The results show that the two-parameters model for the filtration curve can describe the variation characteristics of seepage discharge. Th model parameters own clear physical significances to quantify filter cake forming characteristics such as curvature characteristics of seepage discharge, formation time and forming stages of filter cake. Based on the relationship of power function between parameters a, b and the pore volume v, the quantitative relationship of pore characteristics and the filter cake forming characteristics is established. The research results can determine the reasonable slurry proportion and construction parameters and may provide experimental and theoretical support for guaranteeing the stability of the excavation face for slurry shield tunneling projects excavated in highly permeable sand soils.
Abstract：
A full sandy foundation is prone to be liquefied under an earthquake, explosion or other vibration loadings. As a result, the upper structures such as embankments on the foundation will be damaged. Aiming at the deformation problems of an embankment in a liquefied foundation, the field tests on explosion-induced liquefaction are conducted considering the effects of the embankment materials and reinforcement measures of the embankment foundation on its deformation and cracks. The results show that the subsidence of the embankment mainly occurs within 1 ~ 2 hours after the explosions, which accounts for 84 % ~ 87 % of the total subsidence accumulated in 7 days. The settlement of the embankment with fine aggregate becomes stable after 7 days. The subsidence of the embankment with fine aggregate is 24 % more than that without fine aggregate. The settlement of the foundation reinforced by geogrid and geotextile can be effectively reduced by 10% of the dam settlement in the liquefied ground. The cracks on the embankment body mainly appear in the section with fine aggregate and the interface between two different materials, and they extend along the dam body. The domestic and international specifications for the settlement of structures on a liquefied foundation are summarized. The settlement figure of buildings with shallow footings is adopted as the prediction method for the embankment settlement in this test. The results indicate that the predicted values are close to the measured data.
A full sandy foundation is prone to be liquefied under an earthquake, explosion or other vibration loadings. As a result, the upper structures such as embankments on the foundation will be damaged. Aiming at the deformation problems of an embankment in a liquefied foundation, the field tests on explosion-induced liquefaction are conducted considering the effects of the embankment materials and reinforcement measures of the embankment foundation on its deformation and cracks. The results show that the subsidence of the embankment mainly occurs within 1 ~ 2 hours after the explosions, which accounts for 84 % ~ 87 % of the total subsidence accumulated in 7 days. The settlement of the embankment with fine aggregate becomes stable after 7 days. The subsidence of the embankment with fine aggregate is 24 % more than that without fine aggregate. The settlement of the foundation reinforced by geogrid and geotextile can be effectively reduced by 10% of the dam settlement in the liquefied ground. The cracks on the embankment body mainly appear in the section with fine aggregate and the interface between two different materials, and they extend along the dam body. The domestic and international specifications for the settlement of structures on a liquefied foundation are summarized. The settlement figure of buildings with shallow footings is adopted as the prediction method for the embankment settlement in this test. The results indicate that the predicted values are close to the measured data.
Abstract：
A new high-precise fast indirect boundary element method is developed to solve the two-dimensional scattering problems of high-frequency seismic waves (2D in-plane) based on the fast multi-pole expansion technique. It is verified that this method has high accuracy, efficiency and excellent numerical stability, and can greatly reduce the computational storage. Taking the seismic response of a canyon and a hill in a half space under high-frequency incident plane SV waves as an example, the basic characteristics of high-frequency scattering around the canyon and hill are discussed, and the scattering results of 0~25 Hz broadband waves by large-scale local sites of thousands-meters are illustrated. The numerical results show that under the high-frequency incident SV waves, the amplification effect of the canyon on the horizontal and vertical displacement under high-frequency incident SV waves can be seen clearly at the corner of the canyon, but seems not so pronounced at the bottom of the canyon. The spectral peak of displacement amplitudes reaches up to 5.0 near the top of the hill, while the displacement response at the foot of the hill is constrained. As for the obliquely incident SV waves, the displacement at the canyon surface is more pronounced, but the amplification effect is more significant at the back of hill surface. The numerical results may provide a theoretical basis for the seismic design of large-scale projects constructed in complex local sites.
A new high-precise fast indirect boundary element method is developed to solve the two-dimensional scattering problems of high-frequency seismic waves (2D in-plane) based on the fast multi-pole expansion technique. It is verified that this method has high accuracy, efficiency and excellent numerical stability, and can greatly reduce the computational storage. Taking the seismic response of a canyon and a hill in a half space under high-frequency incident plane SV waves as an example, the basic characteristics of high-frequency scattering around the canyon and hill are discussed, and the scattering results of 0~25 Hz broadband waves by large-scale local sites of thousands-meters are illustrated. The numerical results show that under the high-frequency incident SV waves, the amplification effect of the canyon on the horizontal and vertical displacement under high-frequency incident SV waves can be seen clearly at the corner of the canyon, but seems not so pronounced at the bottom of the canyon. The spectral peak of displacement amplitudes reaches up to 5.0 near the top of the hill, while the displacement response at the foot of the hill is constrained. As for the obliquely incident SV waves, the displacement at the canyon surface is more pronounced, but the amplification effect is more significant at the back of hill surface. The numerical results may provide a theoretical basis for the seismic design of large-scale projects constructed in complex local sites.
Abstract：
The uplift deformation phenomenon occurring in the dam due to reservoir impounding appears rarely. To explore the influences of the hydro-geological structure of the rock mass on the uplift deformation, the mode of the hydro-geological structure inducing the generation of uplift deformation near the dam area is put forward through the researches on the existing literatures. The relative impermeable layer inclination to the downstream under the dam foundation and the relative permeable layer lying beneath the impermeable layer are the necessary hydro-geological conditions for the generation of uplift deformation. A numerical method based on the hydro-mechanical coupling theory is employed to study the influences of the variation of attitude elements on the spatial distribution law of uplift deformation occurring near the dam. It is shown that the shorter the distance between the exposure site and the dam, the greater the uplift deformation value. With the increase of the dip angle of the impermeable layer, the maximum uplift deformation near the dam increases firstly and then decreases. It is also indicated that the spatial distribution position of the uplift deformation is directly determined by the variation of dip direction.
The uplift deformation phenomenon occurring in the dam due to reservoir impounding appears rarely. To explore the influences of the hydro-geological structure of the rock mass on the uplift deformation, the mode of the hydro-geological structure inducing the generation of uplift deformation near the dam area is put forward through the researches on the existing literatures. The relative impermeable layer inclination to the downstream under the dam foundation and the relative permeable layer lying beneath the impermeable layer are the necessary hydro-geological conditions for the generation of uplift deformation. A numerical method based on the hydro-mechanical coupling theory is employed to study the influences of the variation of attitude elements on the spatial distribution law of uplift deformation occurring near the dam. It is shown that the shorter the distance between the exposure site and the dam, the greater the uplift deformation value. With the increase of the dip angle of the impermeable layer, the maximum uplift deformation near the dam increases firstly and then decreases. It is also indicated that the spatial distribution position of the uplift deformation is directly determined by the variation of dip direction.
Abstract：
The limit analysis method is adopted to study deep arch tunnel with straight wall in cohesive strata based on the shear failure theory. Firstly, a theoretical model for ruptured zone in the deep arch tunnel is established based on loading tests. And then a formula for lateral pressure is proposed considering the effect of two sphenoid rupture surfaces beside tunnel sidewalls. Secondly, the approximate solutions to the lateral pressure and the vertical earth pressure are derived. The criteria for the quantitative stability of the deep arch tunnel without supports are developed, and the approximate ultimate load on the ground can be calculated according to the stability of the shear wedge beside the tunnel sidewalls. Finally, the safety factor of the deep tunnel is defined based on the limit analysis method, and the influencing factors are analyzed. The results show that the proposed model for ruptured zone and the solution to the ultimate load are close to the test results. Compared with the traditional theoretical value, the calculated vertical and lateral earth pressure is much smaller according to the proposed formula. The interaction theory between lateral pressure and vertical pressure on arch tunnels is proposed in the good strata. The smaller the vertical earth pressure is, the greater the lateral pressure is, and the greater the lateral pressure or the lateral pressure coefficient is, the smaller the vertical earth pressure is in the limit state. The safety factor of stability increases with increasing the cohesive force and internal friction angle, and it decreases with increasing the span and height of the tunnel. The cohesive force and tunnel span have larger influences on the safety factor of stability than the internal friction angle and the tunnel height respectively. However, the greater buried depth has smaller influences on the stability safety factor of the deep tunnel.
The limit analysis method is adopted to study deep arch tunnel with straight wall in cohesive strata based on the shear failure theory. Firstly, a theoretical model for ruptured zone in the deep arch tunnel is established based on loading tests. And then a formula for lateral pressure is proposed considering the effect of two sphenoid rupture surfaces beside tunnel sidewalls. Secondly, the approximate solutions to the lateral pressure and the vertical earth pressure are derived. The criteria for the quantitative stability of the deep arch tunnel without supports are developed, and the approximate ultimate load on the ground can be calculated according to the stability of the shear wedge beside the tunnel sidewalls. Finally, the safety factor of the deep tunnel is defined based on the limit analysis method, and the influencing factors are analyzed. The results show that the proposed model for ruptured zone and the solution to the ultimate load are close to the test results. Compared with the traditional theoretical value, the calculated vertical and lateral earth pressure is much smaller according to the proposed formula. The interaction theory between lateral pressure and vertical pressure on arch tunnels is proposed in the good strata. The smaller the vertical earth pressure is, the greater the lateral pressure is, and the greater the lateral pressure or the lateral pressure coefficient is, the smaller the vertical earth pressure is in the limit state. The safety factor of stability increases with increasing the cohesive force and internal friction angle, and it decreases with increasing the span and height of the tunnel. The cohesive force and tunnel span have larger influences on the safety factor of stability than the internal friction angle and the tunnel height respectively. However, the greater buried depth has smaller influences on the stability safety factor of the deep tunnel.
Abstract：
The jacking force is the crucial parameter for the design and construction of pipe jacking projects. It is controlled by the lateral frictional resistance of pipe jacking and affected by the characteristics of pipe-soil contact and pipe-mud contact. In order to calculate the frictional resistance of pipe jacking more accurately, it is assumed that the excavation tunnel is stable under mud pressure. The pipes are contacted with soil and mud concurrently. By using the Persson's contact model for coordinated surface to study the pipe-soil contact characteristics, the pipe-soil frictional resistance can be obtained by considering the contact pressure distribution. By using the cylindrical cavity expansion theory in elastic half plane to analyze the thickness of mud screen influenced by the mud pressure, the pipe-mud frictional resistance is calculated by combining the mud thixotropy with the hydromechanical model for parallel plates. And on this basis, the frictional resistance formulae for the straight line and curved pipe jacking are summarized considering the relative position of pipe and excavation tunnel, the sliding friction as the lower limit value during jacking process and the static friction as the upper limit value at restarting of pipe jacking. Compared with the data of project cases, the results show that the lower limit value by the proposed formulae is the closest to the measured one, and thus its applicability is validated.
The jacking force is the crucial parameter for the design and construction of pipe jacking projects. It is controlled by the lateral frictional resistance of pipe jacking and affected by the characteristics of pipe-soil contact and pipe-mud contact. In order to calculate the frictional resistance of pipe jacking more accurately, it is assumed that the excavation tunnel is stable under mud pressure. The pipes are contacted with soil and mud concurrently. By using the Persson's contact model for coordinated surface to study the pipe-soil contact characteristics, the pipe-soil frictional resistance can be obtained by considering the contact pressure distribution. By using the cylindrical cavity expansion theory in elastic half plane to analyze the thickness of mud screen influenced by the mud pressure, the pipe-mud frictional resistance is calculated by combining the mud thixotropy with the hydromechanical model for parallel plates. And on this basis, the frictional resistance formulae for the straight line and curved pipe jacking are summarized considering the relative position of pipe and excavation tunnel, the sliding friction as the lower limit value during jacking process and the static friction as the upper limit value at restarting of pipe jacking. Compared with the data of project cases, the results show that the lower limit value by the proposed formulae is the closest to the measured one, and thus its applicability is validated.
Abstract：
“Soil arching”, as the major medium of load transfer for a piled embankment, has significant influences on the behaviors of load transfer and of embankment fill displacements. Based on the laboratory model tests, a series of numerical models by discrete element method (DEM) are established with the particle flow code PFC2D to investigate the features and evolution of “soil arching” by analyzing the distribution of principal stress direction, contact force chains and embankment fill settlements. The numerical results indicate that the “soil arching” develops gradually with the increase of pile-soil relative displacement and maintains a relatively stable state finally. Meanwhile the relatively stable “soil arching” is roughly parabola-shaped with a height of 0.8 times the clear spacing of cap beam. Additionally, the embankment height has significant influences on the features and evolution of “soil arching”, as well as the load-transfer efficacy. However, the roughness of embankment fill, the clear spacing of cap beam and the width of cap beam have significant influences on the load-transfer efficacy, but not on the “soil arching” features.
“Soil arching”, as the major medium of load transfer for a piled embankment, has significant influences on the behaviors of load transfer and of embankment fill displacements. Based on the laboratory model tests, a series of numerical models by discrete element method (DEM) are established with the particle flow code PFC2D to investigate the features and evolution of “soil arching” by analyzing the distribution of principal stress direction, contact force chains and embankment fill settlements. The numerical results indicate that the “soil arching” develops gradually with the increase of pile-soil relative displacement and maintains a relatively stable state finally. Meanwhile the relatively stable “soil arching” is roughly parabola-shaped with a height of 0.8 times the clear spacing of cap beam. Additionally, the embankment height has significant influences on the features and evolution of “soil arching”, as well as the load-transfer efficacy. However, the roughness of embankment fill, the clear spacing of cap beam and the width of cap beam have significant influences on the load-transfer efficacy, but not on the “soil arching” features.
Abstract：
The carbonate rocks are known to suffer from attack of acid rain due to high content of soluble minerals. In order to simulate the process of deterioration of carbonate rocks, the attack of acid rain on different carbonate specimens is accelerated by increasing the acid strength and rainfall intensity. Under simulated sulphuric acid rain, the variations in Ca2+ concentration, pH value variation, H+ consumption rate of the solution as well as the mass loss are tested. The dissolution mechanism is analyzed and discussed based on the experimental results. Also, the pore characteristics are investigated and analyzed for impact on dissolution process. The results indicate that the dissolution products are removed from rock surface. This leads to mass loss and accelerated removal of the specimens and subsequently changes in micro structures. However, during the dissolution process, parts of micro pores and structures (<0.01 μm) are filled by gypsum crystal, and this can protect the micro pores from further damage. Thus, the porosity and pore characteristics have great influence on the dissolution process. Higher porosity and larger-sized pores are more conducive to the dissolution effect and further erosion. The conclusion may be taken as the scientific support for weathering researches on carbonate rocks and protection of carbonate stone relics.
The carbonate rocks are known to suffer from attack of acid rain due to high content of soluble minerals. In order to simulate the process of deterioration of carbonate rocks, the attack of acid rain on different carbonate specimens is accelerated by increasing the acid strength and rainfall intensity. Under simulated sulphuric acid rain, the variations in Ca2+ concentration, pH value variation, H+ consumption rate of the solution as well as the mass loss are tested. The dissolution mechanism is analyzed and discussed based on the experimental results. Also, the pore characteristics are investigated and analyzed for impact on dissolution process. The results indicate that the dissolution products are removed from rock surface. This leads to mass loss and accelerated removal of the specimens and subsequently changes in micro structures. However, during the dissolution process, parts of micro pores and structures (<0.01 μm) are filled by gypsum crystal, and this can protect the micro pores from further damage. Thus, the porosity and pore characteristics have great influence on the dissolution process. Higher porosity and larger-sized pores are more conducive to the dissolution effect and further erosion. The conclusion may be taken as the scientific support for weathering researches on carbonate rocks and protection of carbonate stone relics.
2017, 39(11): 2068-2077.
DOI: 10.11779/CJGE201711015
Abstract：
A series of shaking table tests on a sawdust-mixed clay site model are conducted under uniform earthquake excitation using the shaking table testing system with 9 sub-tables in Beijing University of Technology. The tests are performed using a rigid prefabricated continuous model box with dimensions of 7.7 meters long, 3.2 meters wide and 1.2 meters high. The test system is subjected to strong ground motions from El Centro record and Tianjin record. Through the horizontal longitudinal and horizontal transverse free-field shaking table tests, the boundary effect of the model box is investigated, and its dynamic characteristics and variation laws are given. The partial test results are analyzed, including the peak ground accelerations and their amplification factors, acceleration time histories and their Fourier spectra for the site responses. The test results indicate that the boundary effect of the model box is small because there is only a slight difference in the accelerations of site responses at the same elevation. With the increase of intensity of the input ground motion, the peak ground acceleration of the site response at the same test point increases, but its acceleration amplification factor decreases, and the main frequency components move from high to low frequency. It means that the soil becomes softer and the soil modulus decreases gradually with the increase of the intensity of input ground motion.
A series of shaking table tests on a sawdust-mixed clay site model are conducted under uniform earthquake excitation using the shaking table testing system with 9 sub-tables in Beijing University of Technology. The tests are performed using a rigid prefabricated continuous model box with dimensions of 7.7 meters long, 3.2 meters wide and 1.2 meters high. The test system is subjected to strong ground motions from El Centro record and Tianjin record. Through the horizontal longitudinal and horizontal transverse free-field shaking table tests, the boundary effect of the model box is investigated, and its dynamic characteristics and variation laws are given. The partial test results are analyzed, including the peak ground accelerations and their amplification factors, acceleration time histories and their Fourier spectra for the site responses. The test results indicate that the boundary effect of the model box is small because there is only a slight difference in the accelerations of site responses at the same elevation. With the increase of intensity of the input ground motion, the peak ground acceleration of the site response at the same test point increases, but its acceleration amplification factor decreases, and the main frequency components move from high to low frequency. It means that the soil becomes softer and the soil modulus decreases gradually with the increase of the intensity of input ground motion.
Abstract：
To study the stability (strength characteristics, deformation laws and long-term stability) of the shallowly buried soft rock (mudstone) tunnel anchorage under heavy load, based on a Changjiang bridge tunnel anchorage project under construction, the 1:10 reduced-scale models are used for field tests on the shallowly buried soft rock (mudstone) tunnel anchorage. According to the research, the shallowly buried soft rock (mudstone) tunnel anchorage still has the bearing capacity. Under the condition of design load or even several times the design load, the tunnel anchorage also has the long-term stability. The failure mode of the shallowly buried soft rock (mudstone) tunnel anchorage is as follows: the surrounding rock at the upper of the anchorage body breaks into blocks first, then the bottom of the anchorage body slides along the contact surface with the surrounding rock. The bottom boundary of the destroyed area is the contact zone of the anchorage body and the surrounding rock. The anchorage body has no damage. Besides, the spatial distribution laws of the creep deformation of surface surrounding rock and the deformation laws of the surface and deep surrounding rocks are also investigated. The research results may provide reference for the similar projects.
To study the stability (strength characteristics, deformation laws and long-term stability) of the shallowly buried soft rock (mudstone) tunnel anchorage under heavy load, based on a Changjiang bridge tunnel anchorage project under construction, the 1:10 reduced-scale models are used for field tests on the shallowly buried soft rock (mudstone) tunnel anchorage. According to the research, the shallowly buried soft rock (mudstone) tunnel anchorage still has the bearing capacity. Under the condition of design load or even several times the design load, the tunnel anchorage also has the long-term stability. The failure mode of the shallowly buried soft rock (mudstone) tunnel anchorage is as follows: the surrounding rock at the upper of the anchorage body breaks into blocks first, then the bottom of the anchorage body slides along the contact surface with the surrounding rock. The bottom boundary of the destroyed area is the contact zone of the anchorage body and the surrounding rock. The anchorage body has no damage. Besides, the spatial distribution laws of the creep deformation of surface surrounding rock and the deformation laws of the surface and deep surrounding rocks are also investigated. The research results may provide reference for the similar projects.
2017, 39(11): 2088-2101.
DOI: 10.11779/CJGE201711017
Abstract：
The seepage force is still one of the most fundamental and controversial concepts in soil mechanics. Based on the literature review, the compatibility between the definition of hydraulic gradient and the mathematical description of Darcy’s law is discussed first, followed by proposing a compatible form of hydraulic gradient definition in the conventional expression for seepage force. By integrating the body force, the general effective stress principle and the total seepage potential equation into the Biot’s consolidation theory, a strict definition of the general seepage force is obtained for various porous media including soils. The general seepage force proposed herein is the dot product of the general effective stress tensor and the total seepage potential gradient, which differs from conventional explanations of seepage force mechanism. Within the scope of Terzaghi's effective stress principle, the general seepage force is reduced to the form of total seepage potential gradient. Other related mechanical properties of seepage force are investigated as well as a further modified formulation for the general seepage force containing seepage velocity. The conclusions may afford profound insight into the concept of seepage force in geotechnical practice.
The seepage force is still one of the most fundamental and controversial concepts in soil mechanics. Based on the literature review, the compatibility between the definition of hydraulic gradient and the mathematical description of Darcy’s law is discussed first, followed by proposing a compatible form of hydraulic gradient definition in the conventional expression for seepage force. By integrating the body force, the general effective stress principle and the total seepage potential equation into the Biot’s consolidation theory, a strict definition of the general seepage force is obtained for various porous media including soils. The general seepage force proposed herein is the dot product of the general effective stress tensor and the total seepage potential gradient, which differs from conventional explanations of seepage force mechanism. Within the scope of Terzaghi's effective stress principle, the general seepage force is reduced to the form of total seepage potential gradient. Other related mechanical properties of seepage force are investigated as well as a further modified formulation for the general seepage force containing seepage velocity. The conclusions may afford profound insight into the concept of seepage force in geotechnical practice.
Abstract：
The hydraulic conductivity of filter cake is important for the stability of tunnel face when the slurry shield is excavated. The behaviors of slurry penetration and filter cake formation are investigated based on the infiltration column tests. The change of hydraulic conductivity of filter cake with time is obtained from tests. The permeable filter cake is formed on the tunnel face during advancing because the filter cake is destroyed by cutter and re-built. The scarcely-permeable filter cake is formed during assembling of linings because the cutter wheel stops rotating. A two-dimensional numerical model considering the permeable filter cake is established for calculating the pore pressure in the filter cake and stratum. The numerical results show the maximum pore pressure when the slurry shield is advancing. The seepage forces on the filter cake and failure zone are computed according to the pore pressure. The distributive ratios of the seepage force in the filter cake to the seepage force in the stratum are obtained under different slurry pressures. The efficiency of stabilizing the tunnel face deteriorates when the slurry pressure increases. For a higher slurry pressure, more pore pressure drops beyond the failure zone. A new calculation approach for the limit slurry pressure is proposed based on the limit equilibrium method. The results show that the limit slurry pressure is overestimated by the existing methods.
The hydraulic conductivity of filter cake is important for the stability of tunnel face when the slurry shield is excavated. The behaviors of slurry penetration and filter cake formation are investigated based on the infiltration column tests. The change of hydraulic conductivity of filter cake with time is obtained from tests. The permeable filter cake is formed on the tunnel face during advancing because the filter cake is destroyed by cutter and re-built. The scarcely-permeable filter cake is formed during assembling of linings because the cutter wheel stops rotating. A two-dimensional numerical model considering the permeable filter cake is established for calculating the pore pressure in the filter cake and stratum. The numerical results show the maximum pore pressure when the slurry shield is advancing. The seepage forces on the filter cake and failure zone are computed according to the pore pressure. The distributive ratios of the seepage force in the filter cake to the seepage force in the stratum are obtained under different slurry pressures. The efficiency of stabilizing the tunnel face deteriorates when the slurry pressure increases. For a higher slurry pressure, more pore pressure drops beyond the failure zone. A new calculation approach for the limit slurry pressure is proposed based on the limit equilibrium method. The results show that the limit slurry pressure is overestimated by the existing methods.
Abstract：
The ground in Xuwei harbor area of Lianyungang Port is the deep soft clay, and its engineering property is poor. On the soft ground the breakwater is proposed to be constructed. Predicting the consolidation settlement of the ground of the breakwater accurately can give a design optimization and provide technical support for the harbor construction. The modified Cam-clay model is used as the constitutive model for soils, and the staged-construction of the breakwater is simulated. A systematic parametric study is conducted, and the results are back-calculated to determine the extent and permeability of smear zone with respect to installation of offshore PVDs. The numerical results reveal that when the extent ratio is 5 and the permeability ratio kh/ks is 5, the in-situ observed results agree well with the numerical ones. In order to meet the requirements of the post-construction settlement of oil pipeline foundation, the construction quality of PVDs should be controlled, and the preloading time of the breakwater should be more than 13 months.
The ground in Xuwei harbor area of Lianyungang Port is the deep soft clay, and its engineering property is poor. On the soft ground the breakwater is proposed to be constructed. Predicting the consolidation settlement of the ground of the breakwater accurately can give a design optimization and provide technical support for the harbor construction. The modified Cam-clay model is used as the constitutive model for soils, and the staged-construction of the breakwater is simulated. A systematic parametric study is conducted, and the results are back-calculated to determine the extent and permeability of smear zone with respect to installation of offshore PVDs. The numerical results reveal that when the extent ratio is 5 and the permeability ratio kh/ks is 5, the in-situ observed results agree well with the numerical ones. In order to meet the requirements of the post-construction settlement of oil pipeline foundation, the construction quality of PVDs should be controlled, and the preloading time of the breakwater should be more than 13 months.
Abstract：
The rock mass rating (RMR14) method can evaluate the mechanical properties and structural conditions of rock mass comprehensively and consider the engineering factors such as the initial ground stress fields and excavation ways. RMR14 is quite suitable for evaluating the surrounding rock of mountaneous tunnels. But the ladder-like rating in RMR14 will cause the fuzzy uncertainty at the interval boundaries. In order to evaluate the quality of rock mass more accurately, the fuzzy membership functions are introduced to solve the problem of fuzzy uncertainty based on the Mamdani fuzzy inference method. A new RMR14 method based on the fuzzy inference is proposed by using the “if - then” reasoning rules to realize multiple input parameters and inference rules of the fuzzy inference. The new method is used to assess the qualities of the surrounding rock mass of Piaoli tunnel of Dushan-Pingtang highway. The results show that the new RMR14 based on the fuzzy inference can draw the existing engineering experience and appropriately solve the boundary problem of fuzzy uncertainty. Compared with the original RMR14, the new RMR14 based on the fuzzy inference can assess the quality of the surrounding rock more accurately and steadily.
The rock mass rating (RMR14) method can evaluate the mechanical properties and structural conditions of rock mass comprehensively and consider the engineering factors such as the initial ground stress fields and excavation ways. RMR14 is quite suitable for evaluating the surrounding rock of mountaneous tunnels. But the ladder-like rating in RMR14 will cause the fuzzy uncertainty at the interval boundaries. In order to evaluate the quality of rock mass more accurately, the fuzzy membership functions are introduced to solve the problem of fuzzy uncertainty based on the Mamdani fuzzy inference method. A new RMR14 method based on the fuzzy inference is proposed by using the “if - then” reasoning rules to realize multiple input parameters and inference rules of the fuzzy inference. The new method is used to assess the qualities of the surrounding rock mass of Piaoli tunnel of Dushan-Pingtang highway. The results show that the new RMR14 based on the fuzzy inference can draw the existing engineering experience and appropriately solve the boundary problem of fuzzy uncertainty. Compared with the original RMR14, the new RMR14 based on the fuzzy inference can assess the quality of the surrounding rock more accurately and steadily.
Abstract：
To improve the anti-seismic performance of tunnels in highly seismic areas with stick-slip fracture, the strong earthquake vibration dynamic tests on the damping layer of large static bond-slip with different thicknesses are performed based on the section of F8 stick-slip fracture in Longxi tunnel of Dujiangyan-Wenchun expressway under earthquake action. The shock absorption technology is used for the damping layer. The results show that under the action of earthquake, the fault rupture dislocation has greater influence scope and degree on the upper part of the tunnel than on the lower part of the tunnel. The plate part is the focus of the earthquake resistance design. The damping effects of 10 cm thick-shock absorption layer of the tunnel are obviously better than those of 5 cm thick and 15 cm thick shock absorption layers. The 10 cm thick shock absorption layer is thus recommended. The research results may provide the technical support for improving the anti-seismic performance of tunnels across dangerous mountains with stick-slip fracture.
To improve the anti-seismic performance of tunnels in highly seismic areas with stick-slip fracture, the strong earthquake vibration dynamic tests on the damping layer of large static bond-slip with different thicknesses are performed based on the section of F8 stick-slip fracture in Longxi tunnel of Dujiangyan-Wenchun expressway under earthquake action. The shock absorption technology is used for the damping layer. The results show that under the action of earthquake, the fault rupture dislocation has greater influence scope and degree on the upper part of the tunnel than on the lower part of the tunnel. The plate part is the focus of the earthquake resistance design. The damping effects of 10 cm thick-shock absorption layer of the tunnel are obviously better than those of 5 cm thick and 15 cm thick shock absorption layers. The 10 cm thick shock absorption layer is thus recommended. The research results may provide the technical support for improving the anti-seismic performance of tunnels across dangerous mountains with stick-slip fracture.
Abstract：
Since the joints between diaphragm wall panels lead to reduction of hoop stiffness in circular excavation, it is necessary to propose a method to quantitatively evaluate the hoop stiffness correction coefficient α. The stress-strain curves of joints are first measured through the mechanical experiments on specimens containing joints and then compared to those of intact concrete cubes, separately considering the two typical locations for diaphragm wall panels with milling joint, in panels and between panels. For reliability purpose, the intact concrete cubes and specimens containing joints are both produced with concrete and slurry applied from a construction site. Then, the trilinear function is introduced to best fit the measured stress-strain curves, thus the formula for the stiffness correction coefficient α is proposed and subsequently applied to a real case analysis. The results show that the value of the stiffness correction coefficient α mainly depends on the average length of panel, the width of joints as well as the stiffness of joints and concrete. The value of α ranges between 0.485~0.514 in case of low hoop stress, and turns into a rise when the hoop stress exceeds 7.37 MPa, and reaches 0.545~0.581 when it rises up to 11 MPa. Further, the calculated results based on α obtained in the site tests indicate that it is efficient when considering the effect of joints by just taking α into account when the hoop stress stays at a low level, but the nonlinear stiffness of joints should be considered when the stress is high. In conclusion, an approach for quantitatively evaluating the hoop stiffness correction coefficient α is proposed and can be further applied for engineering application purpose.
Since the joints between diaphragm wall panels lead to reduction of hoop stiffness in circular excavation, it is necessary to propose a method to quantitatively evaluate the hoop stiffness correction coefficient α. The stress-strain curves of joints are first measured through the mechanical experiments on specimens containing joints and then compared to those of intact concrete cubes, separately considering the two typical locations for diaphragm wall panels with milling joint, in panels and between panels. For reliability purpose, the intact concrete cubes and specimens containing joints are both produced with concrete and slurry applied from a construction site. Then, the trilinear function is introduced to best fit the measured stress-strain curves, thus the formula for the stiffness correction coefficient α is proposed and subsequently applied to a real case analysis. The results show that the value of the stiffness correction coefficient α mainly depends on the average length of panel, the width of joints as well as the stiffness of joints and concrete. The value of α ranges between 0.485~0.514 in case of low hoop stress, and turns into a rise when the hoop stress exceeds 7.37 MPa, and reaches 0.545~0.581 when it rises up to 11 MPa. Further, the calculated results based on α obtained in the site tests indicate that it is efficient when considering the effect of joints by just taking α into account when the hoop stress stays at a low level, but the nonlinear stiffness of joints should be considered when the stress is high. In conclusion, an approach for quantitatively evaluating the hoop stiffness correction coefficient α is proposed and can be further applied for engineering application purpose.
Abstract：
The safety control of blasting vibration under complex condition is the key problem of excavation. The rock mass is frozen under the cold condition, which induces the change of mechanical performance and wave impedance properties, and affects the blasting vibration attenuation characteristics. Firstly, the comparitive experiments are implemented to study the mechanical performance under different conditions. The results reveal that the increase of the elasticity modulus can reach as much as 30%. Based on the test results of the reconstruction project of Fengman Hydropower Station, the characteristics of blasting vibration attenuation for summer and winter are compared. The results reveal that a cut-off point about scaled distance exists, and the magnitude of the relationship changes at the cut-off point. The effect of cold depth is investigated, and a mathematical description for the maximum charge weight versus frozen depth is proposed. Finally, the effect of blasting vibration control standard is studied under cold condition from the angle of spectral characteristics of stress wave and the ultimate strength of the structure. The research results may provide the reference for the blasting safety control in the north cold excavation regions.
The safety control of blasting vibration under complex condition is the key problem of excavation. The rock mass is frozen under the cold condition, which induces the change of mechanical performance and wave impedance properties, and affects the blasting vibration attenuation characteristics. Firstly, the comparitive experiments are implemented to study the mechanical performance under different conditions. The results reveal that the increase of the elasticity modulus can reach as much as 30%. Based on the test results of the reconstruction project of Fengman Hydropower Station, the characteristics of blasting vibration attenuation for summer and winter are compared. The results reveal that a cut-off point about scaled distance exists, and the magnitude of the relationship changes at the cut-off point. The effect of cold depth is investigated, and a mathematical description for the maximum charge weight versus frozen depth is proposed. Finally, the effect of blasting vibration control standard is studied under cold condition from the angle of spectral characteristics of stress wave and the ultimate strength of the structure. The research results may provide the reference for the blasting safety control in the north cold excavation regions.
