2019 Vol. 41 No. 4
Abstract:
The field observations show that the existing widely-used constitutive models tend to underestimate the settlement of dams higher than 200 m. One of the main reasons is the separation of load-induced deformation and creep in such models and the neglecting of creep during construction in numerical simulations. In this study, a constitutive model, taking the stress, strain and time as the basic variables, is proposed in the light of experimental observations to model the loading deformation, creep and stress relaxation in a unified manner. A basic assumption used is the simultaneous occurrence of the load-induced plastic strains and creep strains. Both the change of stresses and the accumulation of time can result in expansion of the yield surface and thus produce plastic strains to obey, however, different flow rules. The model is used to reproduce the experimental results of a rockfill material and a gravelly soil used in an extremely high rockfill dam, and it is further used to study the deformation behavior of the dam. The numerical results show that neglecting the creep during construction may underestimate the settlement by more than 10%. Therefore, for the extremely high rockfill dams, it is necessary to use the unified models to consider the creep and loading behavior for rockfill materials so as to increase the precision of predictions.
The field observations show that the existing widely-used constitutive models tend to underestimate the settlement of dams higher than 200 m. One of the main reasons is the separation of load-induced deformation and creep in such models and the neglecting of creep during construction in numerical simulations. In this study, a constitutive model, taking the stress, strain and time as the basic variables, is proposed in the light of experimental observations to model the loading deformation, creep and stress relaxation in a unified manner. A basic assumption used is the simultaneous occurrence of the load-induced plastic strains and creep strains. Both the change of stresses and the accumulation of time can result in expansion of the yield surface and thus produce plastic strains to obey, however, different flow rules. The model is used to reproduce the experimental results of a rockfill material and a gravelly soil used in an extremely high rockfill dam, and it is further used to study the deformation behavior of the dam. The numerical results show that neglecting the creep during construction may underestimate the settlement by more than 10%. Therefore, for the extremely high rockfill dams, it is necessary to use the unified models to consider the creep and loading behavior for rockfill materials so as to increase the precision of predictions.
Abstract:
The foundation failure of a dam on the eastern margin of the Tibetan Plateau during seepage and chemical processes is studied. By using Cl- tracing, temperature-conductivity tracing and artificial tracing techniques, it is determined that there is leakage through the non-closed concrete cut-off wall, where the reservoir water leaks through the dam foundation, and excretes through the filter layer after the dam area. The acid dissolution experiment and X-ray fluorescence analysis show that the precipitated white particles on the surface after the dam are mainly composited of CaCO3. Combined with the chemical reaction process. It is confirmed that the precipitation comes from the cement of cut-off wall. The CO2 dissolved in the leakage water reacting with Ca(OH)2 in the cement can generate soluble Ca(HCO3)2, which is taken to the surface by the leakage water and decomposed to CaCO3 at normal temperature. Through the measurement and analysis of CO2 content in surface water, it is found that CO2 in the water-rock reaction has other sources except atmospheric precipitation. Considering the local geologic structure, it is inferred that CO2 mainly from the deep carbon pool enters the groundwater and reservoir water through the fault zones, and participates in the water-rock reaction. This results in a faster erosion of the dam. The chemical erosion of the dam and hydraulic structures will be greatly enhanced if the deep CO2 enters the groundwater and participates in the seepage process.
The foundation failure of a dam on the eastern margin of the Tibetan Plateau during seepage and chemical processes is studied. By using Cl- tracing, temperature-conductivity tracing and artificial tracing techniques, it is determined that there is leakage through the non-closed concrete cut-off wall, where the reservoir water leaks through the dam foundation, and excretes through the filter layer after the dam area. The acid dissolution experiment and X-ray fluorescence analysis show that the precipitated white particles on the surface after the dam are mainly composited of CaCO3. Combined with the chemical reaction process. It is confirmed that the precipitation comes from the cement of cut-off wall. The CO2 dissolved in the leakage water reacting with Ca(OH)2 in the cement can generate soluble Ca(HCO3)2, which is taken to the surface by the leakage water and decomposed to CaCO3 at normal temperature. Through the measurement and analysis of CO2 content in surface water, it is found that CO2 in the water-rock reaction has other sources except atmospheric precipitation. Considering the local geologic structure, it is inferred that CO2 mainly from the deep carbon pool enters the groundwater and reservoir water through the fault zones, and participates in the water-rock reaction. This results in a faster erosion of the dam. The chemical erosion of the dam and hydraulic structures will be greatly enhanced if the deep CO2 enters the groundwater and participates in the seepage process.
Abstract:
By using a suction-controllable dynamic triaxial testing apparatus, a series of dynamic deformation tests are carried out on a unsaturated silt subjected to wide suctions imposed by the axial translation technique and vapor equilibrium method with saturated salt solution. The skeleton curves, dynamic elastic moduli and damping ratios are obtained over a wide suction range under almost constant net confining stress and suction. The effect of suction on dynamic deformation characteristics of unsaturated silt is studied. The test results show that at the same net confining pressure, the skeleton curves and dynamic elastic moduli increase with increasing suction, but the damping ratios decrease with increasing suction. As the hydraulic state of specimens changes from the boundary effect zone to the transition zone and then to the residual zone, the increase rates in the skeleton curves and dynamic elastic moduli decrease gradually, the decrease rate in the damping ratios is still obvious, and the decay rate of the dynamic elastic moduli with strain decreases with the increasing suction.
By using a suction-controllable dynamic triaxial testing apparatus, a series of dynamic deformation tests are carried out on a unsaturated silt subjected to wide suctions imposed by the axial translation technique and vapor equilibrium method with saturated salt solution. The skeleton curves, dynamic elastic moduli and damping ratios are obtained over a wide suction range under almost constant net confining stress and suction. The effect of suction on dynamic deformation characteristics of unsaturated silt is studied. The test results show that at the same net confining pressure, the skeleton curves and dynamic elastic moduli increase with increasing suction, but the damping ratios decrease with increasing suction. As the hydraulic state of specimens changes from the boundary effect zone to the transition zone and then to the residual zone, the increase rates in the skeleton curves and dynamic elastic moduli decrease gradually, the decrease rate in the damping ratios is still obvious, and the decay rate of the dynamic elastic moduli with strain decreases with the increasing suction.
Abstract:
The long-term settlement of high-speed railway subgrade usually occurs under the influences of the nearby loads, and it will result in unsafety of the high-speed running trains. A practical computation method for creep is proposed by Yao et al, and it can predict the future deformation according to the limited settlement data. Taking the simulating data from the ABAQUS software which can simulate the increasing subgrade settlement with time as examples, the feasibility of using the practical method for creep to predict the long-term deformation of high-speed railway subgrade is demonstrated, and the relavant method to calculate the effective range of the prediction under certain errors is also given. It is shown that by using the practical method for creep, we can accurately predict the deformation based on the existing data, and also can get the early warning deformation of the railway for the future. It is of great significance to the safe operation management of high-speed railways.
The long-term settlement of high-speed railway subgrade usually occurs under the influences of the nearby loads, and it will result in unsafety of the high-speed running trains. A practical computation method for creep is proposed by Yao et al, and it can predict the future deformation according to the limited settlement data. Taking the simulating data from the ABAQUS software which can simulate the increasing subgrade settlement with time as examples, the feasibility of using the practical method for creep to predict the long-term deformation of high-speed railway subgrade is demonstrated, and the relavant method to calculate the effective range of the prediction under certain errors is also given. It is shown that by using the practical method for creep, we can accurately predict the deformation based on the existing data, and also can get the early warning deformation of the railway for the future. It is of great significance to the safe operation management of high-speed railways.
Abstract:
Bentonite is widely seen as a potential buffer/backfill materials for deeply buried geological disposal system to isolate high-level radioactive waste (HLRW). The swelling deformation and shear strength of the bentonite in saline solutions are very important to the safety of the HLRW depository. A conceptual model is proposed to explain the influences of osmotic suction on the volume change and shear strength of the bentonite in a quantitative way. A new formula for the effective stress is proposed as incorporating with vertical stress (p) and osmotic suction (π), and . The relationship is expressed using a unique curve in the form of for the bentonite in NaCl solutions. The peak shear strength is expressed using a unique curve according to the Mohr-Coulomb criterion for the bentonite in NaCl solutions.
Bentonite is widely seen as a potential buffer/backfill materials for deeply buried geological disposal system to isolate high-level radioactive waste (HLRW). The swelling deformation and shear strength of the bentonite in saline solutions are very important to the safety of the HLRW depository. A conceptual model is proposed to explain the influences of osmotic suction on the volume change and shear strength of the bentonite in a quantitative way. A new formula for the effective stress is proposed as
Abstract:
Energy accumulation and energy release appear evidently during the whole deformation process of jointed rock mass. In order to explore the energy evolution law of jointed rock mass under loading condition, the uniaxial compression experiment and the energy theory are adopted to study the characteristics of energy transformation of total energy U, elastic strain energy Ue and dissipation energy Ud, and to reveal the energy damage evolution mechanism of the jointed rock mass. Based on the change rate of ratio dissipated energy to elastic energy (dK/dε), the criteria for crack propagation and strength failure of non-across jointed rock mass are proposed. The obtained results show that the energy reserve of the rock mass is obviously weakened by the non-across joints, and the total energy and elastic strain energy at the peak point gradually decrease with the increase of the number of joints. The curves of elastic strain energy and dissipated energy of jointed rock mass distinctly appear “step shape” (abrupt decrease or increase), and the values that the elastic strain energy and dissipative energy of double pre-existing flaws abruptly increase or decrease are significantly smaller than those of single pre-existing flaw. The pre- and post-peak continuous mutation (dK/dε alternates between negative and positive) and mutations (dK/dε maintains as positive infinity) of dK/dε are regarded as the crack propagation and the strength failure criteria for the jointed rock mass.
Energy accumulation and energy release appear evidently during the whole deformation process of jointed rock mass. In order to explore the energy evolution law of jointed rock mass under loading condition, the uniaxial compression experiment and the energy theory are adopted to study the characteristics of energy transformation of total energy U, elastic strain energy Ue and dissipation energy Ud, and to reveal the energy damage evolution mechanism of the jointed rock mass. Based on the change rate of ratio dissipated energy to elastic energy (dK/dε), the criteria for crack propagation and strength failure of non-across jointed rock mass are proposed. The obtained results show that the energy reserve of the rock mass is obviously weakened by the non-across joints, and the total energy and elastic strain energy at the peak point gradually decrease with the increase of the number of joints. The curves of elastic strain energy and dissipated energy of jointed rock mass distinctly appear “step shape” (abrupt decrease or increase), and the values that the elastic strain energy and dissipative energy of double pre-existing flaws abruptly increase or decrease are significantly smaller than those of single pre-existing flaw. The pre- and post-peak continuous mutation (dK/dε alternates between negative and positive) and mutations (dK/dε maintains as positive infinity) of dK/dε are regarded as the crack propagation and the strength failure criteria for the jointed rock mass.
Abstract:
The wetting tests of coarse-grained soil under the equal stress ratio path are performed using the basalt rockfill of Gushui concrete face rockfill dam. The tests results show that the wetting deformation of soil under equal stress ratio path is affected by stress state and loading stress path. The stress path is added in the traditional wetting deformation formula, and the volumetric stress and reference stress level are used to simulate the influences of stress state on the wetting deformation. A formula for the wetting deformation under equal stress ratio path is developed. The developed formula can accurately simulate the wetting deformation of the coarse-grained soil under equal stress ratio path.
The wetting tests of coarse-grained soil under the equal stress ratio path are performed using the basalt rockfill of Gushui concrete face rockfill dam. The tests results show that the wetting deformation of soil under equal stress ratio path is affected by stress state and loading stress path. The stress path is added in the traditional wetting deformation formula, and the volumetric stress and reference stress level are used to simulate the influences of stress state on the wetting deformation. A formula for the wetting deformation under equal stress ratio path is developed. The developed formula can accurately simulate the wetting deformation of the coarse-grained soil under equal stress ratio path.
Abstract:
The design of experiments (DOE) is used to study the microscopic parameter calibration for hard rocks of PFC3D model. Firstly, the sensitivity of microscopic parameters to macroscopic responses is analyzed through the Plackett-Burman design. The linear relationship between microscopic parameters and macroscopic responses is established. Then, the interaction between microscopic parameters is investigated by using the response surface method (RSM) and the nonlinear relationship between microscopic parameters and macroscopic responses is obtained. Finally, the problem is transformed into a nonlinear multiobjective mathematical programming problem, and the FGOALATTAIN function in MATLAB software is utilized to solve the problem. It can be found that when using DOE to calibrate the microscopic parameters, the PFC3D model can well reflect the failure process of the rock under uniaxial and low confining compression conditions. However, the fitting results under high confining pressure is unsatisfactory. The method based on the PB design, response surface method and mathematical programming can reflect the sensitivity of the microscopic parameters, and the definite function expressions are obtained. At the same time, it can reflect more characteristics of rock mechanics by adding the constraints condition in the process of solving.
The design of experiments (DOE) is used to study the microscopic parameter calibration for hard rocks of PFC3D model. Firstly, the sensitivity of microscopic parameters to macroscopic responses is analyzed through the Plackett-Burman design. The linear relationship between microscopic parameters and macroscopic responses is established. Then, the interaction between microscopic parameters is investigated by using the response surface method (RSM) and the nonlinear relationship between microscopic parameters and macroscopic responses is obtained. Finally, the problem is transformed into a nonlinear multiobjective mathematical programming problem, and the FGOALATTAIN function in MATLAB software is utilized to solve the problem. It can be found that when using DOE to calibrate the microscopic parameters, the PFC3D model can well reflect the failure process of the rock under uniaxial and low confining compression conditions. However, the fitting results under high confining pressure is unsatisfactory. The method based on the PB design, response surface method and mathematical programming can reflect the sensitivity of the microscopic parameters, and the definite function expressions are obtained. At the same time, it can reflect more characteristics of rock mechanics by adding the constraints condition in the process of solving.
Abstract:
The fracture-compaction mode is the main diffusion mode of grouting process in sand layer. In order to study the diffusion process of fracture-compaction grouting mode in sand layer, a visible grouting simulation test system composed of test frame, stress loading module, dynamic monitoring module and grouting module is designed and developed. As the typical grouted medium, the clayey sand in Qingdao is used in the fracture-compaction grouting simulation tests. The characteristics of the dynamic evolution of the stress and displacement fields of sand layer and the propagation of fracture channel are derived. The influence scope of fracture-compaction grouting in sand layer is obtained. The results show that the initiation and propagation of fracture channel direction are consistent with those of the major principal stress in sand layer. The width of grouting vein decreases remarkably along the propagation direction. Under the influences of grouting, the vertical stress increases from the minor principal stress and decays spatially along the propagation direction. However, the horizontal stress has no obvious changes in the grouting process. The influence scope of fracture-compaction grouting in sand layer is very limited (20~40 cm). Finally, improvement suggestions for fracture-compaction grouting in sand layer are put forward from two aspects (grouting amount of single hole and arrangement of injection hole).
The fracture-compaction mode is the main diffusion mode of grouting process in sand layer. In order to study the diffusion process of fracture-compaction grouting mode in sand layer, a visible grouting simulation test system composed of test frame, stress loading module, dynamic monitoring module and grouting module is designed and developed. As the typical grouted medium, the clayey sand in Qingdao is used in the fracture-compaction grouting simulation tests. The characteristics of the dynamic evolution of the stress and displacement fields of sand layer and the propagation of fracture channel are derived. The influence scope of fracture-compaction grouting in sand layer is obtained. The results show that the initiation and propagation of fracture channel direction are consistent with those of the major principal stress in sand layer. The width of grouting vein decreases remarkably along the propagation direction. Under the influences of grouting, the vertical stress increases from the minor principal stress and decays spatially along the propagation direction. However, the horizontal stress has no obvious changes in the grouting process. The influence scope of fracture-compaction grouting in sand layer is very limited (20~40 cm). Finally, improvement suggestions for fracture-compaction grouting in sand layer are put forward from two aspects (grouting amount of single hole and arrangement of injection hole).
Abstract:
Microbial cementation can effectively improve the strength of soil, but it can also lead to the obvious brittleness at soil failure. In order to balance the adverse effect of brittleness of the bio-cemented soil, a modified method of combining the fiber reinforcement with the microbial cementation is suggested. The polypropylene fibers, with mass fraction of 0%, 0.05%, 0.15%, 0.25% and 0.30%, are uniformly mixed with silica sand, then the soil samples are bio-cemented based on microbial-induced calcite precipitation (MICP). A series of unconfined compression tests are also carried out, the calcium carbonate content in each group is determined by acid pickling, and the morphological structure of fiber surfaces in soil matrix is characterized by using the scanning electron microscopy (SEM). The result shows that: (1) The fiber reinforcement can greatly improve the unconfined compressive strength and residual strength of soil samples, and can significantly improve the toughness of soil failure. (2) The fiber content has important influence on the mechanical properties of bio-cemented soil. The unconfined compressive strength with fiber content shows a trend of decrease after the first increase in general, the optimal fiber content is 0.15%, and the residual strength after peak is monotonically related with the fiber content. (3) The stress-strain curve of the microorganism solidified sandy soil is in a step-down mode, and the wave type relief features are locally exhibited. (4) The fiber reinforcement can improve the precipitation efficiency and yield of microbial-induced calcite, and at the same time, the bio-cementation effect of the calcium carbonate can promote the effect of fiber reinforcement. The combination of fiber reinforcement technology and MICP technology can realize complementary advantages, which has positive significance for improving the safety and stability of construction.
Microbial cementation can effectively improve the strength of soil, but it can also lead to the obvious brittleness at soil failure. In order to balance the adverse effect of brittleness of the bio-cemented soil, a modified method of combining the fiber reinforcement with the microbial cementation is suggested. The polypropylene fibers, with mass fraction of 0%, 0.05%, 0.15%, 0.25% and 0.30%, are uniformly mixed with silica sand, then the soil samples are bio-cemented based on microbial-induced calcite precipitation (MICP). A series of unconfined compression tests are also carried out, the calcium carbonate content in each group is determined by acid pickling, and the morphological structure of fiber surfaces in soil matrix is characterized by using the scanning electron microscopy (SEM). The result shows that: (1) The fiber reinforcement can greatly improve the unconfined compressive strength and residual strength of soil samples, and can significantly improve the toughness of soil failure. (2) The fiber content has important influence on the mechanical properties of bio-cemented soil. The unconfined compressive strength with fiber content shows a trend of decrease after the first increase in general, the optimal fiber content is 0.15%, and the residual strength after peak is monotonically related with the fiber content. (3) The stress-strain curve of the microorganism solidified sandy soil is in a step-down mode, and the wave type relief features are locally exhibited. (4) The fiber reinforcement can improve the precipitation efficiency and yield of microbial-induced calcite, and at the same time, the bio-cementation effect of the calcium carbonate can promote the effect of fiber reinforcement. The combination of fiber reinforcement technology and MICP technology can realize complementary advantages, which has positive significance for improving the safety and stability of construction.
Abstract:
The construction of shield tunnels in upper-soft and lower-hard composite strata is easy to cause cracking diseases of segments, and has a serious impact on their long-term safety. Based on a metro shield tunnel, a large number of field investigations are carried out for the segment cracks during the construction stage. The distribution laws and characteristics of cracking damage of linings are summarized. On this basis, causes for cracking of segments are analyzed by means of the extended finite element method. The results show that the scale of proportion of cracking forms in the shield tunnels from large to small is the spallings in the circumferential region, longitudinal cracks, and cracks at corners and edges. The first two can be described as the structural failure while the last one can be indicated as the material failure. The generation and expansion of longitudinal cracks, which are related to the jack thrust and the unevenness of contact surface, mostly distribute along the thrust interface of jacks. The propagation of cracks is a reciprocating process of energy accumulation and release with a step shape growth. The mechanism of fracture is tensile failure. The spallings in the circumferential region are related to slab staggering between the rings. The critical value of slab staggering for the spallings in the circumferential region is 8 mm, which is the same as the allowable radial displacement of mortise and tenon. During the construction period of shield tunnels with staggered joint assembly in upper-soft and lower-hard composite strata, the segments with tenon should be avoided and the depth of the grooves should be reduced.
The construction of shield tunnels in upper-soft and lower-hard composite strata is easy to cause cracking diseases of segments, and has a serious impact on their long-term safety. Based on a metro shield tunnel, a large number of field investigations are carried out for the segment cracks during the construction stage. The distribution laws and characteristics of cracking damage of linings are summarized. On this basis, causes for cracking of segments are analyzed by means of the extended finite element method. The results show that the scale of proportion of cracking forms in the shield tunnels from large to small is the spallings in the circumferential region, longitudinal cracks, and cracks at corners and edges. The first two can be described as the structural failure while the last one can be indicated as the material failure. The generation and expansion of longitudinal cracks, which are related to the jack thrust and the unevenness of contact surface, mostly distribute along the thrust interface of jacks. The propagation of cracks is a reciprocating process of energy accumulation and release with a step shape growth. The mechanism of fracture is tensile failure. The spallings in the circumferential region are related to slab staggering between the rings. The critical value of slab staggering for the spallings in the circumferential region is 8 mm, which is the same as the allowable radial displacement of mortise and tenon. During the construction period of shield tunnels with staggered joint assembly in upper-soft and lower-hard composite strata, the segments with tenon should be avoided and the depth of the grooves should be reduced.
Abstract:
In order to eliminate the adverse effects of organic matter on the treatment of sludge with cement, the countermeasures to increase the strength of cement-stabilized sludge are put forward, including degrading organic matter, reducing thickness of double electric layers and maintaining pH value steadily. The potassium ferrate and sodium bicarbonate are used as the alkaline oxidant to treat the sludge with cement. Through the unconfined compressive strength tests, the influences of alkaline oxidant on the strength of cement-stabilized sludge are understood preliminarily. The solidification mechanism of alkaline oxidant is studied by means of the organic element tests, electrokinetic potential tests, specific surface area tests and SEM. The experimental results show that the unconfined compressive strength of cement-stabilized sludge can reach 1.536 MPa after curing for 7 days using the alkaline oxidant. The sodium bicarbonate maintains the cement-stabilized sludge in an alkaline environment with the pH value at 9 to 10 by neutralizing organic acids. Under this condition, the potassium ferrate can degrade organic matter effectively. Thus, the removal of the organic matter shell on the surface of clay particles is conducive to free SiO2 and Al2O3 to dissolve into the pore solution, which promotes the formation of cement hydration products. Besides, the high valence ions in pore solution replacing the low valence ions on the clay particles surface lead to a decrease in repulsion between the successive diffused double layers and the flocculation of soil particles.
In order to eliminate the adverse effects of organic matter on the treatment of sludge with cement, the countermeasures to increase the strength of cement-stabilized sludge are put forward, including degrading organic matter, reducing thickness of double electric layers and maintaining pH value steadily. The potassium ferrate and sodium bicarbonate are used as the alkaline oxidant to treat the sludge with cement. Through the unconfined compressive strength tests, the influences of alkaline oxidant on the strength of cement-stabilized sludge are understood preliminarily. The solidification mechanism of alkaline oxidant is studied by means of the organic element tests, electrokinetic potential tests, specific surface area tests and SEM. The experimental results show that the unconfined compressive strength of cement-stabilized sludge can reach 1.536 MPa after curing for 7 days using the alkaline oxidant. The sodium bicarbonate maintains the cement-stabilized sludge in an alkaline environment with the pH value at 9 to 10 by neutralizing organic acids. Under this condition, the potassium ferrate can degrade organic matter effectively. Thus, the removal of the organic matter shell on the surface of clay particles is conducive to free SiO2 and Al2O3 to dissolve into the pore solution, which promotes the formation of cement hydration products. Besides, the high valence ions in pore solution replacing the low valence ions on the clay particles surface lead to a decrease in repulsion between the successive diffused double layers and the flocculation of soil particles.
Abstract:
The swelling behavior of buffer/backfill materials in high-level radioactive waste (HLW) repository is one of the significant indices evaluating their buffer/backfill properties. Due to the limitations of test apparatus, most of the previous swelling tests are performed under two simple extreme boundary conditions of constant volume (CV) and constant mean stress (CMS), which can not effectively reflect the complex stress-strain state in the repository. A new set of multi-boundary conditions dilatometer is designed to carry out a series of swelling tests under CV, CMS and constant stiffness (CS) boundary conditions. Based on this custom-made test apparatus, the swelling behavior of GMZ buffer/backfill materials is tested under complex boundary conditions. The results indicate that the developed test apparatus has the advantages of simple operation and stable performance, which can effectively stimulate various boundary conditions. The boundary conditions have an important influence on the ultimate swelling indices of samples. More specifically, the relationship between various boundary conditions and the corresponding swelling pressure is CV > CS > CMS, and the relationship between these conditions and the corresponding swelling strain is CMS > CS > CV. The swelling equilibrium limit (SEL) curve can be used as a reference for predicting the ultimate swelling strain and pressure of soils during hydration under complex boundary conditions. The test results have certain reference significance for further understanding the swelling behavior and guiding the design of buffer/backfill materials in HLW repository.
The swelling behavior of buffer/backfill materials in high-level radioactive waste (HLW) repository is one of the significant indices evaluating their buffer/backfill properties. Due to the limitations of test apparatus, most of the previous swelling tests are performed under two simple extreme boundary conditions of constant volume (CV) and constant mean stress (CMS), which can not effectively reflect the complex stress-strain state in the repository. A new set of multi-boundary conditions dilatometer is designed to carry out a series of swelling tests under CV, CMS and constant stiffness (CS) boundary conditions. Based on this custom-made test apparatus, the swelling behavior of GMZ buffer/backfill materials is tested under complex boundary conditions. The results indicate that the developed test apparatus has the advantages of simple operation and stable performance, which can effectively stimulate various boundary conditions. The boundary conditions have an important influence on the ultimate swelling indices of samples. More specifically, the relationship between various boundary conditions and the corresponding swelling pressure is CV > CS > CMS, and the relationship between these conditions and the corresponding swelling strain is CMS > CS > CV. The swelling equilibrium limit (SEL) curve can be used as a reference for predicting the ultimate swelling strain and pressure of soils during hydration under complex boundary conditions. The test results have certain reference significance for further understanding the swelling behavior and guiding the design of buffer/backfill materials in HLW repository.
Abstract:
A sinking simulation experiment on a deep and large caisson is carried out based on the main caisson of Shanghai-Nomtong Bridge. Through the analysis of the whole dynamic sinking, it is determined that the effective stress on the sidewall is affected by various factors such as sinking rate, pressure relaxation, inclination, sand-casting and sudden sinking. After stopping sand suction, the effective stress of the sidewall changes from a dynamic distribution to a quasi-static one, the overall trend is decreasing and shows the reduction of extreme points and recovery of pressure relaxation zone. The arrangement of the step can reduce the total side friction of the caisson, and the reduction mainly comes from the vertical section of the caisson. It is found that the sidewall of stepped caisson can be divided into linear zone, step zone, excess zone and pressure relaxation zone. A model for calculating the side friction of the caisson in vertical state is established based on the influences of the step and verified by the on-site monitoring tests on No. 29 caisson of Shanghai-Nantong Bridge. The inclination the of caisson causes a change in the effective stress distribution of the sidewall, and the increased earth pressure generated by the compaction is 3~4 times the active earth pressure of the corresponding side.
A sinking simulation experiment on a deep and large caisson is carried out based on the main caisson of Shanghai-Nomtong Bridge. Through the analysis of the whole dynamic sinking, it is determined that the effective stress on the sidewall is affected by various factors such as sinking rate, pressure relaxation, inclination, sand-casting and sudden sinking. After stopping sand suction, the effective stress of the sidewall changes from a dynamic distribution to a quasi-static one, the overall trend is decreasing and shows the reduction of extreme points and recovery of pressure relaxation zone. The arrangement of the step can reduce the total side friction of the caisson, and the reduction mainly comes from the vertical section of the caisson. It is found that the sidewall of stepped caisson can be divided into linear zone, step zone, excess zone and pressure relaxation zone. A model for calculating the side friction of the caisson in vertical state is established based on the influences of the step and verified by the on-site monitoring tests on No. 29 caisson of Shanghai-Nantong Bridge. The inclination the of caisson causes a change in the effective stress distribution of the sidewall, and the increased earth pressure generated by the compaction is 3~4 times the active earth pressure of the corresponding side.
Abstract:
During the expansion process, the expansive soils will produce expansion stress and vice versa. Therefore, it is very significant to explore the change of the shrinkage stress of the expansive soils during drying. In order to investigate the relationship among internal shrinkage stress of expansive soils, water content and suction during the drying shrinkage process, a series of desiccation tests are conducted under different relative humidity conditions. In the tests, three slurry samples of the initial saturation are prepared and respectively placed in humidifiers with different over-saturated salt solutions and then dried at a constant temperature of 20℃. The change of water content is recorded in real-time. Additionally, the micropressure sensors are embedded in these samples to measure the change of the internal shrinkage stress during drying. The results show that: (1) The evaporation characteristics of soil water are different under different relative humidity conditions. For example, the smaller the relative humidity, the faster the soil water evaporates, and the lower the residual water content of the soils. (2) During the process of drying, the internal shrinkage stress in the soils shows obviously phasic variation. The shrinkage stress increase slowly in the early drying and then begins to increase sharply with drying when the time or soil water content reaches the critical value ws or ts, which decreases with the increase of evaporation rate. (3) After the evaporation, the shrinkage stress has a negative correlation with the environment relative humidity and has a positive correlation with the corresponding suction.
During the expansion process, the expansive soils will produce expansion stress and vice versa. Therefore, it is very significant to explore the change of the shrinkage stress of the expansive soils during drying. In order to investigate the relationship among internal shrinkage stress of expansive soils, water content and suction during the drying shrinkage process, a series of desiccation tests are conducted under different relative humidity conditions. In the tests, three slurry samples of the initial saturation are prepared and respectively placed in humidifiers with different over-saturated salt solutions and then dried at a constant temperature of 20℃. The change of water content is recorded in real-time. Additionally, the micropressure sensors are embedded in these samples to measure the change of the internal shrinkage stress during drying. The results show that: (1) The evaporation characteristics of soil water are different under different relative humidity conditions. For example, the smaller the relative humidity, the faster the soil water evaporates, and the lower the residual water content of the soils. (2) During the process of drying, the internal shrinkage stress in the soils shows obviously phasic variation. The shrinkage stress increase slowly in the early drying and then begins to increase sharply with drying when the time or soil water content reaches the critical value ws or ts, which decreases with the increase of evaporation rate. (3) After the evaporation, the shrinkage stress has a negative correlation with the environment relative humidity and has a positive correlation with the corresponding suction.
Abstract:
The soil arching effect is a redistribution of the stress in soil. It is caused by the rigidity difference between the soil and the structures in the soil. The studies so far about the impact of cyclic loading on the soil arching effect is very limited. A custom-made apparatus is used to conduct plain-strain trapdoor model tests under soil self-weight, static loading and cyclic loading to study the impact of different loading conditions on the soil arching effect. The analogical soil of aluminum rods is used as the fill instead of sand. The parameter of the soil arching ratio is used to evaluate the soil arching effect. The test results from this study are compared with those from the previous studies. It is indicated that both static loading and cyclic loading reduce the existing steady soil arching. The reduction of the soil arching effect increases with the loading magnitude and frequency and decreases with the loading area. Under the same loading level, the cyclic loading reduces the soil arching effect more than the static loading. The difference of reduction caused by them decreases with the loading magnitude and increases with the loading frequency. Besides, the difference under the peak pressure is less than that under the zero pressure. On the whole, the expressions given by Evans[18] can well predict the soil arching ratios in the trapdoor tests under soil self-weight and static loading. However, an improvement of the expressions is necessary for the trapdoor tests under cyclic loading.
The soil arching effect is a redistribution of the stress in soil. It is caused by the rigidity difference between the soil and the structures in the soil. The studies so far about the impact of cyclic loading on the soil arching effect is very limited. A custom-made apparatus is used to conduct plain-strain trapdoor model tests under soil self-weight, static loading and cyclic loading to study the impact of different loading conditions on the soil arching effect. The analogical soil of aluminum rods is used as the fill instead of sand. The parameter of the soil arching ratio is used to evaluate the soil arching effect. The test results from this study are compared with those from the previous studies. It is indicated that both static loading and cyclic loading reduce the existing steady soil arching. The reduction of the soil arching effect increases with the loading magnitude and frequency and decreases with the loading area. Under the same loading level, the cyclic loading reduces the soil arching effect more than the static loading. The difference of reduction caused by them decreases with the loading magnitude and increases with the loading frequency. Besides, the difference under the peak pressure is less than that under the zero pressure. On the whole, the expressions given by Evans[18] can well predict the soil arching ratios in the trapdoor tests under soil self-weight and static loading. However, an improvement of the expressions is necessary for the trapdoor tests under cyclic loading.
2019, 41(4): 733-740.
DOI: 10.11779/CJGE201904017
Abstract:
A series of tests using ureolytic bacteria ATCC 11859 are conducted to reinforce the organic clay, with cementation reagent with different concentrations flowing to the soil under pressures. The reinforcement effect is comprehensively evaluated by comparing the changes of unconfined compressive strength, content of calcium carbonate, permeability, content of organic matter and concentration of ammonia and Ca2+ in the effluent. The results show that it is effective to treat the organic clay using pressure grouting. After treatment, the content of organic matter can be reduced by 1%~4%, the unconfined compression strength can increase by up to 370%, and the permeability coefficient can be reduced by about one order of magnitude. Under this bacteria condition (activity with 9.68 m M-urea hydrolysed•min-1 and concentration with about 108 cell/mL), the concentration of cementation reagent has a significant influence on the treatment effect. The unconfined compressive strength of the soil can be improved significantly by increasing the concentration of urea in 0.25 M-cementation reagent.
A series of tests using ureolytic bacteria ATCC 11859 are conducted to reinforce the organic clay, with cementation reagent with different concentrations flowing to the soil under pressures. The reinforcement effect is comprehensively evaluated by comparing the changes of unconfined compressive strength, content of calcium carbonate, permeability, content of organic matter and concentration of ammonia and Ca2+ in the effluent. The results show that it is effective to treat the organic clay using pressure grouting. After treatment, the content of organic matter can be reduced by 1%~4%, the unconfined compression strength can increase by up to 370%, and the permeability coefficient can be reduced by about one order of magnitude. Under this bacteria condition (activity with 9.68 m M-urea hydrolysed•min-1 and concentration with about 108 cell/mL), the concentration of cementation reagent has a significant influence on the treatment effect. The unconfined compressive strength of the soil can be improved significantly by increasing the concentration of urea in 0.25 M-cementation reagent.
2019, 41(4): 741-747.
DOI: 10.11779/CJGE201904018
Abstract:
Aiming at the phenomenon of salinization diseases in the aeolian sand subgrade in desert areas, the change laws of water, heat and salt in compacted aeolian sand soils are investigated. The self-made aeolian sand column test device with sodium salinite soil as its foundation is based on the real-time monitoring data from natural environment for 6 months (June to December) to analyze the temporal and spatial distributions and migration characteristics of water, heat and salt in the compacted aeolian sandy soil layers. The result shows that affected by the external environmental temperature, the compacted aeolian sandy soil layers affect the severe (0~50 cm) and weak (50~120 cm) areas. As the depth of burial increases, the internal temperature of the soil column tends to be flat, and the temperature of each layer reaching the peak has a hysteresis effect. Under the effect of the concentration gradient of the salinity content of the salt-containing underlay surface and the compaction aeolian sand deposit, the deep area of the compacted aeolian sand soil layer is moved in by salt, and it is composed of nothing. The accumulation of salt in the aeolian sandy soil layer lays the foundation for the further migration of salt. Affected by the external temperature, the shallow layer of soil is affected by the change of water storage and evaporation, which promotes the gradual migration of water carrying salt in the compacted aeolian sand layer. The salt has a "high-low-high" distribution pattern throughout the depth of the soil. Affected by the external environmental loads, the shallow salt content in the compacted aeolian sandy soil layers will gradually accumulate, and from less to more, it will form salinization diseases over time.
Aiming at the phenomenon of salinization diseases in the aeolian sand subgrade in desert areas, the change laws of water, heat and salt in compacted aeolian sand soils are investigated. The self-made aeolian sand column test device with sodium salinite soil as its foundation is based on the real-time monitoring data from natural environment for 6 months (June to December) to analyze the temporal and spatial distributions and migration characteristics of water, heat and salt in the compacted aeolian sandy soil layers. The result shows that affected by the external environmental temperature, the compacted aeolian sandy soil layers affect the severe (0~50 cm) and weak (50~120 cm) areas. As the depth of burial increases, the internal temperature of the soil column tends to be flat, and the temperature of each layer reaching the peak has a hysteresis effect. Under the effect of the concentration gradient of the salinity content of the salt-containing underlay surface and the compaction aeolian sand deposit, the deep area of the compacted aeolian sand soil layer is moved in by salt, and it is composed of nothing. The accumulation of salt in the aeolian sandy soil layer lays the foundation for the further migration of salt. Affected by the external temperature, the shallow layer of soil is affected by the change of water storage and evaporation, which promotes the gradual migration of water carrying salt in the compacted aeolian sand layer. The salt has a "high-low-high" distribution pattern throughout the depth of the soil. Affected by the external environmental loads, the shallow salt content in the compacted aeolian sandy soil layers will gradually accumulate, and from less to more, it will form salinization diseases over time.
Abstract:
Based on the thin annulus element method and the hypothesis of fictitious soil pile, a model for the settlement of an axially-loaded single floating pile in the layered soils is proposed. The soil column beneath the floating pile is regarded as the fictitious pile shaft. The soil-pile system is then divided into the separate thin-layer elements. The stiffness matrices for the soil element and pile element are deduced using the principle of virtual displacements. The global matrices are constructed by considering the continuity and equilibrium conditions between the elements. The vertical displacement of the soil-pile system can be obtained by solving the matrix equation. The comparisons of the results between the proposed model and the available solutions indicate the accuracy of the proposed model. Parametric study shows that the accuracy of the proposed solution depends greatly on the thickness of the thin annulus element and the choice of the displacement function. For the floating pile in the three-layer soil, the bearing capacity of the middle layer around the shaft increases with the increase of its elastic modulus. The tip stiffness of the floating pile increases with the decrease of the layer thickness between the pile tip and bedrock. When this thickness is less than 0.3 times the soil thickness, the pile tip stiffness will increase significantly. The pile tip stiffness is also dependent greatly on the properties of the underlying soil below the tip of the floating pile.
Based on the thin annulus element method and the hypothesis of fictitious soil pile, a model for the settlement of an axially-loaded single floating pile in the layered soils is proposed. The soil column beneath the floating pile is regarded as the fictitious pile shaft. The soil-pile system is then divided into the separate thin-layer elements. The stiffness matrices for the soil element and pile element are deduced using the principle of virtual displacements. The global matrices are constructed by considering the continuity and equilibrium conditions between the elements. The vertical displacement of the soil-pile system can be obtained by solving the matrix equation. The comparisons of the results between the proposed model and the available solutions indicate the accuracy of the proposed model. Parametric study shows that the accuracy of the proposed solution depends greatly on the thickness of the thin annulus element and the choice of the displacement function. For the floating pile in the three-layer soil, the bearing capacity of the middle layer around the shaft increases with the increase of its elastic modulus. The tip stiffness of the floating pile increases with the decrease of the layer thickness between the pile tip and bedrock. When this thickness is less than 0.3 times the soil thickness, the pile tip stiffness will increase significantly. The pile tip stiffness is also dependent greatly on the properties of the underlying soil below the tip of the floating pile.
Abstract:
The carbonate sand is widely distributed in the South China Sea and used as the fill materials for land reclamation. Carbonate sand particles are fragile and can be easily crushed, making the carbonate sand exhibit distinctive mechanical behaviors compared with terrestrial silica sand. Triaxial cyclic shear tests under drained conditions are conducted on a carbonate sand taken from a reef in the South China Sea to investigate the evolution of particle crushing during cyclic shearing process. In the range of the adopted confining pressure, little particle crushing is observed in isotropic consolidation process. In contrast, remarkable particle crushing occurs during the following cyclic shearing process. Angular abrasion is the main form of particle crushing, leading to the increase of the fine particle content in the post-shearing grading. Along with the continuation of cyclic shearing, the amount of particle crushing increases continuously, but the increasing rate decreases gradually. For the cyclic shearing of constant amplitude under constant confining pressure, a logarithmic equation can be used to fit the curve of the relative breakage index versus the number of cycles. Based on with the observed influencing laws of confining pressure and cyclic stress ratio on particle crushing, a mathematical model is proposed to describe the evolution process of particle crushing during cyclic shearing process.
The carbonate sand is widely distributed in the South China Sea and used as the fill materials for land reclamation. Carbonate sand particles are fragile and can be easily crushed, making the carbonate sand exhibit distinctive mechanical behaviors compared with terrestrial silica sand. Triaxial cyclic shear tests under drained conditions are conducted on a carbonate sand taken from a reef in the South China Sea to investigate the evolution of particle crushing during cyclic shearing process. In the range of the adopted confining pressure, little particle crushing is observed in isotropic consolidation process. In contrast, remarkable particle crushing occurs during the following cyclic shearing process. Angular abrasion is the main form of particle crushing, leading to the increase of the fine particle content in the post-shearing grading. Along with the continuation of cyclic shearing, the amount of particle crushing increases continuously, but the increasing rate decreases gradually. For the cyclic shearing of constant amplitude under constant confining pressure, a logarithmic equation can be used to fit the curve of the relative breakage index versus the number of cycles. Based on with the observed influencing laws of confining pressure and cyclic stress ratio on particle crushing, a mathematical model is proposed to describe the evolution process of particle crushing during cyclic shearing process.
Abstract:
The post-construction stability monitoring of breakwaters is one of the key tasks to their operation, and a large amount of measured data needs to be required for the stability and safety analysis. The traditional methods such as leveling and GPS are time consuming and labor-intensive, which only provide sparse points. The PSI technology can obtain the high-density target points and the line-of-sight (LOS) deformations through monitoring the distance changes between the radar sensor and the ground targets (such as artificial buildings, roads, and other infrastructures). PSI is suitable for monitoring the areas where long-term slow deformation occurs. Taking a breakwater (8 km) in Lianyungang as an example, a total of 38 C-band images of Sentinel-1A, from September 29, 2016 to December 29, 2017, are analyzed and 788 persistent scatterers (PS) are extracted. Nine feature points are selected to analyze the deformation pattern. The results show that the cumulative settlement of the breakwater reaches 336 mm, which accounts for 0.8% of the breakwater height, indicating that the dike is safe and stable. Finally, by comparing the PSI with the leveling during the same period, it can be found that the two monitoring results are consistent and can meet the accuracy requirements of the breakwater settlement monitoring, which performs the great potentiality in monitoring the settlement of linear engineering.
The post-construction stability monitoring of breakwaters is one of the key tasks to their operation, and a large amount of measured data needs to be required for the stability and safety analysis. The traditional methods such as leveling and GPS are time consuming and labor-intensive, which only provide sparse points. The PSI technology can obtain the high-density target points and the line-of-sight (LOS) deformations through monitoring the distance changes between the radar sensor and the ground targets (such as artificial buildings, roads, and other infrastructures). PSI is suitable for monitoring the areas where long-term slow deformation occurs. Taking a breakwater (8 km) in Lianyungang as an example, a total of 38 C-band images of Sentinel-1A, from September 29, 2016 to December 29, 2017, are analyzed and 788 persistent scatterers (PS) are extracted. Nine feature points are selected to analyze the deformation pattern. The results show that the cumulative settlement of the breakwater reaches 336 mm, which accounts for 0.8% of the breakwater height, indicating that the dike is safe and stable. Finally, by comparing the PSI with the leveling during the same period, it can be found that the two monitoring results are consistent and can meet the accuracy requirements of the breakwater settlement monitoring, which performs the great potentiality in monitoring the settlement of linear engineering.
Abstract:
In order to solve the problem that the prediction of the buoyancy of underground structures in confined aquifer is inaccurate, a series of model tests on the impact of confined aquifer seepage on the buoyancy of underground structures are conducted. Two cases are studied: (1) the vertical seepage in homogeneous soil layer; (2) the vertical seepage of the confined water through overlying aquiclude. The influences of the seepage on the buoyancy of the underground structure under different hydraulic gradients in both cases are investigated. The results show that the pore pressure distribution and the buoyancy of the underground structures are not consistent with those obtained from hydrostatic pressure when the seepage occurs. And they are usually larger than those obtained from hydrostatic pressure. The raising ratio of the pore pressure and buoyancy to the theoretical value based on hydrostatic pressure equals to the vertical hydraulic gradient in case the vertical seepage occurs in homogeneous soil. The raising ratio of the pore pressure and buoyancy to the theoretical value based on hydrostatic pressure is much larger than the hydraulic gradient in case the confined water seeps in the overlying aquiclude, which is obviously different from that in homogenous soil. In the case herein, the raising ratio is about 2 times the hydraulic gradient.
In order to solve the problem that the prediction of the buoyancy of underground structures in confined aquifer is inaccurate, a series of model tests on the impact of confined aquifer seepage on the buoyancy of underground structures are conducted. Two cases are studied: (1) the vertical seepage in homogeneous soil layer; (2) the vertical seepage of the confined water through overlying aquiclude. The influences of the seepage on the buoyancy of the underground structure under different hydraulic gradients in both cases are investigated. The results show that the pore pressure distribution and the buoyancy of the underground structures are not consistent with those obtained from hydrostatic pressure when the seepage occurs. And they are usually larger than those obtained from hydrostatic pressure. The raising ratio of the pore pressure and buoyancy to the theoretical value based on hydrostatic pressure equals to the vertical hydraulic gradient in case the vertical seepage occurs in homogeneous soil. The raising ratio of the pore pressure and buoyancy to the theoretical value based on hydrostatic pressure is much larger than the hydraulic gradient in case the confined water seeps in the overlying aquiclude, which is obviously different from that in homogenous soil. In the case herein, the raising ratio is about 2 times the hydraulic gradient.
Abstract:
How to obtain the dynamic parameters of rock mass quickly and precisely is a popular and difficult problem in geotechnical engineering, which plays a very important part in the engineering design or construction. Currently, scholars have developed many methods to obtain these parameters, such as in-situ testing method, empirical formula and so on. However, these methods need large investments and long construction period, etc., which cannot obtain the dynamic parameters precisely and quickly in the engineering scale. A new method for estimating the rock parameters based on field blasting vibration signals is proposed. By identifying the arrival times of P and S waves, the propagation velocities of P and S waves are calculated, and the parameters can be obtained. By analyzing the field blasting vibration signals in Fengning pumped-storage power station, the results demonstrate that the dynamic elastic modulus of the rocks inversed by the field blasting vibration signals is higher than its static one given by Beijing Engineering Corporation Limited, i.e. the ratio is about 2.2~2.9, and the inversed dynamic Poisson's ratio is lower than its static one, 0.9~0.975 times the static one. Therefore, the proposed method based on field blasting vibration provides a new and effective way for obtaining the dynamic parameters of rock mass.
How to obtain the dynamic parameters of rock mass quickly and precisely is a popular and difficult problem in geotechnical engineering, which plays a very important part in the engineering design or construction. Currently, scholars have developed many methods to obtain these parameters, such as in-situ testing method, empirical formula and so on. However, these methods need large investments and long construction period, etc., which cannot obtain the dynamic parameters precisely and quickly in the engineering scale. A new method for estimating the rock parameters based on field blasting vibration signals is proposed. By identifying the arrival times of P and S waves, the propagation velocities of P and S waves are calculated, and the parameters can be obtained. By analyzing the field blasting vibration signals in Fengning pumped-storage power station, the results demonstrate that the dynamic elastic modulus of the rocks inversed by the field blasting vibration signals is higher than its static one given by Beijing Engineering Corporation Limited, i.e. the ratio is about 2.2~2.9, and the inversed dynamic Poisson's ratio is lower than its static one, 0.9~0.975 times the static one. Therefore, the proposed method based on field blasting vibration provides a new and effective way for obtaining the dynamic parameters of rock mass.
Abstract:
A high-speed railway subgrade which experiences a continuous and severe heave after construction is investigated through the delamination deformation monitoring, expansive tests and XRD tests. Sulfate attacking on cement-stabilized filler is the main reason of subgrade heave, and the reaction conditions and the development process of sulfate attacking expansion are analyzed based on the mechanism of crystal growth and chemical reaction. The results show that the cement-stabilized filler where the ettringite and thaumasite are identified has obvious swelling deformation, and the subgrade heave relates to the crystal formation of ettringite and thaumasite caused by sulfate attacking on cement-stabilized filler. The relatively humid alkaline environment and existence of sulfate minerals in cement stabilized filler are the necessary condition for such sulfate attacking expansion. The sulfate attacking on cement-stabilized filler exhibits a long-term persistent heave characteristic, and typical expansion of this type may develop for several years to create a critical and dangerous situation.
A high-speed railway subgrade which experiences a continuous and severe heave after construction is investigated through the delamination deformation monitoring, expansive tests and XRD tests. Sulfate attacking on cement-stabilized filler is the main reason of subgrade heave, and the reaction conditions and the development process of sulfate attacking expansion are analyzed based on the mechanism of crystal growth and chemical reaction. The results show that the cement-stabilized filler where the ettringite and thaumasite are identified has obvious swelling deformation, and the subgrade heave relates to the crystal formation of ettringite and thaumasite caused by sulfate attacking on cement-stabilized filler. The relatively humid alkaline environment and existence of sulfate minerals in cement stabilized filler are the necessary condition for such sulfate attacking expansion. The sulfate attacking on cement-stabilized filler exhibits a long-term persistent heave characteristic, and typical expansion of this type may develop for several years to create a critical and dangerous situation.
Abstract:
The three-dimensional swelling pressure tests on highly expensive soil are carried out under different dry densities, initial moisture contents and temperatures using composite modification methods. The results show that the ratio of vertical swelling pressure to horizontal swelling pressure increases with the increase of dry density in the range of test conditions (dry densities of 1.2, 1.35, 1.5 and 1.65 g/cm3, initial moisture contents of 15%, 18%, 21% and 24%, temperatures of 20℃, 35℃, 45℃, 55℃ and 65℃). The ratio is close to one when the dry density is 1.2 g/cm3. Moreover, the dry density is the key factor that determines the swelling pressure equilibrium time. The larger the dry density, the longer the equilibrium time. However, the swelling pressure equilibrium time of all samples is within 3000 minutes. The vertical swelling pressure of the expansive soil increases exponentially with the enlargement of the dry density at room temperature of 20℃ when the initial moisture contents are the same. Under the same temperatures and dry densities, the three-dimensional swelling pressures decrease linearly with the increase of the initial moisture content. Moreover, the slope of the relation curve increases linearly with the dry density. The vertical swelling pressure of the soil decreases by 33.3%, which is treated by 0.8% polyvinyl alcohol (PVA) with 5% potassium chloride (KCl) mixture solution and 0.6% polypropylene fiber, and that of the soil reduces by 16.7%, which is treated by 0.8% PVA with 4% lime (CaO) mixture solution. The vertical swelling pressure of the unmodified soil and the modified sample treated by 0.8% PVA with 4% CaO mixture solution decreases with the rise of the temperature. However, the vertical swelling pressure of the soil samples increases with the rise of temperature, and gradually decreases when the temperature exceeds 45℃ after modification by 0.8% PVA with 5% KCl mixed solution and 0.6% polypropylene fiber.
The three-dimensional swelling pressure tests on highly expensive soil are carried out under different dry densities, initial moisture contents and temperatures using composite modification methods. The results show that the ratio of vertical swelling pressure to horizontal swelling pressure increases with the increase of dry density in the range of test conditions (dry densities of 1.2, 1.35, 1.5 and 1.65 g/cm3, initial moisture contents of 15%, 18%, 21% and 24%, temperatures of 20℃, 35℃, 45℃, 55℃ and 65℃). The ratio is close to one when the dry density is 1.2 g/cm3. Moreover, the dry density is the key factor that determines the swelling pressure equilibrium time. The larger the dry density, the longer the equilibrium time. However, the swelling pressure equilibrium time of all samples is within 3000 minutes. The vertical swelling pressure of the expansive soil increases exponentially with the enlargement of the dry density at room temperature of 20℃ when the initial moisture contents are the same. Under the same temperatures and dry densities, the three-dimensional swelling pressures decrease linearly with the increase of the initial moisture content. Moreover, the slope of the relation curve increases linearly with the dry density. The vertical swelling pressure of the soil decreases by 33.3%, which is treated by 0.8% polyvinyl alcohol (PVA) with 5% potassium chloride (KCl) mixture solution and 0.6% polypropylene fiber, and that of the soil reduces by 16.7%, which is treated by 0.8% PVA with 4% lime (CaO) mixture solution. The vertical swelling pressure of the unmodified soil and the modified sample treated by 0.8% PVA with 4% CaO mixture solution decreases with the rise of the temperature. However, the vertical swelling pressure of the soil samples increases with the rise of temperature, and gradually decreases when the temperature exceeds 45℃ after modification by 0.8% PVA with 5% KCl mixed solution and 0.6% polypropylene fiber.