2012 Vol. 34 No. 11
Abstract:
The stress-strain behaviors of typical rockfill materials under cyclic loading with constant and variable stress amplitudes are analyzed. The stress-dilatancy equations, loading directions, tangential modulus and the representations of plastic modulus are derived based on experimental observations, and a generalized plasticity model is proposed to simulate the dynamic behaviours of rockfill materials. The model treats all the loading-unloading-reloading phases as elastoplastic ones and captures the hardening effects (the influences of the accumulated volumetric strain on the dilatancy and the overall stress strain behaviour) during cyclic loading by incorporating an aging function into the stress-dilatancy equations. There are totally 12 parameters involved in the proposed model, all of which can be determined by the conventional monotonic and cyclic tests. To check the validity of the proposed model, the parameters of two rockfill materials are calibrated based on the experimental data, and the typical cycle triaxial compression tests with constant and variable stress amplitudes are modeled. Satisfactory agreement between the numerical and experimental results under different confining pressures confirms the capability of the proposed model in capturing the hysteretic stress-strain behaviours and the accumulation of permanent deformation under cyclic loading effectively.
The stress-strain behaviors of typical rockfill materials under cyclic loading with constant and variable stress amplitudes are analyzed. The stress-dilatancy equations, loading directions, tangential modulus and the representations of plastic modulus are derived based on experimental observations, and a generalized plasticity model is proposed to simulate the dynamic behaviours of rockfill materials. The model treats all the loading-unloading-reloading phases as elastoplastic ones and captures the hardening effects (the influences of the accumulated volumetric strain on the dilatancy and the overall stress strain behaviour) during cyclic loading by incorporating an aging function into the stress-dilatancy equations. There are totally 12 parameters involved in the proposed model, all of which can be determined by the conventional monotonic and cyclic tests. To check the validity of the proposed model, the parameters of two rockfill materials are calibrated based on the experimental data, and the typical cycle triaxial compression tests with constant and variable stress amplitudes are modeled. Satisfactory agreement between the numerical and experimental results under different confining pressures confirms the capability of the proposed model in capturing the hysteretic stress-strain behaviours and the accumulation of permanent deformation under cyclic loading effectively.
Abstract:
A numerical investigation of the macromechanical behavior and bond breakage of different cemented sands using distinct element method (DEM) is presented. These sand specimens are cemented by means of the contact bond model introduced in a DEM commercial software, PFC2D and the modified bond model without thickness proposed by Jiang et al (modified Jiang model), respectively. First, the modified Jiang model is incorporated into PFC2D; Then, a series of biaxial compression tests are performed on specimens with different bond strengths under various confining pressures, and the DEM results are compared and analyzed. The results show that the Jiang sample, whose bond behavior is controlled by the modified Jiang model, exhibits pronounced strain-softening, and shear-dilatancy, and its peak frictional angle increases with bond strength; the PFC sample, whose bond behavior is controlled by the contact bond model, demonstrates strain-softening, shear-dilatancy with high bond strength, but strain-hardening, and shear-shrink under with low bond strength, and its peak frictional angle decreases with bond strength. The simulated results are in accordance with the experimental ones to some extent, especially the Jiang sample can well capture the major mechanical behavior of bonded sands. In addition, the number of broken bond due to tension is lager than that due to shear in the Jiang sample, while they are the same in the PFC sample. The studies on microscopic parameter of the modified Jiang model show that under low confining pressures, the macromechanical behavior and bond breakage form are sensitive to the ratio of tangential bond strength to normal bond strength.
A numerical investigation of the macromechanical behavior and bond breakage of different cemented sands using distinct element method (DEM) is presented. These sand specimens are cemented by means of the contact bond model introduced in a DEM commercial software, PFC2D and the modified bond model without thickness proposed by Jiang et al (modified Jiang model), respectively. First, the modified Jiang model is incorporated into PFC2D; Then, a series of biaxial compression tests are performed on specimens with different bond strengths under various confining pressures, and the DEM results are compared and analyzed. The results show that the Jiang sample, whose bond behavior is controlled by the modified Jiang model, exhibits pronounced strain-softening, and shear-dilatancy, and its peak frictional angle increases with bond strength; the PFC sample, whose bond behavior is controlled by the contact bond model, demonstrates strain-softening, shear-dilatancy with high bond strength, but strain-hardening, and shear-shrink under with low bond strength, and its peak frictional angle decreases with bond strength. The simulated results are in accordance with the experimental ones to some extent, especially the Jiang sample can well capture the major mechanical behavior of bonded sands. In addition, the number of broken bond due to tension is lager than that due to shear in the Jiang sample, while they are the same in the PFC sample. The studies on microscopic parameter of the modified Jiang model show that under low confining pressures, the macromechanical behavior and bond breakage form are sensitive to the ratio of tangential bond strength to normal bond strength.
Abstract:
Centrifuge tests are carried out to study the embankment supported on the rigid pile composite ground. For all the test models, the upper layer of the ground is soft clay and the lower layer is stiff sand. The cases of using single-row piles and group piles are considered. The deformation behavior, load transfer and failure mechanisms of the piles and embankment are investigated with different pile bending stiffnesses and strengths, piles locations, pile spacings and embedded depth of pile toe into stiff stratum. The centrifuge tests show that in all the cases, the maximum bending moment of piles is located near the interface of soft and stiff strata. For the embankment on group piles, the closer to the embankment center the pile is, the smaller the bending moment is. When the piles have a high bending stiffness and strength, a large spacing, and a sufficient embedded depth into stiff stratum, soil may flow around the piles, which will lead to the overall failure of embankment. In contrast, for the single-row piles with a low bending stiffness and strength, the piles will fail firstly by bending failure mode at the interface of the soft and stiff strata, and then the secondary bending failure may occur at the upper part of the pile before the embankment failure. For the group piles with a low bending stiffness and strength, some piles near the embankment toe would also fail firstly by bending failure mode at the interface of the soft and stiff strata. Besides, the tensile failure may break the pile into two parts. In the final stage, the collapse failure or the secondary bending failure of the pile upper part will lead to the overall failure of embankment. According to the failure mechanisms of embankment, the embankment stability may be enhanced by the measures of increasing pile bending strength, decreasing pile spacing or increasing the embedded depth of pile toe into stiff stratum.
Centrifuge tests are carried out to study the embankment supported on the rigid pile composite ground. For all the test models, the upper layer of the ground is soft clay and the lower layer is stiff sand. The cases of using single-row piles and group piles are considered. The deformation behavior, load transfer and failure mechanisms of the piles and embankment are investigated with different pile bending stiffnesses and strengths, piles locations, pile spacings and embedded depth of pile toe into stiff stratum. The centrifuge tests show that in all the cases, the maximum bending moment of piles is located near the interface of soft and stiff strata. For the embankment on group piles, the closer to the embankment center the pile is, the smaller the bending moment is. When the piles have a high bending stiffness and strength, a large spacing, and a sufficient embedded depth into stiff stratum, soil may flow around the piles, which will lead to the overall failure of embankment. In contrast, for the single-row piles with a low bending stiffness and strength, the piles will fail firstly by bending failure mode at the interface of the soft and stiff strata, and then the secondary bending failure may occur at the upper part of the pile before the embankment failure. For the group piles with a low bending stiffness and strength, some piles near the embankment toe would also fail firstly by bending failure mode at the interface of the soft and stiff strata. Besides, the tensile failure may break the pile into two parts. In the final stage, the collapse failure or the secondary bending failure of the pile upper part will lead to the overall failure of embankment. According to the failure mechanisms of embankment, the embankment stability may be enhanced by the measures of increasing pile bending strength, decreasing pile spacing or increasing the embedded depth of pile toe into stiff stratum.
Abstract:
Large-scale physical model tests are carried out to study the mechanical characteristics of high-rise platform pile groups subjected to cyclic lateral loads such as wave, tidewater and wind, etc. They are conducted in silts, which consist of nine steel-pipes in a closely-spaced arrangement. The response of pile groups and its variation under cyclic loading conditions are discussed. The results indicate that the cyclic loading produces plastic deformation within soil around piles. The lateral stiffness of pile-soil system decreases with cycling, and the lateral load carried by each pile is redistributed during repeated loading. The previous cyclic loading influences the stiffness of piles in the next loading stages. The effect induced by the cyclic loading is more important for group-pile than a single pile, which can be estimated by a multiplier based on thep-y curve method. Finally, a comparison between the calculated results and those obtained by other different methods is presented.
Large-scale physical model tests are carried out to study the mechanical characteristics of high-rise platform pile groups subjected to cyclic lateral loads such as wave, tidewater and wind, etc. They are conducted in silts, which consist of nine steel-pipes in a closely-spaced arrangement. The response of pile groups and its variation under cyclic loading conditions are discussed. The results indicate that the cyclic loading produces plastic deformation within soil around piles. The lateral stiffness of pile-soil system decreases with cycling, and the lateral load carried by each pile is redistributed during repeated loading. The previous cyclic loading influences the stiffness of piles in the next loading stages. The effect induced by the cyclic loading is more important for group-pile than a single pile, which can be estimated by a multiplier based on the
Abstract:
The reinforced-gravel-pile composite ground is a new method for treating soft soil ground. Based on the in-depth studies on the deformation mechanism for reinforced-gravel-pile composite ground under flexible foundation, firstly, according to different relative slips between the gravel piles and the soil around them, the reinforced area can be divided into reinforced-gravel-pile zone, unreinforced-gravel-pile plastic zone and unreinforced-gravel-pile elastic zone. A model for analyzing the settlement of the reinforced-gravel-pile ground under flexible foundation is established. Afterwards, according to different deformation mechanisms in different zones, the corresponding deformation analysis methods for various zones are established. Consequently, a new settlement analysis method for the reinforced-gravel-pile composite ground under flexible foundation is developed. The proposed method can consider the influences of relative slip between pile and soil, and reflect the influences derived from the lateral deformation of gravel piles in the unreinforced-gravel-pile elastic zone. Finally, the proposed method is applied to a practical project. Compared with the other existing methods, the proposed method is more feasible and reasonable.
The reinforced-gravel-pile composite ground is a new method for treating soft soil ground. Based on the in-depth studies on the deformation mechanism for reinforced-gravel-pile composite ground under flexible foundation, firstly, according to different relative slips between the gravel piles and the soil around them, the reinforced area can be divided into reinforced-gravel-pile zone, unreinforced-gravel-pile plastic zone and unreinforced-gravel-pile elastic zone. A model for analyzing the settlement of the reinforced-gravel-pile ground under flexible foundation is established. Afterwards, according to different deformation mechanisms in different zones, the corresponding deformation analysis methods for various zones are established. Consequently, a new settlement analysis method for the reinforced-gravel-pile composite ground under flexible foundation is developed. The proposed method can consider the influences of relative slip between pile and soil, and reflect the influences derived from the lateral deformation of gravel piles in the unreinforced-gravel-pile elastic zone. Finally, the proposed method is applied to a practical project. Compared with the other existing methods, the proposed method is more feasible and reasonable.
2012, 34(11): 2005-2010.
Abstract:
Talus always slides along the bedrock surface. On the basis of straight shear tests on earth-rock aggregate and bedrock, the sliding surface of talus slide is a combination of two media strain-hardening and strain-softening. A shear stress-strain constitutive model for the strain-softening medium is described by the Weibulls distribution law. A shear stress-strain constitutive model for the strain-hardening medium is described by the linear equation. Based on a simplified model for anti-sliding piles in talus slide, the formulas for calculating thrust on anti-sliding piles are derived by considering the above two shear stress-strain constitutive models. How the main parameters of the proposed formulas affect the distribution laws of talus slide thrust on anti-sliding pile is discussed. Finally, the rationality of the proposed formulas is demonstrated based on comparative analysis of the theoretical and the experimental results.
Talus always slides along the bedrock surface. On the basis of straight shear tests on earth-rock aggregate and bedrock, the sliding surface of talus slide is a combination of two media strain-hardening and strain-softening. A shear stress-strain constitutive model for the strain-softening medium is described by the Weibulls distribution law. A shear stress-strain constitutive model for the strain-hardening medium is described by the linear equation. Based on a simplified model for anti-sliding piles in talus slide, the formulas for calculating thrust on anti-sliding piles are derived by considering the above two shear stress-strain constitutive models. How the main parameters of the proposed formulas affect the distribution laws of talus slide thrust on anti-sliding pile is discussed. Finally, the rationality of the proposed formulas is demonstrated based on comparative analysis of the theoretical and the experimental results.
Abstract:
The dynamic relaxation algorithm is used to solve quasi-static problems in numerical manifold method (NMM). However, the mechanism of energy dissipation of NMM is unclear. In this paper, the viscous-type and self-adaptive dampings are adopted to absorb the kinetic energy caused by the oscillation of system. The equilibrium equations containing damping term are derived by minimizing the total potential energy. A new algorithm for simulation of sequential excavation in rock masses is proposed. The excavation faces are considered as the special kind of discontinuity, and the phased excavation process is simulated through setting contact states of excavation faces according to excavation sequences. The convergence criterion including displacement and acceleration criteria are defined to determine whether the system reaches new equilibrium or not. The numerical results show that the improved NMM has good convergence performance and that the results from NMM agree with those from UDEC and analytical solutions.
The dynamic relaxation algorithm is used to solve quasi-static problems in numerical manifold method (NMM). However, the mechanism of energy dissipation of NMM is unclear. In this paper, the viscous-type and self-adaptive dampings are adopted to absorb the kinetic energy caused by the oscillation of system. The equilibrium equations containing damping term are derived by minimizing the total potential energy. A new algorithm for simulation of sequential excavation in rock masses is proposed. The excavation faces are considered as the special kind of discontinuity, and the phased excavation process is simulated through setting contact states of excavation faces according to excavation sequences. The convergence criterion including displacement and acceleration criteria are defined to determine whether the system reaches new equilibrium or not. The numerical results show that the improved NMM has good convergence performance and that the results from NMM agree with those from UDEC and analytical solutions.
Abstract:
The physical nature of frozen soil damage is analyzed. A series of CT scanning during the uniaxial compress test process is conducted on an enhanced triaxial system which works together with the CT system. According to the test results, the yield strain, damage strain critical value, and failure strain critical value of the frozen remolded Lanzhou loess are obtained. Then, the plastic strain is used to describe the damage threshold value. The lower the temperature, the larger the damage threshold value. In the end, the CT value is used to define damage variable, and the damage evolution law is derived based on the results of CT scanning tests. A damage potential function suitable for frozen soil is proposed.
The physical nature of frozen soil damage is analyzed. A series of CT scanning during the uniaxial compress test process is conducted on an enhanced triaxial system which works together with the CT system. According to the test results, the yield strain, damage strain critical value, and failure strain critical value of the frozen remolded Lanzhou loess are obtained. Then, the plastic strain is used to describe the damage threshold value. The lower the temperature, the larger the damage threshold value. In the end, the CT value is used to define damage variable, and the damage evolution law is derived based on the results of CT scanning tests. A damage potential function suitable for frozen soil is proposed.
Abstract:
Water inrush hazard has been a major issue that seriously imperils safe construction of tunnels. In order to achieve real-time monitoring of evolution process of water inrush and disaster warning, the electrical resistivity tomography (ERT) method is introduced for monitoring water inrush in the tunnels. A quantitative evaluation method of ERT is proposed based on the gray correlation theory to quantitatively describe the monitoring information of water inrush. First, the two typical kinds of water inrush hazards caused by geological defects and non-geological defects are generalized, and geoelectric models of different stages during the evolution process of water inrush are acquired, which lays a foundation for numerical forward of real-time monitoring of water inrush of tunnels. Then, the finite element method is adopted to carry out the numerical forward, and the response characteristics of water inrush evolution process are revealed. ERT gray correlation quantitative analysis of water inrush process is done. Sample points whose correlation coefficient is less than 0.5 or 0.3 greatly increase, and the standard deviations of correlation distribution and forward image dramatically increase, which are an important precursory pattern when water inrush happens. Finally, model tests on ERT of water inrush of tunnel are performed, and precursor information of water inrush is captured well and disaster early-warning of water inrush is realized, which shows it is feasible that ERT is applied to real-time monitoring.
Water inrush hazard has been a major issue that seriously imperils safe construction of tunnels. In order to achieve real-time monitoring of evolution process of water inrush and disaster warning, the electrical resistivity tomography (ERT) method is introduced for monitoring water inrush in the tunnels. A quantitative evaluation method of ERT is proposed based on the gray correlation theory to quantitatively describe the monitoring information of water inrush. First, the two typical kinds of water inrush hazards caused by geological defects and non-geological defects are generalized, and geoelectric models of different stages during the evolution process of water inrush are acquired, which lays a foundation for numerical forward of real-time monitoring of water inrush of tunnels. Then, the finite element method is adopted to carry out the numerical forward, and the response characteristics of water inrush evolution process are revealed. ERT gray correlation quantitative analysis of water inrush process is done. Sample points whose correlation coefficient is less than 0.5 or 0.3 greatly increase, and the standard deviations of correlation distribution and forward image dramatically increase, which are an important precursory pattern when water inrush happens. Finally, model tests on ERT of water inrush of tunnel are performed, and precursor information of water inrush is captured well and disaster early-warning of water inrush is realized, which shows it is feasible that ERT is applied to real-time monitoring.
Abstract:
High-precision location is the focus of hot research about microseismic technology, and classification of microseismic events is the basis for the location. Based on the previous studies, the fractal characteristics of microseismic signals are studied, and the range and scale-free fractal box dimensions of the algorithm are established. Using the spectral differences about mine blasting vibration, rock fracture and electromagnetic interference signals, and based on the wavelet analysis and the fractal theory, the microseismic signals are decomposed into 5 layers to gain specified frequency bands using MATLAB software. Then the box fractal dimensions about those specified frequency band reconstructed signals can be calculated. The 23-dimensional values of pattern recognition feature vector can be established. Finally, the support vector machine SVM is adopted to train, classify and recognize 300 sets of data. The results show that three types of signals have obvious fractal characteristics in the specified frequency bands, especially the electromagnetic interference signals. The fractal dimension box of specified frequency bands is close to the whole signal dimension box. The SVM network model with 23 fractal dimension vectors can be well used to recognize microseismic events, and the correct identification rate is 94%, with can meet the needs of the project site, but the efficiency of identification still needs further improvement.
High-precision location is the focus of hot research about microseismic technology, and classification of microseismic events is the basis for the location. Based on the previous studies, the fractal characteristics of microseismic signals are studied, and the range and scale-free fractal box dimensions of the algorithm are established. Using the spectral differences about mine blasting vibration, rock fracture and electromagnetic interference signals, and based on the wavelet analysis and the fractal theory, the microseismic signals are decomposed into 5 layers to gain specified frequency bands using MATLAB software. Then the box fractal dimensions about those specified frequency band reconstructed signals can be calculated. The 23-dimensional values of pattern recognition feature vector can be established. Finally, the support vector machine SVM is adopted to train, classify and recognize 300 sets of data. The results show that three types of signals have obvious fractal characteristics in the specified frequency bands, especially the electromagnetic interference signals. The fractal dimension box of specified frequency bands is close to the whole signal dimension box. The SVM network model with 23 fractal dimension vectors can be well used to recognize microseismic events, and the correct identification rate is 94%, with can meet the needs of the project site, but the efficiency of identification still needs further improvement.
2012, 34(11): 2043-2049.
Abstract:
The permeability coefficient here is expressed in form of complex variable, the imaginary part of which is the variation value. The node head value and its variation value are calculated by self-compiling program according to the solution of linear equations with complex coefficient based on the finite element method. The permeability coefficient of curtain is considered as random variable and to be in uniform distribution by using Monte Carlo stochastic finite element method to calculate the stochastic seepage filed. The head mean value and the head standard deviation are used to represent the statistical values of the calculated results obtained by the Monte Carlo stochastic finite element method. The calculated values of head value and the head variation are compared with the head mean value and the head standard deviation by numerical simulation. The results demonstrate that the seepage calculation with permeability coefficient expressed in complex variable is correct and feasible. This study provides a convenient method for solving large complex problem under large variability conditions.
The permeability coefficient here is expressed in form of complex variable, the imaginary part of which is the variation value. The node head value and its variation value are calculated by self-compiling program according to the solution of linear equations with complex coefficient based on the finite element method. The permeability coefficient of curtain is considered as random variable and to be in uniform distribution by using Monte Carlo stochastic finite element method to calculate the stochastic seepage filed. The head mean value and the head standard deviation are used to represent the statistical values of the calculated results obtained by the Monte Carlo stochastic finite element method. The calculated values of head value and the head variation are compared with the head mean value and the head standard deviation by numerical simulation. The results demonstrate that the seepage calculation with permeability coefficient expressed in complex variable is correct and feasible. This study provides a convenient method for solving large complex problem under large variability conditions.
Abstract:
The distribution and the evolution of three kinds of strain fields are measured for two sand specimens under uniaxial and stress-controlled compression by use of a digital image correlation method with coarse-fine search capability based on the particle swarm optimization algorithm and the Newton-Raphson method. The range of longitudinal strain gradients in a wide non-uniformly deformational zone is determined before strains rise rapidly in a small band with highly localized strain. It is found that the peak values of the positive (or tensile) horizontal linear strain and the negative (or compressive) vertical linear strain fall into the future strain localized band after the loading exceeds a certain value, while the peak of the shear strain can deviate from the band, as can be explained by the differences between linear strain fields and shear strain fields. The former two are inclined and zonal beyond the uniformly deformational stage, while the latter is lumpy. The nearly same range is found for the longitudinal (or vertical) gradients of three kinds of strains, i.e., 0.001~0.002 mm, before the occurrence of the narrow localized band. If the strain gradients are lower than the values mentioned above, rapid strain increases will not occur in a small band within the sand specimen, nor will macroscopic cracks appear. Outside the band, certain plastic strains and gradients exist, as cannot be explained by the nonclassical elastoplastic models.
The distribution and the evolution of three kinds of strain fields are measured for two sand specimens under uniaxial and stress-controlled compression by use of a digital image correlation method with coarse-fine search capability based on the particle swarm optimization algorithm and the Newton-Raphson method. The range of longitudinal strain gradients in a wide non-uniformly deformational zone is determined before strains rise rapidly in a small band with highly localized strain. It is found that the peak values of the positive (or tensile) horizontal linear strain and the negative (or compressive) vertical linear strain fall into the future strain localized band after the loading exceeds a certain value, while the peak of the shear strain can deviate from the band, as can be explained by the differences between linear strain fields and shear strain fields. The former two are inclined and zonal beyond the uniformly deformational stage, while the latter is lumpy. The nearly same range is found for the longitudinal (or vertical) gradients of three kinds of strains, i.e., 0.001~0.002 mm, before the occurrence of the narrow localized band. If the strain gradients are lower than the values mentioned above, rapid strain increases will not occur in a small band within the sand specimen, nor will macroscopic cracks appear. Outside the band, certain plastic strains and gradients exist, as cannot be explained by the nonclassical elastoplastic models.
2012, 34(11): 2058-2065.
Abstract:
Based on the classic Biots consolidation theory, the consolidation equation considering non-Darcian flow is set up and then solved numerically with the aid of the finite element method. Firstly, the well-known Hansbos non-Darcian flow model is incorporated and the selection of the input parameters is analyzed correspondingly. Then, with a further simplification of the hydraulic gradient combining with the flow continuity condition and the force equilibrium condition, the governing equations for the Biots general consolidation theory with non-Darcian flow are formulated. Based on this, the finite element formulations for the spatial eight-node block element are deduced by means of the weighted residual method. A program considering the effect of the non-Darcian flow is developed based on an existing procedure correspondingly. With this modified program, the obtained governing equations can be solved numerically. Finally, the reliability of the numerical method is verified against the established theoretical solutions. The results show that (1) the difference of the calculated results for the two methods increases with the increase of the non-Darcian flow parameters, and the maximum difference is smaller than 10%; (2) the consolidation rate will slow down when considering the effect of non-Darcian flow, and this effect is significant when the property of the non-Darcian flow is apparent.
Based on the classic Biots consolidation theory, the consolidation equation considering non-Darcian flow is set up and then solved numerically with the aid of the finite element method. Firstly, the well-known Hansbos non-Darcian flow model is incorporated and the selection of the input parameters is analyzed correspondingly. Then, with a further simplification of the hydraulic gradient combining with the flow continuity condition and the force equilibrium condition, the governing equations for the Biots general consolidation theory with non-Darcian flow are formulated. Based on this, the finite element formulations for the spatial eight-node block element are deduced by means of the weighted residual method. A program considering the effect of the non-Darcian flow is developed based on an existing procedure correspondingly. With this modified program, the obtained governing equations can be solved numerically. Finally, the reliability of the numerical method is verified against the established theoretical solutions. The results show that (1) the difference of the calculated results for the two methods increases with the increase of the non-Darcian flow parameters, and the maximum difference is smaller than 10%; (2) the consolidation rate will slow down when considering the effect of non-Darcian flow, and this effect is significant when the property of the non-Darcian flow is apparent.
Abstract:
In order to analyze whether the landfill-leachate contaminated soil area and leachate content can be detected quantitatively with electrical resistivity method or not, the resistivity of saturated sand soils polluted by landfill-leachate in lab is measured with 2D or 3D resistivity tomography method and electrical probe. The measured results show that the contaminated area in soil layer has a low-resistivity abnormal character, the boundary of abnormal area in resistivity section corresponds to that of contaminated area, and it varies with landfill-leachate diffusing. The measured resistivity of contaminated sand soil with electrical probe has a quantitative relation with leachate content, and the leachate content can be calculated with the Archie type formula from the measured resistivity. A case study shows that the relative error of the calculated landfill leachate content to the true content is less than 10%. But two experiments are necessary in the process of calculating landfill leachate content with measured resistivity one is the resistivity test experiment of landfill leachate while being diluted, the other is the resistivity test experiment of different saturated sand soil samples.
In order to analyze whether the landfill-leachate contaminated soil area and leachate content can be detected quantitatively with electrical resistivity method or not, the resistivity of saturated sand soils polluted by landfill-leachate in lab is measured with 2D or 3D resistivity tomography method and electrical probe. The measured results show that the contaminated area in soil layer has a low-resistivity abnormal character, the boundary of abnormal area in resistivity section corresponds to that of contaminated area, and it varies with landfill-leachate diffusing. The measured resistivity of contaminated sand soil with electrical probe has a quantitative relation with leachate content, and the leachate content can be calculated with the Archie type formula from the measured resistivity. A case study shows that the relative error of the calculated landfill leachate content to the true content is less than 10%. But two experiments are necessary in the process of calculating landfill leachate content with measured resistivity one is the resistivity test experiment of landfill leachate while being diluted, the other is the resistivity test experiment of different saturated sand soil samples.
Abstract:
In order to compare the seismic responses of netted and packaged reinforced soil retaining walls, large-scale shaking table tests are performed. Based on the earthquake damage investigation, it is found that the failure modes of the reinforced wall are mainly characterized by loose deformation of local blocks under earthquake, and that the overall collapse is rare. Compared with the netted reinforced soil retaining wall, the packaged one has smaller deformation. Under the same magnitude of earthquake, the acceleration amplificatory coefficient of the packaged reinforced soil retaining wall is smaller than that of the netted one, while the value of horizontal peak dynamic earth pressures of the former is substantially larger than that of the latter. It is because the soil is constrained effectively by the wall plate of the packaged reinforced soil retaining wall. Therefore, for the selection of the reinforced soil retaining wall in earthquake-resistance protection zone, especially the buildings in high earthquake intensity regions, the packaged reinforced soil retaining wall will be an optimal choice. Through analysis, for the aseismic design of flexible walls, while maintaining the integral stability of the reinforced soil retaining wall, local-deformation control should be paid attention to, and its the maximal displacement should be less than the allow able displacement under earthquake. In order to maintain the normal use of the road, the deformation exponent of the reinforced soil retaining wall should be smaller than 4%. If the deformation exceeds the allowable value, measures will be taken including increasing the compaction of filling materials and geogrid length as well as the thickness of the wall, and reducing its wall slope can reduce the displacement.
In order to compare the seismic responses of netted and packaged reinforced soil retaining walls, large-scale shaking table tests are performed. Based on the earthquake damage investigation, it is found that the failure modes of the reinforced wall are mainly characterized by loose deformation of local blocks under earthquake, and that the overall collapse is rare. Compared with the netted reinforced soil retaining wall, the packaged one has smaller deformation. Under the same magnitude of earthquake, the acceleration amplificatory coefficient of the packaged reinforced soil retaining wall is smaller than that of the netted one, while the value of horizontal peak dynamic earth pressures of the former is substantially larger than that of the latter. It is because the soil is constrained effectively by the wall plate of the packaged reinforced soil retaining wall. Therefore, for the selection of the reinforced soil retaining wall in earthquake-resistance protection zone, especially the buildings in high earthquake intensity regions, the packaged reinforced soil retaining wall will be an optimal choice. Through analysis, for the aseismic design of flexible walls, while maintaining the integral stability of the reinforced soil retaining wall, local-deformation control should be paid attention to, and its the maximal displacement should be less than the allow able displacement under earthquake. In order to maintain the normal use of the road, the deformation exponent of the reinforced soil retaining wall should be smaller than 4%. If the deformation exceeds the allowable value, measures will be taken including increasing the compaction of filling materials and geogrid length as well as the thickness of the wall, and reducing its wall slope can reduce the displacement.
Abstract:
The connection effectiveness between cutoff wall and composite geomembrane of cofferdam, both of which are usually applied in the cofferdam, plays an important role in seepage control. Based on the site invitation of stage II cofferdam of Three Gorges Project, three centrifuge tests are performed in order to reveal the mechanism of the connecting form of cutoff wall and composite geomembrane by measuring the displacements of the cofferdam fillings and cutoff wall and the strain of the composite membrane. The friction between the composite geomembrane and the cofferdam fillings is so large that the expansion deformation reservation cannot work. Two main influencing factors include the settlement of the weathered sand foundation and the lateral displacement of cutoff wall, which both play a decisive function in the fracture of the composite geomembrane. Furthermore, an improved method of the connecting form, which make the composite geomembrane not be tensioned, is proposed and validated by centrifuge tests. The improved method provides a reliable evidence for the design and construction of the seepage control system of cofferdams.
The connection effectiveness between cutoff wall and composite geomembrane of cofferdam, both of which are usually applied in the cofferdam, plays an important role in seepage control. Based on the site invitation of stage II cofferdam of Three Gorges Project, three centrifuge tests are performed in order to reveal the mechanism of the connecting form of cutoff wall and composite geomembrane by measuring the displacements of the cofferdam fillings and cutoff wall and the strain of the composite membrane. The friction between the composite geomembrane and the cofferdam fillings is so large that the expansion deformation reservation cannot work. Two main influencing factors include the settlement of the weathered sand foundation and the lateral displacement of cutoff wall, which both play a decisive function in the fracture of the composite geomembrane. Furthermore, an improved method of the connecting form, which make the composite geomembrane not be tensioned, is proposed and validated by centrifuge tests. The improved method provides a reliable evidence for the design and construction of the seepage control system of cofferdams.
Abstract:
Joint roughness has an essential in?uence on the shear behavior of rock joints. Fractal dimensions (D ) can be used to effectively reflect the roughness property contributed by the secondary asperity; however, it is not ideal to represent the asperity contributed by the first-order asperity. On the basis of the asperity of a rough joint occurring on many scales, the fractal parameter is used to show the roughness character offered by the secondary asperity, while the degree of a waviness (w d) is used to reflect the roughness property at the first order. In this respect, joint roughness coefficient is represented using the fractal dimensions (D ) and degree of the waviness (w d), and their relationship is established. On the other hand, in virtue of the difficulties in calculating the value of D in engineering practices, a procedure is provided for obtaining the fractal dimension based on the digital image of rock joints. It will be helpful for applying the fractal dimension to describing the JRC in engineering practices.
Joint roughness has an essential in?uence on the shear behavior of rock joints. Fractal dimensions (
Abstract:
Many theoretical criterions have been proposed to predict the shear strength of intermittent joints, of which the Jennings criterion is widely used. In Jennings criterion, the mechanical parameters of joint and rock bridge are simply averaged by a weight coefficient called connectivity rate to calculate the peak shear strength. The main limitation of Jennings shear strength criterion is that it cannot consider the mechanical weakening of rock bridges during the shear tests. The theoretical results by Jennings criterion are usually not consistent with the measured ones. A modified criterion is put forward. The modified criterion can overcome the limitations because it takes the mechanical weakening model of rock bridges. The results show that the predicted values by the modified Jennings criterion is closer to the measured data, compared to the original Jennings criterion, which indicates that the new criterion is better. Finally, the limitations of this new strength criterion are also analyzed.
Many theoretical criterions have been proposed to predict the shear strength of intermittent joints, of which the Jennings criterion is widely used. In Jennings criterion, the mechanical parameters of joint and rock bridge are simply averaged by a weight coefficient called connectivity rate to calculate the peak shear strength. The main limitation of Jennings shear strength criterion is that it cannot consider the mechanical weakening of rock bridges during the shear tests. The theoretical results by Jennings criterion are usually not consistent with the measured ones. A modified criterion is put forward. The modified criterion can overcome the limitations because it takes the mechanical weakening model of rock bridges. The results show that the predicted values by the modified Jennings criterion is closer to the measured data, compared to the original Jennings criterion, which indicates that the new criterion is better. Finally, the limitations of this new strength criterion are also analyzed.
Abstract:
In the analysis of lateral response of adjacent piles due to tunnelling, the elastic-plastic solution of tunnel-pile interaction is given based onp -y curve considering nonlinearity of pile-soil interface. The finite difference method and the concept of limiting force profile are used during calculation. For pile groups, the p -y curve of each pile in the pile groups is obtained using p -mutiplier (f m). Then the results of pile deformation, rotation, bending moment and shear force can be given under axial force using the finite difference method. Moreover, the results obtained by the present method are compared with those from existing tests and case histories. The results show that there is a good agreement between the present computed profiles and test/field data. In the analysis of lateral response of adjacent piles due to tunnelling, the lateral response of the leading row piles in the pile groups is identical to that of the corresponding single pile. The tunnelling effects of the rear piles are lower than those of the front piles due to a positive pile group effect.
In the analysis of lateral response of adjacent piles due to tunnelling, the elastic-plastic solution of tunnel-pile interaction is given based on
Abstract:
The researches on heat conduction characteristics of soft soil under high temperature are performed for further determining the load change on tunnel lining structure surrounded by saturated soft soil and accurately evaluating its safety under high temperature of fire. The experiment adopts the method of thermal needle probe procedure, proposed by American Society for Testing and Materials (ASTM) as the standard method for determination of thermal conductivity of soil and soft rock, by using self-made apparatus for coefficient of thermal conductivity of soil to measure the coefficient of thermal conductivity of Shanghai mucky clay under high temperature. The thermal conductivity of soil samples with different water contents is measured respectively under different temperatures. At the same time, as calibrating instrument, the KD2 thermal conductivity measuring apparatus is used to calibrate the test results. The test results indicate that the thermal conductivity of soil with the same water content increases with the increase of temperatures. When the temperature is low, the increase rate of thermal conductivity of soil is higher, and when the temperature is high, the increase rate of thermal conductivity of soil is lower, which is more serious due to the decomposition of the organic soil under high temperatures. Further, the thermal conductivity of soil with the same temperature increases with the increase of water content.
The researches on heat conduction characteristics of soft soil under high temperature are performed for further determining the load change on tunnel lining structure surrounded by saturated soft soil and accurately evaluating its safety under high temperature of fire. The experiment adopts the method of thermal needle probe procedure, proposed by American Society for Testing and Materials (ASTM) as the standard method for determination of thermal conductivity of soil and soft rock, by using self-made apparatus for coefficient of thermal conductivity of soil to measure the coefficient of thermal conductivity of Shanghai mucky clay under high temperature. The thermal conductivity of soil samples with different water contents is measured respectively under different temperatures. At the same time, as calibrating instrument, the KD2 thermal conductivity measuring apparatus is used to calibrate the test results. The test results indicate that the thermal conductivity of soil with the same water content increases with the increase of temperatures. When the temperature is low, the increase rate of thermal conductivity of soil is higher, and when the temperature is high, the increase rate of thermal conductivity of soil is lower, which is more serious due to the decomposition of the organic soil under high temperatures. Further, the thermal conductivity of soil with the same temperature increases with the increase of water content.
Abstract:
An experimental investigation of Atterberg limits, unconfined compressive strength, secant modulus, pH of soil and microstructure characteristics of cement-treated zinc-contaminated kaolin is presented. The zinc-contaminated soils are artificially prepared with various zinc concentrations (0%, 0.01%, 0.02%, 0.05%, 0.1%, 0.2% and 0.5%), cement contents (8%, 12%, 15%, and 18%), and curing time (7 d and 28 d). The test results of Atterberg limits show that the liquid limit and plastic limit decrease with the increase of the initial zinc concentration in the soils. The unconfined compressive strength decreases with the increase of the initial zinc concentration. The pH values of the soils are significantly affected by the initial zinc concentration. The secant modulus (E 50) decreases as the initial zinc concentration increases. The scanning electron microscope pictures show that as the initial zinc concentration increases, the quantity and morphology of major cement hydration products change significantly. The test results of mercury intrusion porosimetry demonstrate that with the increase of the initial zinc concentration, the quantity of soil pores with diameter of 1 to 10 μm increases, whereas that of pores with diameter of 0.01 to 1 μm decreases.
An experimental investigation of Atterberg limits, unconfined compressive strength, secant modulus, pH of soil and microstructure characteristics of cement-treated zinc-contaminated kaolin is presented. The zinc-contaminated soils are artificially prepared with various zinc concentrations (0%, 0.01%, 0.02%, 0.05%, 0.1%, 0.2% and 0.5%), cement contents (8%, 12%, 15%, and 18%), and curing time (7 d and 28 d). The test results of Atterberg limits show that the liquid limit and plastic limit decrease with the increase of the initial zinc concentration in the soils. The unconfined compressive strength decreases with the increase of the initial zinc concentration. The pH values of the soils are significantly affected by the initial zinc concentration. The secant modulus (
Abstract:
The change rules of the secondary stresses of advanced tunnel segment induced by parallel shield tunnels dirving in close proximity are studied by field tests. The layout of the measured points and the field test methods are introduced, and the change rules of the secondary stresses are analyzed. Furthermore, the relationship between the secondary stresses and the distance of the shield driving face to the measured section are studied. Statistical analyses of the maximum and the stabilized magnitudes of the secondary stresses are carried out. The results of the field tests show that there will be sharp change of the secondary stresses when the hind shield is driven to the measured section, the circumferential secondary stresses in the intersection of horizontal diameter with circumference of the tunnel will be laid, and the magnitudes of the circumferential secondary stresses will be greater than those of the longitudinal secondary stresses. The results may be applied to the quantitative assessment of the construction interaction of shield tunnels in close proximity and is regarded as references to the design and construction of this kind of shield tunnels.
The change rules of the secondary stresses of advanced tunnel segment induced by parallel shield tunnels dirving in close proximity are studied by field tests. The layout of the measured points and the field test methods are introduced, and the change rules of the secondary stresses are analyzed. Furthermore, the relationship between the secondary stresses and the distance of the shield driving face to the measured section are studied. Statistical analyses of the maximum and the stabilized magnitudes of the secondary stresses are carried out. The results of the field tests show that there will be sharp change of the secondary stresses when the hind shield is driven to the measured section, the circumferential secondary stresses in the intersection of horizontal diameter with circumference of the tunnel will be laid, and the magnitudes of the circumferential secondary stresses will be greater than those of the longitudinal secondary stresses. The results may be applied to the quantitative assessment of the construction interaction of shield tunnels in close proximity and is regarded as references to the design and construction of this kind of shield tunnels.
Abstract:
A constitutive model with the double-scale pore structure from macroscopic and microscopic views is proposed for predicting the capillary hysteresis and mechanical behaviours of unsaturated expansive soils based on the BExM and the existing hydro-mechanical models for unsaturated non-expansive soils. The proposed model takes into consideration two factors the interaction between the micro- and macro-structure of expansive soils, and the fully coupled capillary hysteresis with mechanical behaviours. The predictions are performed on the cyclical controlled-suction test results published in public. The comparisons between the measured and predicted results indicate that the proposed model can quantitatively predict the capillary and mechanical behaviours of unsaturated expansive soils.
A constitutive model with the double-scale pore structure from macroscopic and microscopic views is proposed for predicting the capillary hysteresis and mechanical behaviours of unsaturated expansive soils based on the BExM and the existing hydro-mechanical models for unsaturated non-expansive soils. The proposed model takes into consideration two factors the interaction between the micro- and macro-structure of expansive soils, and the fully coupled capillary hysteresis with mechanical behaviours. The predictions are performed on the cyclical controlled-suction test results published in public. The comparisons between the measured and predicted results indicate that the proposed model can quantitatively predict the capillary and mechanical behaviours of unsaturated expansive soils.
Abstract:
The bearing capacity of X-section cast-in-place concrete pile (XCC pile), a special cross-section pile, is significantly influenced by side resistance. The single-piled composite foundation including XCC pile and circular pile is simulated by means of the finite element program ABAQUS. The side resistance and side bearing capacity are compared between XCC pile and circular pile. The distribution of side resistance of XCC pile is analyzed via cross-section plan and depth. The results indicate that both the side resistance and the side bearing capacity for XCC pile are better than those of circular pile, and that the side bearing capacity of XCC pile is significantly larger than that of circular pile at the relative depth of 0.9. At the cross-section plan, because of the soil arching, the side resistance of convex arc is about 1.5 to 3 times as large as that of concave arc for XCC pile. The ratio of the side resistance for convex arc to that of concave arc increases with the relative depth. The results are available for the design of XCC pile.
The bearing capacity of X-section cast-in-place concrete pile (XCC pile), a special cross-section pile, is significantly influenced by side resistance. The single-piled composite foundation including XCC pile and circular pile is simulated by means of the finite element program ABAQUS. The side resistance and side bearing capacity are compared between XCC pile and circular pile. The distribution of side resistance of XCC pile is analyzed via cross-section plan and depth. The results indicate that both the side resistance and the side bearing capacity for XCC pile are better than those of circular pile, and that the side bearing capacity of XCC pile is significantly larger than that of circular pile at the relative depth of 0.9. At the cross-section plan, because of the soil arching, the side resistance of convex arc is about 1.5 to 3 times as large as that of concave arc for XCC pile. The ratio of the side resistance for convex arc to that of concave arc increases with the relative depth. The results are available for the design of XCC pile.
Abstract:
The continuous drainage boundary can correct the pervious and impervious boundaries which are extremely idealized. Based on the Terzaghis one-dimensional consolidation theory and the continuous drainage boundary; an analytical solution of one-dimensional consolidation of undrained symmetry plane under continuous drainage boundary is given. One-dimensional subroutine of continuous drainage boundary is written based on the finite element analysis of ABAQUS. The influence factors of undrained symmetry plane is studied; including boundary permeability; drainage time and permeability coefficient. The variation of the undrained symmetry plane is obtained. Finally; the location of setting sand layers at different depths in saturated soft clay is compared by the method of finite element analysis. The results show that the rate of consolidation is the fastest when the sand layer is set at the location of undrained symmetry plane. The continuous drainage boundary can help to determine the location of undrained symmetry plane. The results and the process of finite element analysis are of practical and important value and significance for the development of soil consolidation theory.
The continuous drainage boundary can correct the pervious and impervious boundaries which are extremely idealized. Based on the Terzaghis one-dimensional consolidation theory and the continuous drainage boundary; an analytical solution of one-dimensional consolidation of undrained symmetry plane under continuous drainage boundary is given. One-dimensional subroutine of continuous drainage boundary is written based on the finite element analysis of ABAQUS. The influence factors of undrained symmetry plane is studied; including boundary permeability; drainage time and permeability coefficient. The variation of the undrained symmetry plane is obtained. Finally; the location of setting sand layers at different depths in saturated soft clay is compared by the method of finite element analysis. The results show that the rate of consolidation is the fastest when the sand layer is set at the location of undrained symmetry plane. The continuous drainage boundary can help to determine the location of undrained symmetry plane. The results and the process of finite element analysis are of practical and important value and significance for the development of soil consolidation theory.
Abstract:
Recently the geologic disasters of landslides with debris flows frequently occur and closely relate to earthquakes and rain storms. Thus it is desired to propose a modified calculation method for seismic force. That is, besides the original consideration of the inertia force of earthquake acceleration, another term of the excess seepage force due to the successive vibration of earthquake waves should be added for it can reduce the effective weight of soil mass and make slope easily slide. An example of an infinite landslide with a parallel straight line plane on the weathered mountain slope is calculated. The formulas for shallow slide under rainy saturated and unsaturated conditions are derived respectively. By comparing the calculated results under different slope gradient earthquake accelerations and excess seepage forces, the calculated safety factor has an error of 25% if the excess seepage force is not considered, and it decreases further one half during slope rainy saturation. The sliding soil mass will induce debris flows under rainstorm run-off. Some measures to prevent landslides with successful illustration in engineering practice are proposed.
Recently the geologic disasters of landslides with debris flows frequently occur and closely relate to earthquakes and rain storms. Thus it is desired to propose a modified calculation method for seismic force. That is, besides the original consideration of the inertia force of earthquake acceleration, another term of the excess seepage force due to the successive vibration of earthquake waves should be added for it can reduce the effective weight of soil mass and make slope easily slide. An example of an infinite landslide with a parallel straight line plane on the weathered mountain slope is calculated. The formulas for shallow slide under rainy saturated and unsaturated conditions are derived respectively. By comparing the calculated results under different slope gradient earthquake accelerations and excess seepage forces, the calculated safety factor has an error of 25% if the excess seepage force is not considered, and it decreases further one half during slope rainy saturation. The sliding soil mass will induce debris flows under rainstorm run-off. Some measures to prevent landslides with successful illustration in engineering practice are proposed.