2010 Vol. 32 No. 7
Abstract：
Centrifuge tests are employed to study the dynamic response of tunnels in dense soil. Three sets of earthquake input motions with different intensities are applied. Meanwhile, a laminar box is used to consider different boundary (flexible and rigid) effects during earthquakes. The earthquake response of free field is also simulated. The responses of ground and tunnel are observed and presented. The results indicate that dynamic responses of tunnels are significantly different from those under the static condition, and that the performance of ground and tunnel depends on the intensities of the excitation motions and the physical boundaries of centrifuge model distinctively. The ground responses are different under free field and non-free field conditions, due to the existence of tunnels affecting the ground responses to some extent.
Centrifuge tests are employed to study the dynamic response of tunnels in dense soil. Three sets of earthquake input motions with different intensities are applied. Meanwhile, a laminar box is used to consider different boundary (flexible and rigid) effects during earthquakes. The earthquake response of free field is also simulated. The responses of ground and tunnel are observed and presented. The results indicate that dynamic responses of tunnels are significantly different from those under the static condition, and that the performance of ground and tunnel depends on the intensities of the excitation motions and the physical boundaries of centrifuge model distinctively. The ground responses are different under free field and non-free field conditions, due to the existence of tunnels affecting the ground responses to some extent.
Abstract：
Soils at different depths are sampled from 11 recent or apt-to-be landslides located in Jingyang County along the Jinghe River. Considering their geological environment and irrigation conditions, triaxial creep tests and direct shear creep tests on these undisturbed soil samples separately under different stress levels and moisture contents are performed to study their creep property. The findings are as follows: the creep deformation consists of 3 stages, namely, constant speed creep, acceleration creep and creep rupture, and the period from the acceleration creep to the creep rupture is significantly short; the rupture of the soil in sliding zone is characterized by plastic destruction; the corresponding strain capacity e of the creep rupture is basically below 10%, and with the increase of the moisture content, ε of the creep rupture accordingly decreases; the results of destruction strain capacity by the direct shear creep tests under different moisture content conditions are all below 4%; and the stress-strain isochronic curves under time (t>0 min) are nearly a bunch of series of curves and exhibit a “normalization” phenomenon, demonstrating that the soil in this sliding zone has consistent creep characteristics. The intrinsic relationship between the creep property and the prediction model of slope failure is also discussed. Experimental studies and theoretical discussions are given for forecasting loess landslides.
Soils at different depths are sampled from 11 recent or apt-to-be landslides located in Jingyang County along the Jinghe River. Considering their geological environment and irrigation conditions, triaxial creep tests and direct shear creep tests on these undisturbed soil samples separately under different stress levels and moisture contents are performed to study their creep property. The findings are as follows: the creep deformation consists of 3 stages, namely, constant speed creep, acceleration creep and creep rupture, and the period from the acceleration creep to the creep rupture is significantly short; the rupture of the soil in sliding zone is characterized by plastic destruction; the corresponding strain capacity e of the creep rupture is basically below 10%, and with the increase of the moisture content, ε of the creep rupture accordingly decreases; the results of destruction strain capacity by the direct shear creep tests under different moisture content conditions are all below 4%; and the stress-strain isochronic curves under time (t>0 min) are nearly a bunch of series of curves and exhibit a “normalization” phenomenon, demonstrating that the soil in this sliding zone has consistent creep characteristics. The intrinsic relationship between the creep property and the prediction model of slope failure is also discussed. Experimental studies and theoretical discussions are given for forecasting loess landslides.
Abstract：
For the discrete element method (DEM), the key to a successful simulation lies in proper micro-parameters. However, the current way to select micro-parameters is often subjective and unreliable, resulting in incomparability among different simulations. Based on the linear contact model, a set of empirical formulas are presented to describe the correlation between macroscopic elastic constants of granular materials and microscopic elastic constants of particles by simulating tri-axial tests with PFC3D and through regression analysis of numerical results. The macroscopic elastic constants include the initial Young’s modulus and the initial Poisson’s ratio, and the microscopic elastic constants include the normal stiffness, stiffness ratio of particle, etc. It is found that the initial Young’s modulus and the shear modulus are approximately proportional to one tenth power of the confining pressure, and inversely proportional to nine tenths power of the particle size. The initial Poisson’s ratio is approximately proportional to one fourteenth power of the stiffness ratio of the particle for the linear contact model, and almost independent of the particle size. It is also found that the rational stiffness ratio of the particle ranges from 2.0 to 20.7 for the common sand.
For the discrete element method (DEM), the key to a successful simulation lies in proper micro-parameters. However, the current way to select micro-parameters is often subjective and unreliable, resulting in incomparability among different simulations. Based on the linear contact model, a set of empirical formulas are presented to describe the correlation between macroscopic elastic constants of granular materials and microscopic elastic constants of particles by simulating tri-axial tests with PFC3D and through regression analysis of numerical results. The macroscopic elastic constants include the initial Young’s modulus and the initial Poisson’s ratio, and the microscopic elastic constants include the normal stiffness, stiffness ratio of particle, etc. It is found that the initial Young’s modulus and the shear modulus are approximately proportional to one tenth power of the confining pressure, and inversely proportional to nine tenths power of the particle size. The initial Poisson’s ratio is approximately proportional to one fourteenth power of the stiffness ratio of the particle for the linear contact model, and almost independent of the particle size. It is also found that the rational stiffness ratio of the particle ranges from 2.0 to 20.7 for the common sand.
Abstract：
The genetic algorithm (GA) and support vector machine (SVM) are applied to analyze the reliability of underground caverns. The explicit form of performance function is established by use of the relative displacement values and relative displacement limit values of surrounding rock. The learning samples of the relative displacement values are built by numerical simulation; then the relative displacement values are predicted by the support vector machine that is optimized by the genetic algorithm. An example in Jinping Hydropower Station is given for illustrating the application of the proposed approach. The new method is proved effective in the reliability analysis of underground caverns.
The genetic algorithm (GA) and support vector machine (SVM) are applied to analyze the reliability of underground caverns. The explicit form of performance function is established by use of the relative displacement values and relative displacement limit values of surrounding rock. The learning samples of the relative displacement values are built by numerical simulation; then the relative displacement values are predicted by the support vector machine that is optimized by the genetic algorithm. An example in Jinping Hydropower Station is given for illustrating the application of the proposed approach. The new method is proved effective in the reliability analysis of underground caverns.
Abstract：
The damage and rupture of fractured rock mass will accumulate and develop simultaneously during the process of its crack occurrence and growth and failure. The damage will be accumulated owing to the rupture. The damage is closely related to the rupture. Assuming the fractured rock mass as contineous mass with damage and considering the crack occurrence and growth under compressive-shear stress state and tension-shear state, the damage evolution is simulated by introducing the initial damage tensors and additional damage tensors. The two tensors are introduced into the user-defined constitutive model DLL (Dynamic Linked Library). The model is used to simulate one tunnel. Through simulation, the damage region is about twice the yield failure region. The maximum displacement of the rock mass is 0.05 m when the damage is not considered. The maximum displacement is 0.1 m when the damage is considered. The bulk and shear moduli are reduced to some extent. The proposed model is applicable for simulating the damage evolution of the rock mass. The simulated results can provide a resonable reacommendation to the engineering practices.
The damage and rupture of fractured rock mass will accumulate and develop simultaneously during the process of its crack occurrence and growth and failure. The damage will be accumulated owing to the rupture. The damage is closely related to the rupture. Assuming the fractured rock mass as contineous mass with damage and considering the crack occurrence and growth under compressive-shear stress state and tension-shear state, the damage evolution is simulated by introducing the initial damage tensors and additional damage tensors. The two tensors are introduced into the user-defined constitutive model DLL (Dynamic Linked Library). The model is used to simulate one tunnel. Through simulation, the damage region is about twice the yield failure region. The maximum displacement of the rock mass is 0.05 m when the damage is not considered. The maximum displacement is 0.1 m when the damage is considered. The bulk and shear moduli are reduced to some extent. The proposed model is applicable for simulating the damage evolution of the rock mass. The simulated results can provide a resonable reacommendation to the engineering practices.
Abstract：
The local settlement of foundation will cause tensile strain of geomembrane and non-woven geotextile in the geosynthetic liner system, and greater tensile strain will lead to tensile failure of the geosynthetic liner system or degradation of its performance. The deformation behaviors of the geosynthetic liner system after local settlement of foundation are investigated by use of strain gauges, earth pressure cells and displacement gauges. The experimental results show that the maximum axial tensile strain occurs in the center of subsidence area after the local settlement of foundation, and that the strain gradually decreases from the center to both sides. The overlying pressure on the geosynthetic liner system within subsidence area changes accordingly. Tensile stiffness has greater influence on the deformation. Finally, some useful conclusions are drawn:different locations have different tensile strains; the relative displacement occurs between interfaces; and the overlying pressures on the geosynthetic liner system are redistributed after the local settlement of foundation.
The local settlement of foundation will cause tensile strain of geomembrane and non-woven geotextile in the geosynthetic liner system, and greater tensile strain will lead to tensile failure of the geosynthetic liner system or degradation of its performance. The deformation behaviors of the geosynthetic liner system after local settlement of foundation are investigated by use of strain gauges, earth pressure cells and displacement gauges. The experimental results show that the maximum axial tensile strain occurs in the center of subsidence area after the local settlement of foundation, and that the strain gradually decreases from the center to both sides. The overlying pressure on the geosynthetic liner system within subsidence area changes accordingly. Tensile stiffness has greater influence on the deformation. Finally, some useful conclusions are drawn:different locations have different tensile strains; the relative displacement occurs between interfaces; and the overlying pressures on the geosynthetic liner system are redistributed after the local settlement of foundation.
Abstract：
The initial ground stress of rock mass is the most basic and original data of the mechanical state of crustal formation. It is a fundamental parameter for the stability analysis of surrounding rock and the design of mining and supporting in mining engineering. Compared with other methods of in-situ stress measurement, the stress relief method of hollow inclusion is more accurate and can get 3D stresses by one-time measurement. The theory of in-situ stress measurement by use of hollow inclusion gauge is discussed in detail. The magnitudes and directions of 3D ground stress at four deep sites in Juji Mine are obtained by means of improved (KX-81) hollow inclusion gauge. Through analysis, the basic rules of the in-situ stress distribution in Juji Mine are as follows: each of the principal stresses is compression stress, and no tensile tress is found; the dip angles of the maximum principal stress are all less than 6o, and their orientation angles range from S10oE to S11oW, that is, the maximum principal stress is close to the horizontal and north-south direction, which shows that the regional geo-stress field of Juji Mine is mainly composed of horizontal tectonic stress; stress regression equations indicate that the vertical stress is not more than the weight of top strata in unit area, and that the values of the principal stresses increase with the increase of depth. The measured results are of great importance to the design of deep mining and the security of production.
The initial ground stress of rock mass is the most basic and original data of the mechanical state of crustal formation. It is a fundamental parameter for the stability analysis of surrounding rock and the design of mining and supporting in mining engineering. Compared with other methods of in-situ stress measurement, the stress relief method of hollow inclusion is more accurate and can get 3D stresses by one-time measurement. The theory of in-situ stress measurement by use of hollow inclusion gauge is discussed in detail. The magnitudes and directions of 3D ground stress at four deep sites in Juji Mine are obtained by means of improved (KX-81) hollow inclusion gauge. Through analysis, the basic rules of the in-situ stress distribution in Juji Mine are as follows: each of the principal stresses is compression stress, and no tensile tress is found; the dip angles of the maximum principal stress are all less than 6o, and their orientation angles range from S10oE to S11oW, that is, the maximum principal stress is close to the horizontal and north-south direction, which shows that the regional geo-stress field of Juji Mine is mainly composed of horizontal tectonic stress; stress regression equations indicate that the vertical stress is not more than the weight of top strata in unit area, and that the values of the principal stresses increase with the increase of depth. The measured results are of great importance to the design of deep mining and the security of production.
Abstract：
By modelling the compressive modulus of soil properties as a spatially random field instead of the traditional random variable and by using the local average theory of random field, a method to calculate the spatial average variance from just the “point” variance is proposed. Then the derivation of an expression for random foundation settlement, which can reflect the effect of the spatial variability characteristics of soil parameters, is accomplished in combination with the common settlement calculation method at present in foundation design, namely the layer-wise summation method. The derivation includes dealing with both single and layered soil. At the same time, the first order second moment (FOSM) analysis of probabilistic foundation settlement is achieved by using the 5-point Gaussian integral based on an assumed autocorrelation function. On this basis, approaches for probabilistic forecast of foundation settlement and reliability analysis of foundation under a given limitation of settlement are recommended. All of those can avoid the complicacy of random field simulation and the expensive computation of Monte-Carlo sampling, and achieve the purpose of compatibility with the common calculation methods, which can be accepted by ordinary designers conveniently. Finally, an example of application is presented in order to demonstrate the analysis procedure and capabilities of the proposed algorithm. It is believed that it has significantly applicable values to risk assessment or reliability analysis in geotechnical engineering.
By modelling the compressive modulus of soil properties as a spatially random field instead of the traditional random variable and by using the local average theory of random field, a method to calculate the spatial average variance from just the “point” variance is proposed. Then the derivation of an expression for random foundation settlement, which can reflect the effect of the spatial variability characteristics of soil parameters, is accomplished in combination with the common settlement calculation method at present in foundation design, namely the layer-wise summation method. The derivation includes dealing with both single and layered soil. At the same time, the first order second moment (FOSM) analysis of probabilistic foundation settlement is achieved by using the 5-point Gaussian integral based on an assumed autocorrelation function. On this basis, approaches for probabilistic forecast of foundation settlement and reliability analysis of foundation under a given limitation of settlement are recommended. All of those can avoid the complicacy of random field simulation and the expensive computation of Monte-Carlo sampling, and achieve the purpose of compatibility with the common calculation methods, which can be accepted by ordinary designers conveniently. Finally, an example of application is presented in order to demonstrate the analysis procedure and capabilities of the proposed algorithm. It is believed that it has significantly applicable values to risk assessment or reliability analysis in geotechnical engineering.
Abstract：
On the basis of a plastic bounding surface model, an anisotropic elasto-plastic model for saturated structured soft clay under cyclic loading is proposed. The effect of damage on structure caused by accumulated plastic strain is modeled with inner variable introduced into the corresponding equations, linking the structure damage with accumulated plastic strain increments. An anisotropic tensor and the rotational hardening law are introduced to reflect the evolution of anisotropy. Besides, by adopting the method of mobile mapping origin, the mapping rule is modified to simulate the elasto-plastic property of soils under cyclic loading. The validity of the model is verified by the cyclic test data of typical structured clay and Shanghai clay.
On the basis of a plastic bounding surface model, an anisotropic elasto-plastic model for saturated structured soft clay under cyclic loading is proposed. The effect of damage on structure caused by accumulated plastic strain is modeled with inner variable introduced into the corresponding equations, linking the structure damage with accumulated plastic strain increments. An anisotropic tensor and the rotational hardening law are introduced to reflect the evolution of anisotropy. Besides, by adopting the method of mobile mapping origin, the mapping rule is modified to simulate the elasto-plastic property of soils under cyclic loading. The validity of the model is verified by the cyclic test data of typical structured clay and Shanghai clay.
Abstract：
Numerical oscillation always occurs when the standard Galerkin method is applied to the pure convection coupled piping model, and it affects the calculation quality and efficiency seriously. The characteristics of piping governing differential equations and the applicability of the standard Galerkin method are analyzed, with the results showing that the nonlinear convection item in the governing differential equations induces the non-self-adjoint equations and unsymmetrical total matrix, and that the optimal approximation character of the standard Galerkin method is destroyed, resulting in the numerical oscillation. The stabilized finite element technique, Streamline Upwind Petrov/Galerkin (SUPG), is employed to improve the coupled piping mathematical model. The results indicate that the SUPG method can eliminate numerical oscillation of pure convection equations effectively, and gain more satisfactory results under the condition of fewer elements compared with the standard Galerkin method.
Numerical oscillation always occurs when the standard Galerkin method is applied to the pure convection coupled piping model, and it affects the calculation quality and efficiency seriously. The characteristics of piping governing differential equations and the applicability of the standard Galerkin method are analyzed, with the results showing that the nonlinear convection item in the governing differential equations induces the non-self-adjoint equations and unsymmetrical total matrix, and that the optimal approximation character of the standard Galerkin method is destroyed, resulting in the numerical oscillation. The stabilized finite element technique, Streamline Upwind Petrov/Galerkin (SUPG), is employed to improve the coupled piping mathematical model. The results indicate that the SUPG method can eliminate numerical oscillation of pure convection equations effectively, and gain more satisfactory results under the condition of fewer elements compared with the standard Galerkin method.
Abstract：
Large-scale triaxial tests are carried out on gabion mesh reinforced red-sandstone granular soil with different reinforcement layers, compaction degrees and water contents. The effects of gabion mesh reinforcement layers, water content and compaction on the stress-strain relationship and strength properties of red-sandstone granular soil are analyzed. The reinforcing effects are evaluated by introducing the strength ratio parameter. A comparison between the geogrid and the gabion mesh reinforced red-sandstone granular soil is conducted. The experimental results show that the peak strength, cohesion and ductility of the red-sandstone granular soil are obviously improved by gabion mesh reinforcements, but its internal friction angle is not greatly improved. The cohesions of the reinforced soil and the plain soil exhibit non-linear relationships with their water content under the same compaction, moreover, there is a maximum value near the optimum water content. The cohesion and internal friction angle increase with the increase of compaction under the same water content. The reinforcing effect relates with the number of gabion mesh layers, water content and compaction of soil, and the effect of the gabion mesh reinforced soil decreases with the increase of the confine pressure.
Large-scale triaxial tests are carried out on gabion mesh reinforced red-sandstone granular soil with different reinforcement layers, compaction degrees and water contents. The effects of gabion mesh reinforcement layers, water content and compaction on the stress-strain relationship and strength properties of red-sandstone granular soil are analyzed. The reinforcing effects are evaluated by introducing the strength ratio parameter. A comparison between the geogrid and the gabion mesh reinforced red-sandstone granular soil is conducted. The experimental results show that the peak strength, cohesion and ductility of the red-sandstone granular soil are obviously improved by gabion mesh reinforcements, but its internal friction angle is not greatly improved. The cohesions of the reinforced soil and the plain soil exhibit non-linear relationships with their water content under the same compaction, moreover, there is a maximum value near the optimum water content. The cohesion and internal friction angle increase with the increase of compaction under the same water content. The reinforcing effect relates with the number of gabion mesh layers, water content and compaction of soil, and the effect of the gabion mesh reinforced soil decreases with the increase of the confine pressure.
Abstract：
More and more large underground power houses are under the geological environment of large buried depth, high ground stress and high pore water pressure. At the same time, it is complicated to analyze the stability of underground houses because of the existence of joints in the rock mass. The joints make intension of the rock mass decrease. According to these characteristics, a new visco-elastoplastic mechanical model is deduced. The model has a comprehensive consideration of damage and rheology. Besides, regarding VC++ as the plat form, the secondary development function of FLAC3D is employed to make a program of this constitutive equation. It is a dynamic link library (DLL) and can be used by the main program of FLAC3D. A new damage rheology FLAC3D rock mass is developed. In the project of Shuangjiangkou large underground power houses, the program model for joins is used to analyze the ability of houses and to guide the design of engineering. The results show that it is quite strong feasible to use this new damage rheology FLAC3d model to simulate the excavation of large underground power houses and analyze the stability. The analytic results can be used to guide the construction of engineering.
More and more large underground power houses are under the geological environment of large buried depth, high ground stress and high pore water pressure. At the same time, it is complicated to analyze the stability of underground houses because of the existence of joints in the rock mass. The joints make intension of the rock mass decrease. According to these characteristics, a new visco-elastoplastic mechanical model is deduced. The model has a comprehensive consideration of damage and rheology. Besides, regarding VC++ as the plat form, the secondary development function of FLAC3D is employed to make a program of this constitutive equation. It is a dynamic link library (DLL) and can be used by the main program of FLAC3D. A new damage rheology FLAC3D rock mass is developed. In the project of Shuangjiangkou large underground power houses, the program model for joins is used to analyze the ability of houses and to guide the design of engineering. The results show that it is quite strong feasible to use this new damage rheology FLAC3d model to simulate the excavation of large underground power houses and analyze the stability. The analytic results can be used to guide the construction of engineering.
Abstract：
The conventional triaxial tests, triaxial tension (extension) tests and combined compression-tension triaxial tests on core clay materials of Nuozhadu and Shuangjiangkou earth-rockfill dams are carried out by using the developed horizontal triaxial tension-compression apparatus. Based on the test results, the stress-strain behaviors of compacted clay under triaxial compression, triaxial tension and compression-to-tension states are investigated. Furthermore, the applicability of Duncan-Chang’s EB constitutive model is extended, so that it can be used to describe the deformation properties of compacted clay from compression state to tension one. This extended Duncan-Chang model can be used for numerical simulation of tensile crack propagation in soil structure.
The conventional triaxial tests, triaxial tension (extension) tests and combined compression-tension triaxial tests on core clay materials of Nuozhadu and Shuangjiangkou earth-rockfill dams are carried out by using the developed horizontal triaxial tension-compression apparatus. Based on the test results, the stress-strain behaviors of compacted clay under triaxial compression, triaxial tension and compression-to-tension states are investigated. Furthermore, the applicability of Duncan-Chang’s EB constitutive model is extended, so that it can be used to describe the deformation properties of compacted clay from compression state to tension one. This extended Duncan-Chang model can be used for numerical simulation of tensile crack propagation in soil structure.
Abstract：
The distribution of earth pressure over a positive rigid culvert buried in non-cohesive soil is studied by means of model tests and theoretical analyses. Based on the measured results from the full-scale physical model tests and the Marston’s theory, a more reasonable analytical model of earth pressure over the culvert is proposed, and the principal parameters are determined based on the test data. The test results with various buried depths on culvert top show that the proposed model can describe the earth pressure distribution over the culvert effectively. The concentration phenomenon of earth pressure on top of the stiff culvert becomes more significant as buried height increases. The normal earth pressure between the soil and the culvert increases first and decreases afterwards with the distribution angle for lower embankment height, whereas it increases monotonically with the distribution angle for high embankment height.
The distribution of earth pressure over a positive rigid culvert buried in non-cohesive soil is studied by means of model tests and theoretical analyses. Based on the measured results from the full-scale physical model tests and the Marston’s theory, a more reasonable analytical model of earth pressure over the culvert is proposed, and the principal parameters are determined based on the test data. The test results with various buried depths on culvert top show that the proposed model can describe the earth pressure distribution over the culvert effectively. The concentration phenomenon of earth pressure on top of the stiff culvert becomes more significant as buried height increases. The normal earth pressure between the soil and the culvert increases first and decreases afterwards with the distribution angle for lower embankment height, whereas it increases monotonically with the distribution angle for high embankment height.
Abstract：
New buildings exert extra load on the ground surface above mined-out areas and may induce overburden failures. A systematic research on the distribution laws of foundation stress after newly increased load on mined-out areas is performed by use of simulation tests on similar materials and numerical methods. The main findings are as follows: the rock stress increases lineally with the depth before the coal mining, while it has a redistribution after the coal worked-out. The additional stress of the foundation decreases gradually with the depth after the newly increased load on the surface. The influence depth is determined by the foundation additional stress caused by newly added load when it is equal to 10% of the rock stress before the coal mining. The influence depths under different loads on the surface before and after mining are obtained, and the study provides an important theory criterion for surface utilization above mined-out areas.
New buildings exert extra load on the ground surface above mined-out areas and may induce overburden failures. A systematic research on the distribution laws of foundation stress after newly increased load on mined-out areas is performed by use of simulation tests on similar materials and numerical methods. The main findings are as follows: the rock stress increases lineally with the depth before the coal mining, while it has a redistribution after the coal worked-out. The additional stress of the foundation decreases gradually with the depth after the newly increased load on the surface. The influence depth is determined by the foundation additional stress caused by newly added load when it is equal to 10% of the rock stress before the coal mining. The influence depths under different loads on the surface before and after mining are obtained, and the study provides an important theory criterion for surface utilization above mined-out areas.
Abstract：
It is the first time to use EPB method in the subway construction in Xi’an loess strata. There is no better work experience for reference. The difference between the observed results and the Peck expressions is larger for the ground settlement trough. The real process of shield tunneling construction is simulated by FEM with Duncan-Chang model, which is programmed with APDL. The Duncan-Chang parameters, moisture content and depth of the tunnel axis are employed to perform quantitative analysis. The law of the ground settlement induced by subway construction with shield tunneling in Xi’an loess strata is studied, and an expressions is put forward to predict a certain measured ground settlement. It shows that the expression is suitable for the investigation of the ground settlement characteristics, and the conclusion is significant for similar projects with shield tunneling in Xi’an loess strata.
It is the first time to use EPB method in the subway construction in Xi’an loess strata. There is no better work experience for reference. The difference between the observed results and the Peck expressions is larger for the ground settlement trough. The real process of shield tunneling construction is simulated by FEM with Duncan-Chang model, which is programmed with APDL. The Duncan-Chang parameters, moisture content and depth of the tunnel axis are employed to perform quantitative analysis. The law of the ground settlement induced by subway construction with shield tunneling in Xi’an loess strata is studied, and an expressions is put forward to predict a certain measured ground settlement. It shows that the expression is suitable for the investigation of the ground settlement characteristics, and the conclusion is significant for similar projects with shield tunneling in Xi’an loess strata.
Abstract：
The shear wave velocity of soils is one of the most widely used parameters in the field of geotechnical earthquake engineering. Based on the measured data of shear wave velocity of soils from lots of boreholes obtained during seismic safety evaluation of engineering sites in China, the relationship between shear wave velocity and depth of soils is qualitatively discussed, and the regression equations of shear wave velocity and depth of conventional soils, including gravelly soil, sandy soil, silt, and cohesive soil, considering site classification and without considering site classification, except of muddy soil, are established by using three statistical models of the first-order linear equation, exponent equation and polynomial in one variable with two-order, respectively. The goodness of fitting is taken as an evaluation index to acquire the recommended models and parameters between shear wave velocity and depth of conventional soils, except of muddy soil. The equations recommended are compared with the formula in the Chinese design code for earthquake resistance of special structures (GB50191-93). In addition, the results of shear wave velocity of some real site predicted using the recommended equations are compared with those of site measurement. The above results show that there is a remarkable relationship between the shear wave velocity and the depth of conventional soils, except of muddy soil, that the fitting accuracy of polynomial in one variable with two-order is the highest on the whole and that the regression equations of shear wave velocity and depth of most of conventional soils are more reliable, which can be used as a reference for sites, whose shear wave velocities are not tested.
The shear wave velocity of soils is one of the most widely used parameters in the field of geotechnical earthquake engineering. Based on the measured data of shear wave velocity of soils from lots of boreholes obtained during seismic safety evaluation of engineering sites in China, the relationship between shear wave velocity and depth of soils is qualitatively discussed, and the regression equations of shear wave velocity and depth of conventional soils, including gravelly soil, sandy soil, silt, and cohesive soil, considering site classification and without considering site classification, except of muddy soil, are established by using three statistical models of the first-order linear equation, exponent equation and polynomial in one variable with two-order, respectively. The goodness of fitting is taken as an evaluation index to acquire the recommended models and parameters between shear wave velocity and depth of conventional soils, except of muddy soil. The equations recommended are compared with the formula in the Chinese design code for earthquake resistance of special structures (GB50191-93). In addition, the results of shear wave velocity of some real site predicted using the recommended equations are compared with those of site measurement. The above results show that there is a remarkable relationship between the shear wave velocity and the depth of conventional soils, except of muddy soil, that the fitting accuracy of polynomial in one variable with two-order is the highest on the whole and that the regression equations of shear wave velocity and depth of most of conventional soils are more reliable, which can be used as a reference for sites, whose shear wave velocities are not tested.
Abstract：
First, a new automatic oil-controlling system for real-time modeling of the excavation of the deep foundation pit with shorings is presented, which can be installed in the 150-ton multi-function centrifuge in the Centrifuge Laboratory of Tongji University, China. Based on the automatic system, the excavation of different layers of soils is simulated by discharging different volumes of special liquid, and the installation of the shorings on a retaining wall after excavating the above soils timely is simulated by locking the shoring of the system. Accordingly, the construction of a deep foundation pit with shorings can be modeled at high-speed flight operations of the centrifuge. Second, the centrifuge modeling of the effect on the adjacent 3 m underpass induced by excavating 32.7 m-deep Yanggao air well of Metro line 9 of Shanghai is introduced. The comparison between the observed data and the modeling data shows that the automatic excavating system of the centrifuge can effectively model a complete excavation with shorings. Finally, the results of the settlement of the underpass by different mixture-pile barrier walls for improving soil between the excavation and the underpass are compared. It is concluded that the barrier walls can effectively decrease the settlement of the adjacent underpass. The barrier wall should be deeper than the excavation depth in order to make an effective barrier, and the length limit of the barrier walls should be the half depth of excavation beyond the excavation zone.
First, a new automatic oil-controlling system for real-time modeling of the excavation of the deep foundation pit with shorings is presented, which can be installed in the 150-ton multi-function centrifuge in the Centrifuge Laboratory of Tongji University, China. Based on the automatic system, the excavation of different layers of soils is simulated by discharging different volumes of special liquid, and the installation of the shorings on a retaining wall after excavating the above soils timely is simulated by locking the shoring of the system. Accordingly, the construction of a deep foundation pit with shorings can be modeled at high-speed flight operations of the centrifuge. Second, the centrifuge modeling of the effect on the adjacent 3 m underpass induced by excavating 32.7 m-deep Yanggao air well of Metro line 9 of Shanghai is introduced. The comparison between the observed data and the modeling data shows that the automatic excavating system of the centrifuge can effectively model a complete excavation with shorings. Finally, the results of the settlement of the underpass by different mixture-pile barrier walls for improving soil between the excavation and the underpass are compared. It is concluded that the barrier walls can effectively decrease the settlement of the adjacent underpass. The barrier wall should be deeper than the excavation depth in order to make an effective barrier, and the length limit of the barrier walls should be the half depth of excavation beyond the excavation zone.
Abstract：
During the construction of tunnels in karst mountain, water inrush and gushing mud (sand) disaster has ranked the first in all the disasters. The exact and timely geological prediction is the critical problem most needed to be studied and solved in the design and construction of tunnels in karst zone. The theories of geology and karstology are used for the geological analysis of the tunnel. A fuzzy mathematical model is put forward to evaluate the risk of different parts along the tunnel, and the risk rank is given. According to the advantages and disadvantages of various geophysical methods and the sensitivity of forecasting karst water, and with the method and principle of the comprehensive geological prediction, the forecast program and process of comprehensive geological prediction are optimized and the accuracy of the position exploration of karst water is improved greatly. And the forecast technology is used during the construction of the Wuchiba tunnel of Shanghai-Chendu Expressway west. During the construction of the tunnel, the comprehensive geological prediction technology is implemented strictly. Through analyzing the geology and the fuzzy mathematical model, different risk rating parts of geological disasters along the tunnel are divided. In different risk rating zones, different programs are enacted. Based on the programs, the karst water is successfully forecast, and the scientificity and feasibility are validated.
During the construction of tunnels in karst mountain, water inrush and gushing mud (sand) disaster has ranked the first in all the disasters. The exact and timely geological prediction is the critical problem most needed to be studied and solved in the design and construction of tunnels in karst zone. The theories of geology and karstology are used for the geological analysis of the tunnel. A fuzzy mathematical model is put forward to evaluate the risk of different parts along the tunnel, and the risk rank is given. According to the advantages and disadvantages of various geophysical methods and the sensitivity of forecasting karst water, and with the method and principle of the comprehensive geological prediction, the forecast program and process of comprehensive geological prediction are optimized and the accuracy of the position exploration of karst water is improved greatly. And the forecast technology is used during the construction of the Wuchiba tunnel of Shanghai-Chendu Expressway west. During the construction of the tunnel, the comprehensive geological prediction technology is implemented strictly. Through analyzing the geology and the fuzzy mathematical model, different risk rating parts of geological disasters along the tunnel are divided. In different risk rating zones, different programs are enacted. Based on the programs, the karst water is successfully forecast, and the scientificity and feasibility are validated.
Abstract：
Secondary consolidation deformation and structure of soil are produced during naturally sedimentary process due to time effect, and then the consolidation yield stress increases. Bjerrum model is not proper for the deformation mechanism of secondary consolidation of structural clays. Based on oedometer tests on Lianyungang structural clays and the concept of void index proposed by Burland, the deformation mechanism of secondary consolidation of structural clays is studied. Change of void ratio index (?Iv) ranges between the intrinsic compression line (ICL) and the compression line of natural sedimentary structural clays. When the consolidation pressure is less than the yield stress (termed as preyield state), the structural strength provides the resistance to the compression potential of void. As a result, ?Iv increases with the consolidation pressure before yielding of soil structure and the secondary consolidation deformation can be ignored during this stage. When the consolidation pressure exceeds the yield stress, ?Iv decreases with the increase of the consolidation pressure mainly due to the collapse of the large void structure. The coefficient of secondary consolidation, Cα, reaches a peak value at the critical yield state and then decreases with the consolidation pressure. It is concluded that the secondary consolidation deformation will not develop infinitely due to the existence of the compression line (SCL) of natural sedimentary clays.
Secondary consolidation deformation and structure of soil are produced during naturally sedimentary process due to time effect, and then the consolidation yield stress increases. Bjerrum model is not proper for the deformation mechanism of secondary consolidation of structural clays. Based on oedometer tests on Lianyungang structural clays and the concept of void index proposed by Burland, the deformation mechanism of secondary consolidation of structural clays is studied. Change of void ratio index (?Iv) ranges between the intrinsic compression line (ICL) and the compression line of natural sedimentary structural clays. When the consolidation pressure is less than the yield stress (termed as preyield state), the structural strength provides the resistance to the compression potential of void. As a result, ?Iv increases with the consolidation pressure before yielding of soil structure and the secondary consolidation deformation can be ignored during this stage. When the consolidation pressure exceeds the yield stress, ?Iv decreases with the increase of the consolidation pressure mainly due to the collapse of the large void structure. The coefficient of secondary consolidation, Cα, reaches a peak value at the critical yield state and then decreases with the consolidation pressure. It is concluded that the secondary consolidation deformation will not develop infinitely due to the existence of the compression line (SCL) of natural sedimentary clays.
Abstract：
A new load transfer model considering friction resistance softening of soils is proposed. Its parameters have clear physical significance, their values are easy to obtain, and the proposed value of softening coefficient c is given in accordance with the engineering experience in Zhejiang province. In this load transfer model, the softening coefficient c decides the degree of softening, the parameter decides the ultimate friction resistance, and the parameter decides the ultimate relative displacement. When the relative displacement is smaller than the ultimate displacement, the friction resistance increases with relative displacement nonlinearly. When the relative displacement is equal to the ultimate displacement, the friction resistance reaches the peak value; when the relative displacement is larger than the ultimate displacement, friction resistance begins to decrease gradually. The normalized analysis of data from case histories and the comparison with the hyperbolic load transfer model indicate that the load transfer model considering strain softening of soils is better than the hyperbolic load transfer model in simulating the stress-strain curve of softening soils.
A new load transfer model considering friction resistance softening of soils is proposed. Its parameters have clear physical significance, their values are easy to obtain, and the proposed value of softening coefficient c is given in accordance with the engineering experience in Zhejiang province. In this load transfer model, the softening coefficient c decides the degree of softening, the parameter decides the ultimate friction resistance, and the parameter decides the ultimate relative displacement. When the relative displacement is smaller than the ultimate displacement, the friction resistance increases with relative displacement nonlinearly. When the relative displacement is equal to the ultimate displacement, the friction resistance reaches the peak value; when the relative displacement is larger than the ultimate displacement, friction resistance begins to decrease gradually. The normalized analysis of data from case histories and the comparison with the hyperbolic load transfer model indicate that the load transfer model considering strain softening of soils is better than the hyperbolic load transfer model in simulating the stress-strain curve of softening soils.
Abstract：
A series of field tests of high energy dynamic compaction (HEDC) are carried out to investigate the effective depth of improvement (EDI) of foundation backfilled with crushed stone, underlain by thin soft layers and high groundwater level in coastal area. The research work includes the following items: for Test Zone A, single-point dynamic compaction tests under energy levels of 14000, 10000 and 8000 kN·m; comparison tests of single-point dynamic compaction under the same energy level (6000 kN·m) and different bowl pressure (weight) of 34 (180 kN), 50 (250 kN) and 90 kPa (460 kN). For Test Zone B, group-point dynamic compaction tests under energy level of 12000 kN·m. For Test Zone C, group-points dynamic compaction tests under energy level of 15000 kN·m. Cone dynamic penetration tests, standard penetration tests and routine geotechnical experiments are performed, and the monitoring results are analyzed. Through comparative analysis on the bearing capacity of foundation before and after dynamic compaction, the EDI of foundation and the revised coefficient of the Menard formulas are presented under different high energy levels. The results provide the parameters for the design, construction, and detection of high energy dynamic compaction in similar regions.
A series of field tests of high energy dynamic compaction (HEDC) are carried out to investigate the effective depth of improvement (EDI) of foundation backfilled with crushed stone, underlain by thin soft layers and high groundwater level in coastal area. The research work includes the following items: for Test Zone A, single-point dynamic compaction tests under energy levels of 14000, 10000 and 8000 kN·m; comparison tests of single-point dynamic compaction under the same energy level (6000 kN·m) and different bowl pressure (weight) of 34 (180 kN), 50 (250 kN) and 90 kPa (460 kN). For Test Zone B, group-point dynamic compaction tests under energy level of 12000 kN·m. For Test Zone C, group-points dynamic compaction tests under energy level of 15000 kN·m. Cone dynamic penetration tests, standard penetration tests and routine geotechnical experiments are performed, and the monitoring results are analyzed. Through comparative analysis on the bearing capacity of foundation before and after dynamic compaction, the EDI of foundation and the revised coefficient of the Menard formulas are presented under different high energy levels. The results provide the parameters for the design, construction, and detection of high energy dynamic compaction in similar regions.
Abstract：
The concept of risk crosses through the society with significance and value that differ greatly in space and time. Mountain tunnel engineering has high risk because of plenty of uncertainties in many of areas related to ground conditions. The uncertainties have characteristics such as complicated geology environment, insufficient basic data and multiple construction methods. Risk management and control, which is effective to reduce risk of mountain tunnel engineering, can be implemented during the construction of mountain tunnel engineering. The residual risk management and control of construction phase can be solved with observation. The case of Huangtian tunnel of Huangshan-Quzhou-Nanping Expressway of Zhejiang Province is taken as an example. A risk management frame, composed of risk management matrix, daily risk report on the basis of observation data and risk warning system, is established according to the potential damage and probability of harmful cases. The risk management frame proves to be effective in dealing with the risk management during the construction phase of Hunagtian tunnel through the instrumentality of field investigation and crown displacement of YK17+660, which is at the right exit end of Huangtian tunnel, and can provide the basis and ideas for similar projects. Moreover, it is concluded that the establishment of the acceptable risk criterion of mountain tunnel engineering should be based on the allowable deformation limits and the reaction time of surrounding rock of tunnels.
The concept of risk crosses through the society with significance and value that differ greatly in space and time. Mountain tunnel engineering has high risk because of plenty of uncertainties in many of areas related to ground conditions. The uncertainties have characteristics such as complicated geology environment, insufficient basic data and multiple construction methods. Risk management and control, which is effective to reduce risk of mountain tunnel engineering, can be implemented during the construction of mountain tunnel engineering. The residual risk management and control of construction phase can be solved with observation. The case of Huangtian tunnel of Huangshan-Quzhou-Nanping Expressway of Zhejiang Province is taken as an example. A risk management frame, composed of risk management matrix, daily risk report on the basis of observation data and risk warning system, is established according to the potential damage and probability of harmful cases. The risk management frame proves to be effective in dealing with the risk management during the construction phase of Hunagtian tunnel through the instrumentality of field investigation and crown displacement of YK17+660, which is at the right exit end of Huangtian tunnel, and can provide the basis and ideas for similar projects. Moreover, it is concluded that the establishment of the acceptable risk criterion of mountain tunnel engineering should be based on the allowable deformation limits and the reaction time of surrounding rock of tunnels.
Abstract：
Firstly, by use of the mechanical testing & simulation system, the trialxial rheology curves of dry carbonaceous shale and carbonaceous shale in pressure water are obtained and the difference of rheology properties is compared and analyzed. Secondly, a linear viscoelastoplastic model is put forward to describe the rheology properties of shale in pressure water, thus the constitutive equation and creep equation are solved. Finally, the parameters of rheology model are achieved. The test results show that the pressure water reduces the yielding strength and raises the rheology characteristics of carbonaceous shale. The comparison between the rheology model and the experiment results shows that the linear viscoelasticoplastic model can reflect the deceleration and constant creep period, and can be used to study the rheology properties of shale or other rock in confined water.
Firstly, by use of the mechanical testing & simulation system, the trialxial rheology curves of dry carbonaceous shale and carbonaceous shale in pressure water are obtained and the difference of rheology properties is compared and analyzed. Secondly, a linear viscoelastoplastic model is put forward to describe the rheology properties of shale in pressure water, thus the constitutive equation and creep equation are solved. Finally, the parameters of rheology model are achieved. The test results show that the pressure water reduces the yielding strength and raises the rheology characteristics of carbonaceous shale. The comparison between the rheology model and the experiment results shows that the linear viscoelasticoplastic model can reflect the deceleration and constant creep period, and can be used to study the rheology properties of shale or other rock in confined water.
