While the moisture content of soil affects significantly the blast impulse of shallow buried explosives,the role of surface-covering water(SCW)on soil in such blast impulse remains elusive.A combined experimental and ...While the moisture content of soil affects significantly the blast impulse of shallow buried explosives,the role of surface-covering water(SCW)on soil in such blast impulse remains elusive.A combined experimental and numerical study has been carried out to characterize the effect of SCW on transferred impulse and loading magnitude of shallow buried explosives.Firstly,blast tests of shallow buried explosives were conducted,with and without the SCW,to quantitatively assess the blast loading impulse.Subsequently,finite element(FE)simulations were performed and validated against experimental measurement,with good agreement achieved.The validated FE model was then employed to predict the dynamic response of a fully-clamped metallic circular target,subjected to the explosive impact of shallow buried explosives with SCW,and explore the corresponding physical mechanisms.It was demonstrated that shallow buried explosives in saturated soil generate a greater impulse transferred towards the target relative to those in dry soil.The deformation displacement of the target plate is doubled.Increasing the height of SCW results in enhanced center peak deflection of the loaded target,accompanied by subsequent fall,due to the variation of deformation pattern of the loaded target from concentrated load to uniform load.Meanwhile,the presence of SCW increases the blast impulse transferred towards the target by three times.In addition,there exists a threshold value of the burial depth that maximizes the impact impulse.This threshold exhibits a strong sensitivity to SCW height,decreasing with increasing SCW height.An empirical formula for predicting threshold has been provided.Similar conclusions can be drawn for different explosive masses.The results provide technical guidance on blast loading intensity and its spatial distribution considering shallow buried explosives in coast-land battlefields,which can ultimately contribute to better protective designs.展开更多
A systematic analysis of southwestern Ordos Basin's sedimentary characteristics,internal architectural element association styles and depositional model was illustrated through core statistics,well logging data an...A systematic analysis of southwestern Ordos Basin's sedimentary characteristics,internal architectural element association styles and depositional model was illustrated through core statistics,well logging data and outcrop observations in Chang 8 oil-bearing group.This analysis indicates that shallow water delta sediments dominated by a fluvial system is the primary sedimentary system of the Chang 8 oil-bearing group of the Yanchang Formation in southwestern Ordos Basin.Four microfacies with fine grain sizes are identified: distributary channels,sheet sandstone,mouth bar and interdistributary fines.According to the sandbody's spatial distribution and internal architecture,two types of sandbody architectural element associations are identified: amalgamated distributary channels and thin-layer lobate sandstone.In this sedimentary system,net-like distributary channels at the delta with a narrow ribbon shape compose the skeleton of the sandbody that extends further into the delta front and shades into contiguous lobate distribution sheet sandstone in the distal delta front.The mouth bar is largely absent in this system.By analyzing the palaeogeomorphology,the palaeostructure background,sedimentary characteristics,sedimentary facies types and spatial distribution of sedimentary facies during the Chang 8 period,a distinctive depositional model of the Chang 8 shallow water fluvial-dominated delta was established,which primarily consists of straight multi-phase amalgamated distributary channels in the delta plain,net-like distributary channels frequently diverting and converging in the proximal delta front,sheet sandstones with dispersing contiguous lobate shapes in the distal delta front,and prodelta or shallow lake mudstones.展开更多
Linear failure criterion is widely used in calculation of earth pressure acting on shallow tunnels. However, experimental evidence shows that nonlinear failure criterion is able to represent fairly well the failure of...Linear failure criterion is widely used in calculation of earth pressure acting on shallow tunnels. However, experimental evidence shows that nonlinear failure criterion is able to represent fairly well the failure of almost all types of rocks, A nonlinear Hock-Brown failure criterion is employed to estimate the supporting pressures of shallow tunnels in limit analysis framework. Two failure mechanisms are proposed for calculating the work rate of extemal force and the internal energy dissipation. A tangential line to the nonlinear failure criterion is used to formulate the supporting pressure problem as a nonlinear programming problem. The objective function formulated in this way is minimized with respect to the failure mechanism and the location of tangency point. In order to assess the validity, the supporting pressures for the proposed failure mechanisms are calculated and compared with each other, and the present results are compared with previously published solutions when the nonlinear criterion is reduced to linear criterion. The agreement supports the validity of the proposed failure mechanisms. An experiment is conducted to investigate the influences of the nonlinear criterion on collapse shape and supporting pressures of shallow tunnels.展开更多
According to the interaction of three shallow tunnels with large section, the analytical solution to rock pressure has been derived and discussed. The load model is given when the bilateral tunnels are excavated. Acco...According to the interaction of three shallow tunnels with large section, the analytical solution to rock pressure has been derived and discussed. The load model is given when the bilateral tunnels are excavated. According to the model, the stresses of three tunnels and single tunnel are calculated and compared to analyze the distribution characteristics, where the stresses are influenced by controlling factors of clear distance, covering depth and inclination angle of ground surface. The results show that, in general, the bias distribution is more serious. Therefore, it is significant to settle down the load model of three shallow tunnels so as to determine the measure of reinforcement and design the structure of support. The model and results can be used as a theoretical basis in designation and further research of the three shallow tunnels.展开更多
Based on the nonlinear failure criterion and the upper bound theorem, the modified tangential technique method was proposed to derive the expression of supporting pressure acting on shallow tunnel. Instead of the same...Based on the nonlinear failure criterion and the upper bound theorem, the modified tangential technique method was proposed to derive the expression of supporting pressure acting on shallow tunnel. Instead of the same stress state, different normal stresses on element boundaries were used. In order to investigate the influence of different factors on supporting pressures, the failure mechanism was established. The solution of supporting pressure, with different parameters, was obtained by optimization theory. The corresponding failure mechanism and numerical results were presented. In comparison with the results using the single tangential technique method, it is found that the proposed method is effective, and the good agreement shows that the present solution of supporting pressure is reliable.展开更多
A method of analyzing the stability of twin shallow tunnels was presented using both limit analysis with nonlinear failure criterion and reliability theory.In the condition of nonlinear failure criterion,the critical ...A method of analyzing the stability of twin shallow tunnels was presented using both limit analysis with nonlinear failure criterion and reliability theory.In the condition of nonlinear failure criterion,the critical clear distancesof twin shallow tunnels were obtained by analyzing the change of surrounding pressure.A reliability model was established based on limit state equation,and the failure probability was solved by virtue of Monte Carlo method.Safety factor and corresponding clear distance of different safety levels were obtained by introducing a target reliability index.The scope of clear distance for different safety levels is described,which can be used as a supplement and improvement to the design codes of tunnels.展开更多
By combining the results of laboratory model tests with relevant flow rules, the failure mode of shallow unsymmetrical loading tunnels and the corresponding velocity field were established. According to the principle ...By combining the results of laboratory model tests with relevant flow rules, the failure mode of shallow unsymmetrical loading tunnels and the corresponding velocity field were established. According to the principle of virtual power, the upper bound solution for surrounding rock pressure of shallow unsymmetrical loading tunnel was derived and verified by an example. The results indicate that the calculated results of the derived upper bound method for surrounding rock pressure of shallow unsymmetrical loading tunnels are relatively close to those of the existing "code method" and test results, which means that the proposed method is feasible. The current code method underestimates the unsymmetrical loading feature of surrounding rock pressure of shallow unsymmetrical loading tunnels, so it is unsafe; when the burial depth is less or greater than two times of the tunnel span and the unsymmetrical loading angle is less than 45°, the upper bound method or the average value of the results calculated by the upper bound method and code method respectively, is comparatively reasonable. When the burial depth is greater than two times of the tunnel span and the unsymmetrical loading angle is greater than 45°, the code method is more suitable.展开更多
Based on the upper bound theorem of limit analysis,the factor of safety for shallow tunnel in saturated soil is calculated in conjunction with the strength reduction technique.To analyze the influence of the pore pres...Based on the upper bound theorem of limit analysis,the factor of safety for shallow tunnel in saturated soil is calculated in conjunction with the strength reduction technique.To analyze the influence of the pore pressure on the factor of safety for shallow tunnel,the power of pore pressure is regarded as a power of external force in the energy calculation.Using the rigid multiple-block failure mechanism,the objective function for the factor of safety is constructed and the optimal solutions are derived by employing the sequential quadratic programming.According to the results of optimization calculation,the factor of safety of shallow tunnel for different pore pressure coefficients and variational groundwater tables are obtained.The parameter analysis shows that the pore pressure coefficient and the location of the groundwater table have significant influence on the factor of safety for shallow tunnel.展开更多
Based on the nonlinear Mohr-Coulomb failure criterion and an associated flow rule,a kinematic admissible velocity field of failure mechanism of the 2-layer soil above a shallow horizontal strip anchor plate is constru...Based on the nonlinear Mohr-Coulomb failure criterion and an associated flow rule,a kinematic admissible velocity field of failure mechanism of the 2-layer soil above a shallow horizontal strip anchor plate is constructed.The ultimate pull-out force and its corresponding failure mechanism through the upper bound limit analysis according to a variation principle are deduced.When the 2-layer overlying soil is degraded into single-layer soil,the model of ultimate pullout force could also be degraded into the model of single-layer soil.And the comparison between results of single-layer soil variation method and those calculated by rigid limit analysis method proves the correctness of our method.Based on that,the influence of changes of geotechnical parameters on ultimate pullout forces and failure mechanism of a shallow horizontal strip anchor with the 2-layer soil above are analyzed.The results show that the ultimate pull-out force and failure mechanism of a shallow horizontal strip anchor with the 2-layer soil above are affected by the nonlinear geotechnical parameters greatly.Thus,it is very important to obtain the accurate geotechnical parameters of 2-layer soil for the evaluation of the ultimate pullout capacity of the anchor plate.展开更多
Based on the nonlinear Mohr-Coulomb failure criterion and the associated flow rules,the three-dimensional(3-D)axisymmetric failure mechanism of shallow horizontal circular plate anchors that are subjected to the ultim...Based on the nonlinear Mohr-Coulomb failure criterion and the associated flow rules,the three-dimensional(3-D)axisymmetric failure mechanism of shallow horizontal circular plate anchors that are subjected to the ultimate pullout capacity(UPC)is determined.A derivative function of the projection function for projecting the 3-D axisymmetric failure surface on plane is deduced using the variation theory.By using difference principle,the primitive function of failure surface satisfying boundary condition and numerical solution to its corresponding ultimate pullout capacity function are obtained.The influences of nonlinear Mohr-Coulomb parameters on UPC and failure mechanism are studied.The result shows that UPC decreases with dimensionless parameter m and uniaxial tensile strength increases but increases when depth and radius of plate anchor,surface overload,initial cohesion,geomaterial density and friction angle increase.The failure surface is similar to a symmetrical spatial funnel,and its shape is mainly determined by dimensionless parameter m;the surface damage range expands with the increase of radius and depth of the plate anchor as well as initial cohesion but decreases with the increase of dimensionless parameter m and uniaxial tensile strength as well as geomaterial density.As the dimensionless parameter m=2.0,the numerical solution of UPC based on the difference principle is proved to be feasible and effective through the comparison with the exact solution.In addition,the comparison between solutions of UPC computed by variation method and those computed by upper bound method indicate that variation method outperforms upper bound method.展开更多
Prediction of the state of roof collapse is a big challenge in tunnel engineering, while the limit analysis theory makes it possible to derive the analytical solutions of the collapse mechanisms. In this work, an exac...Prediction of the state of roof collapse is a big challenge in tunnel engineering, while the limit analysis theory makes it possible to derive the analytical solutions of the collapse mechanisms. In this work, an exact solution of collapsing shape in shallow underwater tunnel is obtained by using the variation principle and the upper bound theorem based on nonlinear failure criterion. Numerical results under the effect of river water and supporting pressure are derived and discussed. The maximum water depth above the river bottom surface is determined under a given buried depth of shallow cavities and the critical depth of roof collapse is obtained under a constant river depth. In comparison with the previous results, the present solution shows a good agreement with the practical results.展开更多
Limit analysis of the stability of geomechanical projects is one of the most difficult problems.This work investigates the influences of different parameters in NL failure strength on possible collapsing block shapes ...Limit analysis of the stability of geomechanical projects is one of the most difficult problems.This work investigates the influences of different parameters in NL failure strength on possible collapsing block shapes of single and twin shallow tunnels with considering the effects of surface settlement.Upper bound solutions derived by functional catastrophe theory are used for describing the distinct characteristics of falling blocks of different parts in twin tunnels.Furthermore the analytical solutions of minimum supporting pressures in shallow tunnels are obtained by the help of the variational principle.Lastly,the comparisons are made both in collapsed mechanism and stability factor with different methods.According to the numerical results in this work,the influences of different parameters on the size of collapsing block are presented in the tables and the limit supporting loads are illustrated in the form graphs that account for the surface settlement.展开更多
The quality changes of shallow groundwater from karst hilly areas in Guiyang region of China impacted by the urbanization were investigated. The results show that the major ions in shallow groundwater from the karst h...The quality changes of shallow groundwater from karst hilly areas in Guiyang region of China impacted by the urbanization were investigated. The results show that the major ions in shallow groundwater from the karst hilly areas are mainly composed of HCO3^-, SO4^2-, Ca^2＋ and Mg^2＋, and the concentrations scopes of NO3^-, Cl^-, K^＋ and Na^＋ of the groundwater in agricultural, residential and industrial areas are 4.5-9.6, 2.8-7.1, 3.9-6.3 and 2.5-4.9 times higher than those in the forest areas, respectively. The concentrations of As, Pb and Cd of shallow groundwater in the industrial areas are also significantly enhanced, followed by those in the residential areas and the agricultural areas. The concentrations of NO3^-, SO4^2- , As, Pb and Cd of the groundwater in the industrial areas and those of NO3^- , SO4^2-, As and Cd of shallow groundwater in the residential areas reach grade Ⅲ of the Groundwater Quality Standard of China （GB/T 14848-93）, while the concentration of NO3^- in the groundwater from the industrial areas exceeds grade V. With the process of urbanization, NO3^- is the key factor to influence the groundwater quality in karst hilly areas, followed by SO4^2-, As, Pb and Cd.展开更多
The Jianpudong No. 4 tunnel is a shallow tunnel, which belongs to Shaoshan County scenic highway in Hunan province, China and whose surrounding rock is weak. According to its characteristics, the field monitoring test...The Jianpudong No. 4 tunnel is a shallow tunnel, which belongs to Shaoshan County scenic highway in Hunan province, China and whose surrounding rock is weak. According to its characteristics, the field monitoring tests and numerical analysis were done. The mechanical characteristics of shallow tunnels under weak surrounding rock and the stress-strain rule of surrounding rock and support were analyzed. The numerical analysis results show that the settlement caused by upper bench excavating accounts for 44% of the total settlement, and the settlement caused by tunnel upper bench supporting accounts for 56% of the total settlement. The maximum axial force of shotcrete lining is 177.2 k N, which locates in hance under the secondary lining. The maximum moment of shotcrete lining is 5.08 k N·m, which locates in the arch foot. The stress curve of steel arch has three obvious stages during the tunnel construction. The maximum axial force of steel arch is 297.4 k N, which locates in tunnel vault. The axial forces of steel arch are respectively 23.5 k N and-21.8 k N, which is influenced by eccentric compression of shallow tunnel and locates in hance. The results show that there is larger earth pressure in tunnel vault which is most unfavorable position of steel arch. Therefore, the advance support should be strengthened in tunnel vault during construction process.展开更多
On the basis of upper bound theorem, non-associated flow rule and non-linear failure criterion were considered together.The modified shear strength parameters of materials were obtained with the help of the tangent me...On the basis of upper bound theorem, non-associated flow rule and non-linear failure criterion were considered together.The modified shear strength parameters of materials were obtained with the help of the tangent method. Employing the virtual power principle and strength reduction technique, the effects of dilatancy of materials, non-linear failure criterion, pore water pressure,surface loads and buried depth, on the stability of shallow tunnel were studied. In order to validate the effectiveness of the proposed approach, the solutions in the present work agree well with the existing results when the non-associated flow rule is reduced to the associated flow rule and the non-linear failure criterion is degenerated to the linear failure criterion. Compared with dilatancy of materials, the non-linear failure criterion exerts greater impact on the stability of shallow tunnels. The safety factor of shallow tunnels decreases and the failure surface expands outward when the dilatancy coefficient decreases. While the increase of nonlinear coefficient, the pore water pressure coefficient, the surface load and the buried depth results in the small safety factor. Therefore, the dilatancy as well as non-linear failure criterion should be taken into account in the design of shallow tunnel supporting structure. The supporting structure must be reinforced promptly to prevent potential mud from gushing or collapse accident in the areas with abundant pore water, large surface load or buried depth.展开更多
In the framework of upper bound theorem of limit analysis, the progressive collapse of shallow rectangular tunnels with double-layer rock mass has been theoretically analyzed based on the three-dimensional (3D) veloci...In the framework of upper bound theorem of limit analysis, the progressive collapse of shallow rectangular tunnels with double-layer rock mass has been theoretically analyzed based on the three-dimensional (3D) velocity discontinuity surfaces. According to the virtual work principle, the difference theorem and the variation method, the collapse surface of double-layer rock mass is determined based on the Hoek-Brown failure criterion. The formula can be degenerated to a single-layer rock collapsing problem when the rock mass is homogeneous. To estimate the validity of the result, the numerical simulation software PLAXIS 3D is used to simulate the collapse of shallow tunnels with double-layer rock mass, and the comparative analysis shows that numerical results are in good agreement with upper-bound solutions. According to the results of parametric analysis, the potential range of collapse of a double-layer rock mass above a shallow cavity decreases with a decrease in A1/A2,σci1/σci2 and σtm1/σtm2 and an increase in B1/B2,γ1/γ2. The range will decrease with a decrease in support pressure q and increase with a decrease in surface overload σs. Therefore, reinforced supporting is beneficial to improve the stability of the cavity during actual construction.展开更多
The analytical solutions for predicting the exact shape of collapse mechanisms in shallow tunnels with arbitrary excavation profiles were obtained by virtue of the upper bound theorem of limit analysis and variation p...The analytical solutions for predicting the exact shape of collapse mechanisms in shallow tunnels with arbitrary excavation profiles were obtained by virtue of the upper bound theorem of limit analysis and variation principle according to Hoek-Brown failure criterion. The seepage force was included in the upper bound limit analysis, and it was computed from the gradient of excess pore pressure distribution. The seepage was regarded as a work rate of external force. The numerical results of roof collapse in square and circular tunnels with different rock parameters were derived and discussed, which proves to be valid in comparison with the previous work. The influences of different parameters on the shape of collapsing blocks were also discussed.展开更多
Based on nonlinear Mohr-Coulomb failure criterion, the analytical solutions of stability number and supporting force on twin shallow tunnels were derived using upper bound theorem of limit analysis. The optimized solu...Based on nonlinear Mohr-Coulomb failure criterion, the analytical solutions of stability number and supporting force on twin shallow tunnels were derived using upper bound theorem of limit analysis. The optimized solutions were obtained by the technique of sequential quadratic programming. When nonlinear coefficient equals 1 and internal friction angle equals 0, the nonlinear Mohr-Coulomb failure criterion degenerates into linear failure criterion. The calculated results of stability number in this work were compared with previous results, and the agreement verifies the effectiveness of the present method. Under the condition of nonlinear Mohr-Coulomb failure criterion, the results show that the supporting force on twin shallow tunnels obviously increases when the nonlinear coefficient, burial depth, ground load or pore water pressure coefficients increase. When the clear distance is 0.5to 1.0 times the diameter of tunnel, the supporting force of twin shallow tunnels reaches its maximum value, which means that the tunnels are the easiest to collapse. While the clear distance increases to 3.5 times the diameter of tunnel, the calculation for twin shallow tunnels can be carried out by the method for independent single shallow tunnel. Therefore, 3.5 times the diameter of tunnel serves as a critical value to determine whether twin shallow tunnels influence each other. In designing twin shallow tunnels,appropriate clear distance value must be selected according to its change rules and actual topographic conditions, meanwhile, the influences of nonlinear failure criterion of soil materials and pore water must be completely considered. During the excavation process, supporting system should be intensified at the positions of larger burial depth or ground load to avoid collapses.展开更多
Numerical simulation using finite differential code was conducted for the single line railway and four-lane road shallow tunnels subjected to unsymmetrical pressure. The mechanical behavior of weak rock mass was studi...Numerical simulation using finite differential code was conducted for the single line railway and four-lane road shallow tunnels subjected to unsymmetrical pressure. The mechanical behavior of weak rock mass was studied considering the influences of stress dilatancy on the failure mechanisms, and the results of numerical simulation were compared with the analytical solutions in specifications. The results show that the dilatancy angle has great influences on the surrounding rock displacement and the shape of failure face for the shallow tunnels. When the dilatancy angle equals zero, the failure face of the surrounding rock forms and extends to the ground surface. With the dilatancy angle increasing, the loose region decreases gradually, and failure surface discontinues. When the dilatancy angle equals the friction angle, the loose region is only distributed in a small range around the crown and sidewalls. On the side of smaller buried depth, the difference of break angle between numerical simulation and the code is less than 10% for single line railway tunnels with the dilatancy angle of zero. However, for the four-lane road tunnels, the difference reaches 20.8%. On the side of larger buried depth, the break angles are smaller than those by the code, the difference reaches 16.8% for single line railway tunnels, and 13.8% for four-lane road tunnels. With the dilatancy angle increasing, especially the dilatancy angle approximating to internal friction angle, it is on safe side to calculate the break angle using the analytical solution method of specifications. Therefore, the influence of stress dilatancy should be considered while determining the failure mechanisms of shallow tunnels subjected to unsymmetrical pressure in weak rocks.展开更多
For shallow tunnels of single-lane railway and four-lane road,numerical simulation using finite differential code was conducted.The mechanical behavior of loose rock masses was studied considering the influences of sh...For shallow tunnels of single-lane railway and four-lane road,numerical simulation using finite differential code was conducted.The mechanical behavior of loose rock masses was studied considering the influences of shear dilation on the shape of the failure surface for the shallow tunnels,and the break angles using numerical simulation was compared with those using the Rankine's analytical solutions and design code.From the comparisons,it is found that the difference between the break angle by numerical simulation and the design code is small when the shear dilation angle is equal to 0,with the maximum relative difference being less than 0.2% in road tunnels.With the dilation angle's increase,the loose plasticity zone area of surrounding rock reduces obviously,and the break angle increases gradually.When the dilation angle is equal to the internal friction angle,the materials follow associated flow rule,and the numerical solution of the break angle is larger than the analytical solution,with the maximum relative difference being greater than 16.7% in road tunnels.Therefore,associated flow rule leads to overestimating break angle,and the dilation angle has an important influence on the failure surface shape for shallow tunnels in the loose surrounding rocks.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.12002156,11972185,12372136)Research Fund of State Key Laboratory of Mechanics and Control for Aerospace Structures(Grant No.MCMS-I-0222K01)。
文摘While the moisture content of soil affects significantly the blast impulse of shallow buried explosives,the role of surface-covering water(SCW)on soil in such blast impulse remains elusive.A combined experimental and numerical study has been carried out to characterize the effect of SCW on transferred impulse and loading magnitude of shallow buried explosives.Firstly,blast tests of shallow buried explosives were conducted,with and without the SCW,to quantitatively assess the blast loading impulse.Subsequently,finite element(FE)simulations were performed and validated against experimental measurement,with good agreement achieved.The validated FE model was then employed to predict the dynamic response of a fully-clamped metallic circular target,subjected to the explosive impact of shallow buried explosives with SCW,and explore the corresponding physical mechanisms.It was demonstrated that shallow buried explosives in saturated soil generate a greater impulse transferred towards the target relative to those in dry soil.The deformation displacement of the target plate is doubled.Increasing the height of SCW results in enhanced center peak deflection of the loaded target,accompanied by subsequent fall,due to the variation of deformation pattern of the loaded target from concentrated load to uniform load.Meanwhile,the presence of SCW increases the blast impulse transferred towards the target by three times.In addition,there exists a threshold value of the burial depth that maximizes the impact impulse.This threshold exhibits a strong sensitivity to SCW height,decreasing with increasing SCW height.An empirical formula for predicting threshold has been provided.Similar conclusions can be drawn for different explosive masses.The results provide technical guidance on blast loading intensity and its spatial distribution considering shallow buried explosives in coast-land battlefields,which can ultimately contribute to better protective designs.
基金Project(SQ2013CB021013)supported by the National Key Basic Research Program of ChinaProject(41002045)supported by the National Natural Science Foundation of China
文摘A systematic analysis of southwestern Ordos Basin's sedimentary characteristics,internal architectural element association styles and depositional model was illustrated through core statistics,well logging data and outcrop observations in Chang 8 oil-bearing group.This analysis indicates that shallow water delta sediments dominated by a fluvial system is the primary sedimentary system of the Chang 8 oil-bearing group of the Yanchang Formation in southwestern Ordos Basin.Four microfacies with fine grain sizes are identified: distributary channels,sheet sandstone,mouth bar and interdistributary fines.According to the sandbody's spatial distribution and internal architecture,two types of sandbody architectural element associations are identified: amalgamated distributary channels and thin-layer lobate sandstone.In this sedimentary system,net-like distributary channels at the delta with a narrow ribbon shape compose the skeleton of the sandbody that extends further into the delta front and shades into contiguous lobate distribution sheet sandstone in the distal delta front.The mouth bar is largely absent in this system.By analyzing the palaeogeomorphology,the palaeostructure background,sedimentary characteristics,sedimentary facies types and spatial distribution of sedimentary facies during the Chang 8 period,a distinctive depositional model of the Chang 8 shallow water fluvial-dominated delta was established,which primarily consists of straight multi-phase amalgamated distributary channels in the delta plain,net-like distributary channels frequently diverting and converging in the proximal delta front,sheet sandstones with dispersing contiguous lobate shapes in the distal delta front,and prodelta or shallow lake mudstones.
基金Foundation item： Project（2013CB036004） supported by the National Basic Research Program of China Project（51178468） supported by the National Natural Science Foundation of China
文摘Linear failure criterion is widely used in calculation of earth pressure acting on shallow tunnels. However, experimental evidence shows that nonlinear failure criterion is able to represent fairly well the failure of almost all types of rocks, A nonlinear Hock-Brown failure criterion is employed to estimate the supporting pressures of shallow tunnels in limit analysis framework. Two failure mechanisms are proposed for calculating the work rate of extemal force and the internal energy dissipation. A tangential line to the nonlinear failure criterion is used to formulate the supporting pressure problem as a nonlinear programming problem. The objective function formulated in this way is minimized with respect to the failure mechanism and the location of tangency point. In order to assess the validity, the supporting pressures for the proposed failure mechanisms are calculated and compared with each other, and the present results are compared with previously published solutions when the nonlinear criterion is reduced to linear criterion. The agreement supports the validity of the proposed failure mechanisms. An experiment is conducted to investigate the influences of the nonlinear criterion on collapse shape and supporting pressures of shallow tunnels.
基金Projects(2013CB036004, 2011CB013800) supported by the National Basic Research Program of ChinaProject(51178468, 50908234) supported by the National Natural Science Foundation of ChinaProject(2011G103-B) supported by the Science and Technology Development of Railway in China
文摘According to the interaction of three shallow tunnels with large section, the analytical solution to rock pressure has been derived and discussed. The load model is given when the bilateral tunnels are excavated. According to the model, the stresses of three tunnels and single tunnel are calculated and compared to analyze the distribution characteristics, where the stresses are influenced by controlling factors of clear distance, covering depth and inclination angle of ground surface. The results show that, in general, the bias distribution is more serious. Therefore, it is significant to settle down the load model of three shallow tunnels so as to determine the measure of reinforcement and design the structure of support. The model and results can be used as a theoretical basis in designation and further research of the three shallow tunnels.
基金Projects(2013CB0360042011CB013800)supported by the National Basic Research Program of China+1 种基金Project(51178468)supported by the National Natural Science Foundation of ChinaProject(2011G013-B)supported by the Science and Technology Development of Railways Department in China
文摘Based on the nonlinear failure criterion and the upper bound theorem, the modified tangential technique method was proposed to derive the expression of supporting pressure acting on shallow tunnel. Instead of the same stress state, different normal stresses on element boundaries were used. In order to investigate the influence of different factors on supporting pressures, the failure mechanism was established. The solution of supporting pressure, with different parameters, was obtained by optimization theory. The corresponding failure mechanism and numerical results were presented. In comparison with the results using the single tangential technique method, it is found that the proposed method is effective, and the good agreement shows that the present solution of supporting pressure is reliable.
基金Project(51378514)supported by the National Natural Science Foundation of China
文摘A method of analyzing the stability of twin shallow tunnels was presented using both limit analysis with nonlinear failure criterion and reliability theory.In the condition of nonlinear failure criterion,the critical clear distancesof twin shallow tunnels were obtained by analyzing the change of surrounding pressure.A reliability model was established based on limit state equation,and the failure probability was solved by virtue of Monte Carlo method.Safety factor and corresponding clear distance of different safety levels were obtained by introducing a target reliability index.The scope of clear distance for different safety levels is described,which can be used as a supplement and improvement to the design codes of tunnels.
基金Project(2014M560652)supported by China Postdoctoral Science FoundationProjects(2011CB013802,2013CB036004)supported by the National Basic Research Program of China
文摘By combining the results of laboratory model tests with relevant flow rules, the failure mode of shallow unsymmetrical loading tunnels and the corresponding velocity field were established. According to the principle of virtual power, the upper bound solution for surrounding rock pressure of shallow unsymmetrical loading tunnel was derived and verified by an example. The results indicate that the calculated results of the derived upper bound method for surrounding rock pressure of shallow unsymmetrical loading tunnels are relatively close to those of the existing "code method" and test results, which means that the proposed method is feasible. The current code method underestimates the unsymmetrical loading feature of surrounding rock pressure of shallow unsymmetrical loading tunnels, so it is unsafe; when the burial depth is less or greater than two times of the tunnel span and the unsymmetrical loading angle is less than 45°, the upper bound method or the average value of the results calculated by the upper bound method and code method respectively, is comparatively reasonable. When the burial depth is greater than two times of the tunnel span and the unsymmetrical loading angle is greater than 45°, the code method is more suitable.
基金Project(51178468) supported by the National Natural Science Foundation of ChinaProject(2010bsxt07) supported by the Doctoral Dissertation Innovation Fund of Central South University,China
文摘Based on the upper bound theorem of limit analysis,the factor of safety for shallow tunnel in saturated soil is calculated in conjunction with the strength reduction technique.To analyze the influence of the pore pressure on the factor of safety for shallow tunnel,the power of pore pressure is regarded as a power of external force in the energy calculation.Using the rigid multiple-block failure mechanism,the objective function for the factor of safety is constructed and the optimal solutions are derived by employing the sequential quadratic programming.According to the results of optimization calculation,the factor of safety of shallow tunnel for different pore pressure coefficients and variational groundwater tables are obtained.The parameter analysis shows that the pore pressure coefficient and the location of the groundwater table have significant influence on the factor of safety for shallow tunnel.
基金Project (51478477) supported by the National Natural Science Foundation of ChinaProject (2016CX012) supported by the Innovation-Driven Project of Central South University,ChinaProject (2014122006) supported by the Guizhou Provincial Department of Transportation Foundation,China
文摘Based on the nonlinear Mohr-Coulomb failure criterion and an associated flow rule,a kinematic admissible velocity field of failure mechanism of the 2-layer soil above a shallow horizontal strip anchor plate is constructed.The ultimate pull-out force and its corresponding failure mechanism through the upper bound limit analysis according to a variation principle are deduced.When the 2-layer overlying soil is degraded into single-layer soil,the model of ultimate pullout force could also be degraded into the model of single-layer soil.And the comparison between results of single-layer soil variation method and those calculated by rigid limit analysis method proves the correctness of our method.Based on that,the influence of changes of geotechnical parameters on ultimate pullout forces and failure mechanism of a shallow horizontal strip anchor with the 2-layer soil above are analyzed.The results show that the ultimate pull-out force and failure mechanism of a shallow horizontal strip anchor with the 2-layer soil above are affected by the nonlinear geotechnical parameters greatly.Thus,it is very important to obtain the accurate geotechnical parameters of 2-layer soil for the evaluation of the ultimate pullout capacity of the anchor plate.
基金Project(51478477)supported by the National Natural Science Foundation of ChinaProject(2016CX012)supported by the Innovation-driven Project of Central South University,ChinaProject(2014122006)supported by the Guizhou Provincial Department of Transportation Foundation,China
文摘Based on the nonlinear Mohr-Coulomb failure criterion and the associated flow rules,the three-dimensional(3-D)axisymmetric failure mechanism of shallow horizontal circular plate anchors that are subjected to the ultimate pullout capacity(UPC)is determined.A derivative function of the projection function for projecting the 3-D axisymmetric failure surface on plane is deduced using the variation theory.By using difference principle,the primitive function of failure surface satisfying boundary condition and numerical solution to its corresponding ultimate pullout capacity function are obtained.The influences of nonlinear Mohr-Coulomb parameters on UPC and failure mechanism are studied.The result shows that UPC decreases with dimensionless parameter m and uniaxial tensile strength increases but increases when depth and radius of plate anchor,surface overload,initial cohesion,geomaterial density and friction angle increase.The failure surface is similar to a symmetrical spatial funnel,and its shape is mainly determined by dimensionless parameter m;the surface damage range expands with the increase of radius and depth of the plate anchor as well as initial cohesion but decreases with the increase of dimensionless parameter m and uniaxial tensile strength as well as geomaterial density.As the dimensionless parameter m=2.0,the numerical solution of UPC based on the difference principle is proved to be feasible and effective through the comparison with the exact solution.In addition,the comparison between solutions of UPC computed by variation method and those computed by upper bound method indicate that variation method outperforms upper bound method.
基金Foundation item： Project（2013CB036004） supported by the National Basic Research Program of China Project（51178468） supported by the National Natural Science Foundation of China Project（2013zzts235） supported by Research Foundation of Central South University, China
文摘Prediction of the state of roof collapse is a big challenge in tunnel engineering, while the limit analysis theory makes it possible to derive the analytical solutions of the collapse mechanisms. In this work, an exact solution of collapsing shape in shallow underwater tunnel is obtained by using the variation principle and the upper bound theorem based on nonlinear failure criterion. Numerical results under the effect of river water and supporting pressure are derived and discussed. The maximum water depth above the river bottom surface is determined under a given buried depth of shallow cavities and the critical depth of roof collapse is obtained under a constant river depth. In comparison with the previous results, the present solution shows a good agreement with the practical results.
基金Project(2017zzts157)supported by the Innovation Foundation for Postgraduate of Central South University,China
文摘Limit analysis of the stability of geomechanical projects is one of the most difficult problems.This work investigates the influences of different parameters in NL failure strength on possible collapsing block shapes of single and twin shallow tunnels with considering the effects of surface settlement.Upper bound solutions derived by functional catastrophe theory are used for describing the distinct characteristics of falling blocks of different parts in twin tunnels.Furthermore the analytical solutions of minimum supporting pressures in shallow tunnels are obtained by the help of the variational principle.Lastly,the comparisons are made both in collapsed mechanism and stability factor with different methods.According to the numerical results in this work,the influences of different parameters on the size of collapsing block are presented in the tables and the limit supporting loads are illustrated in the form graphs that account for the surface settlement.
基金Project(2006CB403205) supported by the National Key Basic Research Program of ChinaProject(40961030) supported by the National Natural Science Foundation of China
文摘The quality changes of shallow groundwater from karst hilly areas in Guiyang region of China impacted by the urbanization were investigated. The results show that the major ions in shallow groundwater from the karst hilly areas are mainly composed of HCO3^-, SO4^2-, Ca^2＋ and Mg^2＋, and the concentrations scopes of NO3^-, Cl^-, K^＋ and Na^＋ of the groundwater in agricultural, residential and industrial areas are 4.5-9.6, 2.8-7.1, 3.9-6.3 and 2.5-4.9 times higher than those in the forest areas, respectively. The concentrations of As, Pb and Cd of shallow groundwater in the industrial areas are also significantly enhanced, followed by those in the residential areas and the agricultural areas. The concentrations of NO3^-, SO4^2- , As, Pb and Cd of the groundwater in the industrial areas and those of NO3^- , SO4^2-, As and Cd of shallow groundwater in the residential areas reach grade Ⅲ of the Groundwater Quality Standard of China （GB/T 14848-93）, while the concentration of NO3^- in the groundwater from the industrial areas exceeds grade V. With the process of urbanization, NO3^- is the key factor to influence the groundwater quality in karst hilly areas, followed by SO4^2-, As, Pb and Cd.
基金Projects(51408060,51208063)supported by the National Natural Science Foundation of China
文摘The Jianpudong No. 4 tunnel is a shallow tunnel, which belongs to Shaoshan County scenic highway in Hunan province, China and whose surrounding rock is weak. According to its characteristics, the field monitoring tests and numerical analysis were done. The mechanical characteristics of shallow tunnels under weak surrounding rock and the stress-strain rule of surrounding rock and support were analyzed. The numerical analysis results show that the settlement caused by upper bench excavating accounts for 44% of the total settlement, and the settlement caused by tunnel upper bench supporting accounts for 56% of the total settlement. The maximum axial force of shotcrete lining is 177.2 k N, which locates in hance under the secondary lining. The maximum moment of shotcrete lining is 5.08 k N·m, which locates in the arch foot. The stress curve of steel arch has three obvious stages during the tunnel construction. The maximum axial force of steel arch is 297.4 k N, which locates in tunnel vault. The axial forces of steel arch are respectively 23.5 k N and-21.8 k N, which is influenced by eccentric compression of shallow tunnel and locates in hance. The results show that there is larger earth pressure in tunnel vault which is most unfavorable position of steel arch. Therefore, the advance support should be strengthened in tunnel vault during construction process.
基金Project(2013CB036004) supported by the National Basic Research Program of ChinaProjects(51178468,51378510) supported by the National Natural Science Foundation of ChinaProject(CX2013B077) supported by Hunan Provincial Innovation Foundation for Postgraduate,China
文摘On the basis of upper bound theorem, non-associated flow rule and non-linear failure criterion were considered together.The modified shear strength parameters of materials were obtained with the help of the tangent method. Employing the virtual power principle and strength reduction technique, the effects of dilatancy of materials, non-linear failure criterion, pore water pressure,surface loads and buried depth, on the stability of shallow tunnel were studied. In order to validate the effectiveness of the proposed approach, the solutions in the present work agree well with the existing results when the non-associated flow rule is reduced to the associated flow rule and the non-linear failure criterion is degenerated to the linear failure criterion. Compared with dilatancy of materials, the non-linear failure criterion exerts greater impact on the stability of shallow tunnels. The safety factor of shallow tunnels decreases and the failure surface expands outward when the dilatancy coefficient decreases. While the increase of nonlinear coefficient, the pore water pressure coefficient, the surface load and the buried depth results in the small safety factor. Therefore, the dilatancy as well as non-linear failure criterion should be taken into account in the design of shallow tunnel supporting structure. The supporting structure must be reinforced promptly to prevent potential mud from gushing or collapse accident in the areas with abundant pore water, large surface load or buried depth.
基金Projects(51478477,51878074)supported by the National Natural Science Foundation of ChinaProject(2017-123-033)supported by the Guizhou Provincial Department of Transportation Foundation,ChinaProjects(2018zzts663,2018zzts656)supported by the Fundamental Research Funds for the Central Universities,China
文摘In the framework of upper bound theorem of limit analysis, the progressive collapse of shallow rectangular tunnels with double-layer rock mass has been theoretically analyzed based on the three-dimensional (3D) velocity discontinuity surfaces. According to the virtual work principle, the difference theorem and the variation method, the collapse surface of double-layer rock mass is determined based on the Hoek-Brown failure criterion. The formula can be degenerated to a single-layer rock collapsing problem when the rock mass is homogeneous. To estimate the validity of the result, the numerical simulation software PLAXIS 3D is used to simulate the collapse of shallow tunnels with double-layer rock mass, and the comparative analysis shows that numerical results are in good agreement with upper-bound solutions. According to the results of parametric analysis, the potential range of collapse of a double-layer rock mass above a shallow cavity decreases with a decrease in A1/A2,σci1/σci2 and σtm1/σtm2 and an increase in B1/B2,γ1/γ2. The range will decrease with a decrease in support pressure q and increase with a decrease in surface overload σs. Therefore, reinforced supporting is beneficial to improve the stability of the cavity during actual construction.
基金Project(2013CB036004)supported by the National Basic Research Program of ChinaProjects(51178468,51378510)supported by National Natural Science Foundation of China
文摘The analytical solutions for predicting the exact shape of collapse mechanisms in shallow tunnels with arbitrary excavation profiles were obtained by virtue of the upper bound theorem of limit analysis and variation principle according to Hoek-Brown failure criterion. The seepage force was included in the upper bound limit analysis, and it was computed from the gradient of excess pore pressure distribution. The seepage was regarded as a work rate of external force. The numerical results of roof collapse in square and circular tunnels with different rock parameters were derived and discussed, which proves to be valid in comparison with the previous work. The influences of different parameters on the shape of collapsing blocks were also discussed.
基金Project(2013CB036004)supported by the National Basic Research Program of ChinaProject(51378510)supported by the NationalNatural Science Foundation of ChinaProject(CX2013B077)supported by Hunan Provincial Innovation Foundation for Postgraduate,China
文摘Based on nonlinear Mohr-Coulomb failure criterion, the analytical solutions of stability number and supporting force on twin shallow tunnels were derived using upper bound theorem of limit analysis. The optimized solutions were obtained by the technique of sequential quadratic programming. When nonlinear coefficient equals 1 and internal friction angle equals 0, the nonlinear Mohr-Coulomb failure criterion degenerates into linear failure criterion. The calculated results of stability number in this work were compared with previous results, and the agreement verifies the effectiveness of the present method. Under the condition of nonlinear Mohr-Coulomb failure criterion, the results show that the supporting force on twin shallow tunnels obviously increases when the nonlinear coefficient, burial depth, ground load or pore water pressure coefficients increase. When the clear distance is 0.5to 1.0 times the diameter of tunnel, the supporting force of twin shallow tunnels reaches its maximum value, which means that the tunnels are the easiest to collapse. While the clear distance increases to 3.5 times the diameter of tunnel, the calculation for twin shallow tunnels can be carried out by the method for independent single shallow tunnel. Therefore, 3.5 times the diameter of tunnel serves as a critical value to determine whether twin shallow tunnels influence each other. In designing twin shallow tunnels,appropriate clear distance value must be selected according to its change rules and actual topographic conditions, meanwhile, the influences of nonlinear failure criterion of soil materials and pore water must be completely considered. During the excavation process, supporting system should be intensified at the positions of larger burial depth or ground load to avoid collapses.
基金Project(200631878557) supported by the West Traffic of Science and Technology of ChinaProject(200550) supported by the Foundation for the Author of National Excellent Doctoral Dissertation of ChinaProject(05-0686) supported by the Program for New Century Excellent Talents in University
文摘Numerical simulation using finite differential code was conducted for the single line railway and four-lane road shallow tunnels subjected to unsymmetrical pressure. The mechanical behavior of weak rock mass was studied considering the influences of stress dilatancy on the failure mechanisms, and the results of numerical simulation were compared with the analytical solutions in specifications. The results show that the dilatancy angle has great influences on the surrounding rock displacement and the shape of failure face for the shallow tunnels. When the dilatancy angle equals zero, the failure face of the surrounding rock forms and extends to the ground surface. With the dilatancy angle increasing, the loose region decreases gradually, and failure surface discontinues. When the dilatancy angle equals the friction angle, the loose region is only distributed in a small range around the crown and sidewalls. On the side of smaller buried depth, the difference of break angle between numerical simulation and the code is less than 10% for single line railway tunnels with the dilatancy angle of zero. However, for the four-lane road tunnels, the difference reaches 20.8%. On the side of larger buried depth, the break angles are smaller than those by the code, the difference reaches 16.8% for single line railway tunnels, and 13.8% for four-lane road tunnels. With the dilatancy angle increasing, especially the dilatancy angle approximating to internal friction angle, it is on safe side to calculate the break angle using the analytical solution method of specifications. Therefore, the influence of stress dilatancy should be considered while determining the failure mechanisms of shallow tunnels subjected to unsymmetrical pressure in weak rocks.
基金Project (200631878557) supported by West Traffic of Science and Technology of ChinaProject (200550) supported by the Foundation for the Author of National Excellent Doctoral Dissertation of ChinaProject (05-0686) supported by the Program for New Century Excellent Talents in University
文摘For shallow tunnels of single-lane railway and four-lane road,numerical simulation using finite differential code was conducted.The mechanical behavior of loose rock masses was studied considering the influences of shear dilation on the shape of the failure surface for the shallow tunnels,and the break angles using numerical simulation was compared with those using the Rankine's analytical solutions and design code.From the comparisons,it is found that the difference between the break angle by numerical simulation and the design code is small when the shear dilation angle is equal to 0,with the maximum relative difference being less than 0.2% in road tunnels.With the dilation angle's increase,the loose plasticity zone area of surrounding rock reduces obviously,and the break angle increases gradually.When the dilation angle is equal to the internal friction angle,the materials follow associated flow rule,and the numerical solution of the break angle is larger than the analytical solution,with the maximum relative difference being greater than 16.7% in road tunnels.Therefore,associated flow rule leads to overestimating break angle,and the dilation angle has an important influence on the failure surface shape for shallow tunnels in the loose surrounding rocks.
