The failure characteristics of thermal treated surrounding rocks should be studied to evaluate the stability and safety of deep ground engineering under high-ground-temperature and high-ground-stress conditions.The fa...The failure characteristics of thermal treated surrounding rocks should be studied to evaluate the stability and safety of deep ground engineering under high-ground-temperature and high-ground-stress conditions.The failure process of the inner walls of fine-grained granite specimens at different temperatures(25–600℃)was analyzed using a true-triaxial test system.The failure process,peak intensity,overall morphology(characteristics after failure),rock fragment characteristics,and acoustic emission(AE)characteristics were analyzed.The results showed that for the aforementioned type of granite specimens,the trend of the failure stress conditions changed with respect to the critical temperature(200℃).When the temperature was less than 200℃,the initial failure stress increased,final failure stress increased,and failure severity decreased.When the temperature exceeded 200℃,the initial failure stress decreased,final failure stress decreased,and failure severity increased.When the temperature was 600℃,the initial and final failure stresses of the specimens decreased by 60.93%and 19.77%compared with those at 200℃,respectively.The numerical results obtained with the software RFPA3D-Thermal were used to analyze the effect of temperature on the specimen and reveal the mechanism of the failure process in the deep tunnel surrounding rock.展开更多
Seismic isolation is an effective strategy to mitigate the risk of seismic damage in tunnels.However,the impact of surface-reflected seismic waves on the effectiveness of tunnel isolation layers remains under explored...Seismic isolation is an effective strategy to mitigate the risk of seismic damage in tunnels.However,the impact of surface-reflected seismic waves on the effectiveness of tunnel isolation layers remains under explored.In this study,we employ the wave function expansion method to provide analytical solutions for the dynamic responses of linings in an elastic half-space and an infinite elastic space.By comparing the results of the two models,we investigate the seismic isolation effect of tunnel isolation layers induced by reflected seismic waves.Our findings reveal significant differences in the dynamic responses of the lining in the elastic half-space and the infinitely elastic space.Specifically,the dynamic stress concentration factor(DSCF)of the lining in the elastic half-space exhibits periodic fluctuations,influenced by the incident wave frequency and tunnel depth,while the DSCF in the infinitely elastic space remain stable.Overall,the seismic isolation application of the tunnel isolation layer is found to be less affected by surface-reflected seismic waves.Our results provide valuable insights for the design and assessment of the seismic isolation effect of tunnel isolation layers.展开更多
To investigate the influence of unloading effect of a circular tunnel face on rockburst process,by innovatively combining rock drilling unloading devices and triaxial systems,the strain rockburst simulation under the ...To investigate the influence of unloading effect of a circular tunnel face on rockburst process,by innovatively combining rock drilling unloading devices and triaxial systems,the strain rockburst simulation under the entire stress path of“high initial stressþinternal unloadingþstress adjustment”(HUS test)was realized for the intact cubic red sandstone samples(100 mm×100 mm×100 mm).Comparative tests were conducted on cubic red sandstone samples with prefabricated circular holes(425 mm)under the stress path of“prefabricated circular hole+þhigh initial stress+stress adjustment”(PHS test),thereby highlighting the influence of internal unloading on rockburst failure.The test results revealed that with an increase in vertical stress,the sidewalls in both the HUS and PHS tests suffered strain rockburst failure.Compared with the PHS test,the initial failure stress in the HUS test is lower,and it is easier to induce sidewall rockbursts.This indicates that the internal unloading influences the sidewall failure,causing an obvious strength-weakening effect,which becomes more significant with an increase in buried depth.The strain rockburst failure was more severe in the HUS test owing to the influence of internal unloading.V-shaped rockburst pits were formed in the HUS tests,whereas in the PHS test,arcshaped rockburst pits were produced.It was also found that strain rockburst failure may occur only when the rock has a certain degree of rockburst proneness.展开更多
Rock burst in a circular tunnel under high in-situ stress conditions was investigated with a numerical method coupled the rock failure process theory (RFPA) and discontinuous deformation theory (DDA). Some numerical t...Rock burst in a circular tunnel under high in-situ stress conditions was investigated with a numerical method coupled the rock failure process theory (RFPA) and discontinuous deformation theory (DDA). Some numerical tests were carraied out to investigate the failuer patterns of circular tunnel under unloading conditions. Compared the results under loading conditions,the shapes of failure zones are more regular under the unloading conditions. The failure pat-terns in the same type of rock mass are clearly different because of non-homogeneity of the rock material. The extension of cracks shows some predictability with an increasing of in-situ stress. When the homogeneity index of rocks (m) is ei-ther relatively high or low and lateral pressure coefficients (λ) is high,the number of regular shear slide cracks decreases and the probability of a rock burst also becomes lower. Our numerical simulation results show that the stability of sur-face rock and the natural bedding stratification of rock material greatly affect rock bursts. Installing bolts with due dili-gence and suitably can effectively prevent rock bursts. However,it is not effective to control rock bursts by releasing the strain energy with normal pre-boreholes.展开更多
To investigate the influence of loading rate on rockburst in a circular tunnel under three-dimensional stress conditions,the true-triaxial tests were conducted on 100 mm×100 mm×100 mm cubic sandstone specime...To investigate the influence of loading rate on rockburst in a circular tunnel under three-dimensional stress conditions,the true-triaxial tests were conducted on 100 mm×100 mm×100 mm cubic sandstone specimens with d50 mm circular perforated holes,and the failure process of hole sidewall was monitored and recorded in real-time by the microcamera.The loading rates were 0.02,0.10,and 0.50 MPa/s.The test results show that the rockburst process of hole sidewall experienced calm period,pellet ejection period,rock fragment exfoliation period and finally formed the V-shaped notch.The rockburst has a time lag and vertical stress is high when the rockburst occurs.The vertical stress at the initial failure of the hole sidewall increases with loading rate.During the same period after initial failure,the rockburst severity of hole sidewalls increased significantly with increasing loading rate.When the vertical stress is constant and maintains a high stress level,the rockburst of hole sidewall under low loading rate is more serious than that under high loading rate.With increasing loading rate,the quality of rock fragments produced by the rockburst decreases,and the fractal dimension of rock fragments increases.展开更多
The present paper aims to establish a versatile strength theory suitable for elasto-plastic analysis of underground tunnel surrounding rock. In order to analyze the effects of intermediate principal stress and the roc...The present paper aims to establish a versatile strength theory suitable for elasto-plastic analysis of underground tunnel surrounding rock. In order to analyze the effects of intermediate principal stress and the rock properties on its deformation and failure of rock mass, the generalized nonlinear unified strength theory and elasto-plastic mechanics are used to deduce analytic solution of the radius and stress of tunnel plastic zone and the periphery displacement of tunnel under uniform ground stress field. The results show that: intermediate principal stress coefficient b has significant effect on the plastic range,the magnitude of stress and surrounding rock pressure. Then, the results are compared with the unified strength criterion solution and Mohr–Coulomb criterion solution, and concluded that the generalized nonlinear unified strength criterion is more applicable to elasto-plastic analysis of underground tunnel surrounding rock.展开更多
The relationship between the Hoek-Brown parameters and the mechanical response of circular tunnels is il-lustrated. Closed-form and approximate solutions are given for the extent of the plastic zone and the stress and...The relationship between the Hoek-Brown parameters and the mechanical response of circular tunnels is il-lustrated. Closed-form and approximate solutions are given for the extent of the plastic zone and the stress and dis-placement fields under axisymmetrical and asymmetric stress conditions. For the same rock masses and under axisym-metrical stress conditions,the radius of the plastic zone in terms of Hoek-Brown criterion is generally an approximation of the radius in terms of the Mohr-Coulomb criterion. The radius in terms of the Hoek-Brown criterion is larger under low stress conditions. For poor quality rock masses (GSI<25),measures (such as grouting,setting rock bolts,etc.) that improve the GSI of rock masses are effective in improving the stability of tunnels. It is not advisable to improve the sta-bility of the tunnels by providing a small support resistance p through shotcrete,except for very poor quality jointed rock masses. Without reference to the quality of the rock mass,the disturbance factor D should not less than 0.5. Meas-ures which disturb rock masses during tunnel construction should be taken carefully when the tunnel depth increases.展开更多
True-triaxial compression tests were carried out on cubic granite samples with a circular through hole using a true-triaxial testing system to investigate the influence of saturated water content(SWC) on the failure p...True-triaxial compression tests were carried out on cubic granite samples with a circular through hole using a true-triaxial testing system to investigate the influence of saturated water content(SWC) on the failure process and characteristics of a circular tunnel of surrounding rocks. The spalling failure under SWC can be divided into four periods: calm period, buckling deformation period, period of rock fragment gradual buckling and exfoliation, and period of formation of symmetrical V-shaped notches. When the horizontal axial and vertical stresses were constant, the spalling failure severity was reduced with the increase in lateral stress. Under natural water content, a strong rockburst with dynamic failure characteristics occurred on the circular hole sidewall. Under SWC, the failure severity was reduced and the circular hole sidewall experienced spalling failure, exhibiting progressive static failure characteristics.Therefore, water can reduce the failure severity of surrounding rocks in deep underground engineering, which has a certain guiding significance for the prevention and control of rockbursts.展开更多
Mechanism of circular tunnel rockburst is that, when the carrying capacity of the centralized zone of plastic deformation in limiting state reduces, the comparatively intact part in rock mass unloads by way of elastic...Mechanism of circular tunnel rockburst is that, when the carrying capacity of the centralized zone of plastic deformation in limiting state reduces, the comparatively intact part in rock mass unloads by way of elasticity; rockburst occurs immediately when the elastic energy released by the comparatively intact part exceeds the energy dissipated by plastic deformation. The equivalent strain was taken as a state variable to establish a catastrophe model of tunnel rockburst, and the computation expression of the earthquake energy released by tunnel rockburst was given. The analysis shows that, the conditions of rockburst occurrence are relative to rock＇s ratio of elastic modulus to descendent modulus and crack growth degree of rocks; to rock mass with specific rockburst tendency, there exists a corresponding critical depth of softened zone, and rockburst occurs when the depth of softened zone reaches.展开更多
A numerical code called Rock Failure Process Analysis (RFPA2D) was em- ployed to investigate the closure,damage and failure behavior of the horizontal tunnels.In this code the time-dependent deformation was described ...A numerical code called Rock Failure Process Analysis (RFPA2D) was em- ployed to investigate the closure,damage and failure behavior of the horizontal tunnels.In this code the time-dependent deformation was described in terms of evolution of meso- scopic structure,leading to progressive degradation of elastic modulus and failure strength of material.In terms of material degradation,a series of numerical simulations were per- formed to study the convergence and subsequent failure in circular tunnels.The numerical results provide a complete illumination for the closure,damage and failure behavior with different loading conditions.It is shown that the depth and the ratio of far field stresses play an important role in the creep behavior of tunnels.Creep failure is expected to occur in the direction of the smallest far field stress component,which means that rheological failure of tunnels is influenced not only by the rock characteristic around the tunnels but also by the orientation and distribution of far field stress on a global scale.展开更多
Underground structures are susceptible to float and move upward during earthquakes when located in a liquefiable soil deposit.There are examples of this phenomenon in past major earthquake events.In this study,the upl...Underground structures are susceptible to float and move upward during earthquakes when located in a liquefiable soil deposit.There are examples of this phenomenon in past major earthquake events.In this study,the uplift of circular tunnels in a liquefiable sand layer was investigated with a series of shaking table tests.The research has focused on the buried depth of the tunnel,tunnel diameter,tunnel weight,liquefaction extent,uplift mechanism,and factor of safety against liquefaction-induced uplift.According to the test results,the shallow buried depth,larger diameter,and lower weight can intensify the tunnel uplift,so the displacement in post-liquefaction time continues at the same rate as during the shaking time.Due to the shear-induced dilation,pore water pressure generation around the tunnel was reduced compared with that of the free field.The excess pore water pressure dissipation in the soil overlying the uplifted tun-nel was significant,which leads to suction in the soil deposit.Furthermore,the acceleration response of overlying soil with the uplifted tunnel was similar to that of the free field.However,the soil acceleration response around the tunnel without uplift was similar to the base motion.展开更多
A series of true-triaxial compression tests were performed on red sandstone cubic specimens with a circular hole to investigate the influence of depth on induced spalling in tunnels.The failure process of the hole sid...A series of true-triaxial compression tests were performed on red sandstone cubic specimens with a circular hole to investigate the influence of depth on induced spalling in tunnels.The failure process of the hole sidewalls was monitored and recorded in real-time by a micro-video monitoring equipment.The general failure evolution processes of the hole sidewall at different initial depths(500 m,1000 m and 1500 m)during the adjustment of vertical stress were obtained.The results show that the hole sidewall all formed spalling before resulting in strain rockburst,and ultimately forming a V-shaped notch.The far-field principal stress for the initial failure of the tunnel shows a good positive linear correlation with the depth.As the depth increases,the stress required for the initial failure of the tunnels clearly increased,the spalling became more intense;the size and mass of the rock fragments and depth and width of the V-shaped notches increased,and the range of the failure zone extends along the hole sidewall from the local area to the entire area.Therefore,as the depth increases,the support area around the tunnel should be increased accordingly to prevent spalling.展开更多
In this study, it was assumed that three-dimensional penny-shaped cracks existed in deep rock masses. A new non-Euclidean model was established, in which the effects of penny- shaped cracks and axial in-situ stress on...In this study, it was assumed that three-dimensional penny-shaped cracks existed in deep rock masses. A new non-Euclidean model was established, in which the effects of penny- shaped cracks and axial in-situ stress on zonal disintegration of deep rock masses were taken into account. Based on the non-Euclidean model, the stress intensity factors at tips of the penny- shaped cracks were determined. The strain energy density factor was applied to investigate the occurrence of fractured zones. It was observed from the numerical results that the magnitude and location of fractured zones were sensitive to micro- and macro-mechanical parameters, as well as the value of in-situ stress. The numerical results were in good agreement with the experimental data.展开更多
The safety assessment of tunnel stability is critical to tunnel construction and requires accurate analysis to obtain a reliable prediction.Strength anisotropy is an important aspect of clay behavior,but it is mostly ...The safety assessment of tunnel stability is critical to tunnel construction and requires accurate analysis to obtain a reliable prediction.Strength anisotropy is an important aspect of clay behavior,but it is mostly neglected in practical stability analyses.In this study,the effects of undrained strength anisotropy and strength nonhomogeneity on the stability of unlined circular tunnels in clays are investi-gated.The static approach of lower-bound(LB)analysis using finite-element and second-order cone programming is employed to exam-ine the aforementioned effects.The anisotropic shear strength of clay is modeled by employing an elliptical yield function under plane strain conditions.A complete set of dimensionless parameters covering the cover depth ratios of tunnels,normalized overburden pressure ratios,normalized strength gradient ratios of clays,and anisotropic strength ratios,are systematically investigated.The new LB solutions indicate that the stability load factor of the problem has a nonlinear relationship with the cover depth ratio and the anisotropic strength ratio,and there exists a linear relationship with the normalized overburden pressure and the normalized strength gradient.Their influ-ence on the predicted failure mechanism is parametrically evaluated.A statistically approximate stability equation of unlined circular tunnels in anisotropic and non-homogeneous clay is proposed for the first time,which contains four new stability factors,namely,con-stant undrained strength,linearly increasing strength gradient,undrained strength anisotropy,and soil unit weight,and it can serve as a fast and accurate tool for predicting the undrained stability of this problem in practice.展开更多
Explicit solution techniques have been widely used in geotechnical engineering for simulating the coupled hydro-mechanical(H-M) interaction of fluid flow and deformation induced by structures built above and under sat...Explicit solution techniques have been widely used in geotechnical engineering for simulating the coupled hydro-mechanical(H-M) interaction of fluid flow and deformation induced by structures built above and under saturated ground, i.e. circular footing and deep tunnel. However, the technique is only conditionally stable and requires small time steps, portending its inefficiency for simulating large-scale H-M problems. To improve its efficiency, the unconditionally stable alternating direction explicit(ADE)scheme could be used to solve the flow problem. The standard ADE scheme, however, is only moderately accurate and is restricted to uniform grids and plane strain flow conditions. This paper aims to remove these drawbacks by developing a novel high-order ADE scheme capable of solving flow problems in nonuniform grids and under axisymmetric conditions. The new scheme is derived by performing a fourthorder finite difference(FD) approximation to the spatial derivatives of the axisymmetric fluid-diffusion equation in a non-uniform grid configuration. The implicit Crank-Nicolson technique is then applied to the resulting approximation, and the subsequent equation is split into two alternating direction sweeps,giving rise to a new axisymmetric ADE scheme. The pore pressure solutions from the new scheme are then sequentially coupled with an existing geomechanical simulator in the computer code fast Lagrangian analysis of continua(FLAC). This coupling procedure is called the sequentially-explicit coupling technique based on the fourth-order axisymmetric ADE scheme or SEA-4-AXI. Application of SEA-4-AXI for solving axisymmetric consolidation of a circular footing and of advancing tunnel in deep saturated ground shows that SEA-4-AXI reduces computer runtime up to 42%-50% that of FLAC’s basic scheme without numerical instability. In addition, it produces high numerical accuracy of the H-M solutions with average percentage difference of only 0.5%-1.8%.展开更多
基金Project(52174098)supported by the National Natural Science Foundation of ChinaProject(2022JJ20063)supported by the Natural Science Foundation of Hunan Province,ChinaProject(2023CXQD011)supported by the Fundamental Research Funds for the Central Universities,China。
文摘The failure characteristics of thermal treated surrounding rocks should be studied to evaluate the stability and safety of deep ground engineering under high-ground-temperature and high-ground-stress conditions.The failure process of the inner walls of fine-grained granite specimens at different temperatures(25–600℃)was analyzed using a true-triaxial test system.The failure process,peak intensity,overall morphology(characteristics after failure),rock fragment characteristics,and acoustic emission(AE)characteristics were analyzed.The results showed that for the aforementioned type of granite specimens,the trend of the failure stress conditions changed with respect to the critical temperature(200℃).When the temperature was less than 200℃,the initial failure stress increased,final failure stress increased,and failure severity decreased.When the temperature exceeded 200℃,the initial failure stress decreased,final failure stress decreased,and failure severity increased.When the temperature was 600℃,the initial and final failure stresses of the specimens decreased by 60.93%and 19.77%compared with those at 200℃,respectively.The numerical results obtained with the software RFPA3D-Thermal were used to analyze the effect of temperature on the specimen and reveal the mechanism of the failure process in the deep tunnel surrounding rock.
基金supported by the National Natural Science Foundation of China[grant number 51991393]support from the Guangdong Provincial Key Laboratory of Earthquake Engineering and Applied Technology and Key Laboratory of Earthquake Resistance,Earthquake Mitigation,and Structural Safety funded by the Ministry of Education。
文摘Seismic isolation is an effective strategy to mitigate the risk of seismic damage in tunnels.However,the impact of surface-reflected seismic waves on the effectiveness of tunnel isolation layers remains under explored.In this study,we employ the wave function expansion method to provide analytical solutions for the dynamic responses of linings in an elastic half-space and an infinite elastic space.By comparing the results of the two models,we investigate the seismic isolation effect of tunnel isolation layers induced by reflected seismic waves.Our findings reveal significant differences in the dynamic responses of the lining in the elastic half-space and the infinitely elastic space.Specifically,the dynamic stress concentration factor(DSCF)of the lining in the elastic half-space exhibits periodic fluctuations,influenced by the incident wave frequency and tunnel depth,while the DSCF in the infinitely elastic space remain stable.Overall,the seismic isolation application of the tunnel isolation layer is found to be less affected by surface-reflected seismic waves.Our results provide valuable insights for the design and assessment of the seismic isolation effect of tunnel isolation layers.
基金This work was supported by the National Natural Science Foundation of China(Grant No.42077244)the Open Research Fund of State Key Laboratory of Deep Earth Science and Engineering(Sichuan University)(Grant No.DESE 202201)the Fundamental Research Funds for the Central Universities(Grant No.2242022k30054).
文摘To investigate the influence of unloading effect of a circular tunnel face on rockburst process,by innovatively combining rock drilling unloading devices and triaxial systems,the strain rockburst simulation under the entire stress path of“high initial stressþinternal unloadingþstress adjustment”(HUS test)was realized for the intact cubic red sandstone samples(100 mm×100 mm×100 mm).Comparative tests were conducted on cubic red sandstone samples with prefabricated circular holes(425 mm)under the stress path of“prefabricated circular hole+þhigh initial stress+stress adjustment”(PHS test),thereby highlighting the influence of internal unloading on rockburst failure.The test results revealed that with an increase in vertical stress,the sidewalls in both the HUS and PHS tests suffered strain rockburst failure.Compared with the PHS test,the initial failure stress in the HUS test is lower,and it is easier to induce sidewall rockbursts.This indicates that the internal unloading influences the sidewall failure,causing an obvious strength-weakening effect,which becomes more significant with an increase in buried depth.The strain rockburst failure was more severe in the HUS test owing to the influence of internal unloading.V-shaped rockburst pits were formed in the HUS tests,whereas in the PHS test,arcshaped rockburst pits were produced.It was also found that strain rockburst failure may occur only when the rock has a certain degree of rockburst proneness.
基金Projects 50639100 supported by the National Natural Science Foundation of China50539100 by the New Century Talents Plan of Education Depart- ment
文摘Rock burst in a circular tunnel under high in-situ stress conditions was investigated with a numerical method coupled the rock failure process theory (RFPA) and discontinuous deformation theory (DDA). Some numerical tests were carraied out to investigate the failuer patterns of circular tunnel under unloading conditions. Compared the results under loading conditions,the shapes of failure zones are more regular under the unloading conditions. The failure pat-terns in the same type of rock mass are clearly different because of non-homogeneity of the rock material. The extension of cracks shows some predictability with an increasing of in-situ stress. When the homogeneity index of rocks (m) is ei-ther relatively high or low and lateral pressure coefficients (λ) is high,the number of regular shear slide cracks decreases and the probability of a rock burst also becomes lower. Our numerical simulation results show that the stability of sur-face rock and the natural bedding stratification of rock material greatly affect rock bursts. Installing bolts with due dili-gence and suitably can effectively prevent rock bursts. However,it is not effective to control rock bursts by releasing the strain energy with normal pre-boreholes.
基金Projects(11972378,41630642)supported by the National Natural Science Foundation of ChinaProject(2019zzts310)supported by the Fundamental Research Funds for the Central Universities,China。
文摘To investigate the influence of loading rate on rockburst in a circular tunnel under three-dimensional stress conditions,the true-triaxial tests were conducted on 100 mm×100 mm×100 mm cubic sandstone specimens with d50 mm circular perforated holes,and the failure process of hole sidewall was monitored and recorded in real-time by the microcamera.The loading rates were 0.02,0.10,and 0.50 MPa/s.The test results show that the rockburst process of hole sidewall experienced calm period,pellet ejection period,rock fragment exfoliation period and finally formed the V-shaped notch.The rockburst has a time lag and vertical stress is high when the rockburst occurs.The vertical stress at the initial failure of the hole sidewall increases with loading rate.During the same period after initial failure,the rockburst severity of hole sidewalls increased significantly with increasing loading rate.When the vertical stress is constant and maintains a high stress level,the rockburst of hole sidewall under low loading rate is more serious than that under high loading rate.With increasing loading rate,the quality of rock fragments produced by the rockburst decreases,and the fractal dimension of rock fragments increases.
文摘The present paper aims to establish a versatile strength theory suitable for elasto-plastic analysis of underground tunnel surrounding rock. In order to analyze the effects of intermediate principal stress and the rock properties on its deformation and failure of rock mass, the generalized nonlinear unified strength theory and elasto-plastic mechanics are used to deduce analytic solution of the radius and stress of tunnel plastic zone and the periphery displacement of tunnel under uniform ground stress field. The results show that: intermediate principal stress coefficient b has significant effect on the plastic range,the magnitude of stress and surrounding rock pressure. Then, the results are compared with the unified strength criterion solution and Mohr–Coulomb criterion solution, and concluded that the generalized nonlinear unified strength criterion is more applicable to elasto-plastic analysis of underground tunnel surrounding rock.
基金Project 50639100 supported by the National Natural Science Foundation of China
文摘The relationship between the Hoek-Brown parameters and the mechanical response of circular tunnels is il-lustrated. Closed-form and approximate solutions are given for the extent of the plastic zone and the stress and dis-placement fields under axisymmetrical and asymmetric stress conditions. For the same rock masses and under axisym-metrical stress conditions,the radius of the plastic zone in terms of Hoek-Brown criterion is generally an approximation of the radius in terms of the Mohr-Coulomb criterion. The radius in terms of the Hoek-Brown criterion is larger under low stress conditions. For poor quality rock masses (GSI<25),measures (such as grouting,setting rock bolts,etc.) that improve the GSI of rock masses are effective in improving the stability of tunnels. It is not advisable to improve the sta-bility of the tunnels by providing a small support resistance p through shotcrete,except for very poor quality jointed rock masses. Without reference to the quality of the rock mass,the disturbance factor D should not less than 0.5. Meas-ures which disturb rock masses during tunnel construction should be taken carefully when the tunnel depth increases.
基金financial support from the National Natural Science Foundation of China (Nos.51904335,41630642)。
文摘True-triaxial compression tests were carried out on cubic granite samples with a circular through hole using a true-triaxial testing system to investigate the influence of saturated water content(SWC) on the failure process and characteristics of a circular tunnel of surrounding rocks. The spalling failure under SWC can be divided into four periods: calm period, buckling deformation period, period of rock fragment gradual buckling and exfoliation, and period of formation of symmetrical V-shaped notches. When the horizontal axial and vertical stresses were constant, the spalling failure severity was reduced with the increase in lateral stress. Under natural water content, a strong rockburst with dynamic failure characteristics occurred on the circular hole sidewall. Under SWC, the failure severity was reduced and the circular hole sidewall experienced spalling failure, exhibiting progressive static failure characteristics.Therefore, water can reduce the failure severity of surrounding rocks in deep underground engineering, which has a certain guiding significance for the prevention and control of rockbursts.
基金Project supported by the National Natural Science Foundation of China (No.50274044) the Educational Committee of Shandong Province of China (No.G04D15) the Natural Science Foundation of Shandong Province of China (No.Y2002-A03)
文摘Mechanism of circular tunnel rockburst is that, when the carrying capacity of the centralized zone of plastic deformation in limiting state reduces, the comparatively intact part in rock mass unloads by way of elasticity; rockburst occurs immediately when the elastic energy released by the comparatively intact part exceeds the energy dissipated by plastic deformation. The equivalent strain was taken as a state variable to establish a catastrophe model of tunnel rockburst, and the computation expression of the earthquake energy released by tunnel rockburst was given. The analysis shows that, the conditions of rockburst occurrence are relative to rock＇s ratio of elastic modulus to descendent modulus and crack growth degree of rocks; to rock mass with specific rockburst tendency, there exists a corresponding critical depth of softened zone, and rockburst occurs when the depth of softened zone reaches.
基金the"973"Program(2007CB209400)the National Natural Science Foundation of China(40638040)the Opening Foundation of State Key Laboratory of Geohazards Prevention and Geoenvironment Protection,Chengdu University of Technology
文摘A numerical code called Rock Failure Process Analysis (RFPA2D) was em- ployed to investigate the closure,damage and failure behavior of the horizontal tunnels.In this code the time-dependent deformation was described in terms of evolution of meso- scopic structure,leading to progressive degradation of elastic modulus and failure strength of material.In terms of material degradation,a series of numerical simulations were per- formed to study the convergence and subsequent failure in circular tunnels.The numerical results provide a complete illumination for the closure,damage and failure behavior with different loading conditions.It is shown that the depth and the ratio of far field stresses play an important role in the creep behavior of tunnels.Creep failure is expected to occur in the direction of the smallest far field stress component,which means that rheological failure of tunnels is influenced not only by the rock characteristic around the tunnels but also by the orientation and distribution of far field stress on a global scale.
文摘Underground structures are susceptible to float and move upward during earthquakes when located in a liquefiable soil deposit.There are examples of this phenomenon in past major earthquake events.In this study,the uplift of circular tunnels in a liquefiable sand layer was investigated with a series of shaking table tests.The research has focused on the buried depth of the tunnel,tunnel diameter,tunnel weight,liquefaction extent,uplift mechanism,and factor of safety against liquefaction-induced uplift.According to the test results,the shallow buried depth,larger diameter,and lower weight can intensify the tunnel uplift,so the displacement in post-liquefaction time continues at the same rate as during the shaking time.Due to the shear-induced dilation,pore water pressure generation around the tunnel was reduced compared with that of the free field.The excess pore water pressure dissipation in the soil overlying the uplifted tun-nel was significant,which leads to suction in the soil deposit.Furthermore,the acceleration response of overlying soil with the uplifted tunnel was similar to that of the free field.However,the soil acceleration response around the tunnel without uplift was similar to the base motion.
基金Projects(41877272,41472269)supported by the National Natural Science Foundation of ChinaProject(2017zzts167)supported by the Fundamental Research Funds for the Central Universities,China。
文摘A series of true-triaxial compression tests were performed on red sandstone cubic specimens with a circular hole to investigate the influence of depth on induced spalling in tunnels.The failure process of the hole sidewalls was monitored and recorded in real-time by a micro-video monitoring equipment.The general failure evolution processes of the hole sidewall at different initial depths(500 m,1000 m and 1500 m)during the adjustment of vertical stress were obtained.The results show that the hole sidewall all formed spalling before resulting in strain rockburst,and ultimately forming a V-shaped notch.The far-field principal stress for the initial failure of the tunnel shows a good positive linear correlation with the depth.As the depth increases,the stress required for the initial failure of the tunnels clearly increased,the spalling became more intense;the size and mass of the rock fragments and depth and width of the V-shaped notches increased,and the range of the failure zone extends along the hole sidewall from the local area to the entire area.Therefore,as the depth increases,the support area around the tunnel should be increased accordingly to prevent spalling.
基金supported by the 973 Project(No.2014CB046903)the National Natural Science Foundation of China(Nos.51325903 and 51279218)the Natural Science Foundation Project of CQ CSTC(Nos.CSTC2013KJRC-1JRCCJ30001 and CSTC2013JCYJYS0005)
文摘In this study, it was assumed that three-dimensional penny-shaped cracks existed in deep rock masses. A new non-Euclidean model was established, in which the effects of penny- shaped cracks and axial in-situ stress on zonal disintegration of deep rock masses were taken into account. Based on the non-Euclidean model, the stress intensity factors at tips of the penny- shaped cracks were determined. The strain energy density factor was applied to investigate the occurrence of fractured zones. It was observed from the numerical results that the magnitude and location of fractured zones were sensitive to micro- and macro-mechanical parameters, as well as the value of in-situ stress. The numerical results were in good agreement with the experimental data.
文摘The safety assessment of tunnel stability is critical to tunnel construction and requires accurate analysis to obtain a reliable prediction.Strength anisotropy is an important aspect of clay behavior,but it is mostly neglected in practical stability analyses.In this study,the effects of undrained strength anisotropy and strength nonhomogeneity on the stability of unlined circular tunnels in clays are investi-gated.The static approach of lower-bound(LB)analysis using finite-element and second-order cone programming is employed to exam-ine the aforementioned effects.The anisotropic shear strength of clay is modeled by employing an elliptical yield function under plane strain conditions.A complete set of dimensionless parameters covering the cover depth ratios of tunnels,normalized overburden pressure ratios,normalized strength gradient ratios of clays,and anisotropic strength ratios,are systematically investigated.The new LB solutions indicate that the stability load factor of the problem has a nonlinear relationship with the cover depth ratio and the anisotropic strength ratio,and there exists a linear relationship with the normalized overburden pressure and the normalized strength gradient.Their influ-ence on the predicted failure mechanism is parametrically evaluated.A statistically approximate stability equation of unlined circular tunnels in anisotropic and non-homogeneous clay is proposed for the first time,which contains four new stability factors,namely,con-stant undrained strength,linearly increasing strength gradient,undrained strength anisotropy,and soil unit weight,and it can serve as a fast and accurate tool for predicting the undrained stability of this problem in practice.
基金the support from the University Transportation Center for Underground Transportation Infrastructure at the Colorado School of Mines for partially funding this research under Grant No. 69A3551747118 of the Fixing America's Surface Transportation Act (FAST Act) of U.S. DoT FY2016
文摘Explicit solution techniques have been widely used in geotechnical engineering for simulating the coupled hydro-mechanical(H-M) interaction of fluid flow and deformation induced by structures built above and under saturated ground, i.e. circular footing and deep tunnel. However, the technique is only conditionally stable and requires small time steps, portending its inefficiency for simulating large-scale H-M problems. To improve its efficiency, the unconditionally stable alternating direction explicit(ADE)scheme could be used to solve the flow problem. The standard ADE scheme, however, is only moderately accurate and is restricted to uniform grids and plane strain flow conditions. This paper aims to remove these drawbacks by developing a novel high-order ADE scheme capable of solving flow problems in nonuniform grids and under axisymmetric conditions. The new scheme is derived by performing a fourthorder finite difference(FD) approximation to the spatial derivatives of the axisymmetric fluid-diffusion equation in a non-uniform grid configuration. The implicit Crank-Nicolson technique is then applied to the resulting approximation, and the subsequent equation is split into two alternating direction sweeps,giving rise to a new axisymmetric ADE scheme. The pore pressure solutions from the new scheme are then sequentially coupled with an existing geomechanical simulator in the computer code fast Lagrangian analysis of continua(FLAC). This coupling procedure is called the sequentially-explicit coupling technique based on the fourth-order axisymmetric ADE scheme or SEA-4-AXI. Application of SEA-4-AXI for solving axisymmetric consolidation of a circular footing and of advancing tunnel in deep saturated ground shows that SEA-4-AXI reduces computer runtime up to 42%-50% that of FLAC’s basic scheme without numerical instability. In addition, it produces high numerical accuracy of the H-M solutions with average percentage difference of only 0.5%-1.8%.
