Understanding the hydromechanical behavior and permeability stress sensitivity of hydraulic fractures is fundamental for geotechnical applications associated with fluid injection.This paper presents a three-dimensiona...Understanding the hydromechanical behavior and permeability stress sensitivity of hydraulic fractures is fundamental for geotechnical applications associated with fluid injection.This paper presents a three-dimensional(3D)benchmark model of a laboratory experiment on graywacke to examine the dynamic hydraulic fracturing process under a polyaxial stress state.In the numerical model,injection pressures after breakdown(postbreakdown)are varied to study the impact on fracture growth.The fluid pressure front and crack front are identified in the numerical model to analyze the dynamic relationship between fluid diffusion and fracture propagation.Following the hydraulic fracturing test,the polyaxial stresses are rotated to investigate the influence of the stress field rotation on the fracture slip behavior and permeability.The results show that fracture propagation guides fluid diffusion under a high postbreakdown injection pressure.The crack front runs ahead of the fluid pressure front.Under a low postbreakdown injection pressure,the fluid pressure front gradually reaches the crack front,and fluid diffusion is the main driving factor of fracture propagation.Under polyaxial stress conditions,fluid injection not only opens tensile fractures but also induces hydroshearing.When the polyaxial stress is rotated,the fracture slip direction of a fully extended fracture is consistent with the shear stress direction.The fracture slip direction of a partly extended fracture is influenced by the increase in shear stress.Normal stress affects the permeability evolution by changing the average mechanical aperture.Shear stress can induce shearing and sliding on the fracture plane,thereby increasing permeability.展开更多
The paper presents an experimental investigation on the strength behaviour of natural rock subjected to polyaxial state of stress. The polyaxial tests were conducted on cubical specimens of sandstone obtained from the...The paper presents an experimental investigation on the strength behaviour of natural rock subjected to polyaxial state of stress. The polyaxial tests were conducted on cubical specimens of sandstone obtained from the Shivpuri district in Madhya Pradesh state of India, The specimens having nominal dimensions of100 mm x 100 mm x 100 mm were tested using a polyaxial testing machine. Twenty-five combinations of intermediate and minor principal stresses were applied and the specimens were loaded till failure occurs. It was observed that the intermediate principal stress has a substantial effect on the strength of the Shivpuri sandstone. A database of rock strength under various combinations of σ_2 and σ_3 was obtained for the Shivpuri sandstone. The database was used to study the predictability of five most commonly used strength criterion. Root mean square error(RMSE), average absolute relative error percentage(AAREP) and coefficient of accordance(COA) were used as indices for the measure of goodness of fit. It was observed that the least error in the prediction was shown by modified Mohr Coulomb criterion followed by modified Weibols and Cook criterion. A probability analysis of the error in prediction was also done.展开更多
In this paper,a nonlinear strength criterion is proposed using the average of intermediate(σ2)and minor(σ3)principal stresses in place of σ3 in Ramamurthy(1994)’s strength criterion.The proposed criterion has the ...In this paper,a nonlinear strength criterion is proposed using the average of intermediate(σ2)and minor(σ3)principal stresses in place of σ3 in Ramamurthy(1994)’s strength criterion.The proposed criterion has the main advantages of negligible variation of strength parameters with confining stress and ability to link with conventional strength parameters.Additionally,a new closed-form solution based on the proposed criterion is derived and validated for Chhibro Khodri tunnel.Further,analytical solutions including Singh’s elastoplastic theory,Scussel’s approach,and closed-form solutions based on conventional and modified Ramamurthy(2007)criteria are compared with the results of proposed approach.It is shown that the in situ squeezing pressure predictions made by the proposed approach are more accurate.Also,a parametric study of the present analytical solution is carried out,which displays explicit dependency of tunnel stability on internal support pressure and tunnel depth.The influence of tunnel geometry is observed to be dependent on the applied support pressure.展开更多
Objective: The aim of this study is to evaluate clinical and radiological outcomes in subjects of atlantoaxial instability which were operated using C1 lateral mass and C2 pedicle screw (Harmes technique). Materials a...Objective: The aim of this study is to evaluate clinical and radiological outcomes in subjects of atlantoaxial instability which were operated using C1 lateral mass and C2 pedicle screw (Harmes technique). Materials and Methods: Twenty patients with atlantoaxial instability were scheduled at our clinic for atlantoaxial fusion using polyaxial C1 lateral mass and C2 pedicle screw between January 2008 and March 2014. Results: Trauma was the most common cause of atlantoaxial instability, seen in 18 (90%) patients. Modes of trauma were road traffic accident (75%) and falling (15%). Osteoarthritis was observed in 10% of cases. Patients were followed up with radiographs and clinical examinations. Satisfactory screw placement and reduction were achieved in all patients with the average union time of 3.5 months. There were no implant failures, nonunions, vertebral artery injuries or C2 nerve root injuries. Only, surgical site infections occur in 20% of cases that are improved with medical treatment. Conclusions: C1 lateral mass and C2 pedicle polyaxial screw fixation is an effective technique for the fusion of the atlantoaxial complex. It provides the highest fusion rates without any risk of vertebral injury.展开更多
Simulations are conducted using five new artificial neural networks developed herein to demonstrate and investigate the behavior of rock material under polyaxial loading. The effects of the intermediate principal stre...Simulations are conducted using five new artificial neural networks developed herein to demonstrate and investigate the behavior of rock material under polyaxial loading. The effects of the intermediate principal stress on the intact rock strength are investigated and compared with laboratory results from the literature. To normalize differences in laboratory testing conditions, the stress state is used as the objective parameter in the artificial neural network model predictions. The variations of major principal stress of rock material with intermediate principal stress, minor principal stress and stress state are investigated. The artificial neural network simulations show that for the rock types examined, none were independent of intermediate principal stress effects. In addition, the results of the artificial neural network models, in general agreement with observations made by others, show （a） a general trend of strength increasing and reaching a peak at some intermediate stress state factor, followed by a decline in strength for most rock types; （b） a post-peak strength behavior dependent on the minor principal stress, with respect to rock type; （c） sensitivity to the stress state, and to the interaction between the stress state and uniaxial compressive strength of the test data by the artificial neural networks models （two-way analysis of variance; 95% confidence interval）. Artificial neural network modeling, a self-learning approach to polyaxial stress simulation, can thus complement the commonly observed difficult task of conducting true triaxial laboratory tests, and/or other methods that attempt to improve two-dimensional （2D） failure criteria by incorporating intermediate principal stress effects.展开更多
Although rock mechanical behaviour has a long record of study,attempts to understand the role of fractures on rock deformation still have unresolved issues.Due to technical and/or economic challenges,natural rock frac...Although rock mechanical behaviour has a long record of study,attempts to understand the role of fractures on rock deformation still have unresolved issues.Due to technical and/or economic challenges,natural rock fractures are often dealt with crudely,without detailed consideration of fracture geometry and heterogeneity in many geoscience applications.Veined rocks that are ubiquitous in the upper Earth crust fall in that category where sustained efforts are needed to offer key information for rock mechanics and geomechanics applications.Following on from a recent study on the rupture of veined rocks(DOI:10.1029/2019JB019052),we further examine stress path constraints on the deformation of veined rocks(i.e.,stress-path-dependent behaviour of veined rocks)under polyaxial conditions.The Discrete Element Method is used to establish a calcite veined model where constant mean stress(σm)and constant least principal stress(σ3)paths that are representative in the subsurface activities are considered.The results reveal the stress-path dependency of brittleness for models under different loading paths.Models tested under constant-σm conditions exhibit no brittleness,compared to cases where constant-σ3 is applied.Sliding along the strike of an inclined vein is evident under constant-σm deformation,irrespective of the level of stress.Shear bands along the dominated(inclined)veins exhibit apparent particle trajectory anisotropy for the constant-σm deformations which is demonstrated by the evident colour contrast of the adjacent rock matrix and the displacement dispersion of the particles forming the shear bands.We envisage that the reactivation of veins is of relevance to Enhanced Geothermal Systems(EGS)development in terms of seismicity mitigation and multiphysics control of fracture and reservoir permeability.展开更多
基金supported by the Knowledge Innovation Program of Wuhan-Basic Research (Grant No.2022010801010159)support from the Helmholtz Association's Initiative and Networking Fund for the Helmholtz Young Investigator Group ARES (Contract number VH-NG-1516)supported by the Swedish Radiation Safety Authority (Project SSM2020-2758).
文摘Understanding the hydromechanical behavior and permeability stress sensitivity of hydraulic fractures is fundamental for geotechnical applications associated with fluid injection.This paper presents a three-dimensional(3D)benchmark model of a laboratory experiment on graywacke to examine the dynamic hydraulic fracturing process under a polyaxial stress state.In the numerical model,injection pressures after breakdown(postbreakdown)are varied to study the impact on fracture growth.The fluid pressure front and crack front are identified in the numerical model to analyze the dynamic relationship between fluid diffusion and fracture propagation.Following the hydraulic fracturing test,the polyaxial stresses are rotated to investigate the influence of the stress field rotation on the fracture slip behavior and permeability.The results show that fracture propagation guides fluid diffusion under a high postbreakdown injection pressure.The crack front runs ahead of the fluid pressure front.Under a low postbreakdown injection pressure,the fluid pressure front gradually reaches the crack front,and fluid diffusion is the main driving factor of fracture propagation.Under polyaxial stress conditions,fluid injection not only opens tensile fractures but also induces hydroshearing.When the polyaxial stress is rotated,the fracture slip direction of a fully extended fracture is consistent with the shear stress direction.The fracture slip direction of a partly extended fracture is influenced by the increase in shear stress.Normal stress affects the permeability evolution by changing the average mechanical aperture.Shear stress can induce shearing and sliding on the fracture plane,thereby increasing permeability.
基金financial assistance obtained from NRDMS Division,Department of Science and Technology,New Delhi(No.NRDMS/11/3067/014(G)
文摘The paper presents an experimental investigation on the strength behaviour of natural rock subjected to polyaxial state of stress. The polyaxial tests were conducted on cubical specimens of sandstone obtained from the Shivpuri district in Madhya Pradesh state of India, The specimens having nominal dimensions of100 mm x 100 mm x 100 mm were tested using a polyaxial testing machine. Twenty-five combinations of intermediate and minor principal stresses were applied and the specimens were loaded till failure occurs. It was observed that the intermediate principal stress has a substantial effect on the strength of the Shivpuri sandstone. A database of rock strength under various combinations of σ_2 and σ_3 was obtained for the Shivpuri sandstone. The database was used to study the predictability of five most commonly used strength criterion. Root mean square error(RMSE), average absolute relative error percentage(AAREP) and coefficient of accordance(COA) were used as indices for the measure of goodness of fit. It was observed that the least error in the prediction was shown by modified Mohr Coulomb criterion followed by modified Weibols and Cook criterion. A probability analysis of the error in prediction was also done.
文摘In this paper,a nonlinear strength criterion is proposed using the average of intermediate(σ2)and minor(σ3)principal stresses in place of σ3 in Ramamurthy(1994)’s strength criterion.The proposed criterion has the main advantages of negligible variation of strength parameters with confining stress and ability to link with conventional strength parameters.Additionally,a new closed-form solution based on the proposed criterion is derived and validated for Chhibro Khodri tunnel.Further,analytical solutions including Singh’s elastoplastic theory,Scussel’s approach,and closed-form solutions based on conventional and modified Ramamurthy(2007)criteria are compared with the results of proposed approach.It is shown that the in situ squeezing pressure predictions made by the proposed approach are more accurate.Also,a parametric study of the present analytical solution is carried out,which displays explicit dependency of tunnel stability on internal support pressure and tunnel depth.The influence of tunnel geometry is observed to be dependent on the applied support pressure.
文摘Objective: The aim of this study is to evaluate clinical and radiological outcomes in subjects of atlantoaxial instability which were operated using C1 lateral mass and C2 pedicle screw (Harmes technique). Materials and Methods: Twenty patients with atlantoaxial instability were scheduled at our clinic for atlantoaxial fusion using polyaxial C1 lateral mass and C2 pedicle screw between January 2008 and March 2014. Results: Trauma was the most common cause of atlantoaxial instability, seen in 18 (90%) patients. Modes of trauma were road traffic accident (75%) and falling (15%). Osteoarthritis was observed in 10% of cases. Patients were followed up with radiographs and clinical examinations. Satisfactory screw placement and reduction were achieved in all patients with the average union time of 3.5 months. There were no implant failures, nonunions, vertebral artery injuries or C2 nerve root injuries. Only, surgical site infections occur in 20% of cases that are improved with medical treatment. Conclusions: C1 lateral mass and C2 pedicle polyaxial screw fixation is an effective technique for the fusion of the atlantoaxial complex. It provides the highest fusion rates without any risk of vertebral injury.
文摘Simulations are conducted using five new artificial neural networks developed herein to demonstrate and investigate the behavior of rock material under polyaxial loading. The effects of the intermediate principal stress on the intact rock strength are investigated and compared with laboratory results from the literature. To normalize differences in laboratory testing conditions, the stress state is used as the objective parameter in the artificial neural network model predictions. The variations of major principal stress of rock material with intermediate principal stress, minor principal stress and stress state are investigated. The artificial neural network simulations show that for the rock types examined, none were independent of intermediate principal stress effects. In addition, the results of the artificial neural network models, in general agreement with observations made by others, show （a） a general trend of strength increasing and reaching a peak at some intermediate stress state factor, followed by a decline in strength for most rock types; （b） a post-peak strength behavior dependent on the minor principal stress, with respect to rock type; （c） sensitivity to the stress state, and to the interaction between the stress state and uniaxial compressive strength of the test data by the artificial neural networks models （two-way analysis of variance; 95% confidence interval）. Artificial neural network modeling, a self-learning approach to polyaxial stress simulation, can thus complement the commonly observed difficult task of conducting true triaxial laboratory tests, and/or other methods that attempt to improve two-dimensional （2D） failure criteria by incorporating intermediate principal stress effects.
基金support from the UK Natural Environment Research Council(NERC,NE/W004127/1).
文摘Although rock mechanical behaviour has a long record of study,attempts to understand the role of fractures on rock deformation still have unresolved issues.Due to technical and/or economic challenges,natural rock fractures are often dealt with crudely,without detailed consideration of fracture geometry and heterogeneity in many geoscience applications.Veined rocks that are ubiquitous in the upper Earth crust fall in that category where sustained efforts are needed to offer key information for rock mechanics and geomechanics applications.Following on from a recent study on the rupture of veined rocks(DOI:10.1029/2019JB019052),we further examine stress path constraints on the deformation of veined rocks(i.e.,stress-path-dependent behaviour of veined rocks)under polyaxial conditions.The Discrete Element Method is used to establish a calcite veined model where constant mean stress(σm)and constant least principal stress(σ3)paths that are representative in the subsurface activities are considered.The results reveal the stress-path dependency of brittleness for models under different loading paths.Models tested under constant-σm conditions exhibit no brittleness,compared to cases where constant-σ3 is applied.Sliding along the strike of an inclined vein is evident under constant-σm deformation,irrespective of the level of stress.Shear bands along the dominated(inclined)veins exhibit apparent particle trajectory anisotropy for the constant-σm deformations which is demonstrated by the evident colour contrast of the adjacent rock matrix and the displacement dispersion of the particles forming the shear bands.We envisage that the reactivation of veins is of relevance to Enhanced Geothermal Systems(EGS)development in terms of seismicity mitigation and multiphysics control of fracture and reservoir permeability.
