During the excavation of large-scale rock slopes and deep hard rock engineering,the induced rapid unloading serves as the primary cause of rock mass deformation and failure.The essence of this phenomenon lies in the o...During the excavation of large-scale rock slopes and deep hard rock engineering,the induced rapid unloading serves as the primary cause of rock mass deformation and failure.The essence of this phenomenon lies in the opening-shear failure process triggered by the normal stress unloading of fractured rock mass.In this study,we focus on local-scale rock fracture and conduct direct shear tests under different normal stress unloading rates on five types of non-persistent fractured hard rocks.The aim is to analyze the influence of normal stress unloading rates on the failure modes and shear mechanical characteristics of non-persistent fractured rocks.The results indicate that the normal unloading displacement decreases gradually with increasing normal stress unloading rate,while the influence of normal stress unloading rate on shear displacement is not significant.As the normal stress unloading rate increases,the rocks brittle failure process accelerates,and the degree of rocks damage decreases.Analysis of the stress state on rock fracture surfaces reveals that increasing the normal stress unloading rate enhances the compressive stress on rocks,leading to a transition in the failure mode from shear failure to tensile failure.A negative exponential strength formula was proposed,which effectively fits the relationship between failure normal stress and normal stress unloading rate.The findings enrich the theoretical foundation of unloading rock mechanics and provide theoretical support for disasters prevention and control in rock engineering excavations.展开更多
Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted r...Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted regarding various initial normal stresses(1e7 MPa)and numbers of shearing cycles(1 e5).The peak shear stress of fractures decreased with shear cycles due to progressively smooth surface morphologies,while increased with both JRC and initial normal stress and could be verified using the nonlinear Barton-Bandis failure criterion.The joint friction angle of fractures exponentially increased by 62.22%e64.87%with JRC while decreased by 22.1%e24.85%with shearing cycles.After unloading normal stress,the sliding initiation time of fractures increased with both JRC and initial normal stress due to more tortuous fracture morphologies and enhanced shearing resistance capacity.The surface resistance index(SRI)of fractures decreased by 4.35%e32.02%with increasing shearing cycles due to a more significant reduction of sliding initiation shear stress than that for sliding initiation normal stress,but increased by a factor of 0.41e1.64 with JRC.After sliding initiation,the shear displacement of fractures showed an increase in power function.By defining a sliding rate threshold of 5105 m/s,transition from“quasi-static”to“dynamic”sliding of fractures was identified,and the increase of sliding acceleration steepened with JRC while slowed down with shearing cycles.The normal displacement experienced a slight increase before shear sliding due to deformation recovery as the unloading stress was unloaded,and then enhanced shear dilation after sliding initiation due to climbing effects of surface asperities.Dilation was positively related to the shear sliding velocity of fractures.Wear characteristics of the fracture surfaces after shearing failure were evaluated using binary calculation,indicating an increasing shear area ratio by 45.24%e91.02%with normal stress.展开更多
Excavation and earth surface processes(e.g.,river incision)always induce the unloading of stress,which can cause the failure of rocks.To study the shear mechanical behavior of a rock sample under unloading normal stre...Excavation and earth surface processes(e.g.,river incision)always induce the unloading of stress,which can cause the failure of rocks.To study the shear mechanical behavior of a rock sample under unloading normal stress conditions,a new stress path for direct shear tests was proposed to model the unloading of stress caused by excavation and other processes.The effects of the initial stresses(i.e.,the normal stress and shear stress before unloading)on the shear behavior and energy conversion were investigated using laboratory tests and numerical simulations.The shear strength of a rock under constant stress or under unloading normal stress conforms to the Mohr Coulomb criterion.As the initial normal stress increases,the cohesion decreases linearly and the tangent of the internal friction angle increases linearly.Compared with the results of the tests under constant normal stress,the cohesions of the rock samples under unloading normal stress are smaller and their internal friction angles are larger.A strength envelope surface can be used to describe the relationship between the initial stresses and the failure normal stress.Shear dilatancy can decrease the total energy of the direct shear test under constant normal stress or unloading normal stress,particularly when the stress levels(the initial stresses in the test under unloading normal stress or the normal stress in the test under constant normal stress)are high.The ratio of the dissipated energy to the total energy at the moment failure occurs decreases exponentially with increasing initial stresses.The direct shear test under constant normal stress can be considered to be a special case of a direct shear test under unloading normal stress with an unloading amount of zero.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.42372326 and 42090054)supported by the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection Independent Research Project(SKLGP2023Z015).
文摘During the excavation of large-scale rock slopes and deep hard rock engineering,the induced rapid unloading serves as the primary cause of rock mass deformation and failure.The essence of this phenomenon lies in the opening-shear failure process triggered by the normal stress unloading of fractured rock mass.In this study,we focus on local-scale rock fracture and conduct direct shear tests under different normal stress unloading rates on five types of non-persistent fractured hard rocks.The aim is to analyze the influence of normal stress unloading rates on the failure modes and shear mechanical characteristics of non-persistent fractured rocks.The results indicate that the normal unloading displacement decreases gradually with increasing normal stress unloading rate,while the influence of normal stress unloading rate on shear displacement is not significant.As the normal stress unloading rate increases,the rocks brittle failure process accelerates,and the degree of rocks damage decreases.Analysis of the stress state on rock fracture surfaces reveals that increasing the normal stress unloading rate enhances the compressive stress on rocks,leading to a transition in the failure mode from shear failure to tensile failure.A negative exponential strength formula was proposed,which effectively fits the relationship between failure normal stress and normal stress unloading rate.The findings enrich the theoretical foundation of unloading rock mechanics and provide theoretical support for disasters prevention and control in rock engineering excavations.
基金support from the National Natural Science Foundation of China(Grant Nos.52174092 and 52104125)the Fundamental Research Funds for the Central Universities,China(Grant No.2022YCPY0202)is gratefully acknowledged.
文摘Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted regarding various initial normal stresses(1e7 MPa)and numbers of shearing cycles(1 e5).The peak shear stress of fractures decreased with shear cycles due to progressively smooth surface morphologies,while increased with both JRC and initial normal stress and could be verified using the nonlinear Barton-Bandis failure criterion.The joint friction angle of fractures exponentially increased by 62.22%e64.87%with JRC while decreased by 22.1%e24.85%with shearing cycles.After unloading normal stress,the sliding initiation time of fractures increased with both JRC and initial normal stress due to more tortuous fracture morphologies and enhanced shearing resistance capacity.The surface resistance index(SRI)of fractures decreased by 4.35%e32.02%with increasing shearing cycles due to a more significant reduction of sliding initiation shear stress than that for sliding initiation normal stress,but increased by a factor of 0.41e1.64 with JRC.After sliding initiation,the shear displacement of fractures showed an increase in power function.By defining a sliding rate threshold of 5105 m/s,transition from“quasi-static”to“dynamic”sliding of fractures was identified,and the increase of sliding acceleration steepened with JRC while slowed down with shearing cycles.The normal displacement experienced a slight increase before shear sliding due to deformation recovery as the unloading stress was unloaded,and then enhanced shear dilation after sliding initiation due to climbing effects of surface asperities.Dilation was positively related to the shear sliding velocity of fractures.Wear characteristics of the fracture surfaces after shearing failure were evaluated using binary calculation,indicating an increasing shear area ratio by 45.24%e91.02%with normal stress.
基金This research was funded by the Fundamental Research Funds for the Central Universities,CHD(Grant Nos.300102210307 and 300102210308)the National Natural Science Foundation of China(Grant Nos.41831286 and 41972297)the Natural Science Foundation of Shaanxi Province(Grant No.2020JQ-369).
文摘Excavation and earth surface processes(e.g.,river incision)always induce the unloading of stress,which can cause the failure of rocks.To study the shear mechanical behavior of a rock sample under unloading normal stress conditions,a new stress path for direct shear tests was proposed to model the unloading of stress caused by excavation and other processes.The effects of the initial stresses(i.e.,the normal stress and shear stress before unloading)on the shear behavior and energy conversion were investigated using laboratory tests and numerical simulations.The shear strength of a rock under constant stress or under unloading normal stress conforms to the Mohr Coulomb criterion.As the initial normal stress increases,the cohesion decreases linearly and the tangent of the internal friction angle increases linearly.Compared with the results of the tests under constant normal stress,the cohesions of the rock samples under unloading normal stress are smaller and their internal friction angles are larger.A strength envelope surface can be used to describe the relationship between the initial stresses and the failure normal stress.Shear dilatancy can decrease the total energy of the direct shear test under constant normal stress or unloading normal stress,particularly when the stress levels(the initial stresses in the test under unloading normal stress or the normal stress in the test under constant normal stress)are high.The ratio of the dissipated energy to the total energy at the moment failure occurs decreases exponentially with increasing initial stresses.The direct shear test under constant normal stress can be considered to be a special case of a direct shear test under unloading normal stress with an unloading amount of zero.
