Microbially induced calcium carbonate precipitation(MICP)technology can induce calcium carbonate crystals with cementation and stable performance in the process of microbial metabolism or enzymization through the regu...Microbially induced calcium carbonate precipitation(MICP)technology can induce calcium carbonate crystals with cementation and stable performance in the process of microbial metabolism or enzymization through the regulation of environmental factors MICP can be used as a cementing agent to cement cohesionless sand particles to form the materials with the characteristics of higher strength,better durability and environmental friendli-ness,as well as a good engineering application prospect.In this paper,the shear strength of sand column was tested by triaxial compression tests,and the strength index was obtained.In order to further study the micro-strength mechanism and the failure process,based on the discrete element method,a numerical model of MICP cemented sand column was established considering the factors of matrix soil particle gradation,particle mor-phology,content ratio of induced calcium carbonate,pore distribution characteristics,inter-particle cementation and so on.The failure process of MICP cemented sand column under load was analysed by numerical simulation,and the reliability of the numerical model was tested by combining with the stress intensity curve of samples under test conditions.The results indicate that compared with the actual triaxial tests of MICP cemented sand column,although there are deviations in stress and strain,cohesion and internal friction angle,the numerical simulation shows similar development law and intensity amplitude,and the same failure trend.The work in this paper verifies the reliability of the numerical model and provides a theoretical basis for the subsequent analysis of the factors influencing the geotechnical mechanical properties of biomineralized materials.展开更多
According to the different stress paths,similar model test and PFC simulation test of tunnel surrounding rock are designed to compare the failure mechanisms at macroscopic and mesoscopic scales.The following conclusio...According to the different stress paths,similar model test and PFC simulation test of tunnel surrounding rock are designed to compare the failure mechanisms at macroscopic and mesoscopic scales.The following conclusions are drawn.1)Excavation unloading will disturb the surrounding rock to form a certain excavation damaged zone.2)Under the loading path,the stress of surrounding rock failure is 1.500 MPa;under the unloading path with initial stress of 60% σ_(Zmax) and 100% σ_(Zmax),the failure stress is 1.583 and 1.833 MPa respectively in the model test.3)In terms of the failure mode of rocks under different stress paths,tensile fractures first appear in two sides of the vertical walls;thereafter,the spandrel and arch foot are loosened due to the stress concentration.The fractures gradually coalesce with those occurring in the vertical walls.4)In the process of excavation unloading,the proportion of shear cracks is 35.3%,and the rock is subject to strong shear effect.The final failure surface is approximately V-shaped.5)The tangential peak stress on the vertical walls at the free face is the lowest;the vertical walls at the free face show the poorest bearing capacity and are easily subjected to tensile failure.展开更多
Understanding the mechanism of progressive debonding of bolts is of great significance for underground safety.In this paper,both laboratory experiment and numerical simulation of the pull-out tests were performed.The ...Understanding the mechanism of progressive debonding of bolts is of great significance for underground safety.In this paper,both laboratory experiment and numerical simulation of the pull-out tests were performed.The experimental pull-out test specimens were prepared using cement mortar material,and a relationship between the pull-out strength of the bolt and the uniaxial compressive strength(UCS)of cement mortar material specimen was established.The locations of crack developed in the pull-out process were identified using the acoustic emission(AE)technique.The pull-out test was reproduced using 2D Particle Flow Code(PFC^(2D))with calibrated parameters.The experimental results show that the axial displacement of the cement mortar material at the peak load during the test was approximately 5 mm for cement-based grout of all strength.In contrast,the peak load of the bolt increased with the UCS of the confining medium.Under peak load,cracks propagated to less than one half of the anchorage length,indicating a lag between crack propagation and axial bolt load transmission.The simulation results show that the dilatation between the bolt and the rock induced cracks and extended the force field along the anchorage direction;and,it was identified as the major contributing factor for the pull-out failure of rock bolt.展开更多
Hydraulic fracturing(HF)technology can safely and efficiently increase the permeability of coal seam,which is conducive to CBM exploration and prevent coal and gas outburst.However,conventional HF fractures tend to ex...Hydraulic fracturing(HF)technology can safely and efficiently increase the permeability of coal seam,which is conducive to CBM exploration and prevent coal and gas outburst.However,conventional HF fractures tend to expand in the direction of maximum principal stress,which may be inconsistent with the direction of fracturing required by the project.Therefore,the increased direction of coal seam permeability is different from that expected.To solve these problems,PFC2D software simulation is used to study directional hydraulic fracturing(DHF),that is the combination of slotting and hydraulic fracturing.The effects of different slotting angles(θ),different horizontal stress difference coefficients(K)and different injection pressures on DHF fracture propagation are analyzed.The results show that the DHF method can overcome the dominant effect of initial in-situ stress on the propagation direction of hydraulic fractures and control the propagation of fractures along and perpendicular to the slotting direction when θ,K and liquid injection pressure are small.When the DHF fracture is connected with manual slotting,the pressure will shake violently,and the fracturing curve presents a multi-peak type.The increase and decrease of particle pressure around the fracturing hole reflect the process of pressure accumulation and fracture propagation at the fracture tip respectively.Compared with conventional HF,DHF can not only shorten the fracturing time but also make the fracture network more complex,which is more conducive to gas flow.Under the action of in-situ stress,the stress between slots will increase to exceed the maximum horizontal principal stress.Moreover,with the change in fracturing time,the local stress of the model will also change.Hydraulic fractures are always expanding to the area with large local stress.The research results could provide certain help for DHF theoretical research and engineering application.展开更多
基金sponsored by the National Natural Science Foundation of China(Grant No.12002173,12262027)Research start-up project of Inner Mongolia University of Technology(No.2200000924)key Lab.of University of Geological Hazards and Geotechnical Engineering Defense in Sandy and Drought Regions,Inner Mongolia Autonomous.
文摘Microbially induced calcium carbonate precipitation(MICP)technology can induce calcium carbonate crystals with cementation and stable performance in the process of microbial metabolism or enzymization through the regulation of environmental factors MICP can be used as a cementing agent to cement cohesionless sand particles to form the materials with the characteristics of higher strength,better durability and environmental friendli-ness,as well as a good engineering application prospect.In this paper,the shear strength of sand column was tested by triaxial compression tests,and the strength index was obtained.In order to further study the micro-strength mechanism and the failure process,based on the discrete element method,a numerical model of MICP cemented sand column was established considering the factors of matrix soil particle gradation,particle mor-phology,content ratio of induced calcium carbonate,pore distribution characteristics,inter-particle cementation and so on.The failure process of MICP cemented sand column under load was analysed by numerical simulation,and the reliability of the numerical model was tested by combining with the stress intensity curve of samples under test conditions.The results indicate that compared with the actual triaxial tests of MICP cemented sand column,although there are deviations in stress and strain,cohesion and internal friction angle,the numerical simulation shows similar development law and intensity amplitude,and the same failure trend.The work in this paper verifies the reliability of the numerical model and provides a theoretical basis for the subsequent analysis of the factors influencing the geotechnical mechanical properties of biomineralized materials.
基金Project(52179104)supported by the National Natural Science Foundation of ChinaProjects(ZR2020ME099,ZR2020MD111,ZR2019BEE051)supported by the Shandong Provincial Natural Science Foundation,China。
文摘According to the different stress paths,similar model test and PFC simulation test of tunnel surrounding rock are designed to compare the failure mechanisms at macroscopic and mesoscopic scales.The following conclusions are drawn.1)Excavation unloading will disturb the surrounding rock to form a certain excavation damaged zone.2)Under the loading path,the stress of surrounding rock failure is 1.500 MPa;under the unloading path with initial stress of 60% σ_(Zmax) and 100% σ_(Zmax),the failure stress is 1.583 and 1.833 MPa respectively in the model test.3)In terms of the failure mode of rocks under different stress paths,tensile fractures first appear in two sides of the vertical walls;thereafter,the spandrel and arch foot are loosened due to the stress concentration.The fractures gradually coalesce with those occurring in the vertical walls.4)In the process of excavation unloading,the proportion of shear cracks is 35.3%,and the rock is subject to strong shear effect.The final failure surface is approximately V-shaped.5)The tangential peak stress on the vertical walls at the free face is the lowest;the vertical walls at the free face show the poorest bearing capacity and are easily subjected to tensile failure.
基金Financial supports for this work,provided by the National Natural Science Foundation of China(No.41974164)the Scientific Research Startup Fund for High Level Talents Introduced by Anhui University of Science and Technology(No.2021yjrc16)the Chinese Government Scholarship(No.201906420030),are gratefully acknowledged.
文摘Understanding the mechanism of progressive debonding of bolts is of great significance for underground safety.In this paper,both laboratory experiment and numerical simulation of the pull-out tests were performed.The experimental pull-out test specimens were prepared using cement mortar material,and a relationship between the pull-out strength of the bolt and the uniaxial compressive strength(UCS)of cement mortar material specimen was established.The locations of crack developed in the pull-out process were identified using the acoustic emission(AE)technique.The pull-out test was reproduced using 2D Particle Flow Code(PFC^(2D))with calibrated parameters.The experimental results show that the axial displacement of the cement mortar material at the peak load during the test was approximately 5 mm for cement-based grout of all strength.In contrast,the peak load of the bolt increased with the UCS of the confining medium.Under peak load,cracks propagated to less than one half of the anchorage length,indicating a lag between crack propagation and axial bolt load transmission.The simulation results show that the dilatation between the bolt and the rock induced cracks and extended the force field along the anchorage direction;and,it was identified as the major contributing factor for the pull-out failure of rock bolt.
基金supported by National Natural Science Foundation of China(52130409,52004291,51874314)the Fundamental Research Funds for the Central Universities(2022YJSAQ03,2022XJAQ02).
文摘Hydraulic fracturing(HF)technology can safely and efficiently increase the permeability of coal seam,which is conducive to CBM exploration and prevent coal and gas outburst.However,conventional HF fractures tend to expand in the direction of maximum principal stress,which may be inconsistent with the direction of fracturing required by the project.Therefore,the increased direction of coal seam permeability is different from that expected.To solve these problems,PFC2D software simulation is used to study directional hydraulic fracturing(DHF),that is the combination of slotting and hydraulic fracturing.The effects of different slotting angles(θ),different horizontal stress difference coefficients(K)and different injection pressures on DHF fracture propagation are analyzed.The results show that the DHF method can overcome the dominant effect of initial in-situ stress on the propagation direction of hydraulic fractures and control the propagation of fractures along and perpendicular to the slotting direction when θ,K and liquid injection pressure are small.When the DHF fracture is connected with manual slotting,the pressure will shake violently,and the fracturing curve presents a multi-peak type.The increase and decrease of particle pressure around the fracturing hole reflect the process of pressure accumulation and fracture propagation at the fracture tip respectively.Compared with conventional HF,DHF can not only shorten the fracturing time but also make the fracture network more complex,which is more conducive to gas flow.Under the action of in-situ stress,the stress between slots will increase to exceed the maximum horizontal principal stress.Moreover,with the change in fracturing time,the local stress of the model will also change.Hydraulic fractures are always expanding to the area with large local stress.The research results could provide certain help for DHF theoretical research and engineering application.
