| In recent years,with the increasing magnitude of ground stress faced by rock tunnel(tunnel)engineering,the stability and maintenance of roadways have become more and more difficult.A higher requirement is put forward for understanding the mechanism of wall rock-support interaction.Theoretical research and engineering practice show that in order to understand the surrounding rock-support interaction mechanism accurately and comprehensively.It is necessary to grasp the deformation and destruction mechanisms of rocks and surrounding rocks under the excavation unloading conditions of roadways(tunnels).This is also the basic subject that needs to be solved urgently in underground rock project.In order to explore the deformation and damage characteristics of roadway surrounding rock under excavation unloading conditions,the paper successfully constructed a test system that can simulate and reproduce the excavation unloading process of a roadway using small surrounding rock specimens(Thick-walled cylinder)(height: 290 mm,outer diameter: 200 mm,inner diameter: 100 to 150 mm)firstly.On the basis of this system,a series of tests were carried out on the small surrounding rock made of organic glass,high strength gypsum and natural sandstone individually under a simulated excavation unloading condition.The deformation and failure law of surrounding rock of roadway were studied.First,the unloading test was carried out on the specimens of plexiglass materials.The plexiglass is a very good elastic-plastic material.Simulation of the roadway excavation unloading process in the elastic stage of the plexiglass surrounding rock specimen is to find out whether the unloading response of surrounding rock in the elastic stage is consistent with the elastic analysis theory of secondary stress state of mature surrounding rock,and at the same time,to verify the reliability and feasibility of the test system.Secondly,the paper simulates the transient and slow unloading of small surrounding rock specimens prepared from high-strength gypsum materials.Because of the brittleness of high-strength gypsum materials,the excavation unloading failure process was simulated.Finally,the unloading tests of natural sandstone surrounding rock specimens were conducted under different unloading rates,different excavating radii,different initial confining pressures and high stress conditions.The deformation and failure characteristics of roadway surrounding rock under different unloading conditions were analyzed.At the same time,the two surrounding rock responses of high strength gypsum and natural sandstone were compared and analyzed under the different unloading rates.The main research findings are as follows:(1)The construction of excavation model test system for surrounding rock of rock roadwayA set of test system which can simulate and reproduce the excavation and unloading path of roadway surrounding rock is developed.The system is mainly composed of three separate systems:(1)system I: SAM-3000 microcomputer controlled electro-hydraulic servo rock triaxial test system;(2)system?:small tunnel roadway rock specimens loading and unloading cavity;(3)system?:the integration test system of sound wave-acoustic emission.Through the integration and debugging of these three systems,the process of simulating and reproducing unloading of roadway/tunnel excavation in the laboratory was realized.The innovative monitoring method and complete set of test technology for excavation unloading test of surrounding rock specimens have been obtained.The test method is simple and practical,the simulation effect is real,and it has wide practicality.(2)Analysis of elastic state of secondary stress state of roadway surrounding rock under excavation unloading conditionAccording to the analysis of the excavation unloading simulation test results of the plexiglass surrounding rock specimens,the monitored deformation of the surrounding rock exhibits elastic characteristics,and the deformation is oriented towards the inside of the cave.The inner deformation of the surrounding rock specimen is greater than that of the outer side,that is,the nearer the wall of the surrounding rock,the greater the deformation of the surrounding rock.According to the knowledge of the strain theory of the elastic stage of the thick-walled cylinder,the theoretical solution about the surrounding rock unloading strain of the plexiglass surrounding rock specimen is calculated and compared with the experimental results.The analysis results show that the test results can correspond well with the theoretical values,and the trend of change is the same.The difference between the two is not significant.The theoretical value is slightly larger than the test value.The comparative analysis results between the experimental values and the theoretical solution on the elastic stage of the secondary stress state of the surrounding rock prove that the performance of the test system constructed in this paper is stable and the test simulation method is feasible.Therefore,the system can be used to simulate excavation of the roadway surrounding rock.The deformation and failure characteristics of roadway surrounding rock are analyzed under simulated excavation unloading conditions.(3)The influence of unloading confining pressure rate on unloading deformation and failure characteristics of surrounding rockThe transient and slow excavation unloading process of the high-strength gypsum wall rock specimen and the natural sandstone wall rock specimen was simulated respectively,and the unloading deformation of the surrounding rock with two different materials was compared and analyzed.During the transient unloading and slow unloading of high-strength gypsum wall rock specimens,the axial and tangential directions of the surrounding rock are basically under pressure.Some of the measuring points are changed in tension and pressure,and the outside axial strain is greater than the inside and the outside.The lateral tangential strain is less than the inner one.When the unloading amount is the same,the deformation of the surrounding rock shows elasto-plasticity during slow unloading and brittleness during transient unloading.The tangential strain at the same measuring point is larger than the axial strain at any time of unloading.The tangential strain response to unloading is more sensitive than axial.The tangential strain of the roadway surrounding rock is mutated at the time of unloading failure,and the failure is along the radial direction,that is,toward the inside of the cave.The failure mode appears as shear failure and is accompanied by vertical alignment.During the transient unloading of the natural sandstone wall rock specimens,the axial and tangential pressures on the outside are subjected to compression,and the axial shear stress on the inside is tangential to the inside.The response of the surrounding rock specimen to transient unloading is not only expanding into the cave wall,but also expanding in the axial direction.It shows that in the actual tunneling/tunnel excavation project,when the blasting method is used,the deformation at the cave wall is spread and expands to the surrounding area.When the sandstone surrounding rock samples are slowly unloaded,the axial strain reflects the unloading process slightly behind the tangential strain.From the beginning of unloading confining pressure,the tangential strain increases sharply,which is obviously greater than the axial strain growth rate,that is,the apparent lateral expansion.The tangential strain is about 10 times that of the axial strain,indicating that the tangential strain that dominates the deformation of the surrounding rock during excavation unloading is tangential strain.In the whole unloading process,the lateral axial strain is greater than the axial inner strain,and the lateral tangential strain is smaller than the lateral tangential strain,which indicates that the small surrounding rock specimen expands and expands inward during the unloading process.(4)The influence of different excavation diameter on unloading deformation and failure characteristics of surrounding rockWhen the natural sandstone wall rock specimen is unloaded at transient and slowly unloading,the unloading deformation of surrounding rock under different excavation diameters is approximately the same as that of internal pressure unloading.The internal and external strains of the surrounding rock specimens increase with the increase of the internal diameter of the excavation.It shows that in the actual excavation process,the deformation of the cave wall and the deformation along a certain point in the surrounding rock along the radial direction gradually increase inward as the inner diameter of the excavation increases.When transient unloading occurs,rebound deformation occurs when the inner diameter is 100 mm.With the same initial confining pressure,the surrounding rock specimens were destroyed when the internal diameter was 150 mm.But the surrounding rock specimens were not destroyed during the transient unloading.The unloading failure of the surrounding rock specimen starts with the middle part of the specimen.The brittleness characteristics during the destruction are obvious.The strong expansion characteristics along the unloading direction are obvious,and the middle part of the surrounding rock specimen protrudes in the unloading direction as a drum.The phenomenon of lumping is observed,and there is a circular crack.The unloading and yielding failure of the surrounding rock specimen shows a tensile failure mode.The transient and slow unloading process of natural sandstone surrounding rock specimens was simulated using FLAC3 D and compared with the experimental results.The comparison shows that the experimental value is 2 to 3 times larger than the numerical solution,but the presented deformation rule is the same.(5)The influence of different initial confining pressure on unloading deformation and failure characteristics of surrounding rockBy changing the initial confining pressure and other conditions unchanged,the influence of initial confining pressure on the unloading deformation of the surrounding rock is obtained.At the same unloading rate,the larger the initial confining pressure,the later the unloading end point is,that is,the larger the confining pressure is before unloading,the larger the unloading energy,the more unloading steps are required,and the larger the deformation.With the increase of the initial confining pressure,the unloading deformation of the surrounding rock becomes larger and larger.In the actual project,the higher the stress of the original rock,the greater the deformation encountered by the surrounding rock during the excavation of the roadway/tunnel,the greater it is prone to damage and the greater the support required.Through the simulation of excavation unloading process under the same conditions by FLAC3D5.0,the numerical solution obtained is larger than the numerical solution,but the rules of both are the same as the increase of initial confining pressure,and the unloading deformation of surrounding rock increases with the increase of initial confining pressure.(6)The study on unloading deformation and failure characteristics of surrounding rock under high stress conditionsThe simulation of roadway/tunnel excavation unloading process was carried out for the surrounding rock specimens with inner diameters of 100 mm and 125 mm under high stress conditions.The deformation of surrounding rock with large inner diameter was large,and the inner strain under the same inner diameter condition was greater than the outer strain.When the inner diameter is 125 mm,the unloading failure phenomenon occurs.When the surrounding rock specimens are destroyed,there are lamellar delaminations on the inner side and there are local lumps in the surrounding rock.When the surrounding rock breaks down,there are many new nascent cracks.From the unloading failure pattern of all natural sandstone specimens,it can be concluded that the higher the initial confining pressure,the stronger the unloading degree and the higher the degree of rock fragmentation.The unloading failures all have the feature of strong expansion along the unloading direction,which is mainly characterized by tensile failure.There are a small number of splitting planes parallel to the axial loading and accompanied by circumferential cracks.The failure mode of surrounding rock specimens changed from brittle to ductile with the increase of confining pressure.The excavation unloading process under the same high stress condition was simulated by FLAC3D5.0,and compared with the experimental results.The numerical solution value is less than the experimental value but the same rules are presented.In the numerical simulation with an inner diameter of 125 mm,the unloading failure of the confining pressure test specimen was also monitored. |
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