Surrounding Rock And Rockbolt Support Macro-Meso Properties Of Anchored-on-rock Crane Beam In Underground Powerhouse

Whether it is the new stage of China's overall construction of a well-off society or the needs to build the "Bridge for Cities" economic zone,it has prompted China's hydraulic and geotechnical underground engineering construction.The safety and stability of underground plants are related to the safe operation of many large-scale projects at home and abroad.As its important structure,the anchored-on-rock crane beam directly withstands crane loads,and suffers large local deformation and complex stress characteristics,affecting the overall safety and stability during the excavation and operation stages.At present,the researches on the crane beam structure are mainly based on experience reference and macroscopic analysis methods.The studies of rock wall surrounding rockmass meso-damage process,rockbolt action meso-mechanism and weak contact surface meso-stress characteristics are still insufficient.It is necessary to introduce new models and methodologies to study and discuss these issuses from a new perspective,and to help provide theoretical support for practical projects.Focused on the key issues of surrounding rockmasses and rockbolt support macro-meso mechanical properties of anchored-on-rock crane beam in underground powerhouse,this paper firstly studies the deformation and damage characteristics of rock wall,the safety factor of weak contact surfaces and the force distribution of rockbolts under different blasting parameters,different excavation conditions and different rockbolt support conditions,using the finite element method and from the perpective of macro-machanics.In order to further research the meso-mechanical behavior of anchored-on-rock crane beam structure during the excavation and operation stage of underground powerhouse,it introduces the discrete element analysis method based on discrete elementary particle flow PFC program,the discrete-continuous coupled calculation method based on PFC-FLAC program,and improved coupled particle model acoustic emission(AE)record technology.The main research content is as follows:(1)On the basis of fine excavation requirements of crane beam rock wall in practical underground projects,it proposes three-dimensional finite element model and analysis method of anchored-on-rock crane beam,and studies the influence of blasting excavation load on the stress distribution and failure characteristics of surrounding rock nearby,results of which show that the stability of surrounding rock in the vicinity of crane beam is extremely sensitive to the blasting parameters.Also it establishes a reasonable anchored-on-rock crane beam over-excavation FE moel and proposes its safety evaluation index,which shows that it increases the surrounding rockmass damage zone and deformation,as wall as reduces bearing capacity and contace surface safety factor.It is harmful to the forming of reasonable stress deformation characteristics and guarantee the operation safety.Therefore,in order to avoid the poor excavation phenomenon and ensure the rock bench is shaped according to design,it has to carry out rock blasting fine excavation control.(2)To simulate the asphalt-coated rockbolt technique used in practical projects,a method is proposed for generating asphalt elements in underground powerhouse finite element model through rewriting the model file with Fortran language:it divides the local element mesh according to the rockbolt direction near crane beam and builds the initial FE model of the underground powerhouse cavern group;new nodes are generated inside the upper row of elements where asphalt-coated bolts pass through;new FE model is regenerated according to the basis point-line-plane relationship of the eight-node hexahedral element.Implicit lever and column elements are introduced to simulate rockbolts with and without asphalt coatings,respectively,and appropriate stiffness matrices and iterative FE calculation formulas are derived.Such a model is applied in the analysis of anchored-on-rock crane beam with asphalt-coated rockbolts in Mengdigou underground powerhouse,which effectively shows that adoption of this technique reduces the surrounding rock damage zone of rock wall,increases the stress of deep bolts inside the surrounding rockmass and thus can improve the mechanical characteristics of rockmass in the vicinity of rockbolts and increase the overall safety of the surrounding rockmass and crane beam,which is also in line with the engineering practice.(3)It proposes PFC particle flow simulation method for rock and rockbolt matertials and discusses ways to describe the media's meso-mechanism of deformation,stress,damage discrimination and failure progress,including relationship between the particle-to-particle contact force chain and the macroscopic mechanical properties,relationship between the micro-crack development and the AE characteristics of the particle model,relationship between the particle model wall servo and the macroscopic stress of the media,etc.It has been applied to PFC numerical test simulations such as rock unconfined compression test,direct tensile test,compression test under different confining pressures and bonded rockbolt pulled out test,with micro-crack propagation morphology and AE characteristics during the rock failure process,and the micro-mechanical characteristics of the bonded rockbolt action analyzed.Thus from the perspective of meso-mechanics,effective particle flow simulation methods for rock and rockbolt materials are explored and successfully used.(4)To solve the problem that usage of the discrete element particle flow PFC program alone cannot fit the scale of underground powerhouse rockmass excavation and rockbolt supporting,the PFC-FLAC discrete-continuous coupled calculation method is used to divide the solution area into two sub-areas:it establishes finite difference element continuousmodel in the FLAG program to maintain the boundary stress during excavation process,and particle flow model in PFC program to carry out the underground cavern excavation.On the basis of data transmission design based on FISH language secondary development between ITASCA programs,the continuous model and the discrete particle model in these two programs can perform effective data exchange and continuous coupled calculation,at the same time of which FLAC model coupled boundary maintains good deformation consistency and stress equivalence with PFC walls.It is used to study the full-length bonded rockbolt action mechanism when supporting actively deforming rockmass in the vicinity of excavation surface,and research the self-balanced "pressure ring" mechanism of excavation boundary surrounding rockmass,the "neutral" feature of full-length bonded rockbolt action and rock-rockbolt joint bearing mechanism,from the perspective of microscopic.The results show that the rockbolt support reduces the thickness of pressure ring around excavation surface,but the value of its single contact force chain increases and interweaves around the rockbolt rod.The parallel cohesive forces between the rockbolt particles are much greater than that of the surrounding rockmass particles,which presents the meso-mechanical bearing behavior of rockbolt and surrounding rockmass.Through the deformation characristics it pictures that the rock displacement close to the excavation surface is greater than that of rockbolts,and the rock displacement far away from the excavation surface is less than that of rockbolts.Where the displacement is equal with each other,there is the "neutral" point,the axial force between the full-length bonded rockbolt particles largest.The percentage reduction of surrounding rock porosity before and after excavation is significantly lower than that without rockbolt supporting,which reflects the important role that rockbolt plays in bettering the loosening effect of surrounding rockmass caused by excavation.(5)The classical PFC-FLAC coupled numerical simulation of PFC AE model does not perform well in the underground cavern areas with large deformation or stress concertration.To solve this problem,it proposes an improved PFC AE model based on bilinear interpolation of node velocity in FLAC model coupled region:it carries out underground cavern stage excavation process and installs supporting system in FLAC model,while establishes PFC AE model sharing the same coordinates with interested area.The particle velocity belongs to FLAC coupled area node velocity based on bilinear interpolation,which allows arbitrary deformation of the coupling area and meets the need of synchronous displacement of PFC particles,PFC walls and FLAC coupled boundary.After verification of a certain underground excavation by stages it shows that this method is well applicable to the analysis of local meso-mechanical characteristics of underground caverns,and then applied to study the local stress,deformation and failure characteristics of anchored-on-rock crane beam structure in the underground powerhouse of Mengdigou hydropower station.A method for judging the depth of rockmass failure zone according to the AE event development is presented,whose results are consistent with the results of finite element model analysis,and also make up for the shortcomings of the latter that the display method of surrounding rockmass failure is single and it is particularly difficult to describe change in the degree of surrounding rockmass damage,proving that the method proposed above has obvious advantage when describing rockmass excavation related problems.Also it is used to research the weak contact surface of the concrete beam and rock wall under conditions of surrounding rockmass property deterioration and crane force overloading,which demonstrates the meso-mechanical response process that the vertical and inclined contact surfaces will respectively produce tensile failure and shear failure.