| Under the action of geological structure,rock mass in nature will have different types of defects such as joints and faults.Cracks of different occurrence are also distributed in large engineering rock mass such as bedrock of hydraulic engineering.Pre-existing cracks are the main reason for rock instability and fracture,which will seriously affect the long-term stability of hydraulic engineering.In recent years,with the implementation of major water conservancy projects such as Bai hetan Hydropower Station and Jinping II hydropower station,more and more engineering practices show that the deformation and failure of rock mass often experience a more obvious aging process.The rock mass is in the unloading state after excavation.With the passage of time,creep deformation and failure will occur,resulting in cracking and inclination of engineering buildings,which will inevitably affect the stability of hydraulic engineering.Based on this,double fractured rock samples with different occurrence(angle,spacing and length)are prepared in this paper;The strength and deformation characteristics of double fractured rock mass under conventional uniaxial,triaxial compression and constant axial pressure are analyzed by means of indoor test,theoretical analysis and numerical calculation;Fracture mechanics analysis of fractured rock mass;The creep damage constitutive model of fractured rock mass is established and the parameters are identified;The precursory characteristics of unloading creep of fractured rock mass are analyzed based on acoustic emission test.The main research results of this paper are as follows:(1)Based on the uniaxial and triaxial compression test results of fractured rock samples,the relationship between fracture occurrence and mechanical parameters is analyzed;Based on the theory of fracture mechanics,the correction factor F of stress intensity factor at the crack tip is deduced;The position coefficient S considering the spatial distribution of fractures is introduced,and the coefficient expression suitable for different fracture occurrence is given;Compared with the results of other empirical formulas,the calculation results of influence coefficient have better rationality and applicability.(2)Based on the theory of rock creep and damage mechanics,the nonlinear viscoelastic plastic damage creep model of fractured rock mass is established by introducing the viscous element considering damage evolution and lade failure criterion.After identifying the model parameters,it is compared with other creep models to verify the applicability and superiority of the creep model.(3)Based on the triaxial unloading confining pressure creep test results of fractured rock samples,the deformation characteristics of fractured rock mass with different occurrence under unloading creep conditions are analyzed;With the increase of fracture occurrence,the creep deformation of fractured rock mass increases,and the more obvious the expansion phenomenon is;At the last level of stress,the percentage of strain in creep strain in decay creep stage and steady-state creep stage gradually decreases,and the proportion of strain in accelerated creep stage and average creep rate gradually increase.(4)Based on acoustic emission monitoring technology,the precursory characteristics of acoustic emission parameters and waveforms in the creep process of fractured rock samples are analyzed;During each creep stage,AE ringing count and energy show the change law of rise low level fluctuation rise;The AE dominant frequency of unloading creep of fractured rock mass presents the characteristics of continuous and tight strip;Based on the cusp catastrophe theory and AE dominant frequency,the creep failure of fractured rock is warned;The appearance of higher frequency and lower frequency bands and the significant increase of the energy proportion of the two bands can be regarded as the precursory characteristics of unloading creep failure of fractured rock. |
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