Passive Triaxial Transient And Creep Mechanical Properties Of Undisturbed Specimens Of Cataclastic Rock Mass

Complete or incomplete rock mass with incomplete or undeveloped structural surfaces often obtain specimens through conventional on-site drilling,and then carry out a series of uniaxial compression,conventional triaxial compression or direct shear tests indoors on this basis,which can obtain more reliable rock strength and deformation parameters,and then provide relatively reliable design parameters for underground engineering support design.However,for the weak fractured rock mass with extremely developed structural surface,low strength and large deformation of the rock block itself,it is difficult to obtain rock samples using the conventional sampling method recommended by the specification,and it is also impossible to carry out experiments according to conventional test methods.In view of this,this paper relies on the diversion tunnel project of a large hydropower station in southwest China,and on the basis of the investigation and analysis of the regional geological environment of the construction site,comprehensively uses theoretical analysis,indoor test of the prototype specimen of the fractured rock mass and numerical simulation analysis to carry out the study of the transient and creep mechanical characteristics of the fractured rock mass.The relevant research work and conclusions are as follows:（1）In the research on the sampling and preparation process of in-situ catastrophic rock,firstly,the lithology and structural characteristics of in-situ catastrophic rock mass are described by the on-site survey.Secondly,the catastrophic sample sampler was developed,and a method suitable for researching the transient and rheological deformation characteristics of cataclastic rock mass,namely the passive triaxial compression test,was proposed.Then a simplified mechanical calculation model of"the fractuated rock sample-confined cylinder-force transfer column"in the experiment is established.Finally,the mathematical relationship expression between passive restraint stress and restraint cylinder structural strain is deduced.（2）In the study of the transient deformation characteristics of catastrophic rock,a compression test under axially graded loading and radial passive restraint stress is designed.Firstly,the axial deformation characteristics of the fractured rock samples are analyzed.Secondly,the mathematical relationship between the radial strain and the axial strain of the fractured rock specimen under three-dimensional stress state is established,and the radial deformation and volume deformation laws of the specimen are discussed accordingly.Finally,the variation law of the deformation parameters of the fractured rock samples under different stress states is discussed.The research results show that the axial stress-axial strain curve of the sample has obvious 4-stage characteristics.In addition,the compressive modulus of the fractured rock sample increases with the increase of the axial pressure,while the poise of the sample increases.The loose ratio decays exponentially with increasing loading stress level.（3）In the experimental study of the creep deformation characteristics of the fractured rock,a passive triaxial loading and unloading creep test is designed.Firstly,the axial creep deformation characteristics of the fractured rock samples are analyzed.Secondly,the characteristics of creep rate curve,isochronous stress-strain curve and visco-elastic-plastic characteristics of the fractured rock samples were studied.The research results show that the creep deformation of the fractured rock undergoes decay creep and stable creep successively,and there is no failure stage due to the strong restraint of the cylinder.In the experiment,the creep rate decays exponentially until it decreases to a certain value.Instantaneous deformation occurred in the initial stage of the application of axial pressure at all levels,and instantaneous elastic recovery deformation also occurred in the initial stage of unloading.The creep model is identified according to the characteristics of the creep test curve of the fractured rock sample,and it is found that the Burges creep model can describe the creep deformation law of the sample well.The creep parameters of the Burges model were inverted by the least-squares method,and the variation law of the model parameters with the increase of the loading stress level was analyzed.But the growth is gradually decreasing.（4）Using the numerical simulation software FLAC^3D,and the numerical simulation research of the passive triaxial creep test was carried out.Firstly,the passive triaxial creep numerical test curve and the indoor passive triaxial creep test curve of the fractured rock sample are compared.Secondly,by changing the stiffness design parameters of the restraining cylinder,the triaxial compression creep simulation under different lateral restraint conditions is carried out.Influence law of passive triaxial creep deformation characteristics of specimens.The research results show that with the increase of the diameter-to-thickness ratio and the elastic modulus of the confining pressure cylinder,the circumferential restraint force distributed along the cylinder wall gradually decreases,while the axial deformation of the specimen increases gradually.In addition,the axial deformation and hoop restraint force of the specimen are not sensitive to the change of Poisson’s ratio of the confining pressure cylinder.