Theoretical And Experimental Research On Mechanical And Seepage Characteristics Of Layered Composite Rock In True Triaxial Stress Conditions

The layered rock is a common geological material in actual engineering practice,due to its complex structure which is composed by natural rock components with different thickness,mechanical and permeability properties in a certain order,its apparent characteristics are obviously different from the single ones.Though these differences have been studied by some scholars in rencent years,their theory and experiment on mechanical and permeability properties of layerd rock were mainly derived and conducted in uniaxial and conventional triaxial conditions which neglect the influence of intermediate principal stress.While in underground engineering,e.g.tunnel excavation and deep mining,the surrounding layered rock mass are always in true triaxial stress conditons(1 2 3? >? >?)and the magnitude of principal stress are changing with the excavation process,it is necessary to carry out more related studies on layered composite rock in true triaxial stress conditons.In view of this,we use the theoretical method combined with the experimental investigations,and several series of true triaxial tests utilizing the self-made “multi-functional true triaxial fluid-solid coupling experiment system” in Chongqing University have been performed.The laboratory tests on mechanical and permeability properties of both pure rock and layerd composite rock mainly include ?3.and b-constant tests,p-and b-constant tests,different ?3 and gas pressure tests,different loading and unloading rate tests,etc.Besides,the anisotropy failure criterion and damage statistical constitutive model for layered composite rock have been obtained by a homogenization procedure on the basis of regarding the rock as an equivalent homogeneous one and the MLC failure criterion.Based on the theory of layed composite rock and the established failure criterion,a series of true triaxial tests under different static stress combination,alternating frequency and amplitude,strain-rate and gas pressure conditions of layered composite coal-rock were conducted study the occurrence condition and behaviour characteristics of compound dynamic disaster of rock burst and coal-gas outburst under static and dynamic loading conditions.The research results are as follows:(1)The intermediate principal stress has obvious wtrengthening effect on the overall strength of rock,compared with the MLC true triaxial failure criterion,M-C failure criterion appears to predict a lower peak strength with the increase of b value.When b value varies from 0 to 0.25,the magnitude of the increase of the peak strength is greater than that of other intervals.Both the strength and deformation are more sensitive to the intermediate principal stress in this interval.Besides,when the intermediate principal stress remains constant,the stress-strain curves are almost coincident with each other with the increase of the minimum principal stress,peak strength of layered composite rock is almost linearly related to the minimum principal stress.Moreover,the characteristics of strength and deformation of layered rock have obvious obliquity effect and rate sensitivity,that is,the larger the loading and unloading rate are,the larger the elastic energy would be released at failure.(2)In ?3.and b-constant tests,volumetric dilatancy onset gradually rise with the increase of b in the interval between 0 and 0.5,intermediate principal stress delays the appearance of nonlinear volumetric strain,when b value exceeds 0.5,the dilatancy onset experienced a downward trend,but this descent degree would not be too evident.On the whole,there is a significant positive correlation between the dilatancy onset and the peak strength of layered composite rock.While in p-and b-constant tests,dilatancy onset appears a monotone decreasing over the constantly increase of intermediate principal stress coefficient b.(3)Utilizing the homogenization theory,the criterion of true triaxial anisotropic failure criterion of layered composite rock based on MLC failure criterion has been established,and the failure locus in octahedral plane has been described.Later,several series of true triaxial test have been conducted to validate the anisotropic failure criterion.Besides,the failure characteristics of the layered composite rock mainly depend on the orientation of the principal stress,the strength parameters of the rock layers and the geometric parameters,several possible cases and mathematical conditions are given in the fourth chapter.(4)Based on the MLC failure criterion and Weibull distribution function,several different anisotropic damage statistical constitutive models for layered composite rock have been set up.By introducing a new correction factor representing the volumetric dilatancy characteristics,the results show that the damage model is in good agreement with the true triaxial tests data,and the deformation and strength characteristics of the layered rock could be well described.(5)Based on the assumption of equivalent continuous medium,the horizontal layered composite coal rock is regarded as transversely isotropic medium,and taking into account the coupling of stress and seepage,a new anisotropic permeability model and the coupling equation of stress,damage and seepage have been established on the basis of the microscopic damage mechanics theory.Then some true triaxial seepage tests for layered coal-rock and roof and floor rock were carried out to validate the created models.The permeability model can be well simulated by using the permeability model expressed by the volumetric strain and the bulk stress.(6)Under true triaxial stress condions,the intermediate principal stress,gas pressure,alternating frequency and amplitude and strain-rate have great influence on the behaviour of compound dynamic disaster.the spalling failure is the main damage mode near the free face of layered composite coal-rock under true triaxial test conditions.Besides,the compound dynamic disaster can be divided into three types by load types: high static-load type,disturbance type and impact type,however,high pressure gas always plays a role of motive force and boosting power in either type of compound dynamic disaster,and both of its potential energy,seepage force and desorption expansion energy make positive contributions to the preparation and occurrence of compound dynamic disasters.