Experimental And Discrete Element Analysis Of F-T Damage And Failure Modes In Filled Jointed Rock Mass

Globally,permafrost regions and seasonal permafrost regions are very wide.The cold regions in China are mainly distributed in the northeast,northwest,and Qinghai-Tibet Plateau.Especially in Northwest China,the temperature difference between morning and evening is too large,which leads to the phenomenon of the F-T cycles of water in rock and soil mass.The repeated frost heaving force causes the deformation of rock mass structure,and this kind of rock mass deformation can not be completely restored.The repeated cycle further aggravates the expansion,shrinkage,damage,and cracks in the rock.However,geotechnical engineering in cold regions often faces not only the problem of the F-T cycle but also different types of fillers in rock joints will also have a great impact on the stability of rock mass.Because the fillers are generally in the rock mass,and the physical and mechanical properties of fillers in joints and engineering surrounding rock are often very different,the former is more vulnerable to the effect of the F-T cycle,so the F-T cycle causes the water in the fillers in rock mass to condense into ice at low-temperature,The frost heaving force produced in this process aggravates the damage of jointed rock mass,and finally threatens the stability and safety of projects in cold regions.Therefore,through a series of indoor tests,this paper studies the F-T damage characteristics and failure characteristics of jointed rock mass from the macro micro-scale and makes an in-depth study on the crack propagation mechanism during uniaxial loading by comparing the results of discrete element numerical simulation.The main work of this paper is as follows:(1)A series of indoor experimental studies on joint specimens with different fillers are carried out,and the F-T damage laws of joint specimens with different fillers are obtained:the F-T damage of non filled joint specimens increases with the increase of F-T cycle times,but the damage to the specimens in the early stage(the first 40 times)of F-T cycle is greater than that in the later stage(the last 40 times).By comparing the surface roughness,particle spalling degree,and peak strength of filled joints and intact rock samples after multiple F-T cycles,it is concluded that the integrity of rock mass can reduce the freeze-thaw damage of rock mass after multiple F-T cycles.(2)The effects of different fillers on the failure mode of specimens are summarized.The failure mode of filled joint specimens is generally "diagonal connection type";Intact rock mass specimens generally show vertical splitting failure.The peak strength of different specimens under uniaxial loading is compared,and the influence of different fillers on the peak strength of specimens is analyzed.It is pointed out that the higher the strength of fillers,the greater the peak stress of specimens.(3)The physical and mechanical parameters such as mass loss rate,porosity increase rate,main pore throat distribution growth rate,peak stress loss rate,peak strain loss rate,and damage variable of each specimen are fitted.The results show that the F-T cycle has different effects on the physical and mechanical indexes of joint specimens with different fillers,that is,it has a great influence on the peak stress of clay and stone powder-filled specimens and on the peak strain of cement and gypsum specimens,The conclusion that the damage to the specimen in the early stage of F-T cycle is greater than that in the later stage is verified again.(4)ESEM tests were carried out on the surface,failure surface,and internal filters of different filled joint specimens.It is observed that after many F-T cycles,the white sandstone and filler appear to have microdamage such as mineral fracture,particle cementation,loosening,and the development of micro-cracks between particles.These phenomena coincide with the macro damage observed in the laboratory test,such as rough surface,loss of filling flow,and particle spalling.By observing the microparticle characteristics such as "uneven section" and "river scratch" on the failure surface,the failure type of the failure surface is analyzed.(5)Using the fish language in PFC2D and based on the thermal module,the relevant programs of F-T cycle and uniaxial compression test are compiled.Using this program,the "frost heaving crack concentration area" formed by each model after the F-T cycle is observed,and the F-T damage rules and frost heaving crack propagation modes of different models are analyzed.The frost resistance of filled joints and complete rock mass models is not different,but the frost heaving crack propagation modes are very different,The filling model will appear obvious "crack prone area" and slender "frost heave zone",and the complete rock mass model will not appear.(6)The indoor test and numerical simulation results of different specimens under uniaxial loading are compared.The results show that the failure modes of filled joints are different from those of the intact rock mass model.The filled joint models show the failure of the "lower left upper right continuous line";The complete rock mass model is basically in "X" failure except that the unfrozen and thawed model is in "lower left upper right continuous line".Each model is divided into several parts by the main crack during failure,which is consistent with the phenomenon of mass falling of each specimen during failure in the laboratory test.(7)The types and number of microcracks in each numerical model during loading are counted,and the causes of failure of each model under uniaxial loading are summarized from the perspective of microcracks.Compared with the indoor test,the crack evolution mechanism of different models is deeply analyzed from the perspective of the stress field.The excessive stress difference between the "stress concentration area" produced in each stage of uniaxial loading and the surrounding area is the reason for the crack initiation,propagation,and penetration.