Study On Mechanical Characteristics And Crack Propagation Mechanism Of Saturated Fractured Rock Mass Under Loading And Unloading

With the advance of the"one belt and one way"strategy,China's high-speed and high-speed rail construction has developed rapidly,and the scale of the road network has been expanding.A large number of construction projects have been extended from plain area to western mountain area.A large number of tunnels,bridges and so on need to be built.Engineering geological conditions have become more complex.Coupling effect of rock mass fissure,rock mass in-situ stress,groundwater and loading and unloading effect of engineering construction caused all kinds of engineering problems.Further research on fractured rock mass can better avoid and deal with these engineering problems.Therefore,it is very important to study mechanical characteristics and crack propagation mechanism of fractured rock mass under loading and unloading.Because there are many factors affecting the mechanical characteristics and crack propagation mechanism of fractured rock mass under loading and unloading,this paper simplifies it to a single crack model with fixed size.In this paper,the loading and unloading mechanical properties and crack propagation mechanism of rock-like specimens with single crack under different fracture angles,confining pressures and initial axial pressures are studied by means of laboratory tests and numerical simulation.Specific work and main results are as follow:?1?According to the selection principle of similar test materials and a large number of tests,the proportion of cement mortar rock materials is determined as follows:425#Portland cement:quartz sand:water=1:0.8:0.35.A rock-like specimen with a single crack is made to study the mechanical properties of saturated fractured rock mass.?2?Conventional triaxial tests were carried out.The results show that:Due to the expansion of prefabricated cracks,there is a fluctuating section in the stress-strain curve of the specimen.With the increase of confining pressure,the fluctuation section increases.With the increase of fracture angle and confining pressure,the peak strength of specimens increases.Deformation modulus of elastic stage increases with the increase of prefabricated fracture angle,and is little affected by confining pressure.When the angle of prefabricated crack is 30 degree,45 degree and 60 degree,the failure forms are similar.Cracks occur at both ends of prefabricated crack and break through.When the angle is 0degree and 90 degree,cracks occur and break through at one end of prefabricated crack.?3?Triaxial unloading test was carried out.The results show that:After the peak strength,the stress-strain curve of the specimens falls off like a cliff.Before the peak strength,there is a platform section of?₃ and?_V.Peak strength increases linearly with the increase of initial confining pressure and initial axial compression level,and exponentially with the increase of prefabricated fracture angle.Both axial and circumferential strain increments decrease with the increase of initial axial and confining pressures,while circumferential strain is more sensitive to initial axial and confining pressures.The axial strain increment increases linearly with the increase of prefabricated fracture angle,while the circumferential strain increment increases with the increase of prefabricated fracture angle,showing a V-shaped change.There are more tension cracks when specimens are damaged.With the increase of initial confining pressure and initial axial pressure,the tension cracks decrease obviously,and the failure modes are both tension-shear failure.?4?The constitutive model of elastic-brittle damage and the corresponding numerical modeling method are proposed and applied to the commercial software Flac3D.The triaxial loading and unloading test is simulated and expanded,and the feasibility of this method is verified by comparing with the test results.The results show that:With the increase of prefabricated crack angle,the development of far-field shear cracks on the upper and lower ends of specimens decreases first and then increases,and the initiation strength changes in U-shape.With the decrease of confining pressure and initial axial pressure,the crack propagation distance of airfoil at the tip of prefabricated crack increases.Under unloading conditions,airfoil crack propagation is dominant.Before crack penetration,crack development is low and crack propagation is more concentrated.?5?Based on the numerical simulation results,the stress state in the process of crack propagation is analyzed,and it is found that the relative position of the stress concentration area is basically fixed.There is a stress weakening zone??₃?along the direction of maximum principal stress at the crack tip,and a stress concentration zone(?₁ and ?_max)along the direction of about 45 degrees at the crack tip.With the crack growth,the stress concentration area becomes more and more obvious,while the stress weakening area gradually weakens.Compared with the test results,the crack growth phenomenon in the test can be well explained.