Study On Creep Mechanical Behavior And Mesoscopic Damage Characteristics Of Granite Fractured By Loading And Unloading

As global energy consumption continues to increase,traditional shallow energy resources have failed to meet demand.Therefore,the development and utilization of deep energy resources are of strategic importance.However,the surrounding rock of deep chambers may experience fracturing under the influence of geological processes and engineering disturbances,resulting in significant differences in mechanical properties compared to shallow rock masses.Specifically,the issue of creep deformation is complex and adversely affects the long-term stability of surrounding rocks.In this paper,the influence of induced fractures on the mechanical properties of granite under two stress paths of loading and unloading was considered.The SAM-2000 rock mechanics testing system was used to simulate loading and unloading stress paths in engineering through indoor mechanical tests,obtaining granite specimens that reflect the surrounding rock stress process with fractures.The conventional and creep mechanical behaviors of intact granite blocks,specimens containing loading-induced fractures,and specimens containing unloading-induced fractures were studied,revealing the significant impact of induced fractures on the creep deformation and long-term strength of granite under loading and unloading stress paths.Then combined with CT scanning technology,through the quantitative analysis of the pore throat parameters and the comparative analysis of the failure modes of the test specimens with loading and unloading fractures,the differences in the mesoscopic characteristics of the two granites with fracture surfaces are revealed.Finally,based on classical combination model,considering the initial damage of fractured rocks,combined with their microscopic damage characteristics and fractional calculus theory,a three-dimensional nonlinear damage creep model of fractured granite that can describe the entire process of creep was established by introducing an extended variable-order fractional S-B element.The main conclusions of this study are as follows:（1）The impact of different morphologies of loading and unloading fractures on the mechanical properties of granite was revealed through conventional triaxial compression tests on intact granite specimens,specimens containing loading and unloading fracture surfaces.The fractures significantly weakened the deformation and strength of granite.The secant modulus E₅₀ and compressive strength of granite with fractures were both lower than those of intact rock.The distribution of the two types of fractures differed significantly,with the unloading fractures being complex,accompanied by multiple joints and microcracks,and the loading fractures being a single shear failure plane.The cohesive force of granite with unloading fractures was smaller than that of granite with loading fractures,nevertheless the internal friction angle was larger,indicating that unloading fractures had a greater weakening effect on the cohesive force of granite,while loading fractures had a greater effect on the internal friction angle of granite.（2）The creep characteristics of granite and the differential effects of different fracture surfaces on its creep behavior were revealed through the triaxial creep tests on intact granite specimens,specimens containing loading and unloading fracture surfaces.All three types of specimens exhibited typical three-stage creep behavior,but intact rock specimens exhibited"negative creep"under low-stress conditions,which was not observed in specimens containing fracture surfaces.The cause of negative creep is related not only to the stress level of the rock,but also to its structure（such as structural surface features）.Additionally,the lateral creep deformation sensitivity of intact rock specimens and specimens containing fracture surfaces was higher than that of axial creep deformation,and lateral deformation exhibited more significant creep characteristics,with lateral creep entering the accelerated creep stage earlier than axial creep.Fracture surfaces have an adverse effect on the deformation of granite,with unloading fracture surfaces having a more significant impact on creep deformation.Under the same deviatoric stress conditions,the creep deformation of intact rock specimens was the smallest,that of specimens containing loading fracture surfaces was larger,and that of specimens containing unloading fracture surfaces was the largest.Specimens containing unloading fracture surfaces exhibited a more unstable creep trend under low confining pressure,with axial and lateral strains of specimens containing unloading fracture surfaces at 5MPa confining pressure being greater than those of specimens containing loading fracture surfaces.Fracture surfaces have a significant impact on the long-term strength of granite,with the long-term strength of specimens containing fracture surfaces significantly lower than that of intact rock specimens.Time has a more significant weakening effect on the strength of specimens containing fracture surfaces.Additionally,the long-term strength of specimens containing loading fracture surfaces is lower than that of specimens containing unloading fracture surfaces.（3）The microstructure differences between granite containing loading and unloading fracture surfaces were revealed through CT scanning and image reconstruction analysis.At a confining pressure of 5MPa,specimens containing fracture surfaces had a more complex and violent failure process,with more interconnected cracks and a more complex sphere-stick model with more pores.Specimens had mainly small pores with a concentrated distribution,with pore radii mainly ranging from 0 to 225μm and mainly concentrated in the range of 0 to 75μm,and pore surface area mainly distributed in the range of 0 to 2.5×10⁵μm².The fracture surface had a significant differential impact on the morphological characteristics and spatial distribution of granite cracks,with the loading fracture surface showing a"wing-shaped"crack distribution and the unloading fracture surface showing a"Y-shaped"crack distribution.In terms of failure mode,the loading fracture surface had a greater influence on the internal friction angle of granite,while the unloading fracture surface had a greater weakening effect on the cohesive force.The specimen containing the loading fracture surface mainly underwent shear failure,while the specimen containing the unloading fracture surface mainly underwent conjugate shear failure.（4）Based on the classical composite creep model,a three-dimensional nonlinear creep damage model was established to describe the granite containing fracture characteristics and nonlinear creep behavior under three-dimensional stress conditions.In order to better describe the complex changes of the rock during creep,the fractional-order theory was introduced into the classical composite creep model framework to obtain a variable-order fractional S-B element.Considering the damage characteristics of granite containing fractures,the damage variable with initial damage was derived.And based on the analysis of the axial and lateral creep behavior of granite under three-dimensional stress conditions in the previous chapter,a nonlinear creep damage model was established from a one-dimensional to a three-dimensional stress state.The experimental fitting results show that the model is reasonable and effective,and can be used to predict the creep deformation caused by excavation unloading and loading,which can provide some reference value for related issues.