| Most of Chinese large open-pit coal mines are located in the northern alpine frozen areas,and the long-term freeze-thaw cycle environment poses a potential danger to the stability of the end slopes of open-pit coal mines that cannot be ignored.A comprehensive understanding of the damage-degradation mechanism and the creep instability law of the coal and rock under freeze-thaw cycles is the foundation for studying the instability mechanism and control methods of open-pit mine slopes in seasonal frozen and alpine region.Although many efforts have been devoted to the investigation of the damage-degradation mechanism of rock subjected to freeze-thaw cycle,and many engineering application achievements have been made,there is still a lack of systematic theoretical support for direct research on coal-bearing rock masses in cold regions.Therefore,the major goal of this dissertation is to reveal the damagedegradation mechanism and the creep law of the coal-bearing sandstone exposed to the freeze-thaw cycle,which will have important academic significance and engineering application value for the safe and efficient mining of open-pit coal mines in cold regions.To obtain the damage-degradation mechanism,the creep law,and the macro-micro failure mechanism of the coal-bearing sandstone,many approaches were adopted herein,including laboratory tests,theoretical analysis,and numerical simulation.Following are some of the innovative findings:(1)The evolutions of the longitudinal wave velocity v_p,volume V,and porosity increment Δ_Φ with freezing temperature T and times of the freeze-thaw cycles N were revealed through experimental tests.It was found that the decrease in the longitudinal wave velocity v_p and increase in volume V of the sandstone sample are the macroscopic damage degradation reactions resulting from an increase in wave propagation impedance and porosity.Given the Lemaitre’s strain equivalence principle,a calculation formula for the freeze-thaw damage factor D_t which was determined on the longitudinal wave velocity and volume variables of the sample was proposed.In addition,an evolution equation for the freeze-thaw damage factor D_t with parameters of freezing temperature T and freeze-thaw cycle number N was established,effectively characterizing the freeze-thaw damage degradation effect of coal-bearing sandstone.(2)A series of uniaxial and triaxial compressive tests on the coal-bearing sandstone were conducted.The changes in elastic modulus E,peak strength σ_c,peak strain ε_c,macroscopic failure mode,and acoustic emission information characteristics of coal bearing sandstone with freezing temperature T and freeze-thaw cycle number N were analyzed and obtained.It was found that not like the peak strain ε_c,the elastic modulus E and strength σ_c of samples reduce with the decreasing of freezing temperature T and times of the freeze-thaw cycle N.Besides,compared with elastic modulus E,the strength σ_c and peak strain ε_c are more sensitive to the damage caused by the freeze-thaw cycle.And times of the freeze-thaw cycles N has a stronger damage and deterioration effect on coal-bearing sandstone than the freezing temperature T;In addition,the confining stress prohibits the strength from degradation,and this is especially obvious when the samples are subjected to fewer times of freeze-thaw cycles N.As the freezing temperature T drops and the freeze-thaw cycle Nimproves,the failure of the specimen gradually switches from tension failure to combined failure mode(tension and shear failure),and the degree of damage to the sample gradually intensifies.Moreover,the cumulative ringing count,cumulative energy,crack initiation threshold,damage threshold,etc.of acoustic emission show a decreasing trend with the increase of times of freeze-thaw cycles N or the decrease of freezing temperature T.This reflects the deterioration effect of freeze-thaw cycles on the deformation resistance and strength of coal-bearing sandstone.(3)Considering the effects of times of freeze-thaw cycles N and freezing temperature T,a local damage constitutive model capable of describing the post-peak residual strength of the coal-bearing sandstone was proposed based on the data obtained from uniaxial and triaxial compressive tests of coal-bearing sandstone under freezethaw cycles.With the help of the modified Weibull distribution function of micro element strength,a damage evolution equation was constructed which can describe the drop characteristics after the peak of the stress-strain curve of the sample,and a method for determining model parameters was provided.The model and experimental results have good confirmatory properties,effectively revealing the inherent nature of freezethaw cycles on deformation and strength degradation of coal-bearing sandstone.(4)The creep law of coal-bearing sandstone under freeze-thaw cycles was studied,and changes in the creep time-history curve,creep rate (?)_r,and long-term strength characteristics of coal-bearing sandstone with times of freeze-thaw cycles N and freezing temperatures T were obtained.It was found that the instantaneous strain ε_e,creep strain ε_e,,and creep rate (?)_r of the sample all increase to varying degrees as the freezing temperature T decreases,the times of freeze-thaw cycles N increases and the stress level increases.Besides,there exists a stress threshold σ_s~2,and when the stress level is higher than this value,the creep strain ε_r,and creep rate (?)_r increase rapidly until the failure of the sample.As expected,the long-term strength of the sample reduces with the increasing of times of freeze-thaw cycles N and the decreasing of freezing temperatures T.However,the variation in the long-term strength is negligible compared to that in the compressive strength,and this provides an important basis for predicting long-term creep strength in engineering.(5)By using SEM scanning and image analysis techniques,quantitative analysis was conducted on the microscopic morphology characteristics(roughness Sa and crack width Da)of the creep fracture surface of coal-bearing sandstone subjected to freeze-thaw cycles.It was found that as the times of freeze-thaw cycles Nincreases and the freezing temperature T decreases,the roughness Sa and crack width Da of the sample fracture increase in a quadratic polynomial function,but their growth rate gradually slows down;Besides,compared with freezing temperature T,the roughness Sa and crack width Da is more sensitive to the variation in the times of freeze-thaw cycles N.These findings reveal the microscopic mechanism of creep failure of the coalbearing sandstone exposed to freeze-thaw cycles effectively.(6)The freeze-thaw creep damage factor Dd,basing on creep damage dissipation energy,was introduced,and its damage evolution equation was established.By analyzing the creep dissipation energy of coal-bearing sandstone under freeze-thaw cycles,the variation law of the damage factor Dd with the number of freeze-thaw cycles N and freezing temperature T was obtained;According to the four stage characteristics of creep behavior,a creep model and damage constitutive equation considering the number of freeze-thaw cycles N and the effect of freezing temperatures T were established;then with the help of the Levenberg-Marquardt method,the identification and experimental verification of the creep model parameters were achieved,the proposed model depicts good agreement with the results obtained from creep tests.The findings of this dissertation are of great importance to the stability analysis of the slope of open-pit coal mines in northern alpine frozen areas.This dissertation consists of 122 Figures,50 Tables,and 219 References. |
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