Research On Physical And Mechanical Properties And Damage Characteristics Of Freeze-thaw Rock In Alpine Mountain Regions

In recent years, with the increasing number of construction projects in our alpine mountain regions, a series of freezing and thawing, freezing issues have been highlighted and taken seriously. However, currently in our country,the related theoretical research and practical experience about freeze-thaw rock especially freeze-thaw hard rock are still relatively lack. Therefore,the research on physical and mechanical properties and damage characteristics of freeze-thaw rock in alpine mountain regions has very important theoretical value and practical significance.This paper relies on tutor's program, takes the dacite of a proposed hydropower station dam area in Tibet as the object of study, focus on the different freeze-thaw cycles, through the freeze-thaw cycle test, uniaxial compression test, triaxial compression test and SEM test etc at indoor water-saturated rock state, takes the method combing experimental research, experimental mechanics, freezing and thawing mechanics and damage mechanics etc, and systematically studies the physical and mechanical properties and damage characteristics of freeze-thaw dacite in alpine mountain regions. The main conclusions obtained by studying the following:(1)The physical properties were studied. With the increase of freeze-thaw cycles, quality changes of saturated water dacites showed first increased and then decreased, and eventually stabilized, but in general a slight increase compared to the initial saturated water quality. With the increase of freeze-thaw cycles, the longitudinal wave velocity of saturated dacite was on the decline in the overall, but the rate of changes showed the trend of first accelerate and then slowing down.Under the influence of Freeze-thaw cycles, dacite while produced the crack propagation degradation mode and the particle precipitation degradation mode,but the former showed a stronger degree, and both effects were limited damage degradation modes, eventually stabilized.(2)The mechanical properties were studied. Under different freeze-thaw cycles, according to the peak strength, the stress-strain curve of the dacite rock sample was divided into two stages. in comparison, the concave segment of Initial curve before the peak was more obvious, but the secondary yield of post-peak curve under triaxial compression was more obvious. The dacite rock samples mostly exhibited shear and splitting failure, or composite failure mode. Confining pressure had a greater impact on failure modes and failure positions of rock samples, and freeze-thaw cycles had an impact on the surface of rock samples, smoothness of damage surface and the number of near cracks. On the one hand, with the increase of freeze-thaw cycles, the peak intensity of dacite rock sample gradually decreased, and the modulus of elasticity Increased after the first decreased, On the other hand, with the increase of confining pressure, the peak intensity of rock samples gradually increased, while the variation of the modulus of elasticity was complex,whose correlation was worse than peak intensity. On the whole, with increase of freeze-thaw cycles, the frost resistance of the dacite rock samples gradually weakened, and with the increase of confining pressure gradually increased, but the freezing and thawing damage to rock samples caused by the freeze-thaw cycles was limited. especially after the first 60 times, the frost resistance of dacite rock samples still remained very high.(3)The damage characteristics were studied. Whether under the conditions of uniaxial compression or triaxial compression, the damage constitutive models of freeze-thaw and loading used by this paper had a certain rationality for the description of the damage evolution behavior of rock samples, but the model failed to consider the influence of rock samples inside fissure tiny crack on the closure stress- strain curves, therefore there were some differences between the theoretical curve and the experimental curve. At the beginning of strain,damage evolution curve substantially remained horizontal, and stress-strain curve was linear elastic stage. but with the increase of strain, the total damage of dacite rock samples caused by freeze-thaw and loading increased, and finally the damage curve of the oretical model approached 1.0. However, in the actual test, the rock samples after the destruction still had a certain carrying capacity, and usually rock samples had been destroyed before they reached the fracture strain in theory. With the increase of confining pressure, reeze-thaw damage and Loaded damage both decrease, and the curve slope of damage evolution also weakened, which showed that under the influence of confining pressure,the cumulative effect that was injury to strain. on the macro,these reflected brittle of rock samples weakened, ductility and shaping enhanced, and its ability to resist deformation and failure increased. However, the influence of freeze-thaw cycles got brittle of rock samples to enhance,and its ability to resist deformation and failure weakened. Dacite belongs to hard rock, which is high strength, and whose internal porosity compact is good, but the influence of freeze-thaw damage was limited. Rock samples mainly damaged because of loading, so they were worse sensitive to the freeze-thaw cycles than confining pressure. From this we can know, the rock mass in dacite areas which was more shallow surface and high weathering degree was more vulnerable to freeze-thaw damage and failure.In addition, these experimental studies have shown: to fresh dacite, the freeze-thaw damage deterioration caused by freeze-thaw factor of alpine mountain regions is limited. In the research area, a lot offragmentation loose rock mass formed mainly by valley cut unloading stress release adjustment, and by the result of the combined effects, such as the lithology, rock mass structure, freeze-thaw, weathering in plateau and crustal stress etc. Therefore, antifreeze design in dacite Strip should also consider the effect of local rock mass' s weathering degree and suffered unloading, ground stress and other factors.