| China’s cold area covers a vast area,about 70% of the frozen area of the country.However,the vast cold region is rich in a large number of mineral resources,resource exploitation and engineering construction in the cold region face frequent freezing and thawing phenomenon.Compared with frozen soil,frozen rock contains a lot of cracks.These cracks in the water under the effect of low temperature will freeze into ice,and volume expansion,the frost heaving force produced by water freezing will lead to the expansion of crack,under the effect of repeated freezing and thawing cycles,the rock crack expanding and eventually affect the occurrence of rock burst,which contain tunnel of fractured rock mass,slope stability,dangerous rock and steep slope rock mass engineering.Therefore,studying the change of mechanical properties of fractured rock mass,especially multi-fractured rock mass,under the action of freezingthawing cycle is helpful to evaluate the possibility of engineering failure of fractured rock mass under the action of freezing-thawing,which is beneficial to avoid disaster accidents and assist rock mass engineering construction.In this paper,the compressive mechanical properties and constitutive model of rock with multiple fractures under freezing-thawing cycles are studied.The main research work and results are as follows:(1)The ratio of cement: fine sand: water volume ratio of 2:1:1 was selected to produce sandstone-like samples.The fracture arrangement scheme is selected as the ordered multi-fracture group,the Angle of the fracture group includes 0°,45° and 90°,and the number of fractures includes 2,4 and 6.A total of 9 groups of samples are selected.Meanwhile,a control group was set as a complete sample.Five samples were prepared for each group,and five different freezing-thawing cycles were tested.Freeze-thaw test,uniaxial compression test and acoustic emission test were carried out for the samples after curing.(2)The apparent changes of multi-crack samples before and after freezing and thawing were recorded,and the multi-crack samples before and after freezing and thawing were weighed under dry and saturated conditions.The results show that two kinds of cracks occur in the freezingthawing cycle: the tensile crack at the crack tip along the Angle direction of the crack and the coplanar connected crack between the cracks along the rock bridge direction.The quality change results show that,on the whole,the quality damage rate of samples is positively correlated with the number of freezing-thawing cycles,and the average growth rate of the quality damage rate of samples with different crack angles and number of cracks is different with the number of freezing-thawing cycles.The results of water saturation rate before and after freezing and thawing showed that the water saturation rate before freezing and thawing was mainly related to the number of cracks,and the more cracks,the greater the water saturation rate of the sample.After freeze-thaw,the water saturation rate increases linearly with the number of freeze-thaw cycles.(3)Uniaxial compression tests were carried out on the freezingthawing samples with multiple cracks,and the effects of fracture Angle,fracture density and number of freezing-thawing cycles on the uniaxial compressive strength and elastic modulus of the samples with multiple cracks were analyzed.The results of uniaxial compressive strength of samples show that the uniaxial compressive strength is negatively correlated with freezing-thawing cycle under the condition of the same crack density.Under the same number of freezing-thawing cycles,the uniaxial compressive strength of cracked specimens decreases compared with that of intact specimens.The results show that under the same crack density,with the number of freeze-thaw cycles increasing from 0 to 10,the elastic modulus of all samples decreases continuously and the decreasing rate gradually slows down.Under the same number of freezing-thawing cycles,the elastic modulus of the multi-crack sample is lower than that of the intact sample.The uniaxial compressive strength and elastic modulus of specimens with different crack angles are basically consistent.(4)Acoustic emission tests were carried out simultaneously in the process of uniaxial compression for samples with multiple cracks.AE test results show that the AE count of the sample can be divided into three stages: the increase period,the quiet period and the failure period.For the complete sample,with the increase of the freeze-thaw cycle,the AE number of the curve peak is also increasing,and the AE activity of the sample is more violent.For samples with multiple fractures,AE phenomenon becomes more obvious with the increase of freezing-thawing cycles.When the density of cracks increases,peak AE count increases faster with the number of freezing-thawing cycles,and the effect of freezing-thawing cycles on AE activities of specimens becomes more significant.(5)Through theoretical deduction,a constitutive model of complete rock compression damage under freezing-thawing cycles was established based on lognormal distribution,and the theoretical results were compared with the experimental results.The results show that the theoretical curve has a similar trend with the experimental curve,indicating that the statistical damage constitutive model is reasonable.At the same time,the sensitivity analysis of the two lognormal distribution statistical parameters shows that: there is a positive correlation with rock strength;Positively correlated with rock ductility.(6)A constitutive model of freezing-thawing compression damage for multi-crack samples was established by introducing the deterioration damage variable of the samples caused by cracks.The degradation of fracture damage variables was put on the lognormal distribution statistical parameters,and the freezing-thawing compression damage constitutive model of multi-crack specimens was further deduced by combining with the complete rock freezing-thawing compression constitutive model,and the model was verified by multi-crack uniaxial compression test results. |
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