Study On The Damage-fracture Mechanics Characteristics Of Rock Mass Under Freeze-thaw Cycle Condition

Toppling or rockfall of rock slopes is one of the most common geological disasters in nature.In cold regions,due to the large fluctuations in temperature between day and night,the mechanical properties of rocks are significantly reduced due to freeze-thaw weathering.Toppling failure is common in the rock masses of the reverse layered structural slope,if the thin-layered rock mass is considered as a cantilever beam,the rock mass may become unstable when external stress exceeds its tensile or compressive strength at the root of the plate beam,which may cause root cracking or pullout of the rock mass or both.Therefore,in order to comprehensively study the mechanical behavior of rock masses at the root of the plate beam under freeze-thaw conditions,this study takes the perilous rock mass in the slope of the G318 national highway in Luding,western Sichuan Tibetan area,as the research background,and uses laboratory experiments and theoretical models to study the bending-tensile evolution mechanism of the rock mass under freeze-thaw cycles,to achieve an understanding of the mechanical characteristics of rockfall and collapse in cold regions.The main research contents and conclusions of this paper are as follows:(1)By pre-fabricating cracks of different lengths in the rock samples and classifying them,different groups of specimens were subjected to freeze-thaw cycling tests and uniaxial compression tests to investigate the degree of deterioration of various mechanical properties of rocks under different numbers of freeze-thaw cycles.The depth of the pre-fabricated cracks in the experiment was analyzed for its influence on the mechanical properties of rocks,and the failure modes of the rock samples after different numbers of freeze-thaw cycles were studied.Based on the splitting creep test,an improved inverse proportion function was proposed to fit the tensile elastic modulus calculated from various parameters after the experiment using the Ye’s improved splitting test method.This function can accurately describe the process of changes in tensile elastic modulus over time.At the same time,the time-dependence of tensile strength of rock samples was studied by the splitting creep test,and the tensile cracking process of the rear edge of the rock mass during tilting was analyzed.Under long-term loading,the tensile strength of rock specimens with more freeze-thaw cycles significantly decreased.(2)The damage state of the rock mass was abstracted into three damage states:baseline,freeze-thaw,and loading.On the basis of the development and extension of Lemaitre strain equivalent criterion,the evolution equation and constitutive modification model of total damage variables of rock mass are derived.The stress-strain curves obtained from freeze-thaw cycles and uniaxial compression tests were compared with the proposed modified theoretical model.The results indicate that the revised constitutive model has high consistency.The parameters of the modified model for rock samples with different prefabricated fracture depths were determined through parameter identification using 1stOpt software.The proposed theoretical model for predicting and analyzing freeze-thaw damage in cold regions provides a theoretical basis for evaluating rock mass stability.(3)This study utilized Geostudio software to establish a time-dependent temperature field.Based on the temperature field,a method was proposed to determine whether the freezing depth could affect the main control structure surface freeze-thaw force generation.This paper introduces the maximum circumferential stress criterion of fracture mechanics,and the study focused on the back-tilted rocky slope with the perilous rock mass at the slope crest.The high-risk rock mass in the slope was divided into n potential unstable rectangular rock masses,and a fracture mechanics-based slope stability calculation method considering the joint action of freeze-thaw force,fissure water pressure,and gravity was established.For the corresponding examples,the safety factor of each divided rock mass was calculated using the proposed method,and it was found that the freeze-thaw force had a certain influence on the stability of the rock mass.The feasibility of the proposed method was verified by combining it with the actual engineering situation.Finally,based on the relevant perilous rock stability evaluation standards,corresponding treatment measures were proposed for different rock mass stability conditions.Through the above research content,research has been conducted on the entire process from the self-damage initiation of the perilous rock mass on the slope under the influence of freeze-thaw weathering to the instability of overturning and collapse,achieving an understanding of the freeze-thaw cycle and long-term strength under the action of load.Established a modified constitutive model of rock mass under freezing and thawing loads,described the model in the damage evolution process of project specific geological background conditions.A theoretical model was established that combines temperature field analysis and is applicable to the stability analysis of perilous rock masses on cold region slopes.Research results to reveal the cold region perilous rock mass damage mechanism provides an important theoretical basis and application value.Perilous rock mass stability evaluation for the cold regions to provide a more reasonable and comprehensive exploration method.