Study On The Physical And Mechanical Characteristics And Deterioration Mechanism Of Fractured Limestone Under Freeze-thaw Cycle

Taking the cold zone engineering along the Sichuan-Tibet Line as the background and limestone as the research object,this paper uses the method of prefabricated fractures,and sets up 7 groups of different inclination fracture limestone specimens such as 0°,15°,30°,45°,60°,75°,90°,etc.,and takes one group of complete limestone specimens as a control,first through consulting data,pre-experiments and other methods to determine the freeze-thaw cycle test parameters,and then through the physical and mechanical tests under freeze-thaw conditions,combined with theoretical analysis of freeze-thaw damage mechanics,frozen soil,rock mechanics and so on.The changes of physical and mechanical properties and deterioration mechanism of fracture-bearing limestone under freeze-thaw cycle were systematically studied,and the main results were as follows:(1)Through the analysis of the quality,water absorption rate and other physical properties of fractured limestone after freeze-thaw cycle test,it is found that the mass loss rate and water absorption rate of fractured limestone increase with the increase of the number of freeze-thaw cycles,but the increase rate is different,the mass loss rate after 40～60 freeze-thaw cycles is faster than 20～40 times and 60～80 times,and the water absorption rate is the opposite,and thus define the two deterioration stages of fractured limestone under the action of freeze-thaw cycle: pore development stage and material peeling stage.(2)In order to explore the deterioration characteristics of the physical properties of fractured limestone after freeze-thaw cycle,the peeling and new pore formation of the specimen were studied,and the new open porosity after the rock freeze-thaw cycle was used to indicate the rock pore deterioration characteristics,which realized the dynamic description of the newly formed open pores of a single specimen in the process of freeze-thaw cycle,avoiding the deficiency of water absorption rate,which can only be described by comparing the water absorption of different specimens and different freeze-thaw cycles.Through the analysis of the apparent deterioration characteristics of fractured limestone specimens,it is found that the fractured limestone has a stratigraphic spalling phenomenon from outside to in,from shallow to deep at the tip of the fracture and the end face of the specimen,and defines it as the local stratigraphic deterioration mechanism of limestone caused by freeze-thaw cycle.(3)Through the analysis of the strength parameters,deformation parameters,frost resistance coefficient and other mechanical parameters of fractured limestone after freeze-thaw cycle test,it is found that the influence of precast fractures on the strength of limestone specimens is greater than that of freeze-thaw cycle,and the effect of freeze-thaw cycle on the formation of new pores of fractured limestone specimens is greater than that of precast fractures,and the existence of precast fractures will promote the formation of new pores of fractured limestone by freeze-thaw cycle.The strength parameters and deformation parameters of the specimen decreased with the increase of the number of freeze-thaw cycles,and the influence of cracks on the strength parameters of the specimen was greater than that of the deformation parameters,and the influence of freeze-thaw cycle on the deformation parameters of the specimen was greater than that of the strength parameters.After the freeze-thaw cycle test,the frost resistance coefficient R of the fractured limestone specimens was still greater than 0.75,indicating that the specimens used in this test had good frost resistance.(4)In order to explore the deterioration characteristics of the mechanical properties of fractured limestone after freeze-thaw cycle,the deterioration of the mechanical parameters of fractured limestone was analyzed,and it was found that the deterioration degree and stage deterioration of each mechanical parameter increased with the increase of the number of freeze-thaw cycles.In the first and second freeze-thaw cycle stages,the deterioration influence sequence is the deformation modulus> elastic modulus >peak strength,and in the third and fourth freeze-thaw cycle stages,the deterioration effect sequence is elastic modulus> deformation modulus > peak intensity.Then,according to the theory of volume expansion and the principle of water migration,the mechanical damage mechanism of fractured limestone after freeze-thaw cycle was analyzed,and it was found that the frost heave force was like cyclic loading in the rock mass,which finally deteriorated the mechanical parameters of fractured limestone.Finally,the macroscopic damage mechanics method is used to derive the damage degradation equation of limestone under freeze-thaw cycle in this experiment,and the results obtained are verified.