Study On Mechanical Characteristics And Damage Constitutive Model Of Freeze-Thaw Fissure Under True Triaxis

With the economic development of China,more and more projects are established in cold areas,but these projects face various geotechnical engineering problems during the construction process,mainly for the stability of cold area rock masses in the construction process.There are many factors affecting the stability of rock engineering in cold areas,and the microscopic fractures and joints inside the rocks have a significant impact on the stability of rock masses.So,for these factors this thesis conducts true triaxial compression tests on sand-like rock specimens containing different geometric conditions after freeze-thaw cycles,and based on the damage mechanics theory,the damage intrinsic model of fractured rock masses under the coupled action of freeze-thaw and load is modified,as follows:(1)The cement mortar was selected as the similar material in accordance with the similarity theory,and the fracture specimens with different geometric characteristics were produced by prefabricating fractures in the cement mortar to simulate the jointed rock mass.The fracture specimens with different geometric characteristics were tested for 0,20 and 40freeze-thaw cycles,and the damage deterioration patterns of the specimens were summarized with three patterns of particle spalling,crack extension and flake shedding.(2)Through the true triaxial compression test after freeze-thawing on the fracture specimens,the influence law of different geometric characteristics of fractures on the mechanical properties of the specimens under different number of freeze-thawing cycles and different intermediate principal stresses were analyzed,and the macroscopic penetration patterns of the freeze-thawing fractured rock masses were compared and analyzed.The results show that with the increase of the number of freeze-thaw cycles,the freeze-thaw damage also deepens gradually,which is macroscopically expressed as the deterioration of mechanical parameters such as compressive strength and elastic modulus.When the fracture dip angle was changed from 30° to 90°,the compressive strength,cohesive force and elastic modulus were characterized by increasing changes,and the freeze-thaw coefficient was reduced,while the internal friction angle and Poisson’s ratio were not showed obvious change patterns.When the fracture depth to height ratio gradually was increased from 1/3 to1,the compressive strength,elastic modulus,cohesion and Poisson’s ratio of the fracture specimen with fracture depth to height ratio of 1/2 were significantly larger than the other two.When the intermediate principal stress gradually was increased from 10 MPa to 20 MPa,the compressive strength,frost resistance coefficient and elastic modulus of the fracture specimens increased significantly,while the Poisson’s ratio was gradually decreased in general.For the fracture specimens,the macroscopic penetration pattern was mainly composed of secondary coplanar cracks,wing cracks and secondary inclined cracks,and the overall macroscopic penetration pattern was not significantly different.(3)In the study of damage instantiation equation,the fine damage of fractured rock body under freeze-thaw and loading conditions is modified from two perspectives,so as to obtain the fine damage instantiation model under true triaxial stress conditions of freeze-thaw fractured rock body;the original macroscopic damage variable equation of fractured rock body is improved,and the macroscopic damage coupling relationship is combined to obtain the macroscopic damage instantiation model of fractured rock body under the coupling effect of freeze-thaw and true triaxial.