Theoretical Damage Models Of Porous Rocks And Hard Jointed Rocks Subjected To Frost Action And Further Experimental Verifications

Frost decay of rocks in cold regions has induced a series of engineering problems. Besides, extreme cold waves and serious snow-ice disasters rage more frequently around the world within the last two decades, causing enormous casualties and property losses. Therefore, study of the frost damage mechanisms of rocks is beneficial to solving freezing-related issues in engineering constructions and to coping with geo-hazards triggered by extremely cold weathers. In the present study, theoretical frameworks of the frost damage mechanisms of both porous rock and hard jointed rocks are constructed, taking sandstone and jointed granite as study samples respectively.To begin with, energy conversion, mass conservation, deformation compatibility and stress balance conditions during water freezing in both closed and connected pores were studied, upon which the theoretical model of crack growth driven by volumetric expansion process. Influence of pore structure of sandstone on the frost damage process was then discussed, a new concept named "Feature Frost Damage Unit" was introduced to describe the above influence; an unified theoretical framework was proposed according to thermodynamic analysis of water freezing in pores, through which frost damage mechanisms of porous rock under different freezing conditions can be distinguished; the above theoretical analysis was verified by frost damage experiments of sandstone. Afterwards, freeze-thaw actions were categorized based on their duration and water availability, and were further equaled to different types of fatigue loads; Fatigue damage theory was then employed to establish a theoretical model describing the damage evolution law of porous rock subjected to repeated frost action. Then, a middle-size model experiment was conducted to investigate the deformation response of crack upon freezing and the underlying mechanisms of propagation under different freezing patterns, the model is a granite block containing an artificial crack. In the end, the process of sealing a crack during frost wedging was analyzed, particularly, growth, deformation and movement of the ice plug; moreover, some key issues in stress corrosion cracking were discussed; a primarily theoretical framework depicting the mechanical processes in frost wedging was proposed in this part.The above study indicates that: ①mechanical work of volumetric expansion during freezing is the energy source of crack propagation within the frame of volumetric expansion mechanism; in closed pores, the higher the elastic modulus and tensile strength of rock are, the bigger frost pressure in elastic stage is, and the faster the decay rate of frost pressure over crack propagating is; while in connected pores, frost pressure has a positive correlation with freezing rate, but a negative correlation with permeability and seepage path.②rocks characterized by ’Main Stem Pore Structure’ are more susceptible to frost damage; freezing rate predominately determines the frost damage mechanisms of sandstone, when freezing rate is low the capillary pressure mechanism and crystallization pressure mechanism lead the damage process; while when freezing rate is high, volumetric expansion mechanism and hydraulic pressure mechanism. ③three types of frost damage can be detected at micro-scale:crack growth, particle detachment and particle rotation, and the first one is the major type; deformation of sandstone samples during freezing is not isotropic, furthermore, frost damage emerges primarily perpendicular to the bedding layer.④In natural environments, saturation degree of rocks controls the intensity of frost damage within diurnal freeze-thaw action, and migration rate of unfrozen water plays that role within annual freeze-thaw action; damage variables defined by flaw area and residual strain can be unified by the change of porosity; tested damage accumulation data of sandstone agrees well with the calculated values through the model. ⑤There are three basic freezing modes in natural environments, namely top-down, inside-out and bidirectional; crack widening under top-down mode is much bigger than under inside-out mode; within diurnal freeze-thaw action, thermal bending of the rock beam and frost wedging process should be responsible for crack propagation under top-down mode, while under inside-out mode, thermal bending of the rock beam and frost pressure induced by both volumetric expansion and ice segregation have the potential to drive crack propagation; within long-term freeze-thaw action under top-down mode, stress corrosion process was assumed as the damage mechanism according to the calculation of ice pressure. ⑥During frost wedging process, strength of the seal, consisting of material seal and mechanical seal, determines the magnitude of frost pressure; the mechanical seal is controlled by the ice plug sliding along crack walls; stress corrosion cracking at subzero temperatures is possible according to thorough demonstrations.